Air Conditioning - Central air basics

Hi all,

I have some basic questions about the operation of a split central air system.

1. Would the entire system be powered by 2 breakers? One for the air handler and one for the condenser?

2. Is it pretty much universal that the power for the thermostat comes from a transformer mounted on the air handler? And powered via the air handlers breaker?

3. Is the operation of the thermostat a simple binary operation, i.e. it's either off or on?

4. Do the air handler and condenser operate in lock step? I.e. they both turn on and off at the same time (via the thermostat)?

5. I would assume that if the condenser were to keep running while the air handler is off, it would be bad for the condenser. Is there and built-in mechanism that causes the condenser to shut off if the air handler is not working?

Thanks
NJ

1. Yes

2. Yes.

3. Yes.

4. Simple answer. Yes. In most new digital t-stats and some condensor units there are time delays mostly for short cycling of the compressor. But that will cause the indoor unit to start and then in a few minutes the condensor will start.

5. If the condensor continues to run without the indoor blower the evaporator will freeze up and send liquid refrigerant to the compressor eventually damaging it. Most lower end units have no safeties but the higher end units will at least have high and low pressure safties. The low pressure safety will come into play if the indoor blower is not operating or if for any other reason the suction pressure would drop below a pre-set pressure.

1.) Two breakers (one for the air handler and one for the condenser) in most cases. There are exceptions, however. Some older systems use three breakers: condenser, electric heat strips, blower motor. Some mini splits use one breaker.

2.) Yes, in most cases.

3.) Thermostat is on or off, nothing in between.

4.) Air handler and condenser are controlled by seperate wires connected the thermostat.

5.) In newer systems the condenser may shut off first and the blower may run a little longer (perhaps a minute or two), but not the other way around. If the condenser continues to run without the air handler blower running, you risk damage to your compressor. In many cases there is no safety device to shut down the condensing unit if the AH blower fails. Even so, one should not rely on it if the system has a known problem.

My condenser outside starts up first with the fan blowing outside. After about 30 secs, the air-handler comes on in my attic and blows the A/C. The shutdown process is opposite, as the condenser drops out first and after 30secs the air-handler shuts off. I think this is normal operation.

hexonx::: I think Id check the blower out for sure. The inside blower should start right away on a call for AC . Many have a time off so the blower will run for a short time.After the AC shuts off. Anytime the power is bad we will put a time delay on the condenser. So it will drop off with a volt drop .Or not try and come on with a high head

I don't remember the model number of the stat but Honeywell does make one where you can set a heating and cooling fan on delay and off delay.

We take care of a medical building with 15 of these on remote sensors. Stats are in one cabinet to keep the Docs, Nurses, and whomever from making adjustments.

If I remember next time I'm there I'll get the model and pass it on.

P.S we set the cooling on delay to 0 seconds.

Chris

mine does that also, the outside unit comes on first, then about 30 seconds or so the inside unit comes on. im not sure what order it shuts down.

My old (16 years) Janitrol operates exactly as Hexonx described. The theory was to give the refrigerant time to get a full flow of liquid to the metering device before bringing on the fan (less warm blow) & to purge all or most of the existing cooling capacity when the condenser shuts down.

Yes, Goodman (Janitrol) has a time delay relay for the blower. Blower operation is delayed about 10 to 30 seconds after stat calls for cooling. Blower shut-off is also delayed by about a minute or so after condenser shuts off.

i have a goodman, so thats good. for a second i thought something was wrong, LOL

You know the old saying, it doesn't matter what the Make, it's the installation that counts. I also have a Goodman system which has been good since I owned the house in 2000. That's 7 years now and the original owners kept all the manuals for everything, so the unit is about 14 years old. I had to reconnect the contactor CAP at the compressor end 4 years ago when it broke off due to high temps outside and lots of AC that year. The wire needed a new terminal crimp and I cleaned out all the cob-webs. ;)

Grady- Do you still have the Janitrol setup or are you referring old as the previous setup. I've been pretty happy with my rectangular old school looking Goodman and hope it will last another 5 years.

Despite the bashing of the Goodman brand, I think they make good equipment. We have installed many of their units and had very few warranty problems. The problems that did occur were minor, such as a bad contactor, bad blower time delay relay, a bad circuit breaker in an electric furnace. I did encounter a couple of leaking evaporator coils, however. But, I would say overall, they make good equipment. The 14 seer condensers use the Copeland scroll compressor with a 10 year warranty. We have installed many of these without any problems.

As far as older equipment, I see it all the time on service calls. Usually with minor problems. I just serviced a 20+ year old Janitrol heat pump. Nothing wrong with the equipment, just a bad digital stat. A friend recently upgraded his old janitrol equipment (25+ years?), nothing wrong with it, just upgraded to a new more efficient Goodman 14 seer.

Install is key, as previously mentioned. We always use nitrogen purge and deep vacuum. We even add an extra filter dryer.

my goodman heatpump is about 14 years old, it does ok. the A coil leaks, and i dont think it cools as good as it should, about a 11 degree drop from going in to coming out. but it does keep it feeling ok in here, so i cant complain to much.

...as a homeowner/DIYer I've heard many HVAC contractors bash since Goodman will sell(parts/units) to anyone off the streets compared to the traditional dealer/contractor only jobbers. FWIW...

My equipment is still in service & has been trouble free. I installed it, checked the refrigerant charge at the begining of the second season, & have not done anything to it since other than hosing down the outdoor coils once or twice each summer & changing the air filters regularly.
Being primarily a service person & working on all brands of equipment, I will tell you that 90%+ of the equipment failures are directly traceable to installation. Some equipment is more forgiving of a bad install than others but any of it, properly installed, is good stuff.

Thanks. I'm glad to hear that. Today I hit the breaker for the central AC to on and started it up. I always pop the breaker during winter just in case it accidentally get's turned on. :) Anyway, it worked and blew cold air but I want to flush out the outside coils as you stated. I only do it once a year but will consider doing it again later in early fall before I wrap it up for the winter. What's your take on condenser unit covers. Some folks it's not good to cover the unit. I think it's better to cover it than to have it open to the harsh elements.

DO NOT COVER. Most manufacturers specify in the homeowner's manual not to cover the condensing unit. Covering the unit can cause condensation & potentially rust out the bottom tray. If you want to do something to keep out leaves etc, a piece of plywood with some bricks on it works well.

Breakers should be turned on 24-48 hours prior to start up. This is to allow the oil in the compressor to warm & expell refrigerant from the oil.

Grady- Okay on the manufacturers info. I happen to have all my information from the previous owners who kept all manuals for everything they owned.
How ironic, condensation in the condenser unit. ;) I agree with you somewhat above freezing since that can be harmful and create rust. How about below freezing winter-time weather and use something like a breathable car cover instead of plastic? I have a cover from Home Depot and cut it so it could breath where the coolant pipe goes into the unit. I have religiously keeped rust away by using POR-15 on the hardware nuts and bolts. I will have to evaluate a better cover system. :( Thanks. I read the condenser install and operation manual and didn't see any cover information. My unit is orientated with the fan torwards the house so the servicable area is easy access. It's on a concrete slab, easy to clean and lots of plants around and the service area is easy to get to. You don't have to step into the jungle per say.

BTW, here is some A/C install trivia for you. I have it all here. The unit is a 10 SEER split system, condenser model CE30-1H and evaporator model A30-XX.
I have a heatloss printout using the software HEATLOSS v3.4. The total installed price was $3445.80 in 1992. Hmmm, sounds a bit high IMO. I think those are still priced around the same a few year ago. It must be a lot more now due to inflation.

It does seem ironic that condensation would be a problem in a condensing unit until one realizes the intended condensation is the refrigerant contained in the tubing. Some manufacturers do say a breatheable cover is ok. Personally, I never cover mine, not even the top.

If the price paid was for a replacement, it does sound high. If it included duct work, pretty reasonable.