Posted by: @cold123456

↑

I had my heat pump installed in February this year and the installation quality itself was okay. They added a buffer tank to the system which then had a lot pipework, a circiulation pump which took a lot more room than was planned for. The circulation pumps (for the heat pump) had to be moved as they were incredibly noisy and we had a 'quieter' put in for the heating system.  We had several teething problems where it was continuously on (2.5KwH) when there was no heat demand but that seemed to be sorted out however I suspect I was seeing the defrost mode in action at the time. This install is purely wall radiators which have been resized. We do have two zones but they are controlled with a Hive system with some bespoke wiring. Even though a heat loss evaluation (5.2kW overall) was done we're struggling to get one room (2nd zone) warm but it might be an insulation problem which is a separate issue.

All good this up until till a few days ago when we've had 0C or temperatures. Every hour (between 30 and 50 minutes apart) the flow temp 50C drops suddenly to 35C which is below the return temp. I'm assuming the defrost mode is coming as there doesn't seem to be any other reason for this but it then takes 30 minutes to get back to 50C. Today this has happened every hour and I'm around 16C in the house with a 40kWh usage so far (20:00) and haven't reached my target temperature the last 4 days.

Is there any other reason for this behaviour apart from defrost mode? (I'm trying to catch the symbol change or go outside to see the ASHP doing this). Can the defrost cycle be tuned at all so it comes on at for shorter periods and at longer intervals?

At the moment I'm considering changing to a hybrid solution and getting a gas boiler back in for when the temperatures are low. I'll be talking to my installer this week but I doubt they will be able to advise much however as far as I'm concerned they've sold me a system which will not heat the house. Even on the design document they provide they do mention (MIS 3005-D 5.5.1 clause F "An air source heat pump system should be able to maintain the internal design temperatures across multiple defrost cycles")

 

I would suggest that you start by posting a photo of the main screen display, since it should then be possible to identify the present mode of operation.

it’s all sounds like normal operation in terms of defrost.  Do you have MELCloud app csn you post pictures of flow temps.

what was your design temp 50c is high and an 8.5 ecodan won’t get to 50C at 0C temps I have the same unit it can get to 40C at best. How are you running the unit eg mine is set to run 24/7 at 21C internal temps.  At 0C I dont set back overnight as the house will never get back to temp.  I’m using 40-50kwh a day at these temps 

Posted by: @gary

↑

it’s all sounds like normal operation in terms of defrost.  Do you have MELCloud app csn you post pictures of flow temps.

what was your design temp 50c is high and an 8.5 ecodan won’t get to 50C at 0C temps I have the same unit it can get to 40C at best. How are you running the unit eg mine is set to run 24/7 at 21C internal temps.  At 0C I dont set back overnight as the house will never get back to temp.  I’m using 40-50kwh a day at these temps 

That is a bold statement Gary, it might be true for your installation, but, for example, it isn't for mine which has been running with flow temps between 40 and 47° for the past couple of days. And this is with a night setback of 5 hrs and OATs for long periods of -2/-3°.

@cold123456 I have been getting a defrost about once an hour in these conditions, your more frequent defrosts will be an issue. The screenshot shows the running flow temperatures and the defrost taking a lot of energy out of the circulating water – it then takes about 15-20 mins for the pump to raise the temperature back up to the flow temperature. Once an hour is ok, I guess, but any more and you end up only putting a modest amount of heat into the house between cycles.

We may be past the worst of this cold snap but next time you could try lowering the target flow temperature to prevent the pump working quite so hard, this may then reduce the need for such frequent defrosts (not anything a setting can directly affect) which would compensate for the lower flow temperature.

I suspect that that it is the design of your CH delivery that is the root cause though, particularly having a buffer tank. Unless your primary pump flow rate and secondary heating flow rates are well balanced it is possible that the water circulating to your radiators is cooler than the output from the heat pump and this relative cooling causes the heat pump to take longer to add the heat to bring it back to the temperature the compensation curve or algorithm is try to get to.

Posted by: @harriup

↑

Posted by: @gary

↑

it’s all sounds like normal operation in terms of defrost.  Do you have MELCloud app csn you post pictures of flow temps.

what was your design temp 50c is high and an 8.5 ecodan won’t get to 50C at 0C temps I have the same unit it can get to 40C at best. How are you running the unit eg mine is set to run 24/7 at 21C internal temps.  At 0C I dont set back overnight as the house will never get back to temp.  I’m using 40-50kwh a day at these temps 

That is a bold statement Gary, it might be true for your installation, but, for example, it isn't for mine which has been running with flow temps between 40 and 47° for the past couple of days. And this is with a night setback of 5 hrs and OATs for long periods of -2/-3°.

 

Yes apologies it was a sweeping generalisation mine is slightly undersized hence it can’t get the flow temps higher than 40C when it’s subzero outside.   

Thanks all, delayed response due to illness. The defrost was running every hour based on the graphs.  The design flow temperature is 50 degrees, external temp at -3.1 with a building heat loss of 5.24 kW.

I do have Hive controllers integrated which allows me to configure the setback periods easily however I was letting the house get too cold overall so I've adjusted them up a lot overnight (16C from 12C) which has helped getting up to the target temperature so far.

According to the flow temps I'm getting any temperature up to 55 degrees (haven't tried higher) depending on what I've got it set to even during the cold snap. I do have WC on and testing out a lower temp outside of the freezing weather to reduce cost further.

I was given several reasons for the buffer tank despite reading that it wasn't necessary in most cases here. 10mm Polybutylene pipework (extra pump on outlet for when the heating is on), increase volume of the system, help with the defrost function, etc.

@cold123456 appreciated this thread dates from a while ago now but did you ever get this resolved ?, we have very similar symptoms with our Ecodan 8.5kw system, it's been an issue for 7 winters now, defrosting every 40-50mins when outside temp is around 0oC and flow temp is around 40-45oC house not able to get 21oC set temperature, sits around 18oC. Due to recovery from defrost looks like only max 30 mins in any hour are getting the full target flow temperature (40 ish). I haven't been able to find an installer/engineer that understands why it happens and how to fix it. If you got this solved on your system it would be great to know how. Thanks !

@alsocold123456 It doesn't sound like there is a problem to fix with your system that is pretty much what mine is doing in this damp cold weather.

Are you using setbacks at night, if so, I would stop that, I run at 21C all day and the heat pump can just about keep up but as you have seen with defrosts you spend more time at a lower flow temp as it trys to recover.

@gary no set backs. Yes see others in various forums have similar experiences. Which while "common" can't be desired or by design, whole point of a heating system is to keep the house nice and warm when it gets cold. We shouldn't have to accept these performance compromises, the claims are that these systems work well down to -20C, my experience is that once we go below zero and it's cold and damp then the house starts to feel chilly and seemingly the cause is the regular removal of heat from the radiator circuit to provide heat for frequent defrost. Each time it defrosts the radiators go colder and switch off (about half the rads are iLife 2 slims), so I think there is an issue with my system, possibly something to do with volume and the solution might be buffer tank, volumiser or a larger volume low loss header. The outside unit sits immediately the other side of the wall from the indoor pre plumbed tank, so I think the primary heat pump loop is very short and low volume, so with each defrost it ends up taking energy from the low loss header and thus the radiator circuit is impacted. Would be interested in any heating engineer's opinions, my installer says it doesn't need buffer tanks etc and I agree outside temp >2-3C and it doesn't but in the zero and below window it just doesn't work. Modern well insulated house, built 2018.

i'm having a similar problem, i'm sure we're not the first. interested in options to help. 

Posted by: @alsocold123456

↑

@gary no set backs. Yes see others in various forums have similar experiences. Which while "common" can't be desired or by design, whole point of a heating system is to keep the house nice and warm when it gets cold. We shouldn't have to accept these performance compromises, the claims are that these systems work well down to -20C, my experience is that once we go below zero and it's cold and damp then the house starts to feel chilly and seemingly the cause is the regular removal of heat from the radiator circuit to provide heat for frequent defrost. Each time it defrosts the radiators go colder and switch off (about half the rads are iLife 2 slims), so I think there is an issue with my system, possibly something to do with volume and the solution might be buffer tank, volumiser or a larger volume low loss header. The outside unit sits immediately the other side of the wall from the indoor pre plumbed tank, so I think the primary heat pump loop is very short and low volume, so with each defrost it ends up taking energy from the low loss header and thus the radiator circuit is impacted. Would be interested in any heating engineer's opinions, my installer says it doesn't need buffer tanks etc and I agree outside temp >2-3C and it doesn't but in the zero and below window it just doesn't work. Modern well insulated house, built 2018.

@alsocold123456  Only the primary circuit runs during the defrost cycle so shouldn't be taking heat out of the water sitting in the secondary heating circuit. I thought there was a minimum volume required for the primary circuit but I can't see any reference to that, only a minimum volume for the heating circuit by which I presume they mean the combined primary and secondary. Are you running in WC mode?

Posted by: @harriup

↑

@alsocold123456  Only the primary circuit runs during the defrost cycle so shouldn't be taking heat out of the water sitting in the secondary heating circuit. I thought there was a minimum volume required for the primary circuit but I can't see any reference to that, only a minimum volume for the heating circuit by which I presume they mean the combined primary and secondary. Are you running in WC mode?

I would be careful with that presumption.  Minimum volume is at least in significant part about defrost, and what matters is the minimum volume available to the heat pump (the fundamental is that the water needs to contain enough energy to melt the ice!). 

If both primary and secondary water pumps run during defrost, that is indeed the sum of the primary and secondary volume, but if the secondary water pump switches off then only the primary volume plus the volume of the LLH is available to the heat pump for defrost which may not be enough.