Power Cut - Heat Pump Evaporator Iced Up

matt_drummer

doodling said:

Hi,

nxdmsandkaskdjaqd said:

Netexporter said:

I'd always assumed that defrost is triggered by a drop in air-flow. I imagine a pitot tube between the coil and the fan (although I am prone to wild imaginings!).

Mine is based on the temperature of the evaporator.  Normally defrosts at -10C.

That makes no sense as the heat pump wouldn't be able to provide heating if the air temperature dropped to -10.  It would make sense for defrost to be a function of the difference between ambient temperature and the evaporator temperature.

Why is that?

I think they can!

QrizB

matt_drummer said:

doodling said:

Hi,

nxdmsandkaskdjaqd said:

Netexporter said:

I'd always assumed that defrost is triggered by a drop in air-flow. I imagine a pitot tube between the coil and the fan (although I am prone to wild imaginings!).

Mine is based on the temperature of the evaporator.  Normally defrosts at -10C.

That makes no sense as the heat pump wouldn't be able to provide heating if the air temperature dropped to -10.  It would make sense for defrost to be a function of the difference between ambient temperature and the evaporator temperature.

Why is that?

Because I'd the defrost cycle always kicks in when the evaporator gets to -10C, at -10C ambient temperature the heat pump would be continuously trying to defrost.

matt_drummer said:
I think they can!

You might be answering a different question.

matt_drummer

QrizB said:

matt_drummer said:

doodling said:

Hi,

nxdmsandkaskdjaqd said:

Netexporter said:

I'd always assumed that defrost is triggered by a drop in air-flow. I imagine a pitot tube between the coil and the fan (although I am prone to wild imaginings!).

Mine is based on the temperature of the evaporator.  Normally defrosts at -10C.

That makes no sense as the heat pump wouldn't be able to provide heating if the air temperature dropped to -10.  It would make sense for defrost to be a function of the difference between ambient temperature and the evaporator temperature.

Why is that?

Because I'd the defrost cycle always kicks in when the evaporator gets to -10C, at -10C ambient temperature the heat pump would be continuously trying to defrost.

matt_drummer said:
I think they can!

You might be answering a different question.

Yes, and asking a different one to the one you answered.

My apologies, I read the comment differently to that intended

matelodave

As I said above, when the HP decides it needs to defrost it will do it for a timed period, possibly up to ten minutes and then hold off for something like 30-60 minutes before it does it again otherwise it could get into an endless defrosting loop.

have a shufti at this - https://energy-stats.uk/heat-pump-defrost/

akwexavante

Not that this is very useful, but my ASHP has iced up solid a couple of times that i am aware of. On both occasions, i had a fire in my multi-fuel stove the evening before, which turned off the heating system.

My guess is that when the ASHP kicks in again in the early hours of the morning when the room temperature has dropped the water in the system is cold, so the system is turning on from a cold start when it's very cold outside and theirs insufficient heated water in the system to do a defrost cycle and becomes stuck trying to defrost and fails.  My saving grace (I think) is that my ASHP is on an east facing wall and gets warmed up by the rising sun. On both occasions by chance i had full sun first thing.

The first time this happened, i phoned for help and advice and was told to switch it off and wait for it to defrost which i did, and it did defrost after which i turned it back on and all was well.  The second time i just left it alone, and it did sort itself out eventually.