I did find myself idly wondering, when watching the COP go down the toilet during the cold snap late november, what the feasibility would be of putting in a small hydronic fan heater (heat supplied by the ASHP's CH circuit, from a small buffer store ) . it goes outside ,  blowing at the evaporator fins -  becomes active only in  defrost conditions - whether it would actually be able stop or reduce the negative effect of defrosts or if its actually a "you cant get something for nothing / energy going round in circles" situation that can't be worked around I don't know! idle musings not  a thought through plan!

I hadn't fully appreciated (until getting a heat pump) just how miserably damp the UK's weather is during these -2C to +2C spells and how painful for performance the consequent frequent defrosts are, so if there is genuinely a way, it'd be worth knowing.

Posted by: @iancalderbank

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I hadn't fully appreciated (until getting a heat pump) just how miserably damp the UK's weather is during these -2C to +2C spells and how painful for performance the consequent frequent defrosts are, so if there is genuinely a way, it'd be worth knowing.

You must be less sensitive to damp than me.  I absolutely hate it when its +4C - ~-1C because of how damp it feels.  Once it goes below zero the water starts to freeze out and it feels much better.  I remember once going to Ottawa when it was -20C, much better IMHO than the UK at +2!

Posted by: @iancalderbank

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I did find myself idly wondering, when watching the COP go down the toilet during the cold snap late november, what the feasibility would be of putting in a small hydronic fan heater (heat supplied by the ASHP's CH circuit, from a small buffer store ) . it goes outside ,  blowing at the evaporator fins -  becomes active only in  defrost conditions - whether it would actually be able stop or reduce the negative effect of defrosts or if its actually a "you cant get something for nothing / energy going round in circles" situation that can't be worked around I don't know! idle musings not  a thought through plan!

I hadn't fully appreciated (until getting a heat pump) just how miserably damp the UK's weather is during these -2C to +2C spells and how painful for performance the consequent frequent defrosts are, so if there is genuinely a way, it'd be worth knowing.

I think that it was over 1 year ago when I suggested a method, primarily to improve overall efficiency, but in the process it may help reduce defrost cycles. It involved collecting thermal energy during daylight hours using solar thermal panels, which is collected in a heat store, this stored energy could then be used to preheat the air being drawn into the heat pump to help improve its efficiency, it would also possibly reduce the need to defrost.

If one thinks about why defrosting becomes necessary. As OAT reduces, the heat loss from the home increases, so the heat pump needs to work harder to supply this additional energy. The refrigerant gas is therefore flowing around the heat pump internals at a greater rate, and in turn is extracting more thermal energy from the air flowing through the evaporator, and thereby cooling this air to a greater extent. As the air cools, any moisture within the air will start to condense upon the evaporator fins and coils, and once the fins and coils become cold enough the condensed water will turn to ice. As the ice forms, I suspect it start to act like an insulator, reducing the quantity of thermal energy being absorbed by the refrigerant gas, and hence causing the heat pump to work even harder. A vicious circle.

So what are the options to at least reduce the frequency of defrosting if not preventing it altogether.

1) A warmer air supply should at least delay the point where defrosting becomes necessary. Collecting thermal energy when it is available, and storing this energy by some means so that it can be used later to preheat the air supply.

2) Increased airflow, which I believe some heat pumps can do by varying the fan speed. More air, even at the same temperature, equates to more thermal energy.

3) Reducing the heating demand, so that the heat pump does not have to work so hard. Improved insulation of the home is one method. Just lowering the desired IAT is another method, or increasing IAT during the warmer daytime period, so that the heat pump does not have to work so hard during the colder period.

4) Ensuring the the heating system is operating in the most optimal manner.

5) Using a secondary heating source during particularly cold and damp weather conditions. Again a method of reducing how hard the heat pump needs to work.

All or some of the above may be useful under different conditions, so this is possibly where trial and error may come into play.

I am pretty sure I saw a report somewhere of a coating which resists icing up, I think by causing the condensing water to firm little balls which freeze and mostly fall off.  Presumably its some kind of low friction low wettability coating (Teflon?).

We aren't the only damp country in the world so hopefully some better solutions will make it to manufacturing in the next few years.

My heat pump frosting up this mormning. Outside air temperature 3.4 C, humidity 94%.

Prepare for large KWh consumption today.

Mike

mine too. didn't think it was quite cold / humid enough but clearly it is. defrost after 4hrs running.

the pink highlight is about 0953, when performance drops noticeably to the extent that its visible in the monitoring, there's a clear 10 minutes before the HP goes into defrost mode at 1002 during which "something" could be done , if there was a something that could be done....

btw the metering calculates the defrost as having cost 0.9kwh so thats the energy budget for the "theoretical something".

@iancalderbank that’s useful to know. Your point on humidity chimes with today’s performance with our heat pump. It’s 2c and our HP is working hard yet it’s a very dry, clear morning. Surprisingly we have had no defrost so far this morning. We typically get a defrost, one per hour when around 3c.

So I was wondering if the intake air was somehow dehumidified before passing through the fins? 🤔 no doubt totally impractical.

This ties in with my post on the 5th.

Air temperature today 2.4C (colder than on Friday) but humidity only 72% (was 94% on Friday).

No frosting of ASHP.

Mike