Here’s a snapshot of my defrost activity over the last couple of days. Outside temp dropped to around zero and stayed in the 0-2deg range for a while. At the same time the weather was very wet. I had defrosting taking place as frequently as every 37 mins during the afternoon yesterday and the peak flow temperatures fell well below target. Room temperatures were also falling throughout the period to between 16 and 18 throughout the house. The whole house was at 21deg on Wednesday morning. Each defrost cycle results in 12-15 minutes of significantly lower flow temperatures which is as much as 40% of my effective heating capability. Clearly the remaining 60% is not enough to keep my house comfortably warm.
I’m wondering what I can do to improve this situation? Would a lower target flow temp reduce the frequency of defrosts and thus increase the total heat energy being delivered?
It seems a bit silly that during the defrost cycle the system continues to pump the rapidly cooling water around the house - I wonder if I could create an automation that recognises the start of the defrost cycle and tells my smart thermostat to stop calling for heat (which would in turn stop the secondary and ufh pumps). Would this allow the emitters to at least stay warm and thus improve the recovery time.
@rupes Here's my pattern for yesterday. Although I was in constant defrost cycles, I was hitting target quite often, but the house temp was a good 1C below where I target it (usually 20-20.5; yesterday it was 18.5-19).
I used to have a target LWT of 50C for zeroC outside, but it was never getting close to that. Since dropping to 45C it does more often reach that target, and then with the lower compressor rate, the defrost is less likely to happen. Although this seems very counter-intuitive (and also with a lower LWT you're less likely to get a decent deltaT i.e. less energy to your home so less likely to reach your target IAT).
Do you have the Grant LLH installed? Is its immersion heater wired up? I'm looking at trying that some time (when it's a bit warmer ... I tried during the cold snap and messed up, so not risking it again for a while) to see the effect that has on defrosts and performance.
Another option I've considered is, if your house is zoned, switch off one of them to boost energy to the other(s). As you can see during your HW cycle (I assume that's at 2pm) the target LWT increases rapidly if there's less energy required by the home, so doing that ought to have an impact on defrost cycles. This isn't what one ought to require in an ideal world, but here we are...
I’m wondering what I can do to improve this situation? Would a lower target flow temp reduce the frequency of defrosts and thus increase the total heat energy being delivered?
almost certainly yes. looks like your target WT is around 47. For comparison here's mine from the sub-zero weather on the 18th. Target WT is 41 at -7, 38 at +1. less frequent defrosts and no issues maintaining WT and house temp. obviously reducing WT means you need to look at whether your emitters are big enough and if not upgrade them.
this is unfortunately a fundamental physics issue of running at a higher temp that isn't made enough of - the defrosts are going to happen more frequently and the WT recovery time is longer.
Adding volume to the system could be another option i.e. adding a volumiser tank inline.
It seems a bit silly that during the defrost cycle the system continues to pump the rapidly cooling water around the house - I wonder if I could create an automation that recognises the start of the defrost cycle and tells my smart thermostat to stop calling for heat (which would in turn stop the secondary and ufh pumps). Would this allow the emitters to at least stay warm and thus improve the recovery time.
I assume you must have a buffer/LLH as you reference secondary side pumps, if you stop those the HP will defrost thought the buffer/LLH and the radiators will cool naturally. during this time. However, the water that will be pushed into the house from the ASHP when it restarts is likely to be really cold. So possibly a delay to restarting secondary a few mins after the defrost cycle has finished may be needed. I don't know for sure if it will improve things , the net gain may not be anything, but its worth a try if you can automate this easily... Do you have a way to detect defrost cycle start?
Here’s a snapshot of my defrost activity over the last couple of days. Outside temp dropped to around zero and stayed in the 0-2deg range for a while. At the same time the weather was very wet. I had defrosting taking place as frequently as every 37 mins during the afternoon yesterday and the peak flow temperatures fell well below target. Room temperatures were also falling throughout the period to between 16 and 18 throughout the house. The whole house was at 21deg on Wednesday morning. Each defrost cycle results in 12-15 minutes of significantly lower flow temperatures which is as much as 40% of my effective heating capability. Clearly the remaining 60% is not enough to keep my house comfortably warm.
I’m wondering what I can do to improve this situation? Would a lower target flow temp reduce the frequency of defrosts and thus increase the total heat energy being delivered?
It seems a bit silly that during the defrost cycle the system continues to pump the rapidly cooling water around the house - I wonder if I could create an automation that recognises the start of the defrost cycle and tells my smart thermostat to stop calling for heat (which would in turn stop the secondary and ufh pumps). Would this allow the emitters to at least stay warm and thus improve the recovery time.
When operating in normal mode a ASHP absorbs thermal energy from the outside air and uses this thermal energy to heat the water being used to transfer this thermal energy into the home.
When operating in defrost mode the refrigerant gas flow is reversed, so thermal energy is being absorbed from the central heating water and being used to melt the ice that has built up on the evaporator. I therefore doubt that stopping the water flow will have any useful effect and may even make matters worse, particularly if there is no anti-freeze in the system.
The cold and damp weather that we frequently experience in the UK is possible the worst operating conditions for the reliable and efficient operation of an ASHP.
The following suggestions may possibly help to reduce the effect of defrost cycles.
1) If possible lower the required LWT setting. This should hopefully reduce the demand on the heat pump and reduce the rate of ice build-up. This may cause lower indoor temperatures, so would need to be a balanced approach.
2) If possible get a secondary heat source for use in damp and cold weather, thereby helping to reduce the demand on the heat pump. A log burner or fan heater may be a simple and cost effective solution.
3) Improve insulation and draft proofing to lower heating demand.
4) Larger heat emitters may help by reducing the LWT, but the heating demand would probably remain the same.
The cold and damp weather that we frequently experience in the UK is possible the worst operating conditions for the reliable and efficient operation of an ASHP.
This is the crux of it @derek-m. I've just come off a call with my colleagues in Oslo.
Right now it is -13degC and 74% RH, somewhat typical winter weather for them. Absolute moisture content: 0.91g/kg of air. Yesterday at home in the UK it was 2degC and 90% RH, absolute moisture content 3.94g/kg of air. That's over 4x the latent duty on the evaporator coil here in the damp and drizzly UK.
Scandinavian climes are often quoted as proof that ASHPs work in lower temperatures, and they do, but the milder damper temperatures we more frequently experience in the UK are more problematical for air source evaporators. There's less enthalpy at the lower air temperature, so the refrigerant process COP is lower, but the much lower frequency of defrost means a more consistent delivery of heat to the indoor space. Doesn't matter if it's a direct A2A refrigerant vapour condensing heat pump or an A2W indirect hydronic condensing heat pump - once the refrigerant evaporating temperature falls much below zero, evaporator coils prefer dry colder air over damp humid air.
@derek-m I live on sufficient area of land for GSHP however way too many trees and roots for horizontal install and vertical was much too expensive. GSHP would have been my preference.
Retrofitted 11.2kw Mitsubishi Ecodan to new radiators commissioned November 2021.
14 x 500w Monocrystalline solar panels.
Thanks for all the replies to my post of this morning - plenty of food for thought.
I might try a lower target flow temp next time the conditions are right for a lot of defrosting and see if that helps.
I do have a LLH so there’s a bit of a thermal store there to support the defrost if the heating circulation was to be stopped. I did have a go at manually stopping the central heating pumps today when I saw a defrost cycle starting but I don’t think I got the timing quite right and ended up disrupting the recovery from the defrost instead which prolonged the downtime - I guess the problem with this approach is that when the defrost finishes - if there is no heating demand then it doesn’t start heating up again. So getting the timing right would be critical and probably quite difficult in the absence of any specific signals. I don’t think I have enough free time on my hands for that challenge.
I have the immersion heater connected up on my LLH and have experimented a bit with this. There are several settings linked to this which are a bit confusing and there are two different ways it can be used - or potentially a combination of both.
1. It can be used as a supplemental heater so it switches on when outside temp is below a set point AND the temperature in the LLH is below a set point. E.g when it’s -2 outside and LLH is below 40 degrees. This one definitely had an impact but also has the potential to be quite inefficient depending what the set points are as it can end up being on for a prolonged period. I didn’t try this for very long but it looked like it was causing the ASHP to throttle back a bit - presumably detecting that the delta between flow and return temperatures was falling so less heat was needed. it did help to bring up the temperature peaks between frequent defrost cycles though and also extended the time between defrosts because it was allowing the heat pump to work less hard.
2. This option is supposed to support the defrost process by providing some extra heat into the LLH while the defrost is taking place but only for a limited time. This is more subtle than option but did seem to help the recovery from defrost to get back to the target temperature. However it didn’t seem to reduce the duration of the down time.
here’s a plot from yesterday showing normal defrosts for the first part and then the impact of the immersion heater in the second part. I didn’t do this very scientifically unfortunately so I can’t remember exactly what settings I had applied in each case
@rupes This is very interesting. I'm trying to wire up my LLH heater atm, but it's a tricky cabling path for both power, and the control switch from the external unit. I want to just "try it" for a while to see the impact, before I see if it's worth the cost of adding a new circuit to my consumer unit.
From what you show, 15:00-19:30 ish is standard defrost (every 40 mins) and you aren't hitting target LWT; then after that the cycles extend to around 1 hour, and you are hitting LWT target. It's whether that improvement is worth the cost of the COP=1 power of the LLH heater.
Can you add external air temp to your graph? Maybe it was just warmer after 19:30? The key temp for a Grant seems to be 3C and lower.
Do you know how much power the LLH is drawing? It's a 3kW element, but the Grant docs suggest it'll only pull 250W in "defrost cycle help" mode. I'm a little sceptical.
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