Dumb air handler electrical question

SteamedInWharton

Homeowner and electrical engineer here...

When Central AC is installed (steam heat, no furnace), how is power (not thermostat/control) provided to the air handler/fan? Does it come off the compressor circuit or compressor disconnect box? Separate 120 V or 240 V circuit to the attic? Hooked to existing circuit in the attic?

mattmia2

Separate usually 120v circuit.

ChrisJ

mattmia2 said:

Separate usually 120v circuit.

Most if not all of the air handlers I've seen including my own are 240V. But yes, an independent circuit just for the air handler, 2 breakers.

They are set up this way so it's easier to use resistance heating with them.

EBEBRATT-Ed

Some air handlers are 240 volt, and some are 120 volt. Either one would require a dedicated circuit. In addition the condensing unit compressor outside would require it's own 240 volt dedicated circuit. The 24 volt power for the thermostat usually comes from the air handler.

SteamedInWharton

Great answers and very helpful esp. the tidbit about the 24 volt source. Next question: can a whole house dehumidifier run on the air handler circuit?

JUGHNE

The couple I have installed just plug into a 120V outlet. Dedicated circuit is good.

Would this be in the basement?

SteamedInWharton

I would prefer to keep all of the AC equipment out of the moldy & musty basement/crawl space area. It's going in the attic.

ChrisJ

SteamedInWharton said:

I would prefer to keep all of the AC equipment out of the moldy & musty basement/crawl space area. It's going in the attic.

I put mine in the attic.
I wish it could've been in the crawl space.


BUT in my case, the crawl spaces are very dry and cool where my attic is brutal.

JUGHNE

Steamed, it sounds like your basement might need a dehumidifier.

SteamedInWharton

The basement has a dehumidifier. The crawlspace just got encapsulated and additional drainage was done in the basement. I haven't had a heavy enough rain to evaluate the effects of the improvements (maybe with today's rain and tonight's rain...). The crawlspace is drier but I think could still use some a dehumidifier of its own.

JUGHNE

From an install and service standpoint, I would always avoid any install of duct in an attic.
No mater how well insulated there is always some heat gain to the AC in the summertime.

Perhaps you have a stand up walk around attic which might justify the install and servicing.
(IIRC ChrisJ has this advantage).

ChrisJ

JUGHNE said:

From an install and service standpoint, I would always avoid any install of duct in an attic. No mater how well insulated there is always some heat gain to the AC in the summertime. Perhaps you have a stand up walk around attic which might justify the install and servicing. (IIRC ChrisJ has this advantage).

Walkup stairs and you can almost stand up but not quite unless you're 5' tall.

Could be worse but it's fun going up there when it's 130+ in the attic.

SteamedInWharton

JUGHNE said:

From an install and service standpoint, I would always avoid any install of duct in an attic.
No mater how well insulated there is always some heat gain to the AC in the summertime.

Perhaps you have a stand up walk around attic which might justify the install and servicing.
(IIRC ChrisJ has this advantage).

I agree, attic is not ideal but mine is walk up and does have some high sections by the gables/peaks. I could sacrifice a closet for the air handler but it would still need to connect to duct work in the attic. I'm wondering if my laundry room would work, except it's also half bath. If it could be sealed up (so it won't suck in bathroom smells) with both supply and return ducts originating from the attic, I'd consider that.

ChrisJ

SteamedInWharton said:

JUGHNE said:

From an install and service standpoint, I would always avoid any install of duct in an attic.
No mater how well insulated there is always some heat gain to the AC in the summertime.

Perhaps you have a stand up walk around attic which might justify the install and servicing.
(IIRC ChrisJ has this advantage).

I agree, attic is not ideal but mine is walk up and does have some high sections by the gables/peaks. I could sacrifice a closet for the air handler but it would still need to connect to duct work in the attic. I'm wondering if my laundry room would work, except it's also half bath. If it could be sealed up (so it won't suck in bathroom smells) with both supply and return ducts originating from the attic, I'd consider that.

Bathroom smells vs what my 150+ year old attic smells like on a hot summer day.....
Hard to decide on that one.

I chose the attic for several reasons. One was I didn't want to lose more useable space and proper ductwork is quite large. DO NOT undersize the ductwork.

For example, according to what I used a 7" is good for about 110 cfm, 8" 160 cfm, 9" 220 cfm and 10" 300 cfm. Of course length, turns etc all come into play, but those are the numbers I used. I used flex duct at the end of all runs for about 5' or so for noise attenuation. It barely effects airflow if pulled tight and 5' is enough to kill most noise.

EdTheHeaterMan

Interesting fact. I quoted a new duct system for air conditioning a ranch home at the Jersey shore. the load calculation came out to 41,000(+/-) so I specified a 3.5Ton system to be installed in the attic.

The thing is, this house was raised about 11 ft after Super Storm Sandy. so the "Crawl Space" is now 12 feet from the dirt floor. I also did the same load calculation with the duct work in the crawlspace. the Calculation came out to 32,000(+/-) and it was a difficult sell to get the customer to agree to use the empty "crawl space". Since the crawl space is in the flood plane, the area can not be used as a basement and the access to the 12 foot high "Crawl Space" is only 24" high and 36" wide, it is not considered living space and can never be used as such

There is enough hight to keep the air handler and duct work high enough to pass inspection. (long story) but the point Im making here is if the crawl space is acceptable for ductwork, it is more efficient for the life of the system.

The alternative is to have the roof spray foam insulated and that will cool the attic down significantly! (ask me how I know?)