Steve W Posted February 24, 2023 Share Posted February 24, 2023 The question is simple, are some makes and models of ASHP more prone to frequent defrost cycles. I have had an ASHP (Daikin Altherma 7kW) fitted in October last year and it works well until you get the -2 to +5 temperatures with high humidity - recently humidity has been in the high 90s. Under the above conditions the unit was going into a defrost cycle every 25 minutes even at 9pm with an outside temperature of +2.5C. The interval between cycles is so little that the unit struggles to get to 35C before the next defrost. Essentially the average temperature is around 27C which doesn't make any impression on heating the house. As I have a mixture of UFH and rads the LWT is set to 45C. Hence the question are some units more prone to icing or are some units more prone that others. Link to comment Share on other sites More sharing options...
dpmiller Posted February 24, 2023 Share Posted February 24, 2023 well that's exactly the conditions when defrosting will be needed so... How badly frosted-up does it look each time? Is there an external air temp sensor on the back of the unit near the fins that's getting iced-up? Are the external and unit temperatures displayed plausible? Can't say about other brands but the Engineering menu on our CoolEnergy unit will allow sensor calibration, time between defrosts, plus various duration/pressure/temp settings specifically for defrost. Our environment sounds pretty similar to yours and I'd see a defrost about every 40-50 mins worst-case. Link to comment Share on other sites More sharing options...
Steve W Posted February 24, 2023 Author Share Posted February 24, 2023 The unit has an external sensor on the North wall of the house to ensure that it shaded from the sun. I have attached the pictures from the 27th January - 9 p.m. (this was the fourth cycle of the morning). The temperature was 2.5C and sunny - the humidity was probably greater than 95% (this was before I bought my weather station). The frost started to be apparent after approximately 8 minutes and by 25 minutes the unit was labouring just before it went into its next defrost cycle. Link to comment Share on other sites More sharing options...
ProDave Posted February 24, 2023 Share Posted February 24, 2023 Frequency of defrosting is also a function of how hard the ASHP is working. My 5kW LG rarely defrosts when it is just driving the UFH but is much more likely to do so when it starts heating the DHW where the load is higher and the delivered water temperature is higher. Mine is also on the S side of the house where it is always a little warmer. In a still cold winter spell of weather, the north side of the house can remain frozen for days on end, but it only takes a little sun to warm the S side of the house. What may be more relevant from one make to another is how the defrost cycle works. On the LG when it needs to defrost it stops the compressor and fan, reverses the 4 way valve, starts the compressor only which now warms up the evaprorator, and only when it has warmed and melted all the ice, does it start the fan to blow the water vapour out. then it stops and swaps the valve back and returns to normal operation. I am always surprised how quick this happens, and if it happens quickly, it can't be removing much heat from the house. If yours is struggling to reach the demand temperature in the 20 minutes or so between defrosts, it sounds like it is working hard. Could the unit be under sized for the job? 1 Link to comment Share on other sites More sharing options...
PeterW Posted February 24, 2023 Share Posted February 24, 2023 7 hours ago, Steve W said: As I have a mixture of UFH and rads the LWT is set to 45C. There is your issue - a 7kW unit is going to be flat out I expect to get to 45°C and maintain it for any period of time without significant defrosts. Try turning it down and running for longer - even at 40°C you will still get heat from the rads but will need to operate longer periods of time. are the rads and UFH on separate zone valves ..?? 1 Link to comment Share on other sites More sharing options...
Steve W Posted February 24, 2023 Author Share Posted February 24, 2023 It would be really wonderful if it managed to get to 40C, depending on the humidity I have seen it to struggle to get to 35C and a really bad day it only just makes 33C. What I really want to know is whether the rumours are correct that the first generation Daikin Altherma is better or worse than any other heat pump. The unit was installed in October last year as stated above and I have been pursuing the installer who is a big name player as to what action they are going to take to remediate it. After all I bought a heat pump that I expect to keep the house warm not a device that may or may not work when the temperature is below 5C and it is humid. When it is not humid the heat pump works well and maintains a good temperature throughout the house. Link to comment Share on other sites More sharing options...
ProDave Posted February 24, 2023 Share Posted February 24, 2023 As an experiment, turn off the heating to part of the building, so it is say only heating the UFH part or heating the radiator part, and see if it achieves the target temperature and how much defrosting it does. This might give a clue if trying to generate more heat than it it capable of is the problem. Link to comment Share on other sites More sharing options...
Steve W Posted February 24, 2023 Author Share Posted February 24, 2023 When the problem has been most acute the only way to bring the house back up to any semblance of normality is run a subset of the zones. It still goes through defrost cycles but by and large achieves the required temperature. Strangely the hot water which is on a fixed time slot (12.30 to 2.00) always achieves 45C with a 250L tank. I live in what in an area that was predominantly Cotton Spinning and cloth manufacture which is known for its above average humidity. I am was one of the first installations installed by the company and they may not have been aware of the potential humidity issues. Do some ASHPs come better with high humidity if not what is the solution? Higher output unit or reinstall my gas boiler that I still have. Suggestions please Link to comment Share on other sites More sharing options...
PeterW Posted February 24, 2023 Share Posted February 24, 2023 What was the heat loss calculation and what did they say for the 5° and -5°C external temperatures ..? Link to comment Share on other sites More sharing options...
Steve W Posted February 25, 2023 Author Share Posted February 25, 2023 I've attached the SAP (as built) summary and the Performance Estimate from the installer. The house overall is A rated, high airtightness (2.38) and 94% MVHR. The heat loss is low and is why it took me several days to realize that the house temp was gradually falling - a bit like a car with a slow puncture. Last night the outside temperature was 1C but the humidity was considerably lower than it has been 83% - the heat pump performed flawlessly and the house is essentially the same as yesterday. Steve W SAP Extract.pdf Performance Estimate - J-669BD82E_Redacted.pdf Link to comment Share on other sites More sharing options...
dpmiller Posted February 25, 2023 Share Posted February 25, 2023 what schedule is the heating on? 24/7 or something less? Link to comment Share on other sites More sharing options...
Steve W Posted February 25, 2023 Author Share Posted February 25, 2023 Going back to my original question. Is this heat pump more prone to icing up than any other heat pump? A 7KW heat pump is appears to be more than adequate for the size of the house given the insulation and air tightness. The original calculations from Polypipe were 5KW but I added more insulation. Two people sitting in the television room with the TV on in the middle of winter raises the temperature of the room. Given the wrong conditions e.g. 0C and high humidity it is estimated that overall the output of the ASHP is essentially 43% - hence slow puncture syndrome. Link to comment Share on other sites More sharing options...
dpmiller Posted February 25, 2023 Share Posted February 25, 2023 never mind all the SCOP numbers, how many kW is it actually rated for at the temp and flow? Link to comment Share on other sites More sharing options...
ProDave Posted February 25, 2023 Share Posted February 25, 2023 A SAP assesment does not tell you the actual heat loss of the building at different temperatures. Be best it does is tell you in the coldest month is uses 976kWh so that's 31kWh per day or an average of 1.3kW continuous. If that is to be believed, the 7kW ASHP should be plenty. Also on that basis, you can probably get enough heat into the house by lowering the flow temperature and running it for longer. That would be the first thing to try and see if it improves the defrosting. Link to comment Share on other sites More sharing options...
Steve W Posted February 25, 2023 Author Share Posted February 25, 2023 Hi Dave - thank you for your help and input. At the moment I'm stuck with a single LWT setting for two conflicting requirements - namely UFH (low temperature) and radiators (higher temperature). At one point I was running at a lower temperature and then when I woke up I would adjust the temperature back up to 45C for a couple of hours. The installer is still working on the problem and has had Daikin onsite - muttering about potentially not enough refrigerant. Daikin have said that they will come back and check the refrigerant but only after the installer has done their bit. The Daikin unit itself is capable of having two LWTs this is controlled by two zero volt contacts on the PCB. Time permitting I will swap to two LWTS, UFH set to 35C and the radiators set to 45C - by which time I will have switched the whole system off for the summer. Link to comment Share on other sites More sharing options...
ProDave Posted February 25, 2023 Share Posted February 25, 2023 The ASHP will only do one LWT at a time, so to do that you would need to run UFH or radiators one at a time, alternating. Is that really going to work better than just keeping the radiators at a low temperature? Link to comment Share on other sites More sharing options...
Steve W Posted February 25, 2023 Author Share Posted February 25, 2023 Agreed, there is only one LWT (at a time). The second 0V contact tells the ASHP to use the second setting which I will set to 45C. I run the system serially Hot Water -> UFH -> radiators and if necessary, which it usually isn't, a period in the afternoon. The downstairs of the house coasts from 6.30 a.m. till the next heating cycle 12.30 a.m. using the thermal mass of the house and slab - the room that I use most, in the evenings, is usually around 21C and rises to 21.5C with the television on. The South side of the house has a very high solar gain as it is mainly triple glazed which is not on the SAP report - this does mean that on a sunny day in winter that the house can get to 24C+. I also have 5 x 3.5 kWh of battery (and 4kW solar + Solar Boost) that I use if the house needs any additional heat. Typically the electricity consumption (heating, lighting, cooking, general use) for the house is 30-35kW at 14p/kW (economy 7). There is a very interesting post https://lamiacasaelettrica.com/daikin-defrost-a-conscious-choice/ where the 25 minute defrost cycle is analyzed in some depth. Although not exactly the same model is of the same technology. Link to comment Share on other sites More sharing options...
JohnMo Posted February 25, 2023 Share Posted February 25, 2023 Are you trying to run the heat pump like a boiler and having shortish periods blasting heating. I also have an A rated house and my UFH pipes are at 300mm centres, which is really wide, so output from the floor is low compared to usual. Even so at -9 my UFH flow temp is 35, any other time it's between 25 and 30. Yet your temp is always 35 for your UFH and 45 for the rads. If would turn your temps down as suggested by @ProDave and run for longer, that my be your fix. Link to comment Share on other sites More sharing options...
Steve W Posted February 25, 2023 Author Share Posted February 25, 2023 The 7kW heat pump I have is more than sufficient for the house and indeed heats the house more than adequately even when the weather is below zero. My original question was trying to determine if other makes / models of heat pump have defrost cycles as low as 25 minutes when the humidity is above 90%. What I can't find anywhere is the fine print that states only to be used in non humid conditions because you won't get an LWT of 45C. Link to comment Share on other sites More sharing options...
dpmiller Posted February 26, 2023 Share Posted February 26, 2023 they defrost when they need to. If it's iced up sufficiently that the fan can't pull fresh air through, it needs a defrost. Simples. You absolutely *can* get your LWT over 45c, but you'll need to reduce your energy demand to allow it to happen... / I can make mine ice up and need defrosts at around the same intervals, yes. But only by starting it from a cold house, in DHW mode, but leaving the CH circuits open too. Link to comment Share on other sites More sharing options...
JohnMo Posted February 26, 2023 Share Posted February 26, 2023 I will make some assumptions Your UFH pipes are on 150mm centres. UFH flow temp is 35. If so your input to floor is around 50W/m2. Assume at temperature of defrost, the output of the ASHP is 7kW. 7000W / 50W/m2 is 140m2. So if your total floor area is at or around 140m2 you are driving the heat pump very hard. If it's about 17 to 80m2 you aren't. Link to comment Share on other sites More sharing options...
Steve W Posted March 10, 2023 Author Share Posted March 10, 2023 Both Daikin and the installer are coming out next week. On a day with 75% humidity, and -2C the house is 21 C in the morning, defrost cycle greater than 60minutes. At 94% humidity and -0.5C the temperature is the same as when it started and the defrost cycle is every 25minutes. The area is about 75sqm and the temperature ask is about 1.5C as in the starting temperature is 19.5C. The temperature here at, 6.30pm 10th March, is -2C and the humidity 85% and the defrost cycle is more than an hour, room temperature is 20.5C. Tonight I am expecting -10C so should be interesting. Link to comment Share on other sites More sharing options...
RichardL Posted April 21, 2023 Share Posted April 21, 2023 On 24/02/2023 at 07:50, Steve W said: has an external sensor on the North wall of the house Is it worth a double check of the installation manuals that this location is appropriate? i.e. on paper exercise. Link to comment Share on other sites More sharing options...
SteamyTea Posted April 21, 2023 Share Posted April 21, 2023 On 10/03/2023 at 18:31, Steve W said: On a day with 75% humidity, and -2C the house is 21 C in the morning, defrost cycle greater than 60minutes. At 94% humidity and -0.5C the temperature is the same as when it started and the defrost cycle is every 25minutes. Because the AH is different. 3.164g/m3 at 75 RH and -2⁰C compared to 4.403g/m3 at 94 RH and 0.5⁰C. So 72‰ more actual water to condense and freeze. Link to comment Share on other sites More sharing options...
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