HVAC Question/problem

[49erinnc]

My central air compressor outside just seems to run 24/7, even when the fan in the attic isn't triggered by the thermostat dropping below a certain point. I'm on a heat pump system and the heater works just fine. But for some reason, the fan outside just keeps going non-stop. I flipped the breaker to kill the power to it and then back on. But it just starts right back up and doesn't quit spinning.

Any idea what the problem is?

 

[FoBoT]

maybe it is a feature

 

[49erinnc]

Originally posted by: FoBoT
maybe it is a feature

I don't think the compressor/fan outside is supposed to continuously run. I don't remember it doing that before. Doesn't really make sense for it to run if the heater fan isn't coming on.

 

[Rubycon]

If you have a heat pump the outdoor fan coil and compressor is used both for heating and cooling. It's normal for the outdoor fan motor to run during a call for heat. If the coil gets ice/frost on it a defrost cycle which temporarily stops the fan and engages the reversing valve so the coil becomes the condenser and gets hot to melt the ice. If it gets too thick it will stop airflow which reduces the efficiency of the unit and raises the effective balance point.

ANYHOW(!) - it appears that you have a stuck relay. Some two pole contactors can have their armatures freeze in the down position keeping the unit running continuously. Sometimes an arc can weld the contacts together, etc. The thermostat itself could be bad too.

If the compressor is running and the indoor fan motor is not, the system head pressure will increase to excessive levels, well over 350 psig for a R22 system. This can be verified by feeling the system liquid line. This is the skinny line that's often not wrapped in foam. (armaflex)

If you're not comfortable dickering with mains voltage (240VAC) in the j-box of your outside unit a service call to a qualified HVAC technician is due without delay.

 

[49erinnc]

Originally posted by: MS Dawn
If you have a heat pump the outdoor fan coil and compressor is used both for heating and cooling. It's normal for the outdoor fan motor to run during a call for heat. If the coil gets ice/frost on it a defrost cycle which temporarily stops the fan and engages the reversing valve so the coil becomes the condenser and gets hot to melt the ice. If it gets too thick it will stop airflow which reduces the efficiency of the unit and raises the effective balance point.

ANYHOW(!) - it appears that you have a stuck relay. Some two pole contactors can have their armatures freeze in the down position keeping the unit running continuously. Sometimes an arc can weld the contacts together, etc. The thermostat itself could be bad too.

If the compressor is running and the indoor fan motor is not, the system head pressure will increase to excessive levels, well over 350 psig for a R22 system. This can be verified by feeling the system liquid line. This is the skinny line that's often not wrapped in foam. (armaflex)

If you're not comfortable dickering with mains voltage (240VAC) in the j-box of your outside unit a service call to a qualified HVAC technician is due without delay.

You're all over it MS Dawn. 🙂 I killed the outside breaker and removed the cover. Sure enough, the relay was stuck. I just tapped on it a little and it freed up. Flipped the breaker back on and the fan didn't come on (good thing). Just turned up the heat to make sure it's working, which it is. Cut the thermostat back down to make sure the fan would cut off this time and it did. And oh, the system liquid line was pretty darn hot. Hopefully no damage was done while the fan had been running (at least a day before I realized it).

With that said, I think I should get a new relay to prevent it from happening again. This one is really old and I could see some carbon (??) buildup just under/around the relay where the wire connects. Probably wouldn't hurt to get a new one in there to avoid having my compressor burn up from running nonstop should the relay get stuck again.

 

[Rubycon]

Originally posted by: 49erinnc

You're all over it MS Dawn. 🙂 I killed the outside breaker and removed the cover. Sure enough, the relay was stuck. I just tapped on it a little and it freed up. Flipped the breaker back on and the fan didn't come on (good thing). Just turned up the heat to make sure it's working, which it is. Cut the thermostat back down to make sure the fan would cut off this time and it did. And oh, the system liquid line was pretty darn hot. Hopefully no damage was done while the fan had been running (at least a day before I realized it).

With that said, I think I should get a new relay to prevent it from happening again. This one is really old and I could see some carbon (??) buildup just under/around the relay where the wire connects. Probably wouldn't hurt to get a new one in there to avoid having my compressor burn up from running nonstop should the relay get stuck again.

The main issue at hand is unnecessary operating cost. There is a high head pressure switch that will prevent the system head pressure from getting too high. Luckily yours is an automatic reset. 🙂

Definitely replace the contactor and make sure the replacement has the same (or greater) amperage rating and obviously the same coil voltage. (most likely 24V)

 

[d3n]

Nice info MS Dawn, I think it helps to get familiar with your cooling and heating system. I had a furnace that wouldn't start one time and it turned out to be a bad draft inducer. Another time after coming back from a deployment my AC quit working. Turns out the filters hadn't been changed in about six months causing the coils to ice up.

Professional HVAC guys are a skilled set of people but I am all for being self informed and being able to get things going again in a pinch. That and saving about 500 to 600 bucks in service calls was nice.