School Me on Adding Air Conditioning to My Forced Air HVAC System

[frorider]

Yeah let’s not get ridiculous here. The green trend in power generation will just continue.

20 yrs living in the mountains, 30-40 F most nights in summer and we’ve always liked getting the house down to 55 - 60 in the morning with windows opened. But spending $3800 to add central AC was totally worth it for the two gnarly smoke summers we’ve had recently. HEPA filter.

The energy consumption is low since we use it for short periods.

The speculation that a modern house with filtered AC has air quality comparable to the ashy nightmare outside is absurd.

[highangle]

There is return air.

AFAIK, it's the same as any other forced air system in that regard.



Awesome info, thanks. There are only like 3-4 companies who do installs here. I've yet to find anyone who installs Trane.



There's a house being built next door this summer. Also, last summer there was a 6 week period of intense wildfire smoke and the drought right now is significantly worse than last year.

I don't want to need to have my windows open to cool down.

You prob know all this already, but forced air AC consists of an outdoors unit with the noisy heat pump or compressor unit & coils [condenser], connected by copper tubing to an indoors evaporator coil connected to a filtered blower + plenum manifold [aka air handler], and thence to your ducts.
There are typically no "return air ducts" in residential homes under about 50,000 sqft.

For various reasons having to do with mold and noise, it's vastly better to slightly undersize a residential AC than to oversize it.

Higher SEER numbers are better, but they charge you more. Dig into it, and do the calcs with your electric rates to see how long a certain SEER will take to pay for itself in your use. Municipal code will likely spec a certain minimum.

Filtration - you can put several types of filters in your air handler. But if you want to filter forest fire smoke get a big carbon filter and fan like the dope growers use, and try to put it blowing towards your air handler blower fan, or into your plenum somehow.... That kind of filtration is beyond what you should ask of your home's HVAC.

Breaker box - make sure yours can handle another 50amp circuit, esp if your home has never had AC.

HVAC hardware is commoditized. No one brand is any better than others of the same spec. Buy the spec, not the brand. That may be hard to believe, but it's true.
If you got cash and know the parts to an AC system, commercial HVAC supply houses are your best prices for these commoditized generic components. The only thing they won't sell you is refrigerant. They can also turn you on to moonlighters during slow seasons.

Cheapest way to get a new AC is catch a licensed HVAC tech doing shadetree work during shoulder season, one that will let you buy the components. Their seasons are the hot part of summer and the cold part of winter when everybody's shit breaks, so chances are good about now.


Source: Pretty much did this over the phone for my mom, and later my brother in Fla, when their ACs went tango. Saved them 50-60% over retail, for not a lot of time/labor input.

[lifelinksplit]

If you can afford to retrofit AC in your new intermountain west home does that make you middle class?

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[Kevo]

You prob know all this already, but forced air AC consists of an outdoors unit with the noisy heat pump or compressor unit & coils [condenser], connected by copper tubing to an indoors evaporator coil connected to a filtered blower + plenum manifold [aka air handler], and thence to your ducts.
There are typically no "return air ducts" in residential homes under about 50,000 sqft.

For various reasons having to do with mold and noise, it's vastly better to slightly undersize a residential AC than to oversize it.

Higher SEER numbers are better, but they charge you more. Dig into it, and do the calcs with your electric rates to see how long a certain SEER will take to pay for itself in your use. Municipal code will likely spec a certain minimum.

Filtration - you can put several types of filters in your air handler. But if you want to filter forest fire smoke get a big carbon filter and fan like the dope growers use, and try to put it blowing towards your air handler blower fan, or into your plenum somehow.... That kind of filtration is beyond what you should ask of your home's HVAC.

Breaker box - make sure yours can handle another 50amp circuit, esp if your home has never had AC.

HVAC hardware is commoditized. No one brand is any better than others of the same spec. Buy the spec, not the brand. That may be hard to believe, but it's true.
If you got cash and know the parts to an AC system, commercial HVAC supply houses are your best prices for these commoditized generic components. The only thing they won't sell you is refrigerant. They can also turn you on to moonlighters during slow seasons.

Cheapest way to get a new AC is catch a licensed HVAC tech doing shadetree work during shoulder season, one that will let you buy the components. Their seasons are the hot part of summer and the cold part of winter when everybody's shit breaks, so chances are good about now.


Source: Pretty much did this over the phone for my mom, and later my brother in Fla, when their ACs went tango. Saved them 50-60% over retail, for not a lot of time/labor input.

This is awesome info. I had a guy swing by today who fits this exact profile. He's an HVAC tech in Jackson and his company only works in WY, but he's licensed here in ID and willing to work on a weekend.

Fingers crossed he comes back with a good quote.

[ötzi]

There are typically no "return air ducts" in residential homes under about 50,000 sqft.

Not sure what to make of this comment.

[Nobody Famous]

There are typically no "return air ducts" in residential homes under about 50,000 sqft.

Not sure what to make of this comment.

Same thought when I read it. One acre is 43,560 sq ft, how may homes have 50k sq ft under roof?

In that part of the country where I live almost every home with forced air has return ducting, it's rare to find only a return register between floors and no ducting beyond the register.

This is awesome info. I had a guy swing by today who fits this exact profile. He's an HVAC tech in Jackson and his company only works in WY, but he's licensed here in ID and willing to work on a weekend.

Fingers crossed he comes back with a good quote.

The usual caution flags apply in this situation, do you get the warranty coverage on the equipment? will he do the annual check ups (refrigerant pressures, etc.), ... on the weekends?, who do you call for break down service, are you willing to wait until weekends? Where I live cooling season starts late May and ends early Sept and people don't want to wait till 'next free weekend' for a service repair call. Counter argument is above comments say cooling season is shorter in your part of the world.

The overall tradeoff is you may save upfront costs and you're putting blind faith in the install. Guys doing work like this aren't known for high quality workmanship or follow up but there are exceptions.

[MyNameIsAugustWest]

Not sure what to make of this comment.

Same here...I've owned a couple of homes with forced hot air heating (none were over 50K sq ft) and they all had cold air return ducting. That's what I used to run Cat5 to all my kids' bedrooms in one house.

[Flounder]

Same here...I've owned a couple of homes with forced hot air heating (none were over 50K sq ft) and they all had cold air return ducting. That's what I used to run Cat5 to all my kids' bedrooms in one house.

Clever way to get the cat5 run.

Our first floor foot print is about 900 sf and it has 3 returns.

[glademaster]

Yup, I can't say I've been in houses with forced air that doesn't have return air ducting, and I'm in a lot more houses than the average TGR member. I've seen returns that use just a stud cavity without ducting, but it's still return air, and that was a corner-cutting measure that hasn't been code compliant in 20ish years.

Nowadays, in most cases you have return air and outside air intake with an ERV/HRV to temper your nasty indoor air.

[Nobody Famous]

That's what I used to run Cat5 to all my kids' bedrooms in one house.

Clever way to get the cat5 run.

Our first floor foot print is about 900 sf and it has 3 returns.

Before everyone does this, it's supposed to be plenum rated cable, as I recall.

[Garbowski]

Before everyone does this, it's supposed to be plenum rated cable, as I recall.

Not doubting you, but why? I am already doing this... I figured it's just room air all the way to the basement.

edit: google says the PVC coatings are toxic in a fire. I can see in a multistory office building where that could be bad, but in my house I'm guessing gasses from one particular cable will not be the top of the list of things that went wrong in a fire. My house is wood and 120 years old so YMMV.

[2bjenny]

I know a good, independent HVAC/electrician out of IF. I know he travels an hour north of IF to do work, not sure if he's willing to travel east, as far as Driggs area. If you need another quote, I can give you his contact info.

[Danno]

The usual caution flags apply in this situation, do you get the warranty coverage on the equipment? will he do the annual check ups (refrigerant pressures, etc.), ... on the weekends?, who do you call for break down service, are you willing to wait until weekends? Where I live cooling season starts late May and ends early Sept and people don't want to wait till 'next free weekend' for a service repair call. Counter argument is above comments say cooling season is shorter in your part of the world.

The overall tradeoff is you may save upfront costs and you're putting blind faith in the install. Guys doing work like this aren't known for high quality workmanship or follow up but there are exceptions.

I second this. I did get a solo-operation guy to do the work, so not exactly a guy doing it on the side but no support backbone that a larger company provides. I got a better deal than a full-fledged local company, but not tons better (maybe $300). And in retrospect, I wish I had picked the local company that "came in second" for the reasons you mention.

[Nobody Famous]

Not doubting you, but why? I am already doing this... I figured it's just room air all the way to the basement.

edit: google says the PVC coatings are toxic in a fire. I can see in a multistory office building where that could be bad, but in my house I'm guessing gasses from one particular cable will not be the top of the list of things that went wrong in a fire. My house is wood and 120 years old so YMMV.

My understanding from many years ago is what you found, any cabling in a space allowing 'air movement' or 'air handling' needs to be of the type that if a fire or any low-level smouldering combustion were to occur then toxic or poisonous fumes are to be eliminated or minimized.

The slow air movement is the killer, it will spread the bad stuff through the living spaces without tripping anyone senses. I remember this from commercial buildings but I'm certain the codes apply to residential.

[ötzi]

Well inside a duct it would pretty much be no air movement or brisk air movement depending if the system was running, but whatever.

[highangle]

Not sure what to make of this comment.

There's grilles to let return air back into the air handler, but typically air's not moving so fast nor are the 1/2hp squirrel cage fans in most residential ACs powerful enough to make pressure differential problems.
Moreover, where do these return air ducts return to? An intake manifold?
There are often floor registers that let air get back down to the mechanical room floor from upper floors, but you don't often see a big hydra of return ducts all leading back to the air handler in a typical residential install, even in high humidity zones.

[Nobody Famous]

I don't want to hijack this thread, ... but for those interested only in indoor air quality there's this DIY filtration box from TOH.

I know a guy, retired owner of an HVAC company, who swears by these. He has part time retirement gig placing these in assisted living facilities and nursing homes to clean up persistent smells and odors.

[Marshall Tucker]

Grew up a bit inland from charleston SC. Dahdie liked to say buy fedders. if it can cool houston it can cool any damn place on earth.

[bagtagley]

There's grilles to let return air back into the air handler, but typically air's not moving so fast nor are the 1/2hp squirrel cage fans in most residential ACs powerful enough to make pressure differential problems.
Moreover, where do these return air ducts return to? An intake manifold?
There are often floor registers that let air get back down to the mechanical room floor from upper floors, but you don't often see a big hydra of return ducts all leading back to the air handler in a typical residential install, even in high humidity zones.

I live in a high humidity zone. My house has two returns that both feed directly upstream of the media filter on the air handler. MIL just had her ducting replaced. 5 cold air returns on that system, all ducted to a trunk line that feeds directly into the air handler.

[old_newguy]

I don't want to hijack this thread, ... but for those interested only in indoor air quality there's this DIY filtration box from TOH.

I know a guy, retired owner of an HVAC company, who swears by these. He has part time retirement gig placing these in assisted living facilities and nursing homes to clean up persistent smells and odors.

Built a version of this last year for smoke. Worked really well. Filters were pretty nasty when the smoke cleared.

[highangle]

I live in a high humidity zone. My house has two returns that both feed directly upstream of the media filter on the air handler. MIL just had her ducting replaced. 5 cold air returns on that system, all ducted to a trunk line that feeds directly into the air handler.

Yeah right. Post pics.
The only thing connected to a functioning air handler [filter blower evaporator coil] will be a plenum manifold, which acts as a box of air and keeps the supply air relatively constant to all attached AC ducts [proportional to duct size].

If you got a bundle of "return air ducts" ending near your air handler's intake, you got a ex post facto jackleg job to dry out an unbalanced system, or you're the victim of a gypsy contractor scheme.

[glademaster]

I live in a high humidity zone. My house has two returns that both feed directly upstream of the media filter on the air handler. MIL just had her ducting replaced. 5 cold air returns on that system, all ducted to a trunk line that feeds directly into the air handler.

This is similar to what we see here in an incredibly dry climate. Primarily for air conditioning and fresh air via an ERV. Most houses here have hydronic radiant heat and central air.

[sidebar not related to forced air] I was in a place recently that had three 2 million BTU boilers to run the radiant heat. Those were just for the radiant, the snowmelt system was run by a separate bank of boilers. 6 inch diameter copper pipe isn't something you see a whole lot of. [\sidebar]

[highangle]

Built a version of this last year for smoke. Worked really well. Filters were pretty nasty when the smoke cleared.

Didn't watch, but guess you somehow mash some duct and magically attach it to to your blower fan? Because if you duct a box made of HEPA filters with much higher resistance to airflow than your standard AC filter to your air handler's intake box, it's not going to intake much air.
If you hepa filter all your intake, air flow across your evaporator coil will be reduced, probably to the extent that your evap coil ices up.

[Flounder]

If you got a bundle of "return air ducts" ending near your air handler's intake, you got a ex post facto jackleg job to dry out an unbalanced system, or you're the victim of a gypsy contractor scheme.

I’m not sure you understand what a trunk line is but it’s not a bundle of return ducts all terminating separately at the intake. What Bag describes for his place and his MIL is standard and nothing like what you’re describing above. Our install has 3 returns, all go into the return trunk line at various points on its way back to the air handler. Also have a duct from the ERV that dumps into the return.

[ötzi]

Honestly highangle is baffling me. There are supply ducts and return ducts in forced-air systems. They essentially make a loop with a gap in it, the gap being the conditioned space. The supply ducts deliver conditioned air to the conditioned space and the return collects air from the conditioned space and returns (hence the name) it to the air handler.

Some smaller systems like an apartment or a quite small house might just have a single return and a short piece of ductwork to the intake on the air handler but even if it's only two feet long it's still ductwork.