Heat Pump - how to force defrost?

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Heat Pump - how to force defrost?

Postby bank5 » Sun Dec 05, 2010 1:20 pm

It snowed a bit last night and my heat pump is slightly iced up. It's not going into defrost mode so I'll probably have someone take a look at it.
In the mean time does anyone know how to make it go into defrost mode? I think it's as simple as pushing on something in the circuit board with a screw driver, I'm just not sure where to push :D I can post a picture of the circuit board if that helps.
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Postby rfburns » Sun Dec 05, 2010 1:53 pm

I can't tell you which button to push as they are all slightly different, but look for a tiny button with small lettering indicating "defrost".

The sure fire way to do it without knowing which button to push is by simply setting the thermostat to cooling mode and turning the temperature to a very low setting. Let it run long enough to defrost all the ice, then switch it back to heating mode.

This is essentially how all heat pumps defrost themselves (with the addition of bringing on the strip heat to temper the air during defrost). You won't be getting the tempered air with the method I've given, so it will be uncomfortably cool until the process is finished.

By all means have it serviced soon as there could be a number of different reasons it is not defrosting on its own. Good luck.
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Postby mithrandir » Sun Dec 05, 2010 2:05 pm

Are you sure it's not going into defrost mode?

In defrost mode the heat pump basically switches to summer A/C mode where it "steals" heat from your house to melt the frozen stuff from its metal fins. It's therefore inefficient. Even if a heat pump looks iced up it's still able to perform its function (albeit at a reduced rate). Your heat pump likely won't be able to rid itself of all ice in one defrost cycle. It will melt the stuff and unless it also evaporates, the water will re-freeze.

Sometimes my heat pump will stay iced up for a few days but I personally have not noticed excessive energy consumption.

I would not play around with the circuit board. Yes, invoking defrost generally does require bridging two points on the board with a screwdriver.
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Postby bank5 » Sun Dec 05, 2010 3:47 pm

Thanks for the responses. Last winter during an ice storm it had a couple inches of ice on the top, inside and outside of the unit. The HVAC company came out 4 times (replaced the sensor and circuit board) and finally said it was fixed. Seems like anytime it sleets or snows though it ices both on the inside and outside of the coils. I'll keep an eye on it to make sure it does ice up too much again. I replaced the thermostat last year so could that have affected anything?

Thanks for the tip on turning on the AC. I'll have to give that a try.

Here's a picture of the circuit board. I didn't see anything that says defrost

Image
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Postby rfburns » Sun Dec 05, 2010 4:21 pm

Do you see the pins labeled "TEST" midway down on the left side? Those are the pins a serviceman would jumper to initiate a manual defrost cycle. It looks like there is a built-in shorting tab on the right side to accomplish the task. Bend it and hold until the defrost cycle begins, then let go.

The unit must be running to initiate defrost mode. Now, be aware that this may or may not work due to the sensors that are connected. If one is bad, then this tells the serviceman to look there instead.

Heat pumps must be elevated and given shelter from above, as in no water run-off from the roof-line (broken gutters) or other impediments.

Last, an undercharged system will cause a freeze-up too. This is because sufficient heat cannot be generated in the reverse cycle to thaw the unit even though it is entering defrost mode.
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Postby bank5 » Sun Dec 05, 2010 6:28 pm

Thanks rfburns! I'll give that a shot next time it freezes up
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