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

[ötzi]

Blue is return in this diagram of a very simple system. In a larger system there would be other return ducts leading to the big blue duct.

[Nobody Famous]

Honestly highangle is baffling me.

Yes, went off the rails in the post about houses under 50,000 sq. ft.

[highangle]

Blue is return in this diagram of a very simple system. In a larger system there would be other return ducts leading to the big blue duct.

That's a heating system, brah. One that has to take air from a 2nd floor down to a basement to allow the system to flow at lower fan speeds before it's lost to the attic.
I'm talking about AC here. If a residential system is designed properly, it doesn't need return air ducts, because the magic of air pressure and the witchcraft of thermal density returns cooled air to the blower without ducts.

[Falcon3]

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.

WTF are you talking about.

I’ve installed dozens of AC units, my dad ran his own HVAC shop for 30 years then went commercial for another 10. I started working with him and installing units at age 12. Grandfather ran a different shop with my uncle for 50 years. HVAC runs in my blood. It’s literally all we talk about at every family function Every residential system with central heat ive installed AC on has multiple “cold” air intakes that are stemmed into the cold air plenum, then hits the filter element, then into the blower of the furnace, then across the evap coil and out into the “hot” air vents (exactly like the diagram shows).

The legs of the system are so named as they’re built/named for the furnace function, but they work the same on cold or hot.

Generally you will have one cold air intake register in every room that has a hot air exhaust to keep the airflow in that room (and the system) even.

[highangle]

WTF are you talking about.

I'm talking about AC, Jonny.

I’ve installed dozens of AC units, my dad ran his own HVAC shop for 30 years then went commercial for another 10. I started working with him and installing units at age 12. Grandfather ran a different shop with my uncle for 50 years. HVAC runs in my blood. It’s literally all we talk about at every family function Every residential system with central heat ive installed AC on has multiple “cold” air intakes that are stemmed into the cold air plenum, then hits the filter element, then into the blower of the furnace, then across the evap coil and out into the “hot” air vents (exactly like the diagram shows).

I'd wager your HVAC experience is quite regional. For instance, there are vast swathes of America where the "Furnace" you mentioned would consist of a heat pump coil or an A-shaped electric heating element in the air handler, and there is one 6" duct per bedroom-sized area and it comes out of the ceiling.

The legs of the system are so named as they’re built/named for the furnace function, but they work the same on cold or hot.

No they do not. Cold air falls, warm air rises. Heat does not make condensation the way AC does. Colder air is denser than warmer air.
Most temperate areas have AC ducts in the ceiling. Heat should come from the floor. Because. ^ Also, you want your air handler in the living level with AC.
Heat pumps run longer on lower temp deltas, iow, the coil usually doesn't get as hot as a furnace or electric element, or as cold as a conventional AC evap coil. Because they push air almost constantly [a good thing] they create their own circulation of return air through the normal human hall and doorways better than you can with your little 6" return air ducts.

Generally you will have one cold air intake register in every room that has a hot air exhaust to keep the airflow in that room (and the system) even.

You're talking about a heating system, probably built around a furnace in a basement, no?

[Falcon3]

Um “adding AC to my forced air HVAC” is literally the thread title.

It appears you are talking about ductless mini-splits. That’s a totally different system than this entire thread is focused on. I’d wager the most common system people will run into in most places with AC in the US is the forced air furnace.

[RShea]

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.

Plenum cable outer PVC covering is higher rating for fire resistance (and lower smoke transmission over cheaper Riser cable) and is required for ALL areas that are considered part of the HVAC return air (so some drop ceilings that have vents for the return air, any area really that is going to have air flow to other parts of the building.

https://en.wikipedia.org/wiki/Plenum_cable Not the best practice to drill into HVAC and run the cable inside the duct work, and probably not NEC to code either (unless the wire is for the HVAC system and part of it. SO, if you want to have a home pass inspection- DO NOT RUN any type of low voltage wiring for TV or data inside of the metal duct work of a home (unless just going perpendicular and through it to the other side) !!
This talks about Coax (but twisted pair is pretty much no different unless again it is run as part of and for the functioning of the HVAC system):
http://www.ncwhomeinspections.com/Lo...AC+return+duct

Here is another write up on this: https://www.ecmag.com/section/codes-...andling-spaces

[RShea]

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

Before everyone does this, even Plenum wire INSIDE of the duct work is not to code!! You can use the same path and be outside the duct work but any low voltage wiring- even Plenum wire inside the hot or cold air returns is NOT allowed by NEC code.

[highangle]

Um “adding AC to my forced air HVAC” is literally the thread title.

It appears you are talking about ductless mini-splits. That’s a totally different system than this entire thread is focused on. I’d wager the most common system people will run into in most places with AC in the US is the forced air furnace.

Heat pump. Flex duct.

[highangle]

Before everyone does this, even Plenum wire INSIDE of the duct work is not to code!! You can use the same path and be outside the duct work but any low voltage wiring- even Plenum wire inside the hot or cold air returns is allowed by NEC code.

Can you crawl around in your sheetmetal ducts like they're always doing in the movies?

[RShea]

Before everyone does this, even Plenum wire INSIDE of the duct work is not to code!! You can use the same path and be outside the duct work but any low voltage wiring- even Plenum wire inside the hot or cold air returns is NOT allowed by NEC code.

Can you crawl around in your sheetmetal ducts like they're always doing in the movies?

I guess it depends on the size of the duct- home or commercial (and is it a home run or not also). Some may even say it could depend on how close or far away you are to the heat source furnace. It may also depend on the contractor and how well they secure the duct work.... Many things in Hollywood can be actually done, or just made for a movie. That could be a separate thread or maybe should go in the Unthreadworthy discussion maybe.

(Edit, forgot to include the 2nd not in the comment...)

[ötzi]

It appears you are talking about ductless mini-splits.

Is that what's going on here? This whole thing is kind of blowing my mind.

[glademaster]

This whole thing is kind of blowing my mind.

This. No idea what the hell is going on.

[dtown]

WTF are you talking about.

I’ve installed dozens of AC units, my dad ran his own HVAC shop for 30 years then went commercial for another 10. I started working with him and installing units at age 12. Grandfather ran a different shop with my uncle for 50 years. HVAC runs in my blood. It’s literally all we talk about at every family function Every residential system with central heat ive installed AC on has multiple “cold” air intakes that are stemmed into the cold air plenum, then hits the filter element, then into the blower of the furnace, then across the evap coil and out into the “hot” air vents (exactly like the diagram shows).

The legs of the system are so named as they’re built/named for the furnace function, but they work the same on cold or hot.

Generally you will have one cold air intake register in every room that has a hot air exhaust to keep the airflow in that room (and the system) even.

I don’t know shit about HVAC but this comment reminded me of this for some reason...carry on

https://m.youtube.com/watch?v=GTPUzRXozF0



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[Buster Highmen]

Is that what's going on here? This whole thing is kind of blowing my mind.

[Djongo Unchained]

Just crack a window for fucks sake.

[ötzi]

Grab the box fan from the server closet, they're not using it.

[highangle]

If you don't have to re-duct your expensive air conditioned air down to your basement, don't do it.
Your oil furnace is fine down there, your central AC or heat pump air handler[s] should be filtering your living levels and needs enough intake air registers near it to keep everything quiet.
Goes without saying that unless you have a finished basement, you don't bother paying to AC it or circulate its radon.


Minis splits ductless - Connect by the same sort of soft copper linesets to an outside compressor [Small line for compressed refrigerant from compressor outside, large line for after the refrigerant has expanded, cooled, and done its job and left the evaporator coil on its way to be compressed again], and are essentially small air handlers. They don't need return air ducts either.

[Kevo]

Well, I've just gotten my first quote back. Still waiting on others.

The company I've heard back from have said I need a new electric air handler because of some kind of incompatibility between the refrigerant in my existing system and the refrigerants now used? That doesn't make a lot of sense to me- why can't I just use the blower fan on the existing air handler??

Anyway, here are the options they've presented. The costs includes install, but I need to pay an electrician to tap a 220 line off my breaker box that luckily has plenty of room and is next to the location of where the AC condenser will go. I have a good electrician locally that could probably get that done for around $100. They told me they can also remove the thermostat cost since I'm already running an ecobee.







They also mentioned that they aren't sure the increased efficiency of the more expensive units is worth it in this climate, but that having a variable speed air handler and heat pump is nice from a comfort standpoint.

Any thoughts, suggestions, etc?

[Nobody Famous]

Three things stand out to me:

1. They are quoting two different manufacturers for the indoor air-handler and the outdoor heat pump unit, Amana and Goodman. The norm is to use the same manufacturer for both devices. There's no law against mixing these but where I live the norm is not to do so. Reasons are one manufacturer means one set of training for their techs, one distributor for warranty parts, all parts are compatable.

2. The sizing. 3.5 tons capacity outdoor, 4 ton indoor. Those sizes are for a big home, or a very air-leaky mid size home. You did not mention the size of your home, and the sq. footage does not in any way dictate the size of the units, and any contractor who bases sizing on sq footage is taking a shortcut.

The accepted and correct way (accepted by industry associations) to size the equipment, both Heating and Cooling is to do what is known as a "Manual J" load calculation for a specific home. Some contractors scoff at doing this, as in "Yea, those calculations take an hour or two to do, and every time I've done them for a X sq. ft. home in this town I always get Y tonnage sizing. So why bother." And in some cases what they cite is correct. In my local/regional experience what the load calculation gives you (assures you of) is in cooling season the equipment does not short-cycle, ... runs on a cycle of cooling let's say 5-10 minutes run time every 60 minutes. The 5 to 10 minute run time is considered a short cycle (as opposed to a 30 or 45 minute run time), it achieves the cooling needed in 5 - 10 minutes but will never achieve the dehumidification needed because the run time is too short, not long enough to dehumidify.

3. Warranty. In my region warranty is usually specified as x years for parts and y years for labor. X and Y could be 10 and 10, which is pretty good (esp. so if you own the house for 10 years), and behind the scenes there is usually a third party involved that provides the warranty coverage for this to the contractor, kinda like extended warranties for new cars. The contractors cost for this is rolled into his pricing to you. For five and more year warranties it's good to ask if the warranty is through the contractor or the equipment manufacturer. In this quote two different equipment manufacturers. If the contractor is Joe and his buddy Bob it's worth thinking whether they will be in business in ten years.

Hope this helps.

[Hopeless Sinner]

If you don't have to re-duct your expensive air conditioned air down to your basement, don't do it.
Your oil furnace is fine down there, your central AC or heat pump air handler[s] should be filtering your living levels and needs enough intake air registers near it to keep everything quiet.
Goes without saying that unless you have a finished basement, you don't bother paying to AC it or circulate its radon.

snip....

You do realize this makes no sense in real life?

The cooled air is recycled through the return ducts to the AH so #1 the air gets continually cooler and drier as it's cycled through the AH multiple times, #2 the energy use and wear on equipment to keep cooling ambient temp/humid air is much higher than using a source of previously cooled/dehumidified air, #3 returns high in the space eliminate pockets of hot, humid stale air, #4 when the air is recycled through a proper filter, MERV 11 or better, it continually reduces the pollen, dust, and other pollutants.

I owned 2 homes for a total of 27 years in SE NY one in which I installed from scratch 1 zone central AC the first month after moving in and a second home in which I replaced all 3 zones and installed returns in 3 bedrooms. My AC went on the day the heat went off, windows were never opened due to severe pollen allergies, and dust intrusion. As long as good quality equipment is used it's very quiet. I will add the cooling coils to my new system here in Idaho if/when the smoke forces me to.

[Nobody Famous]

The company I've heard back from have said I need a new electric air handler because of some kind of incompatibility between the refrigerant in my existing system and the refrigerants now used? That doesn't make a lot of sense to me- why can't I just use the blower fan on the existing air handler??

Legit situation. Search for EPA Puron replacement for Freon. The need to change out older air handler heat exchanger is due to big differences in pressures flow rates and equipment specs.

The real scan here would be if a contractor tries to sell you new equipment (Puron only since...2010 or 2014) and tells you it is compatible with your old equipment (Freon based).

[Hopeless Sinner]

Well, I've just gotten my first quote back. Still waiting on others.


Any thoughts, suggestions, etc?

Goodman built a huge new factory in TX and are an "economy" brand. Amana is a brand built by Goodman. What they have quoted you is a completely new heating and cooling system. It's an electric heat pump w 20 KW electric back up. The first thing I would ask is if natural gas is available at your home because that opens you up to using NG for heat. Get the specific model numbers so you can research reviews online and check warranties of each component. There appear to be rebates on heat pumps available depending on your electricity supplier. I also would check on the viability of heat pumps in your climate. I understand that they have improved recently and are a lot more common in cold climates than they used to be. Edit to add: if the electric companies are rebating on new heat pump installs thats probably all you need to know about their effectiveness and reliability. Then be sure the installers themselves are technically qualified and reputable.

I will say simpler is less prone to need repair so the variable speed air handler is something you might not need, but thats old tech now so prob reliable. 16 SEER is a pretty high rating, but local codes sometimes demand 18 SEER. FYI Goodman has models that are compatible w R-22 and R410. They seem to be using the refrigerant type in order to swing you into a complete new system, but w/out knowing the age, condition, and efficiency of your present system it's hard to know if installing a heat pump is warranted other than the need to add AC. Of course based on electric company rebates it's pretty clear that a heat pump will be more efficient than your present unit,

What the installer wants to avoid is callbacks and having 2 pieces of equipment not specifically engineered to work together requires a LOT more time and thought on their part to make it work and keep it running. They actually have to have training, ability, and experience which is pretty hard to come by in the HVAC world w few exceptions. Of course they are always pressed for time so by doing the whole system they have a pretty good understanding of how long everything should take so they maximize the profit compared to the time spent.

[ötzi]

I don't remember where you live Kevo but if it's in a cold climate definitely look into putting in a condensing gas boiler and central AC instead of a heat pump. Condensing gas boilers are very efficient and heat pumps lose efficiency as it gets colder and when they have to go to resistance heat they're expensive to run, that's basically just straight electric heat.

And when they're in heat pump mode it's "cold heat" - the output air is only a few degrees warmer than the room air and it feels chilly on your skin if you get in the path of the moving air. You'll hear about it from your wife, guaranteed.

[glademaster]

He mentioned that he has a forced air electric furnace for heat currently. My guess is that there isn't natural gas available where he lives.