(Sent here from alt.hvac)
Does a heat pump's defrost control board change the voltage level
seen at the thermostat on the reversing valve lead?
When in heat mode but blowing cold air, the reversing valve
voltage at the thermostat is 0V as opposed to 26.6V when
My heat pump puts out very good heat but it also seems to love to
periodically blow cold *far* more often and for *far* longer than I'd
expect to be due only to the defrost cycle. (The unit does not
have resistive heater strips).
Sometimes it seems to get "stuck" blowing cold for hours and sometimes it
blows cold at least 50% of the time.
I'm trying to determine if the thermostat or the defrost control board
has gone insane.
A tech came out recently and could find nothing wrong with the heat pump.
The refigerant level was perfect and its diagnostic board reported no
problems. It is a trane.
It was of course not misbehaving when he was here.
This is Arizona and therefore only about 50F with low humidity.
Also, the heat pump, during summer months, sometimes does the
opposite. It will blow hot air when it should be blowing cold.
Thanks in advance for any help.
The supply temperature of an air to air heat pump will feel cold because
it's lower than your body temperature. He has no back up heat, so the air
never feels warm. In a typical tract home where the builder hired the low
bid HVAC contractor, the duct work may be too small, so the velocity of air
coming out feels even colder because of a wind chill effect on the body.
However, in this case, it will blow as cold as when in AC mode for
very extended periods ( much longer than defrost cycles ).
So cold for so long that the whole house gets gradually colder instead
of gradually warmer.
I'm really curious if the defrost circuit is bad or if the
thermostat is insane.
When the heat pump desides to start blowing cold, the voltage
at the reversing valve lead at the thermostat goes from
26.6V to 0.0V.
Is this voltage change due to a "command" from the defrost control circuit?
Can the defrost control circuit even drive that voltage level?
If not, It would seem that the cold is due to the thermostat
loosing its mind.
Again, thanks for any help.
Most units (not all) have a board that will only allow so long of a defrost
whether it's done or not. Not saying that it ISN'T getting stuck in defrost,
but not likely. Could be the reversing valve not "opening back up" all the
way when coming out of defrost. Have you looked at the outdoor unit when
it's blowing cold air" If it is getting "stuck" in defrost, the fan won't be
BTW, what kind of outdoor temps are you talking about when it does this?
If the reversing valve is getting power during heat cycle, other than
during the defrost cycle, it is getting that power from one of three
paces: the thermostat, a short in the wiring between the tstat and air
handler or the air handler and condensing unit, or a faulty defrost
board. Anything is possible of course, but I do not recall ever seeing
this particular problem with a thermostat or defrost control. Disconnect
the orange wire from the thermostat and if the problem continues, it is
not the thermostat. Then disconnect the O wire at the condensing unit.
If the problem continues, it would indicate the defrost board. If not,
the wiring. Good luck Larry
Thanks!. This is exactly the information that I was looking for.
* I disconnected the orange wire from the thermostat and turned
on the heat pump.
* It put out good heat and the voltage at the disconnected orange
wire was 26V.
* About ten minutes later, I heard the swooshing sound of the
reversing valve and the unit began to blow cold.
The voltage at the disconnected orange wire at the thermostat
* The heat pump continued to blow very cold continiously
(longer than 30 minutes) and I climbed on the roof and
checked the unit. The external fan *is* running.
* I'll see if I can figure out where the oragne wire at the
condensing unit is.
The OP talks about voltage on the wire to the reversing valve changing
when the unit is blowing cold air vs warm. I assume he is checking from
Red to Orange, as he says it is 24+ when the unit is heating an 0 when
it is blowing cold air. This would iindicate that the valve is getting
power and is most likely OK--luckily, since replacing a reversing valve
is no small job. No matter what is powering the valve when it is not
supposed to be,it is going to be minor compared to replacing the valve.
Thats a nice crystal ball you have there "BooBy".
Too bad you are wrong on this one.
Keep up the bad work though. You pretty decent for a little
entertainment for now.
By the way.......how does that theory work when you have a heat pump
that puts out 99 degrees?
(I cant wait to hear this one)
Also in AZ.
About 15 years ago I owned a Knoell starter home in Chandler
with a Goettl heat pump on the roof. After the heat pump was
about 5 or 6 years old, the first time I would try to fire it up in
heat mode every year, it would be stuck in AC mode. On that
unit, there was a relay in the unit on the roof, controlled by the
thermostat, that applied line voltage to the reversing coil.
Popping off the cover and whacking the relay with the handle
of a screwdriver was enough to break it loose and it would
work for the rest of the heating season.
After about 3 years of that nonsense, got tired of the annual
trip to the roof and replaced the relay. No problems after that.
Don't know if this has anything to do with your problem, but
thought it was worth a mention.
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