Central AC replacement decision

Questions on how we spend our money and our time - consumer goods and services, home and vehicle, leisure and recreational activities
User avatar
magellan
Posts: 3474
Joined: Fri Mar 09, 2007 4:12 pm

Re: Central AC replacement decision

Post by magellan »

IMO, the optimal install cost vs SEER tradeoff depends on personal preference and usage patterns. Do you use setbacks or prefer a set-it-and-forget-it approach? Do you like to open the windows when it's coolish or would you rather just have the system keep your home comfortable without any input from you?

When running all out, modern ac systems generally convert electricity into cooling btus at about the same efficiency, regardless of their SEER. The savings with higher SEER systems is from increased cycling efficiency. If you keep the thermostat at 80 during the day, say in bedrooms or while you're at work, then move it down to 70 at night, the difference in the operating cost of a high SEER unit vs a lower SEER unit will be negligible.

As you indicated in your post, very high SEER systems usually have a long payback time here in the north. We don't use the systems as much compared to folks in hotter climates and many of us let the second floor get hot during the day and only cool it at night, a usage pattern that negates most of the benefit of a higher SEER.

There's nothing wrong with paying a little more to reduce your carbon footprint, even if the math doesn't exactly work out. OTOH, in the north you usually get a bigger carbon bang for the buck on the heating side.

Jim
Last edited by magellan on Sun Aug 03, 2014 8:50 am, edited 1 time in total.
User avatar
4nursebee
Posts: 1565
Joined: Sun Apr 01, 2012 7:56 am
Location: US

Re: Central AC replacement decision

Post by 4nursebee »

My Goodman circa 2003 or so has had less than $700 in service work.
Two Goodmans at rental properties were purchased new over the past few years have required no service work and were installed at 1/2 to 1/3 the price of other brands.
My trane at a rental I purchased used has had a coil repair prior to my ownership and required leakstop infusion for the leak again. $400 in service work in less than one year.

Consumer reports rates Tesla Model S as much better than my VW, it does not mean I ignore financial cost vs benefits and buy a Tesla.
Many people value that which they think name brand/highly rated/higher priced products provide to them. That is okay.
There are also people willing to pay >1% expense ratios to their portfolio managers because of what they know (or dont know) and what they perceive as value.

The money I saved on my original system install would have more than doubled by now allowing for free upkeep and replacement.
Pale Blue Dot
User avatar
walkabout
Posts: 634
Joined: Wed May 25, 2011 8:28 am
Location: Northern Alabama

Re: Central AC replacement decision

Post by walkabout »

Re Goodman...

OTOH, our Goodman (our second unit for our finished basement), installed in 2004 or 2005, suffered a burned up (literally) fan controller board last summer while we were on vacation. The system was stuck in the Fan ON position and could only be turned off via the circuit breaker. Fortunately the damage was contained within the compartment where the circuit boards are mounted. Cost us about $200 to repair.

This is not intended to start an argument about whether or not Goodman is a good brand. I'm sure Goodmans are fine. They do make systems that cost less than other manufacturers. But, from what I have read, when I was researching for the heat pump we bought last summer, I believe that Goodman is less reliable than other brands.

Also, keeping the OP in mind... She is replacing her AC and keeping her relatively new (and probably mid to high end) Lennox furnace. I would never recommend mixing brands in a HVAC system. Such a combination might work just fine, but I think that doing that is just looking for trouble, either from a maintenance or a performance perspective.
User avatar
4nursebee
Posts: 1565
Joined: Sun Apr 01, 2012 7:56 am
Location: US

Re: Central AC replacement decision

Post by 4nursebee »

WWJD?
Pale Blue Dot
arkerr123
Posts: 92
Joined: Fri Mar 09, 2012 5:17 pm

Re: Central AC replacement decision

Post by arkerr123 »

wageoghe wrote:Re Goodman...

OTOH, our Goodman (our second unit for our finished basement), installed in 2004 or 2005, suffered a burned up (literally) fan controller board last summer while we were on vacation. The system was stuck in the Fan ON position and could only be turned off via the circuit breaker. Fortunately the damage was contained within the compartment where the circuit boards are mounted. Cost us about $200 to repair.

This is not intended to start an argument about whether or not Goodman is a good brand. I'm sure Goodmans are fine. They do make systems that cost less than other manufacturers. But, from what I have read, when I was researching for the heat pump we bought last summer, I believe that Goodman is less reliable than other brands.

Also, keeping the OP in mind... She is replacing her AC and keeping her relatively new (and probably mid to high end) Lennox furnace. I would never recommend mixing brands in a HVAC system. Such a combination might work just fine, but I think that doing that is just looking for trouble, either from a maintenance or a performance perspective.
Sorry for the off topic conversation - but a similar thing happened to my Goodman system recently. Compressor was stuck off. Had HVAC company out, and suddenly the fan was stuck "On" too. They came back later and "discovered" a burned up circuit board (looked like a wrench was put on the AC side to DC side of the board). They wanted $1100 to replace it... just the circuit board.
Bought a new board off Amazon for $97 and did it myself.
User avatar
Topic Author
dodecahedron
Posts: 5355
Joined: Tue Nov 12, 2013 12:28 pm

Re: Central AC replacement decision

Post by dodecahedron »

Almost exactly six years after my original post at the top of this thread. It was interesting to re-read this thread as I am now finally ready to replace my central AC. What was a 23 year old AC in 2014 is now a 29 year old AC in 2020.

The old AC (a Trane) is still working reasonably well but clearly laboring with the record string of hot days we have had for week after week this summer.

Time to replace. I was pleasantly surprised to discover there are no supply chain issues and there is plenty of labor available that my longtime HVAC company would like to keep busy.

I had a sales rep come over yesterday. I am interested in getting a purifier/filtration/UV light unit installed to reduce virus and mold transmission. (I share my--fortunately large--home with a young adult daughter whose job has recently become high risk.) I had been keeping windows open at night when we are both home but that is not a sustainable ventilation strategy through our bitter Upstate NY winters.

Not surprisingly, given the long hot summer and supply chain issues, the machine I want is in stock but there are no excess inventories and the 5% manufacturer´s rebate I was offered in 2014 is no longer available. The current mfr discount is tiny and token. But that´s okay, the contractor is giving me a good package deal on installing the new AC, the purification/filtration unit, a winter humidifier unit (air gets very dry in the winter), and a replacement water heater (the old one appears to be in bad shape and ready for replacement). Also a new smart thermostat which will allow me to manage winter humidity as well as temperature year round. My 12-year-old Lennox furnace (which has the blower system) is still performing well.

And I have had time to educate myself about air conditioners and also feel some clarity that I intend to age in place here, as well as having learned more about climate change and the environment in the intervening six years, so I felt ready to make a more fully informed choice.

I am going with the Lennox XC21 (two stage)
https://www.lennox.com/products/heating ... M4EALw_wcB

Everything is scheduled for installation on Monday. They will wear masks and keep their distance from me. I will spend the time when they are working in my basement out on my covered porch.
mervinj7
Posts: 1535
Joined: Thu Mar 27, 2014 3:10 pm

Re: Central AC replacement decision

Post by mervinj7 »

dodecahedron wrote: Wed Jul 29, 2020 8:31 pm Almost exactly six years after my original post at the top of this thread. It was interesting to re-read this thread as I am now finally ready to replace my central AC. What was a 23 year old AC in 2014 is now a 29 year old AC in 2020.

The old AC (a Trane) is still working reasonably well but clearly laboring with the record string of hot days we have had for week after week this summer.

Time to replace. I was pleasantly surprised to discover there are no supply chain issues and there is plenty of labor available that my longtime HVAC company would like to keep busy.

I had a sales rep come over yesterday. I am interested in getting a purifier/filtration/UV light unit installed to reduce virus and mold transmission. (I share my--fortunately large--home with a young adult daughter whose job has recently become high risk.) I had been keeping windows open at night when we are both home but that is not a sustainable ventilation strategy through our bitter Upstate NY winters.

Not surprisingly, given the long hot summer and supply chain issues, the machine I want is in stock but there are no excess inventories and the 5% manufacturer´s rebate I was offered in 2014 is no longer available. The current mfr discount is tiny and token. But that´s okay, the contractor is giving me a good package deal on installing the new AC, the purification/filtration unit, a winter humidifier unit (air gets very dry in the winter), and a replacement water heater (the old one appears to be in bad shape and ready for replacement). Also a new smart thermostat which will allow me to manage winter humidity as well as temperature year round. My 12-year-old Lennox furnace (which has the blower system) is still performing well.

And I have had time to educate myself about air conditioners and also feel some clarity that I intend to age in place here, as well as having learned more about climate change and the environment in the intervening six years, so I felt ready to make a more fully informed choice.

I am going with the Lennox XC21 (two stage)
https://www.lennox.com/products/heating ... M4EALw_wcB

Everything is scheduled for installation on Monday. They will wear masks and keep their distance from me. I will spend the time when they are working in my basement out on my covered porch.
Thanks for the update, OP. Did you consider the Lennox heat pump option as well? Just curious.
https://www.lennox.com/products/heating ... oners/xc21
User avatar
Topic Author
dodecahedron
Posts: 5355
Joined: Tue Nov 12, 2013 12:28 pm

Re: Central AC replacement decision

Post by dodecahedron »

mervinj7 wrote: Wed Jul 29, 2020 9:25 pm Thanks for the update, OP. Did you consider the Lennox heat pump option as well? Just curious.
https://www.lennox.com/products/heating ... oners/xc21
No, I did not give any consideration to a Lennox heat pump.

My high efficiency gas furnace is in great working order and since natural gas prices are low, my cost of heating is very reasonable.

My salesman (who lives in a remote rural area about 20 miles from me) mentioned that he personally is putting in a Lennox heat pump at his home, but he currently relies on trucked in propane which is far more expensive. He said it made economic sense for him but not for me.
softwaregeek
Posts: 564
Joined: Wed May 08, 2019 8:59 pm

Re: Central AC replacement decision

Post by softwaregeek »

I had a Lennox seer 16 filter put in. Only regret was not getting the ultraviolet light that kills germs. It was 600 for the filter install, filter is about five inches thick.
Valuethinker
Posts: 41140
Joined: Fri May 11, 2007 11:07 am

Re: Central AC replacement decision

Post by Valuethinker »

dodecahedron wrote: Wed Jul 29, 2020 8:31 pm Almost exactly six years after my original post at the top of this thread. It was interesting to re-read this thread as I am now finally ready to replace my central AC. What was a 23 year old AC in 2014 is now a 29 year old AC in 2020.

The old AC (a Trane) is still working reasonably well but clearly laboring with the record string of hot days we have had for week after week this summer.

Time to replace. I was pleasantly surprised to discover there are no supply chain issues and there is plenty of labor available that my longtime HVAC company would like to keep busy.

I had a sales rep come over yesterday. I am interested in getting a purifier/filtration/UV light unit installed to reduce virus and mold transmission. (I share my--fortunately large--home with a young adult daughter whose job has recently become high risk.) I had been keeping windows open at night when we are both home but that is not a sustainable ventilation strategy through our bitter Upstate NY winters.
I totally agree with that BUT

It's worth knowing that the heat content of air is far lower than the heat content of solid wall materials (I'd have to look it up but it is something like 1/5th of the energy content or less). So although a room quickly turns cold in winter, the walls & furniture don't lose heat that fast -thermal mass. It is worth, even in winter, opening the window to a bedroom and airing it for say 15-45 minutes - as long as the door is closed your actual impact on your heating bill will be small.

Our grandmothers were not stupid - airing bedrooms every day. The cold air kills a lot of germs and dust mites.

(if insulated, the insulation cuts you off from the thermal mass of the brick or stone behind it - that's good in winter, and usually bad in summer. We have an uninsulated, single pane windows Victorian home, and there can be a 10 degree F lower in the downstairs (no cellar or basement) than the upstairs during a hot summers day (we are due 91 F on Friday - in London, England!).

Not surprisingly, given the long hot summer and supply chain issues, the machine I want is in stock but there are no excess inventories and the 5% manufacturer´s rebate I was offered in 2014 is no longer available. The current mfr discount is tiny and token. But that´s okay, the contractor is giving me a good package deal on installing the new AC, the purification/filtration unit, a winter humidifier unit (air gets very dry in the winter), and a replacement water heater (the old one appears to be in bad shape and ready for replacement). Also a new smart thermostat which will allow me to manage winter humidity as well as temperature year round. My 12-year-old Lennox furnace (which has the blower system) is still performing well.

And I have had time to educate myself about air conditioners and also feel some clarity that I intend to age in place here, as well as having learned more about climate change and the environment in the intervening six years, so I felt ready to make a more fully informed choice.

I am going with the Lennox XC21 (two stage)
https://www.lennox.com/products/heating ... M4EALw_wcB

Everything is scheduled for installation on Monday. They will wear masks and keep their distance from me. I will spend the time when they are working in my basement out on my covered porch.
Thank you for returning and updating us - that's always very valuable.
User avatar
Topic Author
dodecahedron
Posts: 5355
Joined: Tue Nov 12, 2013 12:28 pm

Re: Central AC replacement decision

Post by dodecahedron »

Valuethinker wrote: Thu Jul 30, 2020 4:39 am
dodecahedron wrote: I am interested in getting a purifier/filtration/UV light unit installed to reduce virus and mold transmission. (I share my--fortunately large--home with a young adult daughter whose job has recently become high risk.) I had been keeping windows open at night when we are both home but that is not a sustainable ventilation strategy through our bitter Upstate NY winters.
I totally agree with that BUT

It's worth knowing that the heat content of air is far lower than the heat content of solid wall materials (I'd have to look it up but it is something like 1/5th of the energy content or less). So although a room quickly turns cold in winter, the walls & furniture don't lose heat that fast -thermal mass. It is worth, even in winter, opening the window to a bedroom and airing it for say 15-45 minutes - as long as the door is closed your actual impact on your heating bill will be small.
Thanks for that useful info. I will definitely bear it in mind. It sounds like asking my daughter to open her bedroom window every morning for 15 minutes or so before she leaves for work could be worthwhile. (Her bedroom is small and with the door closed as you suggest, airing it in the morning would not have a big impact on energy use. She would likely be elsewhere--in the kitchen eating breakfast or preparing her lunch during this short time so the blast of cold would not bother her.)
Our grandmothers were not stupid - airing bedrooms every day. The cold air kills a lot of germs and dust mites.
Amusing you should mention grandmothers. My mother always likes to talk about the fact that her father, my grandfather, was a great believer in all members of his family sleeping year round with a bedroom window open. He grew up in Central Massachusetts and survived the Great Influenza and perhaps this belief helped him do that. My mom was not so wild about the idea but had no choice to abide by her dad´s policy. At least they lived in DC at the time, somewhat milder winters than New England. And my grandfather was a superintendent of buildings and grounds with an employer-provided house for his family. The employer may well have provided the heating fuel too, so my grandfather may not have had to be concerned about the impact on heating bills.
(if insulated, the insulation cuts you off from the thermal mass of the brick or stone behind it - that's good in winter, and usually bad in summer. We have an uninsulated, single pane windows Victorian home, and there can be a 10 degree F lower in the downstairs (no cellar or basement) than the upstairs during a hot summers day (we are due 91 F on Friday - in London, England!).
Thanks for that explanation of why the temperature gradient is so much more marked in the summer than in the winter. Yes, my second floor bedroom (with cathedral ceiling and three skylights--lovely view and natural light but so much heat buildup!) is often 10 degrees more than the first floor. And I do have a basement, no thermometer but it can feel like a walk-in freezer there while my first floor is pleasant and my second floor is miserable.

We have definitely been experiencing a lot of hot summers since the unusually mild summer of 2014 when I first posted this. I am sure this particular summer is a record breaker for number of days in the 90s, often in the mid or even upper 90s. We have not yet quite broken the 100 degree mark. June 2020 was bad and July has been even worse. The links go to observations from a local weather station. We have consistently been running several degrees hottter, but they do give an idea of how this year compares to historic averages.

But when we first moved here 30 years ago (and installed the now 29-year-old Trane shortly afterwards), it was common to have entire summers where the temps never broke 90. And in fact our real estate agent thought we were somewhat unreasonable for insisting that we only wanted to look at houses with ductwork that could support central AC. She kept telling us most people got along quite well without it, which was likely true. It was cooler then. But my late husband insisted. With his allergies, we could never open windows, so we did need central AC. I am grateful for that now, especially since I am spending so much of my time at home during the pandemic. (No escaping to air conditioned college campus, public library, or other air conditioned public indoor spaces. No travel so escaping to beach or mountains does not work. At least I have my lovely shady yard and covered veranda.
jharkin
Posts: 2596
Joined: Mon Mar 28, 2016 7:14 am
Location: Boston suburbs

Re: Central AC replacement decision

Post by jharkin »

Mingus wrote: Thu Jul 31, 2014 8:26 pm If your summers are mild, there is no reason to replace a well functioning unit with something new. It might have another 23 years in it.
This is the correct answer. It’s “beyond useful life” is just a high pressure sales tactic by contractors that hope you won’t research.

Yes it might fail tomorrow. Or it might last 20 or even 30 more years.

And the things is when it breaks it’s not necessarily going to leak. A lot of times when people say their unit goes in a heat wave, what actually happened is the compressor start capacitor failed.A $20 part that can be replaced with a screwdriver in 5 minutes.

BTW the really nasty refrigerant was R12. That’s been banned since I was a kid. Your unit (is it 20 years or 30 years? You mentioned 1991) is probably filled with R22. New replacements use something like R410a.
jharkin
Posts: 2596
Joined: Mon Mar 28, 2016 7:14 am
Location: Boston suburbs

Re: Central AC replacement decision

Post by jharkin »

Valuethinker wrote: Thu Jul 30, 2020 4:39 am

I totally agree with that BUT

It's worth knowing that the heat content of air is far lower than the heat content of solid wall materials (I'd have to look it up but it is something like 1/5th of the energy content or less). So although a room quickly turns cold in winter, the walls & furniture don't lose heat that fast -thermal mass. It is worth, even in winter, opening the window to a bedroom and airing it for say 15-45 minutes - as long as the door is closed your actual impact on your heating bill will be small.
Actually thermal mass is exactly that - proportional to mass. The mass of the air in the room is so small it doesn't even register compared to the walls and furniture. Its not 1/5, its .0000000000000000000000000005 or such.

Thing is, heat transfer takes 3 forms - conduction, convection and radiation.
Conduction is when you sit on the couch and feel cold.
Convection is when the fan blows air by you and you feel cooler.
Radiation is that wall of heat you feel from the fireplace. And it works even across vacuum (this is how craft in space heat or cool).

All 3 are at play all the time. US style forced air heating is the worst system comfort wise because what happens is they typically tell you to lower it all day than blast at night when you come home. The air is quickly heated, satisfying the thermostat... but all the furniture and walls take a long time to catch up so you get radiative cooling effect that makes the people always feel cold. (f you want to get really technical, whats actually happening is that YOU are radiatively heating the cold wall :twisted: )

Radiant heating (underfloor, wall radiators) with modulation so the heat is always on low is far more comfortable but never really took of in the states beyond some new construction and retrofits of older systems in the northeast where Steam and hot water where typical years ago.

Valuethinker wrote: Thu Jul 30, 2020 4:39 am Our grandmothers were not stupid - airing bedrooms every day. The cold air kills a lot of germs and dust mites.
Uhh no. Cold air doesn't really kill germs. The Victorians thought it warded off disease and it was fashion to sleep with the windows open in winter in the late 1800s. Because of this homes (in England, and in the USA, here in New England and the upper midwest) built in that era have massively oversize radiators.

This causes a real headache for high efficiency retrofits because you are limited in how much you can downsize the boiler (especially on steam systems - which still exist on that old housing stock). So you are left with a really big boiler that short cycles and doesn't heat anywhere near as efficiently as it could. Case in point, my first house was a antique 1400sq ft cape style that was retrofitted with steam around the year 1900. We had a 2000s vintage modern boiler but it was 140,000BTU because that's what the radiators needed to work properly. House had insulation upgrades so it probably only really needed 40,000-50,000BTU even on a sub freezing day. Heat never ran more than 20 minutes at a time - even in the depth of January.
User avatar
Topic Author
dodecahedron
Posts: 5355
Joined: Tue Nov 12, 2013 12:28 pm

Re: Central AC replacement decision

Post by dodecahedron »

jharkin wrote: Thu Jul 30, 2020 6:32 am
Mingus wrote: Thu Jul 31, 2014 8:26 pm If your summers are mild, there is no reason to replace a well functioning unit with something new. It might have another 23 years in it.
This is the correct answer. It’s “beyond useful life” is just a high pressure sales tactic by contractors that hope you won’t research.

Yes it might fail tomorrow. Or it might last 20 or even 30 more years.

And the things is when it breaks it’s not necessarily going to leak. A lot of times when people say their unit goes in a heat wave, what actually happened is the compressor start capacitor failed.A $20 part that can be replaced with a screwdriver in 5 minutes.

BTW the really nasty refrigerant was R12. That’s been banned since I was a kid. Your unit (is it 20 years or 30 years? You mentioned 1991) is probably filled with R22. New replacements use something like R410a.
Jharkin, my unit was 23 years old when I started this thread six years ago. It is now 29 years old six years later. It was indeed installed in 1991.

And the Mingus post you quoted above (also from 2014) was part of what persuaded me to leave well enough alone back then. Another reason I was content to leave well enough alone was that the summer of 2014 had been mild and it was easy to escape the house on the occasional very hot day. Yet another reason I was content to leave well enough alone six years ago was that at that time I was not sure how much longer I would be staying in the home. (My husband had died a year earlier and I was just starting to mull over whether to age in place or to move elsewhere and downsize.)

Since then it has gotten hotter and hotter, and I have decided to age in place, and I do not have any air-conditioned public places that I can easily escape to during this COVID time.

And my unit did not totally ¨go¨ during this heat wave, but it was clearly laboring (running a lot and very loudly) when we would get day after day after day of consecutive 90 degree temps. It would do fine in the early days of each hot streak of days but by the end it just couldn´t stay on top of it. And yes, it appears that my existing unit does use R-22, which the government has now banned from manufacture as of 2020 and will only be obtainable on the aftermarket.

Humidity has also been unusually bad this summer. The house has just not been as naturally comfortable as it once was. I am concerned about mold issues. I have been running the AC this summer far more than I ever expected to back in 2014.

So all in all, I think it was fine (ex post) to wait six years before replacing, but the future life expectancy of my existing AC is less than *my* life expectancy of living in this home. In expectation, I am going to have to buy another AC before I die, so I might as well do it now, especially since I also want to add features (air purifier and winter humidity control) that I was originally planning just to add on to my existing AC.

So it made sense to install a new more efficient and quieter and more environmentally friendly AC that will hopefully keep up with the demands of increasingly hot summers. And my HVAC contractor, who is reputable and who has installed and serviced our furnaces (including a complicated switch from oil to gas in 2008) has staff currently twiddling their thumbs (apparently because most folks are not installing HVAC systems at this point in the summer) so I figured why not carpe diem.

If this unit also lasts 29 years, I will be 95, if I am still on this earth. Who knows what technologies may be available down the road.

So the answer from Mingus that was right six years ago is no longer the right answer in the light of circumstances today.
tibbitts
Posts: 11869
Joined: Tue Feb 27, 2007 6:50 pm

Re: Central AC replacement decision

Post by tibbitts »

I would not count on 29 years from current technology. If you get more than a dozen year that should be considered reasonable.
User avatar
Topic Author
dodecahedron
Posts: 5355
Joined: Tue Nov 12, 2013 12:28 pm

Re: Central AC replacement decision

Post by dodecahedron »

jharkin wrote: Thu Jul 30, 2020 6:55 am Thing is, heat transfer takes 3 forms - conduction, convection and radiation.
Conduction is when you sit on the couch and feel cold.
Convection is when the fan blows air by you and you feel cooler.
Radiation is that wall of heat you feel from the fireplace. And it works even across vacuum (this is how craft in space heat or cool).

All 3 are at play all the time. US style forced air heating is the worst system comfort wise because what happens is they typically tell you to lower it all day than blast at night when you come home. The air is quickly heated, satisfying the thermostat... but all the furniture and walls take a long time to catch up so you get radiative cooling effect that makes the people always feel cold. (f you want to get really technical, whats actually happening is that YOU are radiatively heating the cold wall :twisted: )

Radiant heating (underfloor, wall radiators) with modulation so the heat is always on low is far more comfortable but never really took of in the states beyond some new construction and retrofits of older systems in the northeast where Steam and hot water where typical years ago.
Thanks--this is educational for me. Given that I am not going to change my forced hot air system, it seems like I can at least mitigate the problem by using an electric fleece blanket to heat my sofa or bed. And possibly a small electric space heater. Our house is so big for two people that it does not make sense to use central heat to keep the entire house cozy on bitter cold days and nights, when a small amount of strategic electric source heat can provide comfort where needed.
jharkin wrote: Thu Jul 30, 2020 6:55 am
Valuethinker wrote: Thu Jul 30, 2020 4:39 am Our grandmothers were not stupid - airing bedrooms every day. The cold air kills a lot of germs and dust mites.
Uhh no. Cold air doesn't really kill germs.
I won´t weigh in on weather cold air kills germs but it does seem clear that dissipation of aerosols carrying viruses into the outdoors (at any temperature) is very much worthwhile.
jharkin wrote: Thu Jul 30, 2020 6:55 am The Victorians thought it warded off disease and it was fashion to sleep with the windows open in winter in the late 1800s. Because of this homes (in England, and in the USA, here in New England and the upper midwest) built in that era have massively oversize radiators.

This causes a real headache for high efficiency retrofits because you are limited in how much you can downsize the boiler (especially on steam systems - which still exist on that old housing stock). So you are left with a really big boiler that short cycles and doesn't heat anywhere near as efficiently as it could. Case in point, my first house was a antique 1400sq ft cape style that was retrofitted with steam around the year 1900. We had a 2000s vintage modern boiler but it was 140,000BTU because that's what the radiators needed to work properly. House had insulation upgrades so it probably only really needed 40,000-50,000BTU even on a sub freezing day. Heat never ran more than 20 minutes at a time - even in the depth of January.
My house has its own idiosyncratic story. When we bought it in 1990, it had ductwork, force hot air and central AC. The original part of the house (60% of it) was built in 1938 and probably had radiators originally. In 1980 (ten years before we acquired it), the previous owners did a major two story addition (the other 40% of it.) The upstairs bedrooms in the older part of the house are weirdly Goldilocks inadequately served by the ducts. One bedroom in the older part of the house is much warmer than the other bedroom in that part of the house for reasons my handy late husband explored addressing but ultimately gave up on. Since there is only one daughter regularly living in that part of the house and she is free to sleep wherever she wants, I no longer waste time thinking about addressing that.

My bedroom and bathroom (the master suite) are in the new part of the house. Beautiful floor to ceiling windows looking out on the shady wooded backyard to the north and cathedral ceiling with three large skylights (that do not open) in my bedroom. Lovely to look up and see the leafy branches of tall trees and blue sky above during the day time and the stars at night, but there is a definite greenhouse effect on sunny days, which we have had a lot of. My bedroom is unavoidably hotter than the rest of the house. A couple ceiling fans do at least mitigate.
Valuethinker
Posts: 41140
Joined: Fri May 11, 2007 11:07 am

Re: Central AC replacement decision

Post by Valuethinker »

jharkin wrote: Thu Jul 30, 2020 6:55 am
Valuethinker wrote: Thu Jul 30, 2020 4:39 am

I totally agree with that BUT

It's worth knowing that the heat content of air is far lower than the heat content of solid wall materials (I'd have to look it up but it is something like 1/5th of the energy content or less). So although a room quickly turns cold in winter, the walls & furniture don't lose heat that fast -thermal mass. It is worth, even in winter, opening the window to a bedroom and airing it for say 15-45 minutes - as long as the door is closed your actual impact on your heating bill will be small.
Actually thermal mass is exactly that - proportional to mass. The mass of the air in the room is so small it doesn't even register compared to the walls and furniture. Its not 1/5, its .0000000000000000000000000005 or such.

Thing is, heat transfer takes 3 forms - conduction, convection and radiation.
Conduction is when you sit on the couch and feel cold.
Convection is when the fan blows air by you and you feel cooler.
Radiation is that wall of heat you feel from the fireplace. And it works even across vacuum (this is how craft in space heat or cool).

All 3 are at play all the time. US style forced air heating is the worst system comfort wise because what happens is they typically tell you to lower it all day than blast at night when you come home. The air is quickly heated, satisfying the thermostat... but all the furniture and walls take a long time to catch up so you get radiative cooling effect that makes the people always feel cold. (f you want to get really technical, whats actually happening is that YOU are radiatively heating the cold wall :twisted: )

Radiant heating (underfloor, wall radiators) with modulation so the heat is always on low is far more comfortable but never really took of in the states beyond some new construction and retrofits of older systems in the northeast where Steam and hot water where typical years ago.

Valuethinker wrote: Thu Jul 30, 2020 4:39 am Our grandmothers were not stupid - airing bedrooms every day. The cold air kills a lot of germs and dust mites.
Uhh no. Cold air doesn't really kill germs. The Victorians thought it warded off disease and it was fashion to sleep with the windows open in winter in the late 1800s. Because of this homes (in England, and in the USA, here in New England and the upper midwest) built in that era have massively oversize radiators.

This causes a real headache for high efficiency retrofits because you are limited in how much you can downsize the boiler (especially on steam systems - which still exist on that old housing stock). So you are left with a really big boiler that short cycles and doesn't heat anywhere near as efficiently as it could. Case in point, my first house was a antique 1400sq ft cape style that was retrofitted with steam around the year 1900. We had a 2000s vintage modern boiler but it was 140,000BTU because that's what the radiators needed to work properly. House had insulation upgrades so it probably only really needed 40,000-50,000BTU even on a sub freezing day. Heat never ran more than 20 minutes at a time - even in the depth of January.
Thanks for all the corrections.

(yes - we are heating the wall ;-))

I read something in New Scientist a while ago, I think, about the merits of airing rooms & hygiene. But I should have checked. What I gathered was that the merits were quite real.

UK does heating in kw (and I am too tired and hot this afternoon to do the conversions ;-)) but having grown up in North America with hot water rads, I can relate the points to my experience. I have never actually heard of steam radiators in Canada (I would think Montreal would be the place, if anywhere) but pre WW2 housing was almost all HW rads. Air conditioning has been retrofitted to most houses - given the much hotter summers we are experiencing (grew up without house AC but cannot imagine my parents' house without it now).

I had not thought about the minimum size for the system to work. UK rads tend to be thin steel panel rads, so maybe it's less of an issue?

As part of its net zero programme, the UK government is going to have to mandate a big switch to heat pumps. Since c 25% of the housing stock is pre 1930, and does not even have air cavity walls*, that's going to be tricky to pull off. A typical HP would be 10 kw, and my Vaillant gas boiler is 32 kw (Victorian house, no wall insulation, single pane windows). Whether the solution will come by dual systems (hydrogen through the gas pipes) or what is not yet clear.

* post about 1930, the typical UK house had concrete block walls with a cavity, and then a brick exterior (sometimes covered in stucco - common style in the 1930s). On the inside lathe & plaster, and, postwar, plasterboard (gyprock). That cavity served as moisture control from damp air coming in, and damp air going out and condensing - keeps that moisture away from things that can rot. There has been a programme to fill the empty cavities with insulation but *that* risks problems for moisture control.
User avatar
Topic Author
dodecahedron
Posts: 5355
Joined: Tue Nov 12, 2013 12:28 pm

Re: Central AC replacement decision

Post by dodecahedron »

For anyone still following this continuing story: in an odd and unexpected postscript to this discussion, today's mail brought me a surprise: a prepaid gift Visa card from Lennox (the manufacturer of my new AC and accessories). It seemed curious that the salesperson never mentioned anything about this unexpected bonus.

The paperwork accompanying the card indicated that I would need to go to a website (prepaidcardstatus.com) to find out how much the balance on the card was. Curious, I attempted to do this but was not successful in getting any information. All that happened was a message telling me I needed to answer their Captcha question, but there was no visible Captcha anywhere on my screen. (Could be due to fact I was using a Chromebook.)

Fortunately, there was also a toll-free phone number I could call to find out my balance. It turns out to be $125.

Nice surprise, but kind of weird that nobody bothered to tell me about this back when they were trying to pitch me on this.

Curious, I did an online search. It seems that Lennox is now offering even bigger gift card rebates, as much as $1,350 to folks who buy a complete system (furnace as well as AC) between Sept 7 and Nov 27. That is probably a slow season for them. And in order to collect that gift card rebate, you actually have fill out some paperwork to get it.

Back when I bought my new AC (late July), there was no info provided about any gift card rebate, though there was a manufacturer's incentive credited directly on my bill.
Post Reply