AC out - anyone know how to troubleshoot a thermostat?

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

lassoharp

Well-known member
Joined
Jan 3, 2009
Messages
2,100
Location
USA
House AC is not blowing cool air.  Heat wave.  Might be a bad thermostat. Don't want to replace it if it's not necessary.  Anyone know how to troubleshoot a thermostat?  It's an older all mechanical type. No manuals online.
 
You could try wiring the AC/FAN relays to the 24VAC line and see if you get cool.


http://home.howstuffworks.com/home-thermostat3.htm

Has the color codes and functions.

Mark
 
we have had many hvac troubles in the past at work... Make sure of these few things

1. the duct work is in tact

2. the unit has not frozen over

3.freon levels are proper

 
Does the condensor turn on outside when you drop the temperature on the thermostat?
Have you had anyone in lately doing service work?
Is the condensor outside dirty?

If the condensor doesn't come on outside, the contactor may not be getting 24vac, or the contactor may not be getting 240v from the breaker box/panel.
You could check the wiring at the furnace terminal strip(control board). Y and R should be secured
to their respective terminals. If they are, it may be a bad stat...or loose wiring behind the stat.
You could remove the stat and check the wiring there. Y and R.

If the condensor does come on, you may have a leak and lost gas.
 
desol said:
Does the condensor turn on outside when you drop the temperature on the thermostat?
Have you had anyone in lately doing service work?
Is the condensor outside dirty?

If the condensor doesn't come on outside, the contactor may not be getting 24vac, or the contactor may not be getting 240v from the breaker box/panel.
You could check the wiring at the furnace terminal strip(control board). Y and R should be secured
to their respective terminals. If they are, it may be a bad stat...or loose wiring behind the stat.
You could remove the stat and check the wiring there. Y and R.

If the condensor does come on, you may have a leak and lost gas.


Checking the circuit board connections is where I'm at right now.  I'm getting a reading of 28VAC from RC(cooling relay) to Ground.  Not sure if that clears a t-stat issue or not.  With the middle panel removed the circuit cannot switch on.  Y and R look good and secure. 

Doesn't look like there's an easy place to check the contactor outside.

I'm not 100% on whether the compressor is coming on - it's difficult to tell with the fan going at the same time.

There is the outside possibility that one of the copper lines was bumped by some plumbers who were under the house a few months ago, causing a small leak of refrigerant.

Another odd possibility relayed to me by a local HVAC repair guy - Earwigs or other bugs infesting/interrupting  switch connections inside the outside unit.  We've had a huge population explosion of earwigs locally over the last few years for some reason. 
 
Not likely to the the stat but worth checking.  There is no absolute color code but typically:
red = hot
green = fan
blue or yellow = cool
white or black = heat

So the air handler fan is blowing.  This 50% rules out a T-stat problem.  You know the furnace is getting power, which is where the T-stat transformer lives.  The T-stat is getting power or the fan wouldn't blow.  It's possible the mercury switch is done for, or the cool contact is out, again... not very likely.

>I'm not 100% on whether the compressor is coming on - it's difficult to tell with the fan going at the same time.

Doh!  At first I thought you meant the circulation blower, but I guess you mean the fan on the outside unit.  Well your T-stat is good.  Put your hand on the 2 refrigerant lines.  The larger uninsulated line should be hot to the touch, and the smaller insulated line should be cold to the touch.  If there's ice on the cold line, you're frozen up.  Shut it off for an hour to let it thaw.  This usually means your inside coils are blocked with dirt, no air can flow across them, therefore no heat exchange which = a freeze up.  If one line is warm and the other is cool (not hot & cold), you're probably just low on freon.  If they are the same temp, your compressor ain't working.  The same relay turns on the compressor and the outside fan so there ain't no point in opening up the outside unit just yet.  Shut the system down for 5 minutes.  Grab a beer and stand by the outside unit.  Have the wifey turn on the AC.  Listen for a struggling compressor.  If you hear it strain for like 5 seconds and then stop, you know your compressor is at best knocking on death's door if not completely done for.  You might be able to get a little more life out of it by replacing the start capacitor with a larger one, nick named a "kick start"  or "hard start" cap.  This is a bandage fix and actually is bad for the compressor, but if it's a 15 years old unit and all you gots is a 20 dollar bill....... well..... you know. 

-Richard
 
To further eliminate the t-stat you can call for heating, just for a minute or two.
Buttery has you on the path to cooling.  You have to suss the compressor outside.
Earwigs- YUCK!
Mike
 
Thanks Richard -

After running the unit for about 5 minutes - both lines are the same temp.

So I guess that's the $5000.00 answer to the question.

People seem to have differing opinions on what can be done as far as trying to replace/fix the compressor or putting in a whole new system.  It doesn't sound like there's much middle ground there.

We'll see what the HVAC guy says.  Of all the times of the year for the AC to give up the ghost . . . . . . .
 
Butterylicious said:
The larger uninsulated line should be hot to the touch, and the smaller insulated line should be cold to the touch.

The hot, high pressure line (liquid)...is actually the small line. The cold, low pressure line is the larger insulated line.

If the lines are the same temp and the outside unit is coming on with no banging or weird sounds...it's pretty much certain that you have a leak. Or the compressor may not be coming on at all.
 
You could still get lucky.  A tech might be able to dump some freon in and get a little more life out of it.  The gauges will properly diagnose the problem.  Right now, if both lines are the same (ambient) temp, ain't nothing happening.  I feel your pain.

A few years back, one of my heat pumps stopped cooling.  I had a business associate who owed me a favor so I had them come out and look at it.  It was a stuck reversing valve.  The favor I mentioned was worth a grand so they billed 2 grand.  I flipped out and they knocked the priced down to a grand which is still a total ripoff.  Less than 1 year later, same deal.... the unit quits cooling.  This time I have a trusted friend look at it.  Stuck reversing valve again.  He showed me how they had obviously gotten it too hot when soldering it in because it was blue.  He also said it was time to replace the unit anyway.  I had him look at the other unit.  He gauged on, whistled, anf gauged off.  He said, "I'm gonna stop right here before you accuse me of braking this unit."  Yup... I got to replace 2 complete systems.  The good news is, my house is now cool and my electric bill is less because I bit the bullet and put in the highest rated systems I could find.  They're Carrier "Puron" systems with a SEER rating of 21.  I'm very pleased with their performance.  TempStar is the generic Carrier brand.  Of course Trane is a big name popular brand.  My old systems were Lennox so I give them a thumbs down.  

I'd suggest not skimping and absolutely replace the inside coil too if you replace the compressor.  If the coil is jammed up with dirt, insufficient air will flow across it thus limiting the heat exchange which puts unnecessary load on the new compressor.  
 
>The hot, high pressure line (liquid)...is actually the small line. The cold, low pressure line is the larger insulated line.

Doh!  You are correct.  It was late last night when I wrote that.
 
Yep. I like Carrier too. Hopefully the tech can solve the leak for lasso and re-gas...as well, it's illegal (not sure in the states) to re-gas a system with a 'known' leak...so he can't just fill it up...and leave. I'm assuming it's the same laws there.
 
desol said:
Yep. I like Carrier too. Hopefully the tech can solve the leak for lasso and re-gas...as well, it's illegal (not sure in the states) to re-gas a system with a 'known' leak...so he can't just fill it up...and leave. I'm assuming it's the same laws there.


Yes, there are and it complicates things for the techs.  They try and avoid that if possible. If there is a leak I'm curious to see if there's reasonable evidence that it was caused by the plumbers.  I know the exact area under the house where they were working. I don't know what the odds are of making a claim in court but I'll certainly pursue it if there is.  Too many stories of carelessness causing big problems  If it's a single spot leak they may be able to patch it but there's likely excessive regulations to deal with.
 
Good news.  Bad starting cap.  Wish I'd known to check it.  Still 170.00 is better than 5 grand!

Gotta clean the coils. . . .

cool again  8)
 
Excellent. This was the remote chance i was hoping for lasso... :)

Much better than a leak. So, the compressor side of the cap must have been bad...but the fan side ok.
Fan was coming on...compressor not..
 
Yup  - me too  ;D

Actually used two big cans - one for comp and one for fan. Fan was ok. 
 
Back
Top