Power Cut - Heat Pump Evaporator Iced Up

nxdmsandkaskdjaqd

We had a power cut last night and 1 hour later power was restored.  The heat pump was iced prior to the power going out.  Once power was restored the unit powered up again and after 30 minutes I became nervous that the evaporator was at -20C, fully iced up, hardly any air flow across the evaporator, so I have shut it down.

How do you deal with the above situation and a iced up evaporator?

What would of happened if I had let it run in this iced up situation?

Did I do the right thing by turning off, or should I have let it run?

BTW, R32 refrigerant.

matelodave

Have you got TRVs and or thermostats indoors that are controlling the heatpump.

Its not air over the evaporator that defrosts it. Generally heatpumps reverse the flow through the system and extract heat from the circulating water to enable defrosting.  However external controls, especially TRV's, zone controls or that stop the circulating pump can reduce the volume of circulating water and will prolong the defrost cycle (or even prevent it from completing)

My unit tended to ice up and defrost quite a lot before I disabled the external controls and let the unit use weather compensation to control the house temperature. It now runs longer at a lower flow temperature and generally only produces ice when its really cold and damp outside. (we are surrounded by fields in the fens and it gets very damp and misty). We just now see puff or two of vapour from the outside when the system defrosts itself rather than a thick coating of frost or ice.

Normally you are better off letting the unit doing its own thing rather than panicking and shutting it down because all you've done is stopped all flow through the system and allowed it to stay iced for longer.

nxdmsandkaskdjaqd

The system is underfloor with a fully open manifold, with one thermostat controlling. Under normal circumstances a defrost occurs every 35 to 60 minutes.  The process of defrosting is as you have described by reversing the flow and extracting heat from the circulating water.

At the time of the power cut the evaporator was already iced up.  Upon power being restored and the starting of the heat pump it was still iced up and it didn't defrost upon start up.  It kept running in this iced state which is when the evaporator was at -20C and I shut it down.

matelodave

How long did you leave it before you shut it down again.

I still think you should just leave it to get on with it as it will probably go into heating mode when it turns back on and maybe not defrost until the next cycle is due.

I dont know but I think defrosting is activated by both time and temperature so the unit may well have to run for a certain period before it decides to defrost and then have a minimum run time before defrosting again.

As in all things it depends on your system and how its set up. Mine is now well over 15 years old and so doesn't have lots of parameters to read or adjust so I dont get panicked into doing stuff by trying to guess what is happening.

 Have you got access to the installation and maintenance info to see whether you can tweak the defrost parameters and to see how it acheives defrosting

Netexporter

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!).

matelodave

Not as far as I know, there is a temperature sensor but I think its mainly a combination of temp and time. Not airflow.

What machine have you got (Make and model) to see if there's info on t'interweb (some manufacturer's have quite a lot of info and others not much at all)

https://youtu.be/jjl4CqZGmOE

nxdmsandkaskdjaqd

matelodave said:

How long did you leave it before you shut it down again.

I left switching back on till the morning, the outside temperature was 9C then so it defrost itself.

nxdmsandkaskdjaqd

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.

Netexporter

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.

Makes more sense to use a sensor that is already there, and pretty bombproof, with no exposure to external environmental factors. That's why I'm not a heat pump designer.

matelodave

https://www.renewhq.co.uk/how_defrost_cycles_work_heat_pumps/

a bit more reading around the subject suggests that there are different ways that manufacturers implement defrosting. Some use a crude thermostat and others possibly a combination of thermostats/sensors and pressure sensors.
However most of them have timers as well to avoid excessive defrost cycling so when a defrost event is required, the system may start a timer which may hold off the defrost for anywhere up to an hour or more and even then may limit the defrost time to only 5-10 minutes.

So its quite possible that upon the restart that the heatpump went into normal heating mode for whatever the minimum time may be - possibly 30mins to an hour before the defrost function activated.

As I said above, each manufacturer does it differently so it sbest to understand what is happening

doodling

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.