Installer, long gone 6yrs ago. No support at all. Inherited system with new build purchase. Made improvements over the years with local companies (Inc resizing several rads etc) but not managed to find any one yet that has been able to convince me they really understand the system holistically.

Posted by: @davidnolan22

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@gary

i'm not sure what it will do, but Mitshubishi engineer said the 11.2 ecodan needs to have the secondary flows closed during defrost.

That seems reasonable if you have a good sized buffer tank between primary and secondary. but not if you have a plate heat exchanger or low loss header.   In the latter two cases the primary alone (or primary plus LLH) are unlikely to have sufficient volume in them to complete the defrost operation.

Obviously if your heat pump is directly connected with no LLH, PHE or buffer the question doesnt arise and the system pump , which circulates through the emitters, needs to keep going!

If its a std mitusbishi preplumbed cylinder then it doesn't usually have a buffer they do have what Mitsubishi call a Low Loss Header but it has minimal volume as its just a pipe that separates the primary and secondary sides to prevent flow errors if heating circuits turn off.

Posted by: @gary

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If its a std mitusbishi preplumbed cylinder then it doesn't usually have a buffer they do have what Mitsubishi call a Low Loss Header but it has minimal volume as its just a pipe that separates the primary and secondary sides to prevent flow errors if heating circuits turn off.

Yuk (comment on the LLH)

In this case surely the secondary must be kept running during defrost, otherwise the only water volume available for defrost is the small volume in the primary, which is unlikely to be sufficient (unless the Mitsubishi has a defrost heater built in).

According to this: 

 there is no heater but the compressor passes the hot refrigerant via a reversing valve back through the coil to defrost. 

@gary yep that's it

Thanks @abernyte - I remember seeing this video before, the technicalities are still beyond me but the general idea is useful to understand.

Posted by: @jamespa

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Posted by: @davidnolan22

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@gary

i'm not sure what it will do, but Mitshubishi engineer said the 11.2 ecodan needs to have the secondary flows closed during defrost.

That seems reasonable if you have a good sized buffer tank between primary and secondary. but not if you have a plate heat exchanger or low loss header.   In the latter two cases the primary alone (or primary plus LLH) are unlikely to have sufficient volume in them to complete the defrost operation.

Obviously if your heat pump is directly connected with no LLH, PHE or buffer the question doesnt arise and the system pump , which circulates through the emitters, needs to keep going!

Like @gary I have decent amount of pipework between the unit and the tank, about 15m in total for flow and return plumbed in 42mm copper. I think that's about 18 litres in volume in the primary, well above the 12 litres the unit requires. As the flow rate is also about 18 lpm that's five times round the primary circuit and dropping about 15° to complete the defrost in the five minutes it takes. So 300Wh or so of energy (if my hasty internet-informed calculations are right) taken from the water circulation (YMMV) – thats not a great deal of energy. I am sure my system only uses the primary circuit as the return temp drops as quickly as the flow temp, looking on heatpumpmonitor at some of the other Ecodans as well as Gary's post upthread it is clear that in systems using the combined heating circuit the return temp drops later and by not as much as the flow temp.

I can't see any setting or switch that informs the FTC about system volumes and whether to use the secondary pump, the only ref in the manual concerning these states: If the interlock operation of primary and secondary pump is not available, please ensure required additional water in only primary circuit according to the table 3.6.1. Whatever an interlock operation is?

Yeah, it’s all very complicated. My installer has put over 1000 Ecodans, so I’m leaving it to them. Both they and the mitshi engineer said my secondary circuits should be closed during defrost. I was getting 6 to 8 minutes of colder water flowing though the screed every 30 mins. 

I don’t know how much it will help to change this, but next Friday it’s getting changed so I’ll update here

@davidnolan22 would be good to get some detail from your installer on how closing the secondary circuits is achieved I look forward to your update

They are putting in a new low loss header and a zone valve controlled but the FTC that will close the secondary flows during defrost 

Just to update here on the above. My installers came to hopeful sort my heat pump system out.  They swapped a piped header for a bigger low loss header. and put in some zone valves that were meant to help the defrost.  Well after the change: everything is worse..... I now have a blending problem at the bigger Low loss header that was not there with the smaller one, the defrost look the same, if not a bit longer than before. And the UFH pumps that were swapped, we'll I've still not worked that out. 

I think I might be coming to the end of my heat pump journey, I've had enough over the last 18 months. I've heard NAPIT are pointless, I've called MCS and they just said call NAPIT, I've called HIES, they said nothing they can do, call NAPIT..

If feel totally stuck. Do I just pay a competent installer out to see if they can do the best with what's there? What do others do in similar situations? 

Posted by: @davidnolan22

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Just to update here on the above. My installers came to hopeful sort my heat pump system out.  They swapped a piped header for a bigger low loss header. and put in some zone valves that were meant to help the defrost.  Well after the change: everything is worse..... I now have a blending problem at the bigger Low loss header that was not there with the smaller one, the defrost look the same, if not a bit longer than before. And the UFH pumps that were swapped, we'll I've still not worked that out. 

I think I might be coming to the end of my heat pump journey, I've had enough over the last 18 months. I've heard NAPIT are pointless, I've called MCS and they just said call NAPIT, I've called HIES, they said nothing they can do, call NAPIT..

If feel totally stuck. Do I just pay a competent installer out to see if they can do the best with what's there? What do others do in similar situations? 

Im really sorry to hear that the installer didnt sort the problems.

Ecodans are renowned as good pieces of kit, so there must be something specific about your system that is causing the problem.  I dont know whether the reputation is both for ecodans installed with the pre-plumbed cylinder and its LLH (which seems like an abomination to me) or without, or indeed both.  But the basic unit is sound.

If I were in this situation I would first draw a system diagram which I would examine and think about as well as posting it for others to comment.  Sadly self-help is often best and this comments is by no means restricted to heat pumps.

If you post a system diagram I am sure that people (including me) will have suggestions.  Maybe at the same time post

• a summary of the problem with any evidence/measurements and when/conditions when it is observed 
• your calculated house loss
• the model of your heat pump
• Your house size and construction

I know all of this is above but sometimes putting it altogether helps and also removes ambiguity.