After three months of winter heating, it seemed like a good point to assess how the new heating and hot water system was performing compared to the old gas combi boiler. Because of the changes in solar panels and tariffs a direct comparison in terms of monthly cost might be difficult, so initially a comparison in terms of kWh hours would seem more useful. The amount of gas used in 2022 was used to provide heating and hot water, with a small amount used for cooking on a gas hob. The gas hob was replaced with an induction hob and I wasnt monitoring how much electricity was being used by that, so could not account for it. However the effect would be small.
Ignoring that difference, the number of kWh used between the two systems was profound, with the new system using between 28%-53% of the gas figure. The wide variation in the size of the reduction was a little confusing, but I think this is down to several factors between the two years, including:
I know from my monitoring system , that my heat pump is running at a COP of between 4.6 and 3.6 during the winter so far. The lowest COP occuring during a cold snap where the temperature dropped to about 5 degrees C. I fully expected that the heat pump would be more expensive to run in very cold weather, than gas heating. However I also expected that in the shoulder seasons and summer this effect would reverse. Removing gas from my property, meant that I no longer had to pay a standing charge for it and getting an Electric Vehicle enabled me to access much cheaper off peak tariffs and these factors have helped with reducing my electricity bill. In addition my extra solar panels would also reduce bills and provide an income in the summer. So far comparing my total energy bill with 2022 (adjusting for inflation), I had saved £109.31 for the period Oct-Dec. A modest difference and not enough on its own to justify the costs of the improvements made. However I expect this to improve as the the newer EV tariff starts to kick in and solar export starts to increase.
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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. 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. After giving up trying to contact OVO, I finally opened a complaint with OFFGEM. Within a couple of days OVO had transferred my account to Octopus as instructed!
After looking at the various tariff options available to me, I decided to go for Octopus's Outgoing Export (which gives 15 pence per kWh for exported energy) coupled with Cosy (which gives a cheap rate import window twice a day). My rationale for doing this was that on sunny winter days I would still have some export, and that having only a small 5kWh battery, I may well need to charge it twice a day if there wasnt much sun to power the Air Source Heat Pump. I also planned to heat my hot water cylinder via the immersion during the cheap night rate. Having made some enquires online with people who know more about the issues than I, I came down on the side of replacing the pump I had bought, with a replacement that did have PWM control. In fairness I did get differing views about how useful this would be and the decision wasn't clear cut, but at least now I would be able to measure if the PWM pump was more efficient than the pump I already had. I managed to find a Grundfos UPM3 FLEX AS 25-75 130 AZA on eBay at a very good price, this would replace my Grundfos UPM3 AUTO 25-75 130. You can see how easy it is to make a mistake. :-) Another thing the learned online population managed to help me out with was sorting out the issue with my heat meter. In the end it was surprisingly easy to diagnose. I had put the meter on upside down! While I did feel a bit of an idiot, at least it was an easy fix.
I decided to wait until my new pump arrived and then drain down the system and do both jobs at once. After some further research and fault finding, I discovered that my problem with the E912 error wasn't to do with a sticky relay. I had purchased a Grundfos UPM3 AUTO pump because the Samsung ASHP can use a communication signal called PWM (Pulse Width Modulation) to talk directly with the pump and modulate its output very accurately depending on conditions. The problem I had was that this pump doesnt actually support PWM. In my defence, it has the communications plug socket for it and other pumps in the same range do support it, but mine didn't! To get my pump to work properly, I had to change its connections in the Samsung control unit from 1 and 6 to 7 and 8, remove the PWM lead and change a setting in the controller's FSV values. This was value 4051 which should be set to 0 (Not Used). Once I had done this, the error disappeared and the pump turned off when it should :-)
The dilemma now was whether to just suck up the fact I had made a mistake and live with it, or shell out another £100, get the correct pump and put the old one on ebay! The heat pump worked perfectly after starting it up and all the rooms became lovely and warm. It was a sunny day and the 5 kWh battery was full so the heating was on till 11pm and cost nothing to run :-) I decided to turn the heating off when we went to bed. There is an argument that a heat pump should be left on 24/7 for maximum efficiency, but I knew my battery would be totally drained shortly and even if the heat pump was less efficient while starting up in the morning, I would likely have free solar to cover it. I woke at 6am in need of the toilet and to my horror saw an error message on the heat pump controller. Once I was awake I looked it up, E912 a flow switch error. Luckily there was a forum entry on buildhub.org (https://forum.buildhub.org.uk/topic/25596-samsung-gen-6-ashp-e912-flow-switch-off-and-water-pump-on-error-e912/). The error can be caused by a sticky relay on the control board that causes the pump to stay on even when there is no call for hot water. Lo and Behold my pump was on continually. Something else to contact the heat pump supplier about on Monday! I added some CT clamps to monitor the electricity through the heat pumps outdoor and indoor units to gain some insight into how much it was going to cost the run. The OpenEnergyMonitor system didn't let me down. This graph shows my solar output in blue, the heat pumps power usage in orange and the outside temperature in red. Sadly it was a very dull day.
While thinking about my problem overnight, I thought that I might be able to use a USB endoscope on my phone to have a better look at the circuit board. This proved not to be the case and I was just about to unscrew the casing of the circuit board when I realised the plastic top was just clipped on. Removing it revealed the back of the circuit board with labels! My suspect port was labelled CNP901 BLDC-FAN, Eureka!
Connect fan, re-attach covers and then test run again. Bingo. Now to set up the control unit using the wired display. Youtube proved very handy and with the exception of one issue of which way around to enter the required numbers, weather compensation was set and the heating switched on. After draining down the system, I cut into the combi boiler's central heating pipes and reconnected them to the heat pump flow and return pipes. This was not without its problems, with some residual water spoiling a few joints. However with the help of my grumpy mate Mark the pipework was finished in half a day. The rest of the day was spent connecting the wiring to the control unit. and then having a test run. Initially the test run caused an error E911. On looking it up as a low flow rate error, I realised that most of the radiators were turned off so I opened the valves and tried again. This time a E458 error came up. this was an outdoor unit fan error. After a bit of research on the www, I found out where to look for the fan circuitry, the lead from the fan would be connected to the socket labelled CN90 or CN901 and from there I could test the fan with a multimeter. I decided to remove the top cover of the outdoor unit to see what I could. On doing this this problem seemed to be self evident! Amazingly the fan had not been connected to the circuit board !!! The problem I had now was how to be sure where the correct socket was. I located a promising one of the right size but couldnt make out any labelling on it even with the help of a mirror. At this point I resigned myself to a night without central heating and called it a day.
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AuthorEx Radiographer, Information Analyst, Teacher and Self-builder. Now retired Archives
December 2023
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