Oversized HVAC?

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XtremeSki2001
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Oversized HVAC?

Post by XtremeSki2001 » Mon Aug 24, 2015 8:16 am

Hi All,

Always appreciate the opinions of our knowledgeable user base. Would love some input / thoughts on this situation.

Had a installer out in December 2014 to install a new Carrier HVAC system (furnace and AC). Some small install issues that were quickly remediated. During the summer months I noticed the AC short-cycling ... often running for 6-8 minutes, off for the same, back on, etc. This occurs four times an hour and seems to occur independent of the outdoor temp, humidity, or time of day. Fortunately, the temp / humidity levels indoors have been maintaining.

Long story short, I've escalated the issue to Carrier and now Carrier and the installer are coming to my home to inspect the system. My premise is that the system is oversized, resulting in short-cycling, which is energy inefficient and may lead to increased repair costs due to wear and tear. The installers premise is that nothing is wrong and Carrier will support them. Their confidence concerns me to say the least.

All of my correspondence has been documented and I've gone so far to recap phone conversations that I send to them for record keeping. Unfortunately, the only documentation I have to prove my premise is their load calc. The installer used Philadelphia outdoor design temps of 0 (winter) / 95 (summer) when they should have used 15/89 per the ACCA Manual J table.

Unfortunately, I have no idea whether these two numbers are significant enough to swing a size difference in the equipment I was sold. They came to 3.64 Tons based on the incorrect numbers. As a result, I purchased a 4-ton (two stage) unit as carrier does not make 1/2 ton sizes for a two-stage. My house is 2,700sqft two-story colonial (w/ basement) built in 1992.

I spent ~$12k for the entire system. My gut is telling me the installer should have put in a 3-ton (two-stage), but I have no way to prove it. This is not to mention they installed a 100k BTU furnance, which I also now believe is oversized.

The installer came highly recommended to us by more than one person and I can't help but feel we were taken advantage of ... now I'm worried we're stuck. I feel the likelihood of installing the properly sized equipment and refunding me the difference is extremely unlikely and this will end up in court.

Suggestions are welcome!
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cheese_breath
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Re: Oversized HVAC?

Post by cheese_breath » Mon Aug 24, 2015 8:44 am

Of course the installers will say nothing is wrong. But right now you admit you have no evidence to the contrary and are judging by your gut. So I'd wait to see what Carrier says. Hopefully it's a malfunction. Maybe the thermostat? If Carrier backs up the installer I suppose you could get opinions from a couple other installers, but I fear that won't get you too far in court. Good luck.

edit: Where do you live? Is it an exceptionally hot area? My home in southern MI is about 100 sq. ft. smaller than yours with a basement and 4 ton Carrier, and my A/C works fine.
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Re: Oversized HVAC?

Post by jebmke » Mon Aug 24, 2015 8:50 am

I'd give the installers and Carrier time to check things out. We installed a new 4-ton 2-stage system in a 2,500 ranch (Maryland) a couple years ago. Initially there was a bit of a short cycle problem but they found that when they had installed the new thermo, it was backed up to the return plenum so it wasn't reading correctly. The insulated the back of it, sealed it from the plenum and made a couple tweaks in the control settings on the air handler.
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Re: Oversized HVAC?

Post by XtremeSki2001 » Mon Aug 24, 2015 8:54 am

cheese_breath wrote:edit: Where do you live? Is it an exceptionally hot area? My home in southern MI is about 100 sq. ft. smaller than yours with a basement and 4 ton Carrier, and my A/C works fine.


Thanks. Live just outside Philly.

I'd give the installers and Carrier time to check things out. We installed a new 4-ton 2-stage system in a 2,500 ranch (Maryland) a couple years ago. Initially there was a bit of a short cycle problem but they found that when they had installed the new thermo, it was backed up to the return plenum so it wasn't reading correctly. The insulated the back of it, sealed it from the plenum and made a couple tweaks in the control settings on the air handler.


Thanks. Installers have been out twice, sans Carrier. They did find the same issue with the tstat that you had, but their fix didn't make a difference. Hopefully they're overlooking something ... I'd rather that then a rip out / reinstall!
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edge
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Re: Oversized HVAC?

Post by edge » Mon Aug 24, 2015 9:06 am

If it is two stage it really makes no sense that it short cycles. Sounds like a misconfiguration of the settings, not a mis-sizing.
Last edited by edge on Mon Aug 24, 2015 9:22 am, edited 1 time in total.

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Re: Oversized HVAC?

Post by cheese_breath » Mon Aug 24, 2015 9:11 am

Just pulled out my paperwork on my furnace, and it's 100K too. So I think your equipment is sized correctly based on where you live and your size house compared to where I live and my size house.
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Re: Oversized HVAC?

Post by XtremeSki2001 » Mon Aug 24, 2015 9:24 am

Thanks, Cheese. Fortunately, the furnace is a modulating one so even if it was oversized it shouldn't be a significant issue.

edge wrote:If it is two stage it really makes no sense that it short cycles. Sounds like a misconfiguration.


Stage One runs @ 2.7 tons so I'm inclined to agree. Even on design temp days it can cool the house on Stage One via short-cycling. The system doesn't use Stage Two ... at least that I've witnessed.
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Watty
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Re: Oversized HVAC?

Post by Watty » Mon Aug 24, 2015 9:31 am

How does that compare to the size of your old system?

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Re: Oversized HVAC?

Post by XtremeSki2001 » Mon Aug 24, 2015 9:35 am

Watty wrote:How does that compare to the size of your old system?


Old system was a 3.5 ton one-stage. New one is 4-ton (2-stage).
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jebmke
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Re: Oversized HVAC?

Post by jebmke » Mon Aug 24, 2015 9:40 am

Sounds like a control issue to me. Either a settings problem or perhaps a defective component in the controls.

Most of the time our system runs at low stage with blower fairly low volume.
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Re: Oversized HVAC?

Post by smackboy1 » Mon Aug 24, 2015 11:07 am

We have a Carrier system installed in 2010. Very happy with it and it keeps the house comfortable 365 days a year.

If you want to check the installers work, use an online ACCA Manual J calculator and run your own calculations using your own design conditions. Ask the installer for a copy of their Manual J so you can input all their data points. I paid $50 for a calculator and performed my own Manual J before interviewing installers. There maybe free ones available. My numbers matched the Carrier installer very closely, but obviously the design conditions can make a big difference in the loads calculated.

The design temps are supposed to capture 99% of the cold winter temps and hot summer temps for 30 year average. You can look these up for your location. Sometimes installers will slightly alter the design conditions based on experience because they do not want angry customers complaining on the on average 3-4 days coldest/hottest days/year that are beyond HVAC's capacity. If the HVAC has variable speed blower and staged cooling/heating the homeowner will hardly notice it.

E.g. We use Trenton, NJ. For our area the summer design temp is 90F. I can tell you that we've had many more than 3-4 days of 90F+ days this summer. More like 14 days above 90F. Our installer used 91F as the design temp. I'm glad he did.

On our system I do notice that on very humid days, the AC will run just to dehumidify the air even if the air temp inside the house is at the set point. That's normal.
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Re: Oversized HVAC?

Post by Spirit Rider » Mon Aug 24, 2015 1:56 pm

I agree with jebmke, that this sounds more like a configuration or control problem than an oversizing problem. A two stage system is designed to be able to compensate for an oversized second stage. The first stage handles the efficiency and humidity control issues and the second stage can provide additional capacity.

smackboy1 is also correct that contractors tend to have a built in bias to slightly oversize (maybe by 1/2 ton), to minimize callbacks. Most Americans do not understand that with a properly sized system, there should be a few days where the A/C runs continuously and maybe doesn't quite keep up.

They want the A/C to be able to drop 10 degrees in an hour and turn their house into a walk-in cooler on the hottest days. They can't accept that 75 degrees with low humidity feels just fine when the outside is 95 - 100 degrees and high humidity.

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Re: Oversized HVAC?

Post by Jim180 » Mon Aug 24, 2015 8:51 pm

Your unit might be oversized but here's something also to think about. When you got the new unit they probably installed a new thermostat. Are you able to adjust the on/off temperature span on the thermostat? What temperature does it come on at and what temperature does it turn off at? When I got my new unit they installed a Honeywell thermostat. Honeywell's are preset at the factory and have a very narrow on/off span which can cause short-cycling. If you have a Honeywell set at 74 degrees the AC will come on at 75 and off at 74. I went and bought a Hunter thermostat and manually adjusted the span to come on at 75 and off at 73. That eliminated my short-cycling problem. You might want to try something like that to see if you still have the problem.

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Re: Oversized HVAC?

Post by XtremeSki2001 » Tue Aug 25, 2015 7:18 am

Thanks all for the feedback. A few responses ...

jebmke wrote:Most of the time our system runs at low stage with blower fairly low volume.


Ours runs on low stage even on days when the temperature is at or above the design temp. Even then it only runs 6-8/off for 7-8 minutes and does this four (4) times an hour.

smackboy1 wrote:If you want to check the installers work, use an online ACCA Manual J calculator and run your own calculations using your own design conditions. Ask the installer for a copy of their Manual J so you can input all their data points. I paid $50 for a calculator and performed my own Manual J before interviewing installers. There maybe free ones available. My numbers matched the Carrier installer very closely, but obviously the design conditions can make a big difference in the loads calculated.

[snip]

On our system I do notice that on very humid days, the AC will run just to dehumidify the air even if the air temp inside the house is at the set point. That's normal.


We did check the calculation and their design temps were not compliant with the ACCA design temp standard. Even on the most hot/humid days, our unit will not run for long than 6-8 minutes. Somehow, the humidity in the house is still low.

[quote"Spirit Rider"]I agree with jebmke, that this sounds more like a configuration or control problem than an oversizing problem. A two stage system is designed to be able to compensate for an oversized second stage. The first stage handles the efficiency and humidity control issues and the second stage can provide additional capacity.

smackboy1 is also correct that contractors tend to have a built in bias to slightly oversize (maybe by 1/2 ton), to minimize callbacks. Most Americans do not understand that with a properly sized system, there should be a few days where the A/C runs continuously and maybe doesn't quite keep up.

They want the A/C to be able to drop 10 degrees in an hour and turn their house into a walk-in cooler on the hottest days. They can't accept that 75 degrees with low humidity feels just fine when the outside is 95 - 100 degrees and high humidity.[/quote]

We've had two different techs inspect the install. One was the installation tech and the other was the master tech. They've assured us the system is operating normal and what we're experiencing is not short cycling. They essentially don't think anything is wrong.

Our unit does not go into the second stage unless I decrease the set temp suddenly by 3+ degrees. Our system has never run for more than 10 minutes.

Jim180 wrote:Your unit might be oversized but here's something also to think about. When you got the new unit they probably installed a new thermostat. Are you able to adjust the on/off temperature span on the thermostat? What temperature does it come on at and what temperature does it turn off at? When I got my new unit they installed a Honeywell thermostat. Honeywell's are preset at the factory and have a very narrow on/off span which can cause short-cycling. If you have a Honeywell set at 74 degrees the AC will come on at 75 and off at 74. I went and bought a Hunter thermostat and manually adjusted the span to come on at 75 and off at 73. That eliminated my short-cycling problem. You might want to try something like that to see if you still have the problem.


We have Carrier's Infinity tstat, which is Carrier's top line of tstats. However, there is no way for me to adjust the sensitivity of the tstat. My thoughts are a properly sized HVAC should take more than 7 minutes to reach the set temp, without the tstat tricking it to do so.
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Re: Oversized HVAC?

Post by CassiusRex » Tue Aug 25, 2015 8:27 am

I got a 3 ton (Rheem) replaced last year in Florida in the middle of summer for $4400. If I spent 12 grand, I'd expect it to run like a Rolex (not that my system has had any issues).

You might make sure your thermostat is in a good spot. My old one was practically in the kitchen so they moved it when the new system went in. Didn't want it kicking on just because the oven was on.

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Re: Oversized HVAC?

Post by XtremeSki2001 » Tue Aug 25, 2015 8:48 am

CassiusRex wrote:I got a 3 ton (Rheem) replaced last year in Florida in the middle of summer for $4400. If I spent 12 grand, I'd expect it to run like a Rolex (not that my system has had any issues).

You might make sure your thermostat is in a good spot. My old one was practically in the kitchen so they moved it when the new system went in. Didn't want it kicking on just because the oven was on.


Thank you! To be clear, we did get an AC, Furnance, Humidifier, etc. Their quote was a little higher, but not too far outside the other two quotes we received.

I think moving the tstat would be interesting or somehow adding two.
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Re: Oversized HVAC?

Post by 4nursebee » Tue Aug 25, 2015 9:16 am

What is the indoor humidity?
Is the humidity inside the same now as with the old system?
How many times per hour did the old system go on and off?

6-8 minute run times, four times an hour seems okay to me. What qualifies you to judge this as short cycle?
If the house is cooling and moisture is being pulled out of the air, all is good.
An oversized system should cool but not make it comfortable as it does not run long enough to pull the moisture out.

I wish we could link this discussion to all the other HVAC discussions. I can buy and have a 3.5ton Goodman delivered to my doorstep this week for less than 3k, installed for another k.
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Re: Oversized HVAC?

Post by XtremeSki2001 » Tue Aug 25, 2015 9:27 am

4nursebee wrote:What is the indoor humidity?
Is the humidity inside the same now as with the old system?
How many times per hour did the old system go on and off?

6-8 minute run times, four times an hour seems okay to me. What qualifies you to judge this as short cycle?
If the house is cooling and moisture is being pulled out of the air, all is good.
An oversized system should cool but not make it comfortable as it does not run long enough to pull the moisture out.

I wish we could link this discussion to all the other HVAC discussions. I can buy and have a 3.5ton Goodman delivered to my doorstep this week for less than 3k, installed for another k.


Indoor humidity is 45-50%. I did not have a way to measure humidity with the old system, but it "seems" more comfortable in the home after installing the new system (at least downstairs, upstairs seems the same or worse). The old unit would stay on for longer periods of time, but I honestly never paid much attention to it.

The reason I was surprised by 6-8 minutes is because the salesman told me on the hottest days the AC will often run for most of the day. Energy efficiency with AC units increases over longer run times. Shorter run times increases wear and tear and reduces the life of the equipment.

Consider this article from energy star, which actually mentions the problem with shorter run times. http://www.energystar.gov/ia/home_improvement/home_sealing/RightSized_AirCondFS_2005.pdf
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Re: Oversized HVAC?

Post by 4nursebee » Tue Aug 25, 2015 9:49 am

45-50%? I'd be happy. Relax.
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Re: Oversized HVAC?

Post by Jim180 » Tue Aug 25, 2015 10:17 am

XtremeSki2001 wrote:We have Carrier's Infinity tstat, which is Carrier's top line of tstats. However, there is no way for me to adjust the sensitivity of the tstat. My thoughts are a properly sized HVAC should take more than 7 minutes to reach the set temp, without the tstat tricking it to do so.

Well, I will say this. When my unit starts running on a hot day it only takes about 7 minutes to drop 1 degree (from 75to 74 in my case), so mine would also only run 7 minutes with a 1 degree temperature span. However when I use a 2 degree temperature span (75 to 73) my unit runs 30-40 minutes on one cycle on a hot day. My unit may be oversized by perhaps a half ton. I initially had a 2 ton unit installed because I live in a small home (960 sq.ft.). The unit could not keep up so they came back and put in a 3 ton. In your case you say they do not have half ton sizes. Wasn't there any label on the old unit that showed the size of that one?

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Re: Oversized HVAC?

Post by snodog » Tue Aug 25, 2015 10:22 am

If low stage is 2.7 tons and it sounds like it only runs about 1/2 the time on design temperature days which means maybe you only need 1.5 tons of cooling. You must have a very well insulated house. Are they sure it is running on low stage? But it is possible that is all you may need because I'm about an hour west of you and my 2 ton unit only runs half the time on the hottest days and my house is 1740 sq. foot. And my 40k furnace only runs half the time on the coldest days.

You have a right to be upset because you paid extra for a two stage that really works the same as a one stage unit. If it was me and since your humidity isn't a problem I would ask for a refund of the price difference between a two stage and single stage. That is the least they can do IMO. Hope it all works out for you.

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Re: Oversized HVAC?

Post by XtremeSki2001 » Tue Aug 25, 2015 11:03 am

snodog wrote:If low stage is 2.7 tons and it sounds like it only runs about 1/2 the time on design temperature days which means maybe you only need 1.5 tons of cooling. You must have a very well insulated house. Are they sure it is running on low stage? But it is possible that is all you may need because I'm about an hour west of you and my 2 ton unit only runs half the time on the hottest days and my house is 1740 sq. foot. And my 40k furnace only runs half the time on the coldest days.

You have a right to be upset because you paid extra for a two stage that really works the same as a one stage unit. If it was me and since your humidity isn't a problem I would ask for a refund of the price difference between a two stage and single stage. That is the least they can do IMO. Hope it all works out for you.


House is a Toll Brother's home, built in 1992, and faces South. To the South we have three Sycamore trees blocking a lot of sun. Have R-19 in the attic. Fans in every room. It drafty a bit in the winter, but in the summer it does a good job. In fact, the salesman commented when he did a walkthrough of our home that he couldn't believe the minimal temperate difference between first floor / second floor. Energy audit says otherwise, but I know what I feel :happy

Old unit was a 3.5 ton. I know it's running on low because I can hear the difference and my tstat tells me what stage it's in. Had they installed a 3 ton two-stage then stage 1 would be 1.7 tons and stage 2 would run most the day in stage 2, which is appropriate for a given design temp.

My primary reason for being upset is (1) incorrect design temps used in load calc, (2) poor communication, and (3) being told everything is fine/nothing is wrong.

Thanks for the input - I like your idea about the two-stage refund. That's been one of my issues all along. Why did I pay for a two-stage if it's not needed?
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Re: Oversized HVAC?

Post by FlyingMoose » Tue Aug 25, 2015 11:07 am

If you have an electronic thermostat, read the manual. They use pulse width modulation, and you can set the pulse width (cycle time) in most of them. If yours can't you can buy one that can.

It's not the system's fault, it's just doing what the thermostat tells it.

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Re: Oversized HVAC?

Post by FlyingMoose » Tue Aug 25, 2015 11:10 am

Also, if it's not doing two-stage, that's usually a setting in the thermostat as well...

If you don't have the manual, you can probably find it online.

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Re: Oversized HVAC?

Post by Grt2bOutdoors » Tue Aug 25, 2015 11:17 am

Jim180 wrote:Your unit might be oversized but here's something also to think about. When you got the new unit they probably installed a new thermostat. Are you able to adjust the on/off temperature span on the thermostat? What temperature does it come on at and what temperature does it turn off at? When I got my new unit they installed a Honeywell thermostat. Honeywell's are preset at the factory and have a very narrow on/off span which can cause short-cycling. If you have a Honeywell set at 74 degrees the AC will come on at 75 and off at 74. I went and bought a Hunter thermostat and manually adjusted the span to come on at 75 and off at 73. That eliminated my short-cycling problem. You might want to try something like that to see if you still have the problem.


Same, I have a Honeywell thermostat and just as you say, if you set it to 74, it will come on at 75 and go off at 74, but you can program to go on at 75 and off at 73 - I must say though, 73 in my house would be like an icebox, I have a well-insulated home.
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Re: Oversized HVAC?

Post by Grt2bOutdoors » Tue Aug 25, 2015 11:18 am

XtremeSki2001 wrote:
snodog wrote:If low stage is 2.7 tons and it sounds like it only runs about 1/2 the time on design temperature days which means maybe you only need 1.5 tons of cooling. You must have a very well insulated house. Are they sure it is running on low stage? But it is possible that is all you may need because I'm about an hour west of you and my 2 ton unit only runs half the time on the hottest days and my house is 1740 sq. foot. And my 40k furnace only runs half the time on the coldest days.

You have a right to be upset because you paid extra for a two stage that really works the same as a one stage unit. If it was me and since your humidity isn't a problem I would ask for a refund of the price difference between a two stage and single stage. That is the least they can do IMO. Hope it all works out for you.


House is a Toll Brother's home, built in 1992, and faces South. To the South we have three Sycamore trees blocking a lot of sun. Have R-19 in the attic. Fans in every room. It drafty a bit in the winter, but in the summer it does a good job. In fact, the salesman commented when he did a walkthrough of our home that he couldn't believe the minimal temperate difference between first floor / second floor. Energy audit says otherwise, but I know what I feel :happy

Old unit was a 3.5 ton. I know it's running on low because I can hear the difference and my tstat tells me what stage it's in. Had they installed a 3 ton two-stage then stage 1 would be 1.7 tons and stage 2 would run most the day in stage 2, which is appropriate for a given design temp.

My primary reason for being upset is (1) incorrect design temps used in load calc, (2) poor communication, and (3) being told everything is fine/nothing is wrong.

Thanks for the input - I like your idea about the two-stage refund. That's been one of my issues all along. Why did I pay for a two-stage if it's not needed?


R-19 in the attic seems light? Is that what is recommended for your geographic region?
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Re: Oversized HVAC?

Post by Kosmo » Tue Aug 25, 2015 11:30 am

I was advised against a 2 stage system for a 2 story house because of short cycling. If the thermostat is on the bottom floor (and especially near a register) it will short cycle the 1st stage because the air temp is getting to the commanded value quickly. Have you tried closing all the 1st floor registers to force the air upstairs?

I just had a new furnace and AC installed in June and have a 3.5 ton single stage.

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Re: Oversized HVAC?

Post by dbCooperAir » Tue Aug 25, 2015 11:42 am

A little more food for thought.

Not enough return air always seems to be problem, more so in older homes. You may trying to heat 2,700 sq ft but if you have supply air in the basement with return air (fairly common around here) you can have some issue as you described.

I replaced my furnace last year with the exact same size that came out, house was built in the mid 80's. New furnace was so starved for return air that it would go out on high limit. I added a 10" round return air in the lower level and it made it 10 times better. The newer furnace seems to have a longer run time as well. Went so far to back down the gas pressure (to the end of the furnace spec) in the new furnace to knock some of the heat down. In end after a few tweaks it runs like a champ.

I don't think the newer furnaces can handle as much static pressure as the older ones, add that to the fact most contractors seem to squeeze down the return air can create some issues with a furnace replacement.

As far as the A/C I undersized it a tad to get a longer run time but we only cool up hear a few weeks a year.
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Re: Oversized HVAC?

Post by XtremeSki2001 » Tue Aug 25, 2015 1:38 pm

Grt2bOutdoors wrote:R-19 in the attic seems light? Is that what is recommended for your geographic region?


It is light ... we should have ~R49. That's a separate project. :D

Kosmo wrote:I was advised against a 2 stage system for a 2 story house because of short cycling. If the thermostat is on the bottom floor (and especially near a register) it will short cycle the 1st stage because the air temp is getting to the commanded value quickly. Have you tried closing all the 1st floor registers to force the air upstairs?

I just had a new furnace and AC installed in June and have a 3.5 ton single stage.


Wouldn't short-cycling still occur, but you'd just save money on not buying the two-stage?

Never tried closing all the registers. Might be worth trying.

dbCooperAir wrote:A little more food for thought.

Not enough return air always seems to be problem, more so in older homes. You may trying to heat 2,700 sq ft but if you have supply air in the basement with return air (fairly common around here) you can have some issue as you described.

I replaced my furnace last year with the exact same size that came out, house was built in the mid 80's. New furnace was so starved for return air that it would go out on high limit. I added a 10" round return air in the lower level and it made it 10 times better. The newer furnace seems to have a longer run time as well. Went so far to back down the gas pressure (to the end of the furnace spec) in the new furnace to knock some of the heat down. In end after a few tweaks it runs like a champ.

I don't think the newer furnaces can handle as much static pressure as the older ones, add that to the fact most contractors seem to squeeze down the return air can create some issues with a furnace replacement.

As far as the A/C I undersized it a tad to get a longer run time but we only cool up hear a few weeks a year.


I had the same company do an energy audit and they were quite comprehensive. They installers have read it and haven't noted any significant issues that would drive the issues we're saying. Maybe they need to look again.
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Re: Oversized HVAC?

Post by jebmke » Wed Aug 26, 2015 9:10 am

Out of curiosity I did a time test last evening. It was a moderate day yesterday - mid 80s. I got the following results ~ 7-8PM

System ran for 6-7 minutes
Off for 9-10
Ran for 6-7
Off for 9-10

I assume it was running on first stage; the fan did not run at high speed. But it rarely does in cool mode except in very sustained hot weather in the late afternoon (we have western exposure with a fair amount of glass (low-e). Normally as it gets warm, the cycle time extends out.

Our system is a 2-stage heat pump. I do recall that the tech who sized it said that in our area they size for the heat load and the resulting A/C is normally on the high side of needs.

Our house is very tight. We sealed a lot of air leaks, added a ton of insulation in the attic and replaced most of the glass on the western side with new low-e windows and sliding doors.
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Re: Oversized HVAC?

Post by XtremeSki2001 » Wed Aug 26, 2015 9:45 am

jebmke wrote:Out of curiosity I did a time test last evening. It was a moderate day yesterday - mid 80s. I got the following results ~ 7-8PM

System ran for 6-7 minutes
Off for 9-10
Ran for 6-7
Off for 9-10


According to Energy Star, from the article I quoted above:

"It is common for contractors to install oversized air conditioners because these units provide cooling more quickly, thus avoiding any chance of not meeting the cooling demand. However, oversized air conditioners "short-cycle" or run for shorter periods of time than engineered for optimum operation. The efficiency of air conditioners is low when they first start and increases gradually, reaching peak efficiency in about 10 minutes. As shown in Figure 1 below, when operating time increases from 5 to 9 minutes, efficiency improves 17 percent. In this example, the energy efficiency ratio (EER) increases from 6 to 7. In addition, bursts of cold air from oversized units can trick the thermostats into shutting off the system before the whole house is cool. Moreover, short operation times do not allow the system to effectively remove humidity with serious repercussions on both home comfort and durability."


My system does four-cycles per hour, around the clock, independent of whether it's 75 or 95.

Here's what a more ideally sized HVAC should look like from a run-time perspective. Although this individual had a system he purposely oversized by .5 tons. You'll note during the hottest parts of the day when the outdoor temperate is at or above the design temp the AC runs for over 30-minutes. My unit never runs for more than ~8 minutes, which tends to be evidence (according to energy star) that the unit is bigger than required. This would suggest my unit is significantly oversized.

Image

Here's the cylce times just for perspective. You'll note the maximum on-time was ~15 minutes, versus a minimum on-time of ~7 minutes. Where as my HVAC has a max of ~8 minutes consistently.

Image
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Re: Oversized HVAC?

Post by JohnF » Wed Aug 26, 2015 11:42 am

Knowledge is power. I suspect you’re right on sizing, but as a retired engineer I wonder –

1) When the system runs for 7 minutes or whatever duration how long is the compressor (outside unit) actually running? And on a hot day what percent of time is the compressor running and at what level(s)?

2) What is the temperature swing in tenths of degree at the thermostat during the cycles? (If you don’t have one, Amazon has cheap digital thermometers that register in tenths.)

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Re: Oversized HVAC?

Post by XtremeSki2001 » Wed Aug 26, 2015 12:33 pm

JohnF wrote:Knowledge is power. I suspect you’re right on sizing, but as a retired engineer I wonder –

1) When the system runs for 7 minutes or whatever duration how long is the compressor (outside unit) actually running? And on a hot day what percent of time is the compressor running and at what level(s)?

2) What is the temperature swing in tenths of degree at the thermostat during the cycles? (If you don’t have one, Amazon has cheap digital thermometers that register in tenths.)


You're right in that there is mini-cycles for each cycle I reference above. For example, the condenser will run for 1-2 minutes to cool the water and the fan will blow for ~5-6 minutes after that. My timing has focused on the entire process, not just the active cooling portion.

Unfortunately, my tstat does not show tenths of degrees nor does it allow me to determine (or change) how sensitive it is. If the unit is not oversized then certainly the tstat is to blame.
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Re: Oversized HVAC?

Post by ralph124cf » Wed Aug 26, 2015 10:10 pm

If you haven't already tried it, you might consider setting the fan to constant on. This constant air turnover keeps the temp throughout the house more even, and will reduce the perception (but not the fact) of the AC cycling.

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Re: Oversized HVAC?

Post by RCL » Thu Aug 27, 2015 11:58 am

I would consider moving the Tstat to a different location, possibly to one that is not in a direct air-flow location.
I don't know anything about your Tstat, but the "deadband" setting makes a big difference.
A chance your Tstat is mal-functioning?
Consider finding a retailer with a good return policy and purchase a different Tsat. Try it out, if it doesn't do better, return it.
Out here in Calif., those of us that have 2 story houses, like to use multiple Tstats, one upstairs, one downstairs that are bluetooth linked to help average the house temperatures.

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Re: Oversized HVAC?

Post by Grt2bOutdoors » Thu Aug 27, 2015 12:18 pm

XtremeSki2001 wrote:
JohnF wrote:Knowledge is power. I suspect you’re right on sizing, but as a retired engineer I wonder –

1) When the system runs for 7 minutes or whatever duration how long is the compressor (outside unit) actually running? And on a hot day what percent of time is the compressor running and at what level(s)?

2) What is the temperature swing in tenths of degree at the thermostat during the cycles? (If you don’t have one, Amazon has cheap digital thermometers that register in tenths.)


You're right in that there is mini-cycles for each cycle I reference above. For example, the condenser will run for 1-2 minutes to cool the water and the fan will blow for ~5-6 minutes after that. My timing has focused on the entire process, not just the active cooling portion.

Unfortunately, my tstat does not show tenths of degrees nor does it allow me to determine (or change) how sensitive it is. If the unit is not oversized then certainly the tstat is to blame.


Cool the water? :confused You mean cool the Puron or other compatible gas? This is the first I'm hearing of an A/C condenser cooling water. Usually the A/C if it has a de-humidifier built in will remove the water from the atmosphere and pump it to a drain or depending on location directly into a gutter away from the home.
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Re: Oversized HVAC?

Post by XtremeSki2001 » Thu Aug 27, 2015 12:26 pm

Grt2bOutdoors wrote:Cool the water? :confused You mean cool the Puron or other compatible gas? This is the first I'm hearing of an A/C condenser cooling water. Usually the A/C if it has a de-humidifier built in will remove the water from the atmosphere and pump it to a drain or depending on location directly into a gutter away from the home.


I have no idea what the condenser cools and I shouldn't be trying to articulate what it does :happy
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Re: Oversized HVAC?

Post by Green Nut » Fri Aug 28, 2015 7:05 am

Two things I've done that has made my comfort greater during the summer and satisfied my gut feeling for a/c run-time.

#1 I installed a better t-stat where I had better control of cycles per hour (CPH). My CPH was originally set for 6, way too many on/off cycles for me, greatest stress on the unit is on/off. I could also set the temp differential for when the second stage kicks in. I'm set for 2 CPH and it is near perfect, my highest electric bill this summer has been about $85, last year was ~$120 - I know temps and rates make that comparison fuzzy.

#2 I was able to buy wireless sensors that I located around the house, the t-stat averages all the temps from all the sensors; I have 3 which includes the t-stat as a sensor. Before the sensors the room with the t-stat reached temp before the rest of the house and the a/c shut down. I would've thought the coldest room before the sensor additions would be an ice box now, but the whole house is a nice even temp now. This works great for my mod/con heating system too.

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Re: Oversized HVAC?

Post by XtremeSki2001 » Tue Sep 01, 2015 6:00 am

Thought I'd post a follow-up.

The Carrier rep visit my home and could not explain why the unit was running for 10 minutes a pop on a day where the temp was at design. He said it should be running in the second stage and running nearly continuously. Carrier would not assert whether the equipment was sized correctly.

The installer ran an updated manual J using the ACCA-compliant design temperatures. The result demonstrated they oversized my unit by nearly a ton (I say nearly, because it's a two-stage).

I had a third-party come and re-perform a manual J as well as a manual S for sizing. The 3-ton two-stage version of my 4-ton unit meets all the cooling requirements. They also identified I only needed 72k btus of heating, but the installer told me I needed a 100k unit.

Working with the installer now to replace the units with properly sized equipment. I suspect I'll see at least $3k back in my pocket as well. It's disappointing the amount of work and research I had to do to prove to the "experts" they were wrong.
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Re: Oversized HVAC?

Post by Boglenaut » Tue Sep 01, 2015 6:14 am

Thanks for the update. I've been following the thread but did not make a post because I am not familiar with 2-stage units.

Our house was built with a 3.5 ton unit (one stage). When it died a few years ago, both estimates came back and recommended a 3 ton unit (one stage), so that is what we got.

Between the unit being new, having a better efficiency rating, and being smaller, our cooling bills went way down. It cools the house just fine and is more comfortable. For us, the 3 ton unit is much better than the 3.5 ton unit. I think it is most noticeable on the second floor. The first floor is still a lot cooler than the second, but not as much now.

Air conditioners are counter-intuitive in the it is better to be a bit undersized vs oversized (if you cannot hit it exactly). Most things work better when they have excess capacity (no one complains that their computer is too fast or their car can easily go uphill fast), but the opposite is true in cooling. It is not only more efficient, it actually works better.

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Re: Oversized HVAC?

Post by XtremeSki2001 » Thu Oct 22, 2015 3:12 pm

Long story short, but our installer has agreed to remove our 4-ton 2-stage unit for the same model in a 3-ton 2-stage unit. We had two independent opinions and both confirmed our equipment was over sized. The over sizing resulted from an improper manual J by the original installer and they never performed a manual S.

I encourage owners in a similar position to consult the ACCA's Residential Quality Installation Checklist during the quote/estimate process. The contractor should provide a Manual J and Manual S and provide it to you for review prior to selecting them to do an equipment replacement.

https://www.acca.org/communities/community-home/librarydocuments/viewdocument?DocumentKey=8410020b-17e9-4a7a-a12d-e95a7c1c72f1

The longer version of the story is although contractors freely use ACCA on their website, the ACCA does not monitor compliance even when a dispute is raised. Most townships have building codes requiring the performance of Manual J and Manual S, but my township doesn't request the documentation when approving the HVAC permit. Although many contractors are Factory Authorized Dealers, the factory (e.g., Trane, Carrier, etc.) does not get involved in sizing disputes and the factory reps are the same people the contractors by equipment from so they're indebted to the contractor not the consumer.
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