@johnmo Well thats very clear, just wondering really if you had a big tank it could take 2 hours to heat. on a cold wet day it would seem likely that the HP could ice up. Out of curiosity I might start a thread just to see if different HPs have different strategies. Oh the joys of being retired with time on your hands 🤣

Posted by: @bontwoody

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@johnmo Thanks for clearing up the matter of defrosts during DHW cycles. I was just guessing really. Wouldnt it be more likely though that the HP would need deforsting during the DHW cycle given the higher flow temperature? What happens it it cant heat the hot water anymore untill it defrosts? Especially it it was less than zero outside.

It may be useful to try to understand what is probably happening within the heat pump system.

The main culprit which causes ice to form on the evaporator coil and fins is the amount of moisture within the outside air. Warmer air is less dense, so each cubic metre will contain less water molecules. As the air cools it becomes more dense, so is likely to have a higher water content. Below an ambient temperature of probably -3C, there is likely to be less moisture in the ambient air, because it turns to ice on anything that it comes into contact with that is below 0C. So the higher humidity levels are likely to occur at ambient temperatures between +3C and -3C.

The refrigerant gas entering the evaporator could be at a temperature of -40C, so it quickly absorbs thermal energy from the metal of the evaporator coil and fins. The amount of thermal energy being absorbed will be dependent upon the loading of the heat pump.

Let's consider a situation where a 12kW heat pump is producing 9kW of thermal energy, at a LWT of 40C, and an OAT of 2C, operating at a COP of 3. The heat pump will be drawing 3kW of electrical energy and absorbing 6kW of thermal energy from the outside air.

If the heat pump now commences a DHW heating cycle, where it is now supplying 3kW of thermal energy, at a LWT of 55C, and a COP of 2. The heat pump should now be drawing 1.5kW of electrical energy and absorbing 1.5kW from the outside air.

If my assumptions are correct, the loading on the heat pump during the DHW cycle could be much lower than when in CH mode, so would be less likely to cause icing on the evaporator.

@derek-m Thanks Derek, thats an interesting way of looking at it.

@bontwoody Work expands to fill the time available. 😄

@toodles I dont know how I ever managed to find the time to fit work in! 😆

@bontwoody Of course, since retirement, we all suffer the 50% time taxation - 12 hours per day is hardly enough to contemplate working as well as living! Toodles.

Well this has all been surprisingly stressful so I'm going to take a week out and go to Plymouth and back via some interesting stopovers. If all goes well it'll take only two days out and a leisurely return by water, I may even get to sail some of the way!  All your help and explanations make me feel like I am getting a good understanding of the process thanks.  

So after signing in Sept in two weeks the install may start (my choice to delay). I say may as last week it turned out Octoes sent me the wrong plan!! I had stripped a flower bed and dug a trench. Although I am not happy I have gone along with the new plan which will put the HP on the path not the flower bed. My preparations continue, I have fitted new upgraded emitters in the bedrooms and run pipework for the extra Emitters. I was pleased to see the old emitters were not full of black debris at all. Oh my poor knees! Thankfully my son ,Liam, came down to provide help. I will use the trench to run the supply to the Zappi. My coordinator told me to "now fill in the trench" so I will but only when it suits me, cheek!!