Now that the heating season has started, I am now able to make some comparisons between last year when I was using a gas combi-boiler and this year when I was using my heat pump.
Last year between the 26/09/22 and the 25/10/22, I used 570 kWh of gas to provide my space heating and hot water. During the same period this year, I used 197 kWh of electricity. The 2022 figure is probably an underestimate because I had recently fitted my wood burning stove last year and I know we where using it in October, whereas we hadnt used it at all this year. I also know I could reduce the 2023 figure further by not heating my water with an immersion heater, but by buying a heat exchanger and using the heat pump instead. Adding up how much electricity I used to heat my water and calculating how much I would get if I exported it instead came to about £14 for October. If I could get a COP of 2.5 for water heating, then using the immersion had acutually cost me just over £8. This should be relatively consistant over the year, so using the immersion instead of the heat pump costs about £100 a year. This would be about a 4 year payback term on buying the heat exchanger. Looking at my bills, last October cost £61.49 as I was on a good fixed tariff, at todays prices that would be £87.41, this October my bill was £38.80. So a saving of £48.61 but this included all the improvements I had made including the extra solar panels and battery. A modest saving so far compared to the costs of the improvements, so it will be interesting to how how this changes over the year and what period would be needed to recoup the investment I had made.
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As the colder nights started to draw in I decided that it was time to do some tinkering with my heat pump settings to try and get the maximum efficiency. Heat pumps work most efficiently at lower flow temperatures, so the trick was to use the lowest flow temperature I could whilst still heating the house sufficiently. Obviously as outdoor temperatures get lower, the house will lose heat more quickly and the flow temperature will need to be increased to keep up. The way heat pumps automate this is called weather compensation. All that this involves is entering two set points in the the software for outside temperature and leaving flow temperature from the heat pump. Initally mine was set up so that an outside temperature of 15C, the flow temperature was 38C and at an outdoor temperature of -2C my flow temperature would be 45C. For all outdoor temperatures in between the software would calulate a flow temperature in between my set points. I felt that I could probably reduce the 38C flow to perhaps 35C to get better efficiency and so over a few days tried doing just that and studying the results. What I found was that as I dropped the flow temperature, the heat pump started to turn itself off periodically, as shown in the graph below: This behaviour is called cycling and is generally thought that it is best to be avoided if possible, as it puts extra wear on the compressor and might lead to premature failure of it. The question my mind was, why had it started doing this at lower flow temperatures.
After some research and help on forums, I came to the conclusion that the problem was that the heat pump could not get rid of enough heat into the house, fast enough and so was turning off until needed. This initally seemed counter-intuitive because I was using a lower flow temperature and so trying to put less heat into the house! The answer was that because my flow temperature was lower, the difference in temperature between it and the house was lower and so less heat was being lost by the radiators! The only way I could avoid this was either to use bigger or more radiators, or increase my flow temperature again. As my extension was planned for next year, this would add extra radiators to the circuit, so in the mean time, I decided to raise the flow temperature again and also change one of my set points. This had the desired effect, and the cycling stopped. My new set points were 38C @17C and 45C @ -2C. When the very cold weather came I would have to see if 45C flow was enough. |
AuthorEx Radiographer, Information Analyst, Teacher and Self-builder. Now retired Archives
December 2023
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