HVAC Guys - Hot A/C, Advice Needed

onewhippedpuppy · 09-18-2008, 08:15 AM

Both units in my home are original, 1989. Upstairs unit started blowing hot air with the A/C on. Low side line is much hotter than high side, compressor unit outside is operating but the air being blown out feels cool. Heater burner is not operating on the furnace. Not sure of the model number, but it's a Coleman with natural gas heater, installed in an attic. I had the A/C system charged (slightly low) about one month ago, and have a record of the a-coil being replaced in 1999.

So, suggestions? I've replaced a-coils and compressors before, but don't have enough experience to properly diagnose the actual problem. Upstairs unit hasn't ever blown particularly cold, hence the charging about one month ago.

rick-l · 09-18-2008, 08:33 AM

When you hooked your pressure gauges up what did they say?

onewhippedpuppy · 09-18-2008, 08:39 AM

I don't have a set of my own. I need to pick some up, in the past I've been lazy and just borrowed some. I thought maybe I'd get some educated guesses from my observations.

rick-l · 09-18-2008, 09:12 AM

Are you sure the compressor is running and not just the condenser fan?

rattlsnak · rattlsnak's Garage

Without gauges, it would be just that, a guess.

scottmandue · 09-18-2008, 11:34 AM

Have you checked the filters?

+1 on is the compressor running?

onewhippedpuppy · 09-18-2008, 01:22 PM

I'll double check, but I'm pretty certain the compressor is running. Otherwise I can't imagine why the lines would be much hotter than ambient.

sammyg2 · 09-18-2008, 01:53 PM

Sounds to me like you need to move the condenser inside and the evaporator outside

Brian 162 · 09-18-2008, 07:23 PM

Sound like you have a heat pump, and the reversing valve is stuck in heating mode.

If it isn't a heat pump the only other thing that I can think is that the compressor us running backwards. A set of gauges would help diagnose the problem

billybek · billybek's Garage

Reciprocating compressors always turn the right way. They pump regardless of direction of rotation. Single phase motors are set for direction when manufactured.
It could be that the compressor is seized and is just heating up and tripping on thermal safeties inside the compressor.
What do the lines feel like at the A coil? If no heat the chances are good that you need a compressor.

Tobra · Tobra's Garage

tripped breaker, low freon can give you those symptoms

That is a pretty old unit, if you replace it, you will save money on your electric bill

onewhippedpuppy · 09-19-2008, 06:35 AM

Not a heat pump, or it would make sense. Filter is brand new. Lines at the A-coil are the same as outside, the low side line is hotter than the high side. Both are much hotter than ambient. I'll try to pick up a set of gauges and get some real data.

billybek · billybek's Garage

Shut off the power to the condensing unit, pull top/ fan housing off centre, the fan wires will be attached so just move it out of the way and feel how warm/hot/very hot the compressor is. If you can fry eggs on it you have a bit of a problem.
Leave the power off until it cools down. Turn cooling off at T'stat.
When compressor cools down re assemble condensing unit, restore the power to the unit, then have a helper turn the cooling on while you listen to the compressor start. If you have gauges, all the better, put them on before starting, hi and low side.
You should hear the compressor start. It may hum and drop on thermals. It may not make any pressure while running.
An amp clamp would be helpful to know if the compressor is sitting on locked rotor amps (LRA) or if it is drawing the correct rated load amps (RLA).

scottmandue · 09-19-2008, 04:11 PM

It has been light years since I have worked HVAC.

Any ice on the coils?

onewhippedpuppy · 09-22-2008, 06:19 PM

A bit of a head scratcher here. My upstairs unit seems to be a heat pump converted to a forced air gas furnace/A/C. Both heat pump control units have been cut loose, and a new unit put in place to control the gas ignition and gas valve. Outside unit has been modified, with some wiring work done, as well as a new contactor.

So, I checked the a-coil and found it to be clean. I cleaned it with a foaming a-coil cleaner anyway. Outside unit I found that the compressor not only operates correctly, but that the lines are cold. Until they get to the reversing valve. Lines coming out of the unit are hot. Low side is hotter than high side, air blowing out of the unit is cool. Basically, the outside unit is acting like a heat pump in heat pump mode.

After doing some reading, I think the reversing valve is stuck in heat mode. The solenoid has voltage, I cut it and found no change. Changing the valve itself requires a LOT of soldering work, I think replacing the unit might be more logical in the long run.

Brian 162 · 06:27 PM

Quote:

Originally Posted by onewhippedpuppy

A bit of a head scratcher here. My upstairs unit seems to be a heat pump converted to a forced air gas furnace/A/C. Both heat pump control units have been cut loose, and a new unit put in place to control the gas ignition and gas valve. Outside unit has been modified, with some wiring work done, as well as a new contactor.

So, I checked the a-coil and found it to be clean. I cleaned it with a foaming a-coil cleaner anyway. Outside unit I found that the compressor not only operates correctly, but that the lines are cold. Until they get to the reversing valve. Lines coming out of the unit are hot. Low side is hotter than high side, air blowing out of the unit is cool. Basically, the outside unit is acting like a heat pump in heat pump mode.

After doing some reading, I think the reversing valve is stuck in heat mode. The solenoid has voltage, I cut it and found no change. Changing the valve itself requires a LOT of soldering work, I think replacing the unit might be more logical in the long run.

Lucky guess on my part. For a 20 year old unit you are better off replacing the complete system. The upfront cost may be a little high but newer units are more efficient and quieter.
I'd get an A/C not a heat pump.

wilke3169 · 09-22-2008, 08:11 PM

You can check the coil at the reversing valve with a screw driver. With voltage on it there should be a magnetice field. When you turn power off to the solenoid it should lose the magnetism.
It does sound like you are stuck in heat but if you are going to repair the unit no need to just replace parts. I was in HVAC business for a long time and never found a bad reversing valve. Controls yes, valve no.

onewhippedpuppy · 09-24-2008, 12:05 PM

I had an HVAC guy out to confirm. Reversing valve coil is shot. PN is NLA, so I'm up a creek. I tried everywhere I could think of locally, as well as online, with no luck. Fortunately it's "only" 80s here, so I think we'll tough it out until spring and replace the entire heater/A/C system. I have no doubt that a modern high-efficiency system will save us money on bills when compared to a 20 year old heat pump converted to a gas furnace/A/C.

Rot 911 · 09-24-2008, 12:07 PM

Matt, I know finances are tight, but you might save money over the winter if you replace now. A more efficient furnace will save you on heating bills over the winter.

onewhippedpuppy · 09-24-2008, 12:10 PM

Kurt, I don't argue with you. However, this is a 2nd floor only unit. We have a large open foyer to the second floor, so the upstairs always stays fairly warm. Along with good sunlight in the winter, our upstairs heater really doesn't run much. We also plan to get a pellet stove in our basement this year, so that will only help the issue.