Cold air - hot air - cold air

by Robert
(Dudley, MA)

My 4-ton central ac unit is 8 years old. The Ac unit works normally for up to an hour then starts to blow warm to hot air.


If I shut it down for several hours it might start up cold again. If I leave it overnight it does start up cold then turn warm.

The Tech thought adding a kick-start cap might help. It did not. I get my system cleaned every season. Is there a relay or switch in an HVAC system that could be faulty? Or compressor?

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Reversing valve
by: Anonymous


The reversing valve is energized in the heating mode for a few minutes and then turned to the cooling mode automatically. After these courses repeat two or three times, compressor stops.

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The Fix - Stuck open valve
by: Bob

The problem was found more-or-less by accident when the service person touched the hot water pipe on the hydro system and found it to be very hot. The Eureka moment.

The hot water valve to the hydro air system was stuck open. So the AC unit was trying to cool the house while fighting the heat-water. The air was only cool coming out - maybe 67F until there was a call for hot water heating (separate tank).

Then the flow of hot water would also go to the AC unit overwhelming the cooling and turning to heat. The valve was electrically disconnected, manually shut, and the air temp dropped to 53F and the AC is back to normal.

Valve on order. This is a TACO valve and it's not that old - less than 4yrs probably. Taco is supposed to be a good brand and I know these valves get used in some very high-reliability areas in manufacturing. Disappointing to see that as the failure item.

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Central AC
by: Anonymous

Thanks Shawn - it's a Central AC unit. The fan is working when it turns hot. I don't think its coolent either. It seems like a switch or relay issue to me as well. I can't see any freeze ups. I just hate to have a tech "shotgun" a compressor. That's a costly non-repair. Friday is my appt day. I'll post the end solution once its really ready to go!

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Is this Heat Pump??
by: Shawn

Is this Heat Pump or regular central air conditioner units?

If this is regular central ac units, does the condenser fan (outside fan) works when it produces hot air? It sound like the compressor has internal safety switch kicking in.

"can low coolant pressure be to blame?" Central air conditioner will have low pressure and high pressure switch. I doubt its low on refrigerant. I meant a tech look at it. He will know if it low on refrigerant.


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Started this season
by: Bob

The problem started with this cooling season. It worked fine the first couple of warm days in limited action during April's 90F weather. Then back to heat for a while. This week it went cold to hot.

It worked for 1 hr last night and 30mins this morning before blowing hot again. Tech coming on Friday...can low coolant pressure be to blame? It seems if the compressor works - it works. So something else has to be flipping the system over.

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Cold air - hot air - cold air
by: Anonymous

Is this unit behaving like this from day one. OR have you done gas charging recently after which this problem started?

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