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Author Topic: Should my Heat Pump defrost itself?  (Read 1723 times)
rogermunns
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« on: February 02, 2015, 04:32:46 PM »

Bit of a general question, but I am a bit mystified about the Mitsubishi ASHP I have. Earlier this Winter everything seemed good - it would run fine at low ambient temperatures (-5deg.-ish), albeit at reduced efficiency.
I've been away for 2 weeks, everything seemed fine with the ASHP when I got back.
But last night, again a few negative degrees. The Mitsubishi froze up (heavy hoar frost completely covering the fins).

I always thought that these units defrosted themselves when necessary?
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knighty
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« Reply #1 on: February 02, 2015, 04:45:23 PM »

was it just frost, or solid ice ?

if it's just frost, that's normal, and oddly water turning to ice gives up quite a bit of heat energy into the fins.... which is cancelled out as soon as it needs to defrost.. but it's not the end of the world


so... frost is normal, ice is bad :-)
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rogermunns
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« Reply #2 on: February 02, 2015, 04:51:02 PM »

Thanks, Knighty, reassuring............
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JonG
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« Reply #3 on: February 02, 2015, 09:36:02 PM »

The Mitsubishi should defrost itself, not sure which FTC you have, but the WPAR21 digital display will show defrost when it occurs, pipework usually cools irrespective of which mode it is in HW/SH and generally a cloud of expelled evaporate signifies the end of a defrost.

Generally defrosts occur from plus 5 down to minus 2 or 3, below this the humidity levels drop and defrosts are less frequent.

We have seen issues with heat pumps that have no buffer and small or insufficiently sized heating circuits which don't hold enough heat energy to effect a full defrost. This is usually pretty obvious though because the heat pump becomes an ice cube as successive aborted attempts to defrost build up layers of ice.

Worst case scenario these can smash or break a fan blade. In one case where the system was too small and there was no room for a buffer we had to install trace heating to control the ice build up.

Despite what the Japanese manufacturers recommend, we always fit a buffer.

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phoooby
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« Reply #4 on: February 03, 2015, 12:07:44 AM »

Quote
We have seen issues with heat pumps that have no buffer and small or insufficiently sized heating circuits which don't hold enough heat energy to effect a full defrost. This is usually pretty obvious though because the heat pump becomes an ice cube as successive aborted attempts to defrost build up layers of ice.

No buffer as in no stored heat and small circuit as in a small CH circuit / few rads to store heat?.

Sorry for butting in on this thread, just wanted to improve my understanding. From the above, am I correct in assuming heat pumps use the heat they have generated in a CH or HW circuit to defrost?. The way I understood it (as explained to me several years ago so possibly out of date), ASHP use an electric heating element to defrost in cold weather, hence the efficiency drops off rapidly at low/sub zero temperatures. Has this tech now been superseded or did I get the wrong end of the stick a few years ago?.
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JonG
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« Reply #5 on: February 03, 2015, 06:35:14 AM »

I don't think any ASHP's use resistance based heating to defrost, they may use immersions to boost the output at low ambients, but the drop off in the cold is due to the lower source temperature, and harder work required by the refrigerant circuit to achieve output.
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