Posted by: @heat-pump-newbie

I suspect a drop in the COP of my heat pump may to be due to a previous loss of refrigerant. I can't prove this but I've seen bands of frost forming on my condenser unit during cold damp weather. Without doing a time lapse video of the frost forming I can't prove that the bands are always there, or are only formed during the freezing or melting of the frost.

This only becomes visible of course in cold damp winter weather, but it's just dawned on me that if I could monitor the operation right now using a thermal camera I may well see those bands all the time.

Has anyone done this eg with a Flir ? @admin didn't you have one of those ?

Hi Newbie,

I think that you will always get a frost buildup on the condenser coils, particularly during damp weather with an air temperature between +5C and 0C. This is normal, because the refrigerant gas flowing through the condenser could be as low as -40C, which of course will cause moisture in the air to condense and then freeze when it comes into contact with the metal surface of the condenser. When the air temperature falls below 0C, some of this moisture will condense and freeze elsewhere, so there will be less to freeze at the heat pump. The heat pump will also be working harder since the heat demand of your home will be greater due to the lower ambient air temperature. WE THEREFORE REQUIRE MORE GLOBAL WARMING, BUT ONLY DURING THE WINTER MONTHS.

As you suggest it would be interesting to see the temperature variations at the condenser under varying atmospheric conditions.

Oh yes ? A vote for global warming ?? Should we report you to the climate police ?

It's the bands of frost I'm interested in, they suggest there's not enough refrigerant throughout the whole matrix of the coils, which may be reducing my COP. I'll dig out a photo...

Posted by: @heat-pump-newbie

Oh yes ? A vote for global warming ?? Should we report you to the climate police ?

It's the bands of frost I'm interested in, they suggest there's not enough refrigerant throughout the whole matrix of the coils, which may be reducing my COP. I'll dig out a photo...

Hi Newbie,

I have just realised that it is the evaporator to which you refer, rather than the condenser. 

If you look closely at your evaporator, you will see central pipework with numerous fins attached, the idea being to achieve as much thermal contact as possible with the air flowing through. You will probably get more ice formation in the pipework where the refrigerant gas (in liquid form) enters the evaporator, since it will be at its lowest temperature. As the refrigerant absorbs energy from the metal it cools the metal to below 0C, causing any moisture in the air to condense and freeze when it makes contact. A good flow of warmer air will help prevent the buildup of ice, but not stop it altogether. Another reason for the buildup of ice in particular areas could be restricted air flow, which will probably increase as the ice builds up and restricts air flow even further.

A further reason for excessive ice buildup occurring in a particular area could be that this is the point within the pipework where the refrigerant starts to change from a liquid to a gas, and therefore absorbs more energy during the phase change.

The air flow through the evaporator will not be uniform, the rate of refrigerant gas flowing through the evaporator will vary dependent upon how hard the heat pump is working, as will the buildup of ice will vary dependent upon the relative humidity of the ambient air. All these factors will have an effect upon ice buildup.

What makes you think that your heat pump does not have sufficient refrigerant gas?

Ice 11th Feb this year.

Unfortunately I saw it escape when an installer was moving the machine ! Refrigerant guy called in to check there was no further leak but was happy that the unit still had enough to work ok. However, I've never had as good a COP since then. Difficult to pin it down to that incident as it was done during the summer 2021 so not running heating.

@heat-pump-newbie

As far as I am aware, the only true way to ensure that a heat pump has the correct quantity of refrigerant gas, is to remove all the gas into a specified container using a pump, then refill the system with the specified quantity. This of course needs to be carried out by a trained professional.

Having said that, I do believe there are tests which can be carried out on a heat pump during operation which would indicate that it is working correctly and has sufficient refrigerant gas. There are various U-tube videos on the subject.

So @derek-m do you think the bands in my photo are where the warmed refrigerant is piped back into the unit during a defrost cycle, rather than not freezing there due to a lack of refrigerant?

I haven't stood watching the unit during a time when it's freezing, because, well, it's freezing !

Hence being interested in doing that with a thermal camera in more clement weather...

@heat-pump-newbie

I don't understand how a quantity of refrigerant could leak out just by moving the unit. Where did the leak occur and was it liquid or gas that you saw? How much do you think leaked out? How much should be in your unit?

How did the refrigerant guy tests to clarify that there was sufficient refrigerant still in the unit? What type of refrigerant does your unit use?

I was surprised to see how much ice was on your unit. Was it similarly covered in the main area at the back of the unit? When you took the picture was the unit still in heating mode or carrying out the defrost cycle?

By all means try it, but I'm not certain what a thermal camera used when the outside air temperature is warmer will show, since as the ice builds up the air flow through the unit will change. Do you know anyone nearby with an ASHP, that you could compare notes?

I suspect in the end that your may have to don your thermals, and watch the unit operate through a number of cycles.

I'll pm you for more info on the leak... Thermals are there ready for the winter!

Sorry I got the date wrong on the photo - it was 21st March at 06:26 - obviously a chilly and damp morning ! I don't know if it was doing the defrost cycle. As my controller is in the loft I haven't been able to see what's displayed there to indicate if it's defrosting, too blooming cold! Maybe I could set up a time lapse camera on a frosty morning this coming winter and see how the frost forms then melts. Yes the whole thing gets iced up like the side, except the bands.

@heat-pump-newbie, I’d be surprised if those bands on the evaporator were as a result of escaped refrigerant, but they do look odd. I’ll circulate the photo among installers to get some thoughts.

@heat-pump-newbie, does your heat pump use R410 refrigerant?