AC unit

[Bubblegum Bandit]

Ok, so we get a call today that a realtor wants to show our house. Its in the low 80s outside and we have the windows open. Thermostat says its 78 in the house. Strangely enough, with the breeze outside, it doesn't feel that hot. Nevertheless, we decide we should kick the AC on so that its obvious to the home buyers taht we have AC. I walk to the thermostat, slide the lever to cool, and bump the target temp to 72. 30 minutes later the AC has still not kicked on? So, we open the windows again to get the breeze going through for the prospective buyers. I get home, do the same thing, slide to AC, set temp at 72, it has been 2+ hours now and it still hasn't kicked on? This is our first home, and we just had AC put in last summer, so I've never had to go from winter to summer in this situation. Please tell me I'm an idiot and that I need to flip a switch or something on the AC so that it is "ready" for the summer? I'm not in the mood to think that my 1 year old AC unit is bad.

 

Troubleshoot process so far:

 

1: Checked Breaker Panel. Everything is good.

2: Closed and reopened the furnace trip right next to the unit.

3: Thermostat wires are good.

4: Replaced Thermostat. Replaced Thermostat Batteries

[shep48]

large knife switch next to unit outside ?

[Bubblegum Bandit]

large knife switch next to unit outside ?

 

It doesn't have its own disconnect box, its routed straight from the fuse panel into the AC unit. Reset the breaker, although it was never tripped. Still nothing.

 

One thing I'm worried about though. I looked up the unit online. In the specs it says "Maximum Breaker Size = 20A" and its tied to a 40A. Put a call in to the guy who installed it, but I'm sure I won't hear from him until Monday.

 

5. I also double checked that the furnace was getting power....check

6. I checked the kill switch inside the furnace (the one that trips if you take the panel off).....check

 

Another note. We tried going the other route, and bumping up the temp to see if the heat would run and it wouldn't. Looks like a power to the unit issue. I'm confused though, because of #5 above. The kill switch (light switch next to the unit) has power to it and out of it if it is switched on. That's what led me to make sure the panel kill switch was still good. I have no clue from there....seems like could be a circuit board problem or something inside the unit.

[Bubblegum Bandit]

Next step.

 

7. Pulled the thermostat wires and left a fish tape behind (lol weedeater line, my fish tape was too fat). Checked continuity on all four wires. All good.

 

8. Tested the AC/DC converter where it switches 110AC to the low voltage for the thermostat and I'm getting nothing. So, I've pulled the converter off the box and I'm looking for a new one to replace it.

 

****Edit**** Talked to a buddy of mine who knows wiring etc better than me. Was actually doing an apprentice job before he went back to school for civil engineering. Anyway, he said that that is just a low voltage transformer and that its still AC power but that it should be either 12 or 24 volts. So, wired it back up and its good at the transformer. Checked the wires that I thought were coming into the furnace from that transformer, no power. Chase them back, and they don't even go to that transformer. They go out to the Air Conditioning unit outside. Go out, take cover off, 240 volts into the unit. I notice it has a 24 V step down transformer in it with the two wires coming off of it. No power coming off of them. Not sure what's going to happen here.

 

So I'm trying to figure this out? Should there be 24V out of the AC unit ALL the time, or just when the thermostat is sending a signal for the AC to come on?

[Bubblegum Bandit]

I guess I'm talking to myself anyway, but I figured out the problem. There is a 120V -> 24V 40VA step down transformer in the furnace that powers the thermostat and thus relays to the heating/cooling system. $15 at the local Grainger and about 5 minutes to replace and the house is nice and cool!!!

[stutters]

<points to the sign above the door "Please tell me I'm just an idiot">

 

uh, so i saw the sign on the outside. sounds good to me!

 

bub, you're just an idiot.

 

<scoots>

[Shazz]

Bub ftw!!

[appalachian_fox]

Excellent sleuthing! I came in here attempting to help, but I guess you got your questions answered

 

GG!

[Clueless]

There is usually a small box near the unit, either on the outside of the house, or where in comes into the house, could maybe be inside the unit i guess, but there are a couple tube like fuses, or breakers in there that blow out alot, especially after sitting awhile unused.

 

If you have power going beyond that point, into the unit, there is a solenoid switch that can blow out as well, they are cheap, should be less than 25 bucks.

 

The other thing to check, which would require a pro, would be the coolant level, or have the lines purged and recharged. Even a small amount of air in the lines can cause big problems.

 

I am not an HVAC guy, but I did have a house with a junk A/C unit that always needed worked on, lol.

 

Good luck!!!

[Brew]

I just graduated from a tech school in HVAC this spring and im disappointed that i didnt see this thread earlier

 

Anyways the transformer was the the problem. GJ. it supplies 24V to the furnace board, thermostat, compressor/condensor contactor relay coil, and fan relay coil. So you were not getting power to the control side of both systems.

So I'm trying to figure this out? Should there be 24V out of the AC unit ALL the time, or just when the thermostat is sending a signal for the AC to come on?

Only when the stat is calling for cool. it energizes the contactor relay coil that gives power to the compressor and/or condensor fan motor

 

 

 

for the future, thermostat wiring color code is: Red=hot(24v), White=heat, Yellow=cool, Green=fan

 

to check power to the stat: one lead of voltmeter on Red and the other to GROUND (should be 24Voltish)

 

if your calling for cooling: check voltage at R and Y at your stat (or the terminal block by the furnace board). If the voltmeter say "0" the switch is closed and it is calling for cool (open would be 24volts).

 

this works for FAN ON (Red to Green)

and HEAT (Red to White)

 

 

REMEMBER: a thermistat is a "switch" NOT a "load"!

 

 

One question Bubblegum: didnt you need a business account with Grainger to buy a 115/24 Transformer? normally you need to say what shop you work for. wouldnt you?

 

Anyways, you tha man!!

 

If anybody has any questions about a HVAC just let me know and ill try to help ya out

Edited May 16, 2006 by Brew

[Bubblegum Bandit]

My Engineering company has an account there. So I just paid a cash ticket on their account.

 

Most of what you summed up there I figured out just through common sense and testing as well as pulling stuff off the web. There was something behind the 220 power block on the AC unit that said 24V and I assumed it was a transformer, but I guess it was a relay? Like you said, when the thermostat sends the signal that tells the AC to turn on. Makes sense.

 

What confused me was that there's a step down transformer on the wall above the furnace kill switch that didn't even go into the furnace unit. Therefore I assumed the low voltage must have been coming from the AC unit. It wasn't until I was laying in the floor looking for a fuse that might be blown that I realized there was a transformer in the furnace as well. The one on the wall was for the door bell.

 

After I found that transformer and realized I had 110 coming in and nothing coming out that I was pretty sure it was to energize the low voltage circuit for the thermostat and that it must be the problem since everything else was getting power.

 

Thanks for the reply to clear a few things up though. I know where to go if I have more trouble. LOL

[appalachian_fox]

Learn something new every day! Brew, BGB, you guys rule.

[Brew]

The one on the wall was for the door bell.

hehe

 

 

Most of what you summed up there I figured out just through common sense and testing as well as pulling stuff off the web.

ya, the stuff is not rocket science.. but its one of those things that people won't put their hands on.

[NOFX]

good troubleshooting man.... what really gets me is how people don't want to take their precious time into figuring things out. It wasn't all that difficult, but I know many people, that would rather call someone out there and pay 100 dollars rather than check it out for themselves.