t 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.
Have you got any plots from melcloud showing how it works during recovery from defrost.
I have heard an argument that, if you have lots of fancoils, a volumiser should be fitted in the flow (the normal argument is to fit them in the return). The argument goes that this prevents an immediate rush of cold water to the fancoils during defrost. That said Ilife slims automatically shut down if the water temp is <30C, so I cant quite see how the house goes cold unless the system is failing to recover properly from defrosts (hence the question about graphs from MelCloud.
Just checking but are you running in any kind of 'low noise' mode. If I do that on my Vaillant it cripples recovery from defrost once the temperature gets below about -2C.
LLHs or buffer tanks are a bad idea in most domestic installations, so that wont be helping. Do you know why one was fitted? You may indeed need a volumiser.
Hives (or any other external control) are also a bad idea unless the set temperature is well above the desired temperature. Ditto TRVs other than on a minority of radiators.
More info needed to diagnose. Happy to try to give it a try if you post some, starting if you have them with some plots and some pictures of the area where all the plumbing and in particular the LLH is.
Im not a Mitsubishi expert, my heat pump is a Vaillant, but many of the potential problems/solutions are independent of the manufacturer and anyway more to do with all the parts of the heating system other than the heat pump itself.
This post was modified 2 months ago 3 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
Here is an example of what my 8.5kw Ecodan did last night when it was very damp and 1C outside.
It defrosted every 50 mins, that takes 5 mins and then takes 20 mins to recover back to the set flow temp, which means only 25 mins at that flow temp before the next defrost. So you could say the average flow temp is several degrees less than it should be at that temp, hence your house will not be able to keep up with heat loss if the Weather comp curve is set correctly.
If you have setbacks or turn off your heat pump during the day this is when its really going to struggle as you have no thermal mass in the house to help out in these conditions.
In my experience you need to keep the house toasty before this happens or you won't be able to recover until the weather warms up and the defrosts stop.
You can achieve this in 2 ways, manually offset the flow temp by a few degrees until the weather changes or add in a increased gradient curve for low single digit temps where defrosts are likely.
Thats really interesting. My Vaillant takes (I think) a different approach and whacks the compressor up to 100% modulation immediately after a defrost, so the recovery time is shorter. However this is at the expense of an increase in noise, accompanied by some tonality, which is tolerable but far from ideal. Im actually trying to find a way to 'slow it down' a bit as it could stand a slightly more relaxed recovery without materially compromising performance, and the tonality occurs only when the compressor gets above 80%, which only happens during defrost recovery. This is obviously quite a challenging set of trade offs for ASHPs!
If the system had a backup heater there might be something to be said for switching that on for the recovery period in addition to the compressor. I don't know if any do that but as an option to manage recovery it would be an additional variable. For comparison here it is at the coldest part of the recent weather
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
@jamespa, I can't check modulation but it will be running flat out but I think my is slightly undersized so don't really have any spare capacity to cope with defrosts and no volumiser which may help or just make things worse all year round.
I have learnt to manage around it house never drops below 21C, just have to run flat out 24/7 in cold snaps and will use 50kwhs a day but those days are few and far between
@gary Hi, my graph looks the same, but defrosts every 30 mins, takes 15 mins to recover the temp that's not high enough, then defrosts again. The house can cool from an average temp of 20 to 16 or so. this equals an unhappy family that complain at me as to why they can't "just turn the heating up; are you being tight?". I have to explain that I can't turn the heating up any more, its on max, and its costing me/us a fortune to get you to 16 degrees...... then the question "Dad, why did you get the stupid heat pump". Trickier question to answer!
And, I don't understand why it seem to restrict the power to 50% for the first 5 mins or so after a defrost which seems to compound the problem. I see other brands do the opposite and increase the power to recover. Seems a flaw to the ecodan, but there must be a logical reason.
This post was modified 2 months ago by davidnolan22
Are you operating any kind of noise reduction mode
Have you got all rads permanently on, no trvs
When it's cold could you heat dhw using an immersion
House cooling 4degrees is a lot, over what period of time?
What was the design outside temperature
This might be a case where a bit of management might solve the problem, or it might be just outside the design window (my system was designed for -2, last week it was -5, so I had no right to expect it to keep up, as it happens it did, but not by design).
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
3) yeah, suppose I could, but not really how the system was sold to me
4) heat loss of about 11 at -3, the defrost doom happens about +2 when all zones open at once, I'd understand cooling below -3. but my design was 21 in the main areas at -3, so I'll happily take 18 or 19 when its really cold, not many days like that.
I've tried to run it fully open all the time removing any zoning, but the same things happens. When I expose the pump to the fully system volume, its seems to really struggle. But, I've seen it pull 5000w for short periods in cold weather, which must be putting out 13-15kw at COP of high 2s to 3, which seems a lot for an 11.2Kw unit, this seems to be when the unit quickly ices up. I've currently turned the pump down a bit on the primary loop to seem it calms the compressor down a bit. I've also stopped calling for temps that it can't achieve to see if that helps. But its got mild again, so hard to tell.
Installer is coming next week and we're going to sit down and double check all calculations, but when it goes into defrost mode, it really does cut the power a lot so its hard to see how it could possibly heat the house.
no buffer, no volumiser, piped 35mm Low loss header with one feed off to 2 UFH manifolds, and 1 rad run. Total system volume I have worked out as about 150-175 litres.
I've attached a fairly typical graph for fully open system at temps close to zero. I need something more consistently closer to 40 degrees.
no buffer, no volumiser, piped 35mm Low loss header with one feed off to 2 UFH manifolds, and 1 rad run. Total system volume I have worked out as about 150-175 litres.
I've attached a fairly typical graph for fully open system at temps close to zero. I need something more consistently closer to 40 degrees.
How well balanced is the LLH . Unless perfectly balanced they have the potential to cost 15% or more in efficiency and 5C in flow temperature.
If you have the means to measure temperature Id measure flow in to the LLH from heat pump and flow out to the emitters. If there is any significant difference then this is probably sufficient to cause (or fix, depending on how you look at it) your problem.
i'd need to check. i'm not sure when the temp sensors are for the display i see in mel cloud. i assume they are the ASHP side of the header. the temp at the radiator inlet is the same as what the app shows me.
i'd need to check. i'm not sure when the temp sensors are for the display i see in mel cloud. i assume they are the ASHP side of the header. the temp at the radiator inlet is the same as what the app shows me.
Worth measuring, some ashps have a temperature offset setting which could be fooling the app.
Also worth measuring the other two ports on the llh.
Do you have any idea how the installer went about balancing it or whether you heat pump controls the speed of either or both of the primary or secondary pumps.
During defrost does the secondary pump stop or keep running?
Llhs make diagnosis difficult unfortunately as well as having the potential to create an efficiency hit.
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
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