New construction home - HVAC in attic?!

Ugh. Glad I'm not your builder.....

 

My handler is in the attic. Just ridge and soffit vents. Insulation is just for the interior ceilings. I had the house built and requested to put the handler up there to use unused space. Only trouble I've had was the circulation fan motor bearings seized up. So I replaced the motor. My duct work looks similar to what's in your pics. Yes it gets real hot up there in the summer and real cold in the winter. Sometimes I wish I had gable vents/fans installed just to MAYBE help the cooling/life of the shingles.

 

I have two AC units and each has an air handler in a vented, unconditioned attic space. House is 2400 sq ft. Neither of those spaces gets insanely hot in the summer. I have grey shingles. It can be windy where I am on the coast, maybe that helps, no idea. But my electric bills are about $125 a month. One of my systems lasted 19 years, the other is still chugging along at 20.

 

I was curious about putting the HVAC system in the garage, apparently it's not allowed anymore. Unless this was just a suggestion:
https://basc.pnnl.gov/resource-guides/no-ducts-or-air-handlers-located-garage

 

I'm really not trying to make this difficult but I want the home to be efficient. The pics you're looking at are of some buildings already finished. The one I'm going to get is still slab right now. My only requests were fairly easy - 2 addtl lights in the garage, move thermostat from 3rd floor to 2nd floor, and spray foam the attic while leaving the attic floor uninsulated. Really not that big of a deal. I was going to request an additional HVAC return vent, but I think the 20x30 return upstairs is enough for 1550 sqft, that would have also been a much more involved request than simply adding a couple of fixtures and spray foaming.

It seems I've ruffled some feathers because this is "how it's been done" for so long, and people think there's nothing wrong with doing it the old way. But immediately conducting research on the very topic of HVACs in the attic, I was bombarded with articles and research about how this is not good for efficiency and the best way to do it is to spray foam the attic rafters/roof decking and moving the attic inside the envelope. If the air is moving through a more temperature regulated space, there will be a lot less energy loss.

It's kinda like trying to cool your engine without having a radiator. Or having your radiator tucked behind your engine rather than in front. Or to be even more on the same topic, how about moving your AC condenser to the rear of the engine bay and sticking your evaporator in the engine bay. Your truck's AC system would be essentially useless at that point. It just doesn't make any sense. Why would you want to run cool air through an oven before it goes into the home? The R value of the ducting is usually fairly low to begin with.. and as soon as the AC shuts off, that ducting is going to heat up quickly.

 

OP is attempting to build to a higher building envelope standard than minimum code . I don't think many posters understand the concept of a conditioned attic space . All that is really happening is moving the insulation to the rafter bays from the ceiling joists , which then puts the attic within the building envelope , ie inside the conditioned building space instead of essentially outside .

Doing this is no different than framing a catherdral (vaulted) ceiling/roof where the insulation is in the rafters and there is no attic space .

 

Here's a paper from the DOE on it..https://www.nrel.gov/docs/fy10osti/48163.pdf there are many other papers if people really want to you know, LEARN. https://www.osti.gov/

 

Unless you're going full "net zero" construction, conditioning your attic and doing the spray foam thing is Rube Goldberg. Making sure your builder uses good duct material and installs it right is cheaper and more effective.

 

Now we have to remember that different areas of the country and Canada build differently. I am not really familiar with HVAC units in the attic of residential buildings, so I won't make suggestions. I can tell you that in the Mountain states our hi-efficiency furnaces have two 3" or 4" PVC (depends on length of run) pipes that run from the furnace to the exterior. One of the runs is the fresh air intake and the other run is the exhaust. I am weak on my HVAC skills and this may not be the case for the southern US.



3" PVC Intake and Exhaust and 3/4 inch PVC condensate drainline

4BYTOY

 

That's what the furnace does in this unit also. It's called a sealed combustion system, which is unlike atmospheric systems in the days of old. I am not entire sure if this system will draw in "fresh" air into the home this way or if this air is only used for the heating portion of the HVAC. I don't know enough these mechanics of HVAC systems.

 

I do agree that the newer hi-efficiency systems are a sealed combustion unit, nothing like the old systems that took their combustion air from inside the house.

The furnace pictured above is a 95% efficient unit described as "The higher-efficiency models are called “condensing gas furnaces.” They run exhaust gases through a second heat exchanger to extract and use available heat that’s otherwise exhausted. These models pull out nearly all of the heat, sending cool exhaust out and leaving behind condensed water. This condensate, 5 to 6 gallons per day, is drained or pumped away by a condensate pump connected to the appliance." Hence the need for the 3/4 inch PVC drain.

I believe that the furnace pictured only draws in enough fresh air for combustion, not enough for a complete air exchange for the house. I'm wondering if you may have to look into an air exchanger to provide whole house air exchanges.

Curious as to what you find out as far as whole house air exchange

4BYTOY

 

This one is 95.5. So I guess it's a condensing gas furnace

 

What's so big a deal about moving the thermal envelope from the ceiling joists to the rafters ?

 

I see why you would want to have the attic conditioned if you want to use it as a bonus room or something
I used to work in HVAC. When it's got really cold out I live in north ga so only a little warmer and just as humid as Nashville. Those high efficiency furnaces the exhaust pipe would freeze up with condensate.
They suck in air in the intake for combustion and it goes out the exhaust or flue pipe.
We never did any air exchangers that put outside air into the conditioned space or air conditioning that pulled in fresh air or furnaces either. They all pulled in from the house. In the south an a/c that pulls in fresh air from outside wouldn't work to great since it will have have to use energy to condensate the humidity out of the air instead of cooling it.
I've seen air conditioners that pulled in fresh air (Aaon unit comes to mind) but they were only on big warehouses and plants and I think it was mostly to circulate air through the building like your talking about but also to cut down on humidity and keep the ceiling from sweating.

 

Might be a cheaper alternative to request that they use a higher than normal r-value duct
Also request the plenums on the duct be metal wrapped with insulation instead of cut out of duct board.

 

OP the air handler will pull the air from inside the house and cycle that back into the house.

Are you thinking that the air is pulled directly from the attic and into the airhandler?

 

It would certainly be easier to have them add this vs foaming the attic (probably, I'm not sure how resistant they will be to these types of things as usually appliance and fixture "upgrades" are more in their ballpark). Might be able to get the duct work directly foamed later, I've seen that as another alternative for HVAC attics

 

Houses need some amount of outside air sucked in to replenish lost oxygen and to help with things like dryer and bathroom vents. With most homes this is done just through small cracks here and there but more sealed homes need some kind of venting it seems. I'm not talking about the return venting.

 

You want me to really mess with your head about efficiency?
An electric heater is considered 99 or 100% efficient because all of the power from electricity is used directly inside the building that is being heated. Everybody knows electric head will run up the power bill.
A heat pump is considered 85 or 90% efficient or whatever because some of the electricity used to run it is heating up the compressor and condenser fan outside the building. But a heat pump will convert 2.5 to 3x as many BTU's per watt of electricity used compared to an electric heater.

 

Just have them install 2 fart fans in the bathroom and put one in upside down.