Central Air Sizing Question - Buying a Condo

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Central Air Sizing Question - Buying a Condo

Postby Ketawa » Fri Jul 26, 2013 8:20 pm

I am currently in the process of buying a condo in Norfolk, VA. Some relevant details:

The building was built around 1904.
The conversion to condos was completed sometime in the 90s.
My unit is on the third floor and is about 1400 sq ft. This is the top floor.

During the inspection several weeks ago, the inspector noted that the air handler inside the building was not matched to the compressor outside at ground level. The air handler was 8 SEER and the compressor was 13 SEER. In addition, the temperature differential was on 4 degrees, when the recommended range is 14-22. I asked the seller to replace the air handler to match the compressor, as well as fix the rest of the minor issues in the home, expecting that they would decline replacing the handler and take care of the smaller issues. No harm in asking, right?

I expected the seller to decline replacing the air handler and fix everything else. To my surprise, he offered to replace the air handler if I would take care of everything else. This is the current system in the condo, it's 1.5 tons.
Goodman GSH13 Heat Pump - installed in 2008.
Goodman ARUF SmartFrame Air Handler - new.

During the walkthrough today, cooling performance was very poor in my (amateur) opinion. The outdoor temperature was about 82, and the thermostat had been set to 80. I set the thermostat to 70 and it took 25 minutes to cool 1 degree. It took about 3 hours running constantly to cool 6 degrees to 74.

Is this performance expected of a system like this? My initial Googling leads me to believe that a 1.5 ton system might simply be too small for a 1400 sq ft condo on the 3rd floor of a building, and I will have to deal with it. I might look into upgrading both down the road, but at this point, I will not let something like that make the deal fall through. I highly doubt the seller would be willing to replace both the heat pump and air handler now.

An additional issue was that there was constant dripping at the corner of the air handler around the drainage pipe. This leads me to believe the installation was poor workmanship. The seller is having the contractor who installed the system fix that issue (haha, but at least he'll fix his work).

Should I see if I can get someone to look at the temperature differential of the system? Are there any other measures I should take?

Any advice from the esteemed Bogleheads would be greatly appreciated. Thanks!
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Re: Central Air Sizing Question - Buying a Condo

Postby LadyGeek » Fri Jul 26, 2013 9:03 pm

This thread is now in the Personal Consumer Issues forum (sizing central air capacity).
To some, the glass is half full. To others, the glass is half empty. To an engineer, it's twice the size it needs to be.
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Re: Central Air Sizing Question - Buying a Condo

Postby Ketawa » Fri Jul 26, 2013 9:14 pm

Ah, thanks. I forgot to post in the correct forum. :oops:
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Re: Central Air Sizing Question - Buying a Condo

Postby rfburns » Fri Jul 26, 2013 9:21 pm

I expected the seller to decline replacing the air handler and fix everything else. To my surprise, he offered to replace the air handler if I would take care of everything else. This is the current system in the condo, it's 1.5 tons.
Goodman GSH13 Heat Pump - installed in 2008.
Goodman ARUF SmartFrame Air Handler - new.

Wow! That raises red flags. Your link to that 2008 heat pump shows an R-22 system.
Your link to the AHU shows the new R-410A. What is important is that the indoor coil and outdoor unit match in btu's, refrigerant type, and SEER and HPSF ratings.

Now the bomb. IMHO 1.5 tons of cooling is inadequate given your description. I would guestimate 2 tons minimum and perhaps half a ton more if your space is old and lacking insulation.
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Re: Central Air Sizing Question - Buying a Condo

Postby Saving$ » Fri Jul 26, 2013 9:29 pm

To figure out the size of unit you need to do a manual J calculation. 1.5 tons is probably plenty if the condo does not have 4 outside walls, and has a conditioned space below it.
http://www.loadcalc.net/
READ THE INSTRUCTION before you start entering data. Read them carefully.

My guess on why it is not cooling properly is that the indoor and outdoor units are not matched. Perhaps the lineset is too small. I'm not clear if the new air handler is what the seller just installed, or if you are looking for one of the two parts you listed to be replaced. Your best bet is to head over to http://hvac-talk.com/vbb/forum.php and post your question. Include as much info as you can, such as the model numbers below, the size of the lineset between the two units, etc.
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Re: Central Air Sizing Question - Buying a Condo

Postby Ketawa » Fri Jul 26, 2013 9:38 pm

rfburns wrote:Wow! That raises red flags. Your link to that 2008 heat pump shows an R-22 system.
Your link to the AHU shows the new R-410A. What is important is that the indoor coil and outdoor unit match in btu's, refrigerant type, and SEER and HPSF ratings.

Now the bomb. IMHO 1.5 tons of cooling is inadequate given your description. I would guestimate 2 tons minimum and perhaps half a ton more if your space is old and lacking insulation.


This is the same air handler on Amazon, and it seems to say that it can be used in both R-22 and R-410 systems.

http://www.amazon.com/1-5-Ton-Goodman-Air-Handler/dp/B008L4V0DS

Amazon Listing wrote:Brand new re-designed air handler for use in R-22 and R-410 split systems.
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Re: Central Air Sizing Question - Buying a Condo

Postby Ketawa » Fri Jul 26, 2013 9:52 pm

Saving$ wrote:To figure out the size of unit you need to do a manual J calculation. 1.5 tons is probably plenty if the condo does not have 4 outside walls, and has a conditioned space below it.
http://www.loadcalc.net/
READ THE INSTRUCTION before you start entering data. Read them carefully.

My guess on why it is not cooling properly is that the indoor and outdoor units are not matched. Perhaps the lineset is too small. I'm not clear if the new air handler is what the seller just installed, or if you are looking for one of the two parts you listed to be replaced. Your best bet is to head over to http://hvac-talk.com/vbb/forum.php and post your question. Include as much info as you can, such as the model numbers below, the size of the lineset between the two units, etc.


Unfortunately, the full Manual J calculation may be beyond my means at the moment. If I upgrade the system after purchasing, I will definitely get a professional to do it.

The condo has 3 outside walls, and the space below is conditioned. I can't speak to the insulation above it.

The indoor and outdoor units are matched correctly. The outdoor unit was replaced in 2008. The indoor unit was just replaced 2 weeks ago to match the outdoor unit.

Thanks for the link to that forum, it should be very helpful.
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Re: Central Air Sizing Question - Buying a Condo

Postby wageoghe » Fri Jul 26, 2013 10:21 pm

I agree that the only to really know if the unit is correctly sized is to perform a Manual J. It is not obvious that the unit is too small.

Also, the amount of time it takes to bring the temperature down is not an adequate measure of how effective the system is. A better measure, generally, is how well does it maintain the desired temperature, especially on high demand days.

The HVAC forum linked above is truly an excellent resource. If you have questions about your system, you can ask there. Another good forum is the HVAC forum at http://ths.gardenweb.com/forums/hvac/
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Re: Central Air Sizing Question - Buying a Condo

Postby Spirit Rider » Sat Jul 27, 2013 12:28 pm

Ketawa wrote:I am currently in the process of buying a condo in Norfolk, VA. Some relevant details:

During the walkthrough today, cooling performance was very poor in my (amateur) opinion. The outdoor temperature was about 82, and the thermostat had been set to 80. I set the thermostat to 70 and it took 25 minutes to cool 1 degree. It took about 3 hours running constantly to cool 6 degrees to 74.

Is this performance expected of a system like this? My initial Googling leads me to believe that a 1.5 ton system might simply be too small for a 1400 sq ft condo on the 3rd floor of a building, and I will have to deal with it. I might look into upgrading both down the road, but at this point, I will not let something like that make the deal fall through. I highly doubt the seller would be willing to replace both the heat pump and air handler now.

Should I see if I can get someone to look at the temperature differential of the system? Are there any other measures I should take?

Any advice from the esteemed Bogleheads would be greatly appreciated. Thanks!

It is hard to say if the system is undersize without doing a manual J calculation as has been stated. This is because there are many variables in addition to your climate and being on the third (top) floor. Is the floor below you conditioned?. Since you are the top floor, what is the insulation in the ceiling? How many walls are abutted by conditioned spaces vs. outside walls? What is the insulation on the outside walls? How much window glass area is there and what compass direction are they? So a rule of thumb based on climate and square footage is not enough.

There are many things other than sizing that can affect the cooling capability of an air conditioning system. As has already been mentioned there may be a mismatch between the condenser and the evaporator. Specifically, depending on the size and the metering type of the evaporator the flow of the output of the condenser needs to be matched. This is because a single condenser can be used with more than one evaporator. Usually, a kit comes with the condenser to match to the evaporator. It is possible that the tech failed to correctly do this.

Also, there may be other problems. You would hope that the tech properly balanced the charge in the system after replacing the air handler. Also, you didn't say how old the old air handler was. If this was a case of an even older system where a old failing SEER 8 condenser was replaced with a new SEER 13 condenser, at a minimum the refrigerant should have been entirely recovered and new refrigerant added. I know many techs who will not guarantee the replacement of a condenser without also replacing the evaporator/air handler.

It has been five years since the condenser was replaced, if it hasn't happened in the interim,, the coil probably needs a good cleaning. There could be design or operational problems with air handling. The duct work could be poorly designed or undersized. Are all the vents AND the return open and not blocked. Are the supply vents low and the return vent(s) high. Very often there are speed taps on the blower of the air handler that allow for two or three fan speeds. Maybe the air speed is insufficient.

You don't need someone else to check the temp differential, it is pretty easy to do it yourself. No need to use an HVAC tool. Get a digital meat thermometer. Set it near a return vent and check the ambient temperature. Then drop it in a cooling vent and check the cool air temperature. You should see 20 degrees +- a few degrees.

It very well mean that you need to have a professional come in and do an evaluation. I hope you have friends and family who can give you a reference to an honest and competent HVAC professional. Unfortunately there are a significant minority of crooks and/or incompetent individuals in the HVAC industry.
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Re: Central Air Sizing Question - Buying a Condo

Postby Timoneer » Sat Jul 27, 2013 1:54 pm

Ketawa wrote:During the walkthrough today, cooling performance was very poor in my (amateur) opinion. The outdoor temperature was about 82, and the thermostat had been set to 80. I set the thermostat to 70 and it took 25 minutes to cool 1 degree. It took about 3 hours running constantly to cool 6 degrees to 74.

Is this performance expected of a system like this? My initial Googling leads me to believe that a 1.5 ton system might simply be too small for a 1400 sq ft condo on the 3rd floor of a building, and I will have to deal with it.


I had a 1.5 ton central AC for a small house of approx the same size, and the performance was similar to what you describe. It was definitely a bit undersized, but because it ran on a longer duty cycle to cool the house, it did a very effective job of removing humidity, and we generally were quite comfortable.
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Re: Central Air Sizing Question - Buying a Condo

Postby Calm Man » Sat Jul 27, 2013 7:45 pm

I wouldn't buy it. If anything drips, I let the current owner enjoy it not me. Who knows what else lurks if they can't even have an AC system functioning properly.
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Re: Central Air Sizing Question - Buying a Condo

Postby lws6772 » Sat Jul 27, 2013 8:44 pm

When I was a kid my dad did hvac work and his rule of thumb for sizing was 1 ton per 500 square ft. Of course back then houses weren't as tight and hvac guys tended to err on the side of too big because it was safer(less chance of call backs from disgruntled customers). That being said, our current house is about 1200 sq. ft. and it only has 1.5 tons, but it is an energy star house, so that helps. But personally I would have sized it a little bigger. The drip might be a simple fix. I believe there is a condensate pan below the A-coil(evaporator) and if its drain line is stopped up it will overflow and leak out of the air handler. Or like you said, it could be due to sloppy workmanship, although again possibly a simple fix.
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Re: Central Air Sizing Question - Buying a Condo

Postby Ketawa » Sat Jul 27, 2013 10:19 pm

It turns out the drip was an easy fix - the contractor installed the drainage pipe so it had an upward slope out of the air handler. I should have noticed that right away :oops: . Whether that affects the cooling performance, I'm not sure. I will be checking it out again Monday morning.
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Re: Central Air Sizing Question - Buying a Condo

Postby Spirit Rider » Sat Jul 27, 2013 11:15 pm

Ketawa wrote:It turns out the drip was an easy fix - the contractor installed the drainage pipe so it had an upward slope out of the air handler. I should have noticed that right away :oops: . Whether that affects the cooling performance, I'm not sure. I will be checking it out again Monday morning.

The leak of the drain flow would have no affect on the cooling capacity of the system. As I said with my previous post there could be many reasons for poor performance other than just sizing. However, regardless of how the performance of the system is effected, it is manifested in only two ways, rate of air flow and temperature differential. At a minimum I would use my method of checking the temperature differential.

Other than the base dynamics of the environment of the structure, there is the possibility of some significant air infiltration problems. This could cause serious performance problems. You should check around all windows, doors, and whatever entrance there is to the attic to make sure you are not cooling the outdoors.

Also, if the air handler and duct work is in the attic and is not insulated that could cause significant problems. Also, maybe there was a significant installation problem with the new air handler. For all you know the air flow is being significantly blocked or your conditioned air is being blown into the attic.
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Re: Central Air Sizing Question - Buying a Condo

Postby Call_Me_Op » Sun Jul 28, 2013 8:28 am

I would think you need at least 2 tons for 1400 sq ft.
Best regards, -Op | | "In the middle of difficulty lies opportunity." Einstein
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Re: Central Air Sizing Question - Buying a Condo

Postby wageoghe » Sun Jul 28, 2013 3:07 pm

lws6772 wrote:When I was a kid my dad did hvac work and his rule of thumb for sizing was 1 ton per 500 square ft. Of course back then houses weren't as tight and hvac guys tended to err on the side of too big because it was safer(less chance of call backs from disgruntled customers).


Not to criticize, but a word of advice to any HVAC shoppers who might happen upon this thread...

Do not accept "rule of thumb" sizing. The only way to correctly size HVAC is a Manual J calculation. The calculation takes into account key factors of your home: size, insulation, windows, location, orientation on lot, etc. Even if you are replacing an existing system, insist on a Manual J. It is not necessarily the case that your existing system was correctly sized for your house in the first place.

Also, beware of erring on the size of too big. If your HVAC is too big (more tonnage than you really need), your air conditioning will short cycle (run in relatively short bursts). This will cool your house quickly, but the short run time does not allow for as much humidity to be removed as could be with a longer run time. If you can remove more humidity (with longer run time), you can actually probably maintain a higher temperature than you would expect, and still be comfortable.

As a customer, what complaint could you have that would possibly be addressed by a "too large" system? One common complaint that I see in forums is that a customer feels that his/her HVAC does not cool his/her house quickly enough. More tons will cool faster, but a better judge of how your HVAC performs is the ability to maintain temperature. Theoretically, you would be better off (certainly comfortwise, and probably efficiencywise) maintaining a constant temperature (even when at work). If you set your temperature to, say 78 while you are at work and then to 74 when you are home and then 72 at bedtime, you are asking your unit to drop the temperature 6 degrees in 4-5 hours (assuming 5pm home from work, 10pm to bed) during just about the hottest part of the day. When it is 85-95 outside, that is a tall order for any system.

The bottom line, when considering HVAC sizing:
1. Insist on Manual J.
2. Bigger (than necessary) is NOT better (when considering cooling).

Again, this is not meant as a criticism, just some helpful information for people shopping for HVAC today.
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Re: Central Air Sizing Question - Buying a Condo

Postby Spirit Rider » Sun Jul 28, 2013 7:19 pm

To amplify what wageoghe has said, oversized systems are far more common then undersized systems. Americans always tend to think bigger is better and that just isn't true when it comes to A/C systems.

Unfortunately, HVAC contractors have learned to oversize systems just to save themselves grief on callbacks from displeased customers. So it is very common for contractors to size a system 1/2 ton larger than properly needed. This is so they don't hear "hey my system is running all the time" Uh YEAH, a properly sized system should have a 90%-95% duty cycle at design temperature.

This also means there may be those rare days when a peak temperature will cause the system to be unable to keep up. For example, in northern New England, systems are usually designed to be able to provide a 25 degree outside to inside differential. We maybe average a couple of days above 95 and 100 only occurs a few times a decade. On that rare day that it is 100, your system will only be able to cool your house to 75. You know what, when it is 100 outside, 75 and 50% humidity is still really nice.

Personally, I never set my house to less that 75-77 degrees. Some people expect their house to be a walk-in cooler no matter what the outside temperature is. Many want the system to be able to drop the temperature 5 degrees per hour. All of this causes the contractors to oversize the systems so they don't hear this grief.

However, those people are the same ones who then complain when it is few degrees above their desired temperature with oppressive humidity. Any oversized system simply will not run with enough of a duty cycle to be able to pull the moisture from the air.

Bigger is most definitely not always better when it comes to A/C
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Re: Central Air Sizing Question - Buying a Condo

Postby edge » Mon Jul 29, 2013 9:38 am

Oversized isn't a problem if you have a multiple stage compressor.
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