Having now had a heat pump working over a whole winter, It was time to see if in fact a heat pump was more expensive to run than a gas combi boiler. I had made many improvements to the house in addition that would reduce costs, so I did the comparison twice. Once as a straight like for like cost difference assuming the same factors as last year, then again including the cost savings through the extra solar panels and tariff prices I was able to obtain. Column J shows the standard comparison assuming things were the same as last year. It was interesting to note that the heat pump was slightly more expensive to run than a gas boiler between October and December. This was most likely due to this being the coldest dullest months when its COP and solar output would be lowest. However this effect was dwarfed by the savings in the other months, leading to a total saving of £178 over the winter period. This agreed with the measurement of the heat pumps SCOP of around 3.8 during the period which would make it cheaper than gas given the price differential between the two fuels.
Column K shows the comparison after all the changes I had made. Having extra solar panels probably didnt help that much on the dullest winter days, but getting an EV allowed access to much better overnight tariffs, which allowed the charging of the battery, hot water cylinder and general washing duties at a much reduced rate. The total saving taking all this into account was £363 over the winter period. The difference of £185 between the two analyses, shows that you dont need a lot of solar panels to make a heat pump cheaper to run than gas in the winter. The downside for me was that the small saving also didnt justify the extra expense that the new panels and battery had cost me. For a reasonable payback period, I was going to have to pin my hopes on a substantial income from electricty export during the summer months.
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Having now experienced a wide range of outside temperatures, I could begin to assess how my heat pump installation was performing. It was possible to create a table of Outside Air Temperature (OAT) v Coefficient of Performance (COP) and estimate how much 14 hours of heating would cost. Using my standard Octopus tariff rates, I could see that my crossover point between gas being cheaper than the heat pump was at an OAT of 8C. Colder than that gas would be cheaper, warmer and the heat pump was cheaper. This was useful to know, but I also knew that my SCOP would improve as I did more of the house renovations, which would allow me to run at a lower flow temperature. An additional variable was the cost of the electricity. For comparison I had used the standard rate on my tariff, but in fact I was making use of an excellent night time rate of 7.5p where possible and this was lowering my unit cost overall. Lowering my average cost to 25p per unit changed the calculation markedly The crossover point had now moved to an OAT of 5C. To calculate which would have been cheaper to run all winter, all that would be needed was the average temperatures of each heating day. Above 5C I was winning and below 5C I would be losing. A graph of average daily temperature, showed that the majority a values were well above this level. Comparing bills between this and last year, the heat pump was coming out on top.
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. So with the heat pump now working well it was time to get the Mixergy cylinder installed in the same small cupboard. Since it had been sitting around since last year for various reasons, I was keen to finally get it in and working. Since there were no official Mixergy installers in my area, I convinced a local plumber to have a go under the proviso that I would do all the electrical and internet connection work. Everything went smoothly and by the afternoon of the second day, the combi boiler had gone, gas was capped off and the cylinder was in and charging from the Myenergi Eddi PV Diverter for free. As it was a sunny afternoon, we had two welcome showers and a full tank of hot water from the electricity I would have previously exported.
Initially I had decided to just heat the cylinder via an immersion heater, while this is nowhere near as efficient as using the heat pump, I estimated that I only needed about 3 kWh of hot water each day and this could be supplied from my excess PV, certainly for most of the year. I was going to monitor this and if it occured that I was having to import a lot to do this, then I would reassess buying the heat exchanger needed to hook up the heat pump for domestic hot water heating. 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. Having installed a Sontex 789 heat meter in my pipework, now was the time to use it to monitor how my heat pump was performing. The meter has an MBUS output and by using an MBUS to USB adaptor it could be connected to my EmonPi. The Sontex 789 is a little fiddly to get installed on the EmonPi and it was useful to have had a bit of experience working with Raspberry Pis'. Once I had managed it though, I had my first chart of how things were going. To my surprise it indicated that my heat pump was performing very badly, but that was contrary to fact that it was heating the house using only 700W of power. After some troubleshooting it appeared that the heat meter wasn't measuring flow rate correctly and might need to be taken off and examined. For now however I just created a scaling factor based on the flow rate reported by the Samsung controller, to give a rough idea of what was going on. The red and green lines show the flow and return temperatures of the heat pump, showing a difference of about 3 degrees C once it reaches steady state. The orange line shows the amount of heat produced in Watts and the blue line the amount of electrical power in Watts needed to create it. Dividing the Heat output by the electrical input gives the Coefficient of Performance (COP) of the heat pump at that moment. The purple line at the bottom shows the temperature outside.
My graph shows that with a flow temperature of about 39 C, my COP is about 4. In other words its about 400% efficient when the temperature outside is 9 C This might sound crazy, but its possible because the heat pump isnt using electricity to create heat like an electric heater does. Its using the electricity to move heat, from the outside to the inside of your house. As a preliminary figure, I was very happy with this and it would indicate that if I ran the heating all day at this outdoor temperature my electricity use would be about 24 hours x 0.7 kW = 16.8 kWh. In reality I might not run it all day and some or all of the electrical energy would come from my solar panels. Following the palaver I had getting the heat pump to work, I contacted The Heat Pump Warehouse by email, regarding the problem of the unconnected fan and compensating me for my time diagnosing and correcting it.
On the Tuesday after the weekend, I received a phone call from an Adrian Thompson, Regional Sales Manager for Joule UK. Although I was expecting a call from The Heat Pump Warehouse, I had noticed a Joule sticker on the heat pump so guessed they had supplied it. Instead, as I expected, opening the conversation with an apology for the trouble it had caused me, he instead launched into an aggressive attack, criticising me for not registering the product, not being a qualified heat pump installer, not having a G3 qualification, using antifrost valves instead of antifreeze solution (as per there instructions), not needing to put the central heating on as it was a balmy 10 degrees up where he was and he didnt need it and finally insinuated that I might have removed the fan connector myself! I was a bit taken back by all this, but pointed out that: 1. I didnt need a G3 qualification as I hadn't installed a cylinder 2. that antifrost valves were accepted as a viable alternative to antifreeze by most heat pump installers and manufacturers and anyway there were no Joule instructions supplied with it 3. that I wasnt really bothered about the Joule warranty (which judging by their reviews, isn't worth a great deal anyway-I didnt say that bit) 4. that I was probably a soft Southern B*st who needed excessive warmth. 5. I didnt like being accused of sabotage and fraud He took on my point that I hadnt been supplied with a Joule manual, and I took his that the standard thing to do would be to call them and get an engineer down to fix it. However I did stress it was the weekend and I needed the heating and that I had saved them the cost of an engineers visit. The conclusion to the call was that he said he would get back to me. A couple of days later I had an email from Joule, suggesting that I get in touch with Samsung. Moral of this story: Dont touch Joule UK with a barge pole! 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! |
AuthorEx Radiographer, Information Analyst, Teacher and Self-builder. Now retired Archives
December 2023
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