Posted by: @abernyte

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"we are briefly going to steal some heat from the house or DHW to defrost the HP"

... Isn't great advertising!  Or is that too cynical?

Posted by: @abernyte

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Thank you gentlemen/people (can't be too careful now) perfectly reasoned arguments that fit the observed evidence.  The only fly in this ointment is why the manufacturer does not mention this in it's otherwise clear and technical explanation of defrost. Taking a "tiny bit of heat from the HE at the end of the process" is the only mention.  One would think if they were setting out to make such a video then the simplest route would be to say "we are briefly going to steal some heat from the house or DHW to defrost the HP" But they don't. 

Sigh....split jury, should never have studied law!

@abernyte - you really can’t be too careful.... alas there is possibly another complication in the evidence..... @davidnolan22 has a Low Loss Header fitted and he has been discussing poor recovery after defrosts (presumably the poor recovery is of room temps via the heating circuits and isn’t a failure of the heat pump)

A Low Loss header  is entirely capable of masking the size of the cooldown during a defrost if the flow rates are not balanced on both sides of the LLH.

If the exit flow to his rad circuit and his 2 UFH circuits does not match the flow rate of his primary pump then it is entirely possible that the heat pump just sees a smaller cool down temperature than is actually happening. A small distortion might not be a problem But it mightn’t be worth investigating.

If the chilled water from the defrost was sent in to the secondary circuit (radiators and UFH then there could be SLIGHTLY hotter water  sent straight back to the return thermistor to give a false reading.

This graph shows our heat pump during a defrost and you can see quite a high temperature drop compared to David’s graph

This graph is very clear and our system has no LLH or buffer or separate circuits. In other words there is no opportunity for distortion (or mixing)

here is the same graph where ive tried to sketch the DT as an overlay to show what the flow Andy return temps would have been like had ther not been a defrost. Hopefully you can better see the true drops in temps for both flow and return.

As was said earlier these defrosts could be repeat events every hour which could significantly upset the heating continuity of the home.

ive posted David’s graph below for comparison and I hope it can be seen the temp drop does not look that severe, when in reality it is likely to be just as large a temp drop as my defrosts.

So @davidnolan22  this was one thought I had and perhaps you’ve done it. Regarding recovery after multiple defrosts- Have you checked with thermostats what the temperatures are on all your connection ports on your Low Loss Header? This might give you an indication where the dominant circuit is or if the circuits are well balanced.

@sunandair 

Hi, ive read that a few times, I think I get what you mean

Yes, was definitely struggling to heat the house this winter at several points.  But, I'm not sure if it was the defrosts that were the problem are the system design and how I was running it.  My system was "designed" to run closed loop with fixed flow of 45 degrees.  But my house would drop in temperature intolerably when it got close to freezing.  When all the zones were calling for heat and the fixed flow was set to 45, it struggled like mad.  The house dropped and I could never get it back. 

But having worked through the "design" it seems that my emitter power was quite different in each zone. Overall I have 17Kw of emitter power at DT25 with an 11.2Kw heat pump.  When all the emitters were open at the same and I was calling for 45 degrees, it could never get close, it sent the compressor to 120hz constantly and defrosted every 20 mins.  And as I was running zones, the fabric of the house never got properly warm and the rooms were constantly going up and down in temp, and if the house got stuck too cold in minus weather, I could never get it back. 

As for defrosting. On the old set up: all the systems flows kept running with defrost.

I then changed how I ran the system, all stats set to max and opened it all up all the time, only called got 28-34 degrees, but ran it constantly, and the house heated a lot better and held its heat.  Ive only had a few minus days or periods since, but its been no problems with those temps with flows of 32 to 35, when its 3-5 degrees I'm running 26-28 degrees, no major cycling issues.  With calling for lower flow temps it did defrost less and settled the whole system down, ran much calmer and less noisy and used less power.

However, the installer made some changes to try to help defrosts. but we, and I include the installer who designed everything, we're still trying to get me to 45 degrees.  The changes they made were to swap the initial 35mm piped header for a larger volume low loss header and zone valve off this to close the secondary flows if possible with defrosts. This was (I think) to increase the volume in the primary circuit so the pump had more energy to defrost and not need to use the secondary flows. 

I spoke to Mitsubishi, who said that the secondary flows should close and the pump should be OK to defrost of the primary loop as it only needs 9 litres or so to do this in our climate. 

Anyhow, after the changes nothing has changed other than a small blending problem that has irritated me more than ruined the system.  But now I think what's happening is that as I'm always running very cool water, the pump needs more energy than is currently in the primary circuit to defrost and has to keep the house flows open. 

So in summary, I still don't full know if the Mitsubishi heat pump is meant to defrost on the primary loop if its got sufficient energy or not.  I need to wait until next winter to full tests my system to see if the deforesting issues I had re surface, or I can ride through them.  If I can't then I'm going to have to come up with a plan, a 3 piped buffer, or volumiser placed somewhere. 

Do you think your drops look so much as your running at 40 in the picture and I'm running at 30. And, you retuns drop, but I've got 170l system volume and I get about on lap of the full circuit in a defrost so the return water does not drop until after the defrost finishes and its stole some from the UFH mainly.  I think I've got this system pretty well balanced across the header now... Wow... long post.  Time for a beer.