Hi all, my outside heat pump has a hige fan that runs very nice and
smooth. But once an hour or so, just after the main fan stops I think,
some kind of screaching noise comes out from there somewhere, and I
can't see what. It sounds like a small motor that's worn out but still
Its a trane system.
Sounds like your unit is going through defrost,
3 things happen:
1) The reversing valve switches into cool mode. This makes a loud
whoosh!! sound. This makes the outdoor coil warm to melt ice or frost
on the outdoor coil. The indoor coil gets cool.
2) The outdoor fan shuts off while the compressor keeps running. This
keeps the outdoor air from being cooled by the outdoor air while the
unit is defrosting.
3) The strip heat comes on. This tempers the supply air so it doesn't
blow cold air during defrost.
Note that when your unit goes through defrost, water will run off the
outdoor unit (melted ice), and a white vapor cloud will rise above the
outdoor unit. This is water vapor, not smoke. If you get black vapor,
it is smoke, then you can panick.
The other possibility is the bearings need oiled in the outdoor fan
motor. If it does not have oil ports, you may need a new fan motor.
Hope this helps.
The compressor is in the basement, right? Thanks for the description.
Bt what is the noise? The fan is smooth when its running. I took a look
but its not too clear, and I can't see any other motor in there, but I
don't know these things at all. I didn't even know they have a defrost.
The compressor is outside. Look straight down into the unit, and you'll see
a black (normally) thing. That's the compressor. If the noise happens when
the fan is stopping, it could be the fan motor bearings.
Screeching noises on heat pumps can also be caused by the indoor
blower turning off when the compressor is still running. Lots of
things can cause this, none of them good. The most recent one I worked
on had the thermostat set in gas mode instead of electric. This was
causing the indoor blower to turn off before it should have. Indoor
blower goes off, head pressure shoots way up, check valve opens,
Good point. The poster should find the noise before calling for service. You
don't want to pay a tech by the hour to find the noise. Without a service
contract, it's usually cheaper if you can narrow it down some.
Why do we need to keep outdoor air from cooling outdoor air? :-)
Why not turn off the compressor and leave the outdoor fan running, wasting
less energy, if the outdoor air is above freezing? We can melt about
1000(36-32)/144 = 28 lb/h of ice with 1000 cfm of 36 F air.
Or turn off the compressor and keep the fan running BEFORE ice forms...
">2) The outdoor fan shuts off while the compressor keeps running.
Sorry Nick, I mistyped. The outdoor fan shuts of so the outdoor air
will not cool the OUTDOOR COIL excessively. This way the coil will
"Why not turn off the compressor and leave the outdoor fan running,
less energy, if the outdoor air is above freezing? We can melt about
1000(36-32)/144 = 28 lb/h of ice with 1000 cfm of 36 F air. "
That is per hour Nick. We want to finish defrost in 10 minutes MAX.
The other trouble is that with a severely frosted coil, the fan driven
air flow through the coil will be much reduced. Then there is the
practical matter of heat transfer. You are assuming 100% on the heat
in the air transfers into the frost with only 4 degrees TD. Maybe you
should start manufacturing heat pumps. I'm sure your superior
technology and engineering will blow the competition away!
The reason the fan alone is not used to thaw the coil is it takes too
long, especially if the outdoor temperature is below 32 degrees F. I'm
sure if your schemes would work, some heat pump manufacturer would have
tried it by now.
Look at it this way...
In cooling mode, the units cooling the home, removing heat, and transferring
In heat mode, its trying to cool the world, and moving that heat into your
Due to the basic physics of the operation in heat mode, and remembering that
you have alot more "evaporator coil" area and air flow in heat mode
(remember, the coils operations change) you are going to have a lower
pressure of refrigerant and lower temps=lower pressure still.
The coils going to normally operate close to, and far below, freezing.
Depending on outdoor temps, humidity, and time of operation, you will get a
light layer at the least of frost on the coil. Makers have overcome this by
simply allowing the reversing valve to open, or close, depending on if its
got a B or an O line, the fan stops, or slow to half speed, and the unit
takes heat from indoors and melts the frost or ice off rapidly. On demand
defrost helps to insure that the coil does not completely freeze over. Timed
defrost modules will insure that if it DOES, it will at some point, be
Ice is avoidable, but if your system were completely screwed up and
you wanted to melt 10 pounds pound of ice off a coil, would you rather
a) move 36 F outdoor air through the coil with a 2470 cfm 90 watt fan
for T hours, where (36-32)2470T = 10x144, so T = 0.145, ie 9 min, using
13 watt-hours of electrical energy while removing no heat from the house, or
b) run your heat pump with a COP of 3, stealing 10x144 = 1440 Btu of house
heat and wasting another 10x144/3/3.412 = 141 Wh of electrical energy?
And it wont demand defrost at 36F either...it WILL go into a timed automatic
Now...THIS is HVAC sarcasm:
Gee Nick...your so smart. Dont you think that you need to get a job with
Now, you keep using 36F...is it air temp, or coil temp? You seem to be stuck
What would *I* rather do? Id rather have it switch over to cool, drop the
fan, run 90F or higher refrigerant into the coil, watch the ice melt in 90
seconds, and get back to the business of heating.
And you are not *wasting* anything.
Might be to a point. But then, even when I was just a tech, I had no
problems understanding why it worked the way it did.
In the calc above, melting ice with the fan takes 13 Wh. The compressor
uses 141 Wh and steals 1440 Btu from the house, which takes 141 Wh to
replace, so it uses 242 Wh, ie 19 times more energy than fan-only.
A coil temp.
That could be more fun to watch, but suppose you are paying the bill? :-)
That would waste 229 watt-hours of energy, compared to the fan alone.
The how seems easier than the why, which may have more to do with marketing
decisions, eg selling a lower cost system that uses more energy, and trying
to brainwash people into thinking its efficiency cannot be improved.
Running the coil at 20 F on a 50 F day might extract latent heat from
the air and increase the coil's effective conductance and capacity, but
it looks like fan-only de-icing uses a lot less energy than compressor
de-icing, even if it requires more sensors or smarter software.
I took a really nice, high dollar ThermoPride oil burner out and put in my
heat pump when I bought this old place.
Im about to upgrade again, and the one I use is only a few years old.
Not for energy savings, but for the fact that the model that I have, has
been dropped by York, and its not real smart to be a dealer and have
I AM however, going to try a higher SEER model this time, altho I have some
serious issues with running a 19SEER unit in this climate.
So...you are honestly suggesting that the makers of heat pumps drop the
defrost mode, just run that 1/2 HP fan to defrost the
Right now, no ones brainwashing anyone. Units efficiency ratings are
climbing yearly. So is the cost. I suppose you could tell GM and Chrysler
and the rest that if their cars and trucks would have a sensor in them that
would allow the engine to shut off while coasting and start back up again
when demand was there, they could add a few more MPG points to each
Some units, even the ones I sell, have an outdoor air temp sensor, a line
sensor, and the software to know if its going to need a demand defrost, or a
timed defrost, or neither.
So, given that, the possibility exists that at some point during the cycle
of the unit, a defrost would be close to being needed, however the
thermostat would be satisfied in teh home, the unit would shut down, and the
defrost would occur due to outdoor ambients being above freezing anyway, so
that would save even more eh? Its already there. Running the fan motor
alone, in an attempt to thaw a coil is pointless, particularly if the coil
will not pass air due to being a sold block of ice....and that happens. You
have to find a way to insure that the unit will thaw, no matter what,
quickly, to allow the unit to get back to a frost free condition, where its
making more heat than it will when the coils frosted.
Taking THAT into consideration, the units run time would be shorter still,
the need for the fan to continue to try to thaw the coil is eleiminated
together, and IF the unit DID need to go into a defrost cycle, at this
point, lets consider that the strips DONT come on, you eliminate that load,
the fan stops, you eliminate that load, the compressor uses NO more power
than it was before, it runs for only a few moments in defrost, and the units
output when it goes back into heat mode is increased, therefore, you are
gaining more than you used.
Dont forget the fact that overall run time, NOT those few moments in defrost
mode are not being taken into consideration with your figures. In other
words, you seem to forget that you get more with a heat pump than you pay
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