@jamespa

House is 258m2, recently renovated. done a good as job as possible with insulation: stone build but most external walls old and new have insulation. 

11Kw heat loss at -3

Unknown system volume, But  the UFH is about 60L, and 6 big rads and then pipes to and from.

11.2Kw ecodan.

2 UFH manifolds, 1 rads. No blending valves. 

Problem: was designed to 45 degree at -3. Originally 9 zones, I consolidated to 5 zones (2 on each manifold and a rads). Was working OK while mild and not all the zones calling for heat.  When is got cold and all the zones called for heat, the pump could not get close the the 45 degree flow and defrost kicked in every 30 mins and the house did not get to temp. I was running it on/off on the stats. 

I then ran everything open calling for heat all the time and again, still could not make the temp due to defrosts when it got cold. 

But then I looked my emitter spec, I've got 16kw of emitters at DT 25. which is quite a lot.  So, I reduced the temp I was calling for to 32-34 degrees.  The compressor calmed down. it defrosted less and the house was warming up.  We also then changed a DIP switch (SW2 number 7) on the advice of Mitsubishi from On to Off, and I'm not sure what that did, but it was starting to work.  I got it so the house was holding nice temps at flows of 34 degrees in below freezing weather. 

My installer had come out due to my complaints.  They diagnosed that the defrost cycle was running with the secondary loops open. they said that these should close during defrost and it should just use the primary circuit.  At that time it was a thin pipe 35mm distribution header. But on that header, If my primary circuit was 32 degrees, is was 32 at each manifold and the rads, all of them even.

They came this week and swapped the thin pipe header for a bigger LLH, put in Zone valves in so I though the FC6 would close them during a defrost.  Anyhow, since the swapped to the bigger volume header, I now get different temps at each of manifold and rads, and every the rads are no longer balanced. And, it does not defrost any differently.  So I’m getting lower temps through the secondary circuits now.  I spent 4 hours last night trying to play with pumps and loops nobs trying to balance it, but I can’t get it back to how it was.

My concern is that I’m quite tight on my heat loss. I need every bit of efficiency I can get.  When the primary loop is on 33 deg, and my rads are 29, it wont be enough, and, it wasn’t like that before.

I bought all this up with the Plummer who did it, and I got a nod but a slightly blank state and “It will be fine now mate”

Posted by: @davidnolan22

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We also then changed a DIP switch (SW2 number 7) on the advice of Mitsubishi from On to Off, and I'm not sure what that did, but it was starting to work

This turns off 2 zone control so depending on what thermostats you have wired to the FTC it would have presumably disabled one of the circuits? Are you using the main controller as a thermostat or do you have wireless mitsubishi thermostats or 3rd party ones?

I would presume that the rads are one circuit controlled by a thermostat and the UFH is another controlled by a separate thermostat otherwise SW2-7 wouldn't have been enabled

@gary yeah, I have one wireless stat doing the rads, and then 3rd party ones which i just have turned to max now as they are a total  pain in the ass.

OK  Its a complex system (5 pumps!), how is it controlled:

eg weather compensation only without room influence, zone valves?, TRVs?, external thermostats?  Above you refer to wireless stats, doing what?  Please describe the control system fully including how/by what the pumps are controlled

operated 24x7?

how is DHW controlled (scheduled or always 'on' so controlled by the hysterisis)?

What flow temperatures were the radiators and UFH designed for?

What is the estimated volume of the primary circuit (excluding the LLH)

During a defrost does the secondary close down or not?

Not sure what the point was of installing a bigger LLH, if volume is needed for defrost install a volumiser in the primary, or open up the secondary to defrost.  What was the logic of this change (or were you not given one)

Assuming you have decent insulation 11kW should suffice, my house is 200sq m, 1930s build with piecemeal fabric upgrades, 7kW.

The table below is min system volume I think.  Note the difference between warmer and cooler climate.  I think we are 'warmer' but we are also very wet so I would err on the larger volume side!

The comment 'case 2' is telling and clearly does NOT say that only the primary should be used for defrost.  What it says is that, if the heat pump is not controlling both circuits, the primary alone must have sufficient water.  Thats very different (and also logical unlike some other statements made)

I reckon your problem is somewhere in the way its controlled.  

@jamespa 

At the minute I have all stats turned to max and running weather compensation. Its running 24/7. all the rads are fully open, nothing is shut off

DHW is tank filled schedule  x 2 a day, 1-2pm and 4-5am 

They were “designed” for 45 degrees. But I can’t get close to that. But it seems like I don’t need to.

Primary circuit is about 16m of 32mm PEX pipe, so I guess about 10-12 litres.

Defrost: before the changes, the secondary circuits stayed open. After the changes they still stay open, but the point was they close, they do not close.  

I do not know the logic or why the LLH was changed. 

Am I agree, I think i've got a system design problem, I used to think I have a too small heat pump problem. 

Posted by: @davidnolan22

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@jamespa 

At the minute I have all stats turned to max and running weather compensation. Its running 24/7. all the rads are fully open, nothing is shut off

DHW is tank filled schedule  x 2 a day, 1-2pm and 4-5am 

They were “designed” for 45 degrees. But I can’t get close to that. But it seems like I don’t need to.

Primary circuit is about 16m of 32mm PEX pipe, so I guess about 10-12 litres.

Defrost: before the changes, the secondary circuits stayed open. After the changes they still stay open, but the point was they close, they do not close.  

I do not know the logic or why the LLH was changed. 

Am I agree, I think i've got a system design problem, I used to think I have a too small heat pump problem. 

 

OK nothing obvious here so more questions Im afraid

What controls the pumps

Are you sure that the secondary remains open (and pumped) during defrost.  Can you tell whether the water crosses between the two (eg from temperatures)

When you say you 'cant' get to 45, whats stopping you (defrost?) or dont you know

You posted a plot a couple of pages ago, was that a typical defrost and the time axis is it in minutes

What are your weather compensation settings, is there a maximum target flow temperature specified somewhere in the settings

Do you have any info on flow rate in the secondary loops

Is the LLH correctly plumbed

@jamespa 

What controls the pumps

Wireless stats control the rad and 3rd party stats the UFH, all of them calling for heat all the time

Are you sure that the secondary remains open (and pumped) during defrost.  Can you tell whether the water crosses between the two (eg from temperatures)

yes, I've sat and watched the pumps and measured the temp drop at the rads and UFH 

When you say you 'cant' get to 45, whats stopping you (defrost?) or dont you know

Not just defrosts, If i put in 45 degrees and leave it, it will top out after a few hours at 36-37, even in mild weather if all zones are open. I think this is because my emitter sizing is big I have a lot of surface area. My heat loss is 11kw, but my emitters to 50% bigger than that, and no evenly so which is annoying.  When it defrosts, this makes it a bit worse 

You posted a plot a couple of pages ago, was that a typical defrost and the time axis is it in minutes

yes

What are your weather compensation settings, is there a maximum target flow temperature specified somewhere in the settings

weather comp 38 at -3 to 25 at 12.  It can get to 45 degree flow no problem if I start closing zones off, so the machine is capable of getting there

Do you have any info on flow rate in the secondary loops

only on the wilo pumps, which is 30% out on the primary pump so I ignore, 

Posted by: @davidnolan22

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What are your weather compensation settings, is there a maximum target flow temperature specified somewhere in the settings

weather comp 38 at -3 to 25 at 12.  It can get to 45 degree flow no problem if I sort closing zones off, so the machine is capable of getting there

if 38 at -3 is specified as an end point on the curve, the target temp will never go above 38.  If however you shut zones down then it may overshoot because it cant modulate down.  Is it just this that is the reason you cant get above 38? 

Posted by: @davidnolan22

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Not just defrosts, If i put in 45 degrees and leave it, it will top out after a few hours at 36-37, even in mild weather if all zones are open. I think this is because my emitter sizing is big I have a lot of surface area. My heat loss is 11kw, but my emitters to 50% bigger than that, and no evenly so which is annoying.  When it defrosts, this makes it a bit worse 

Emitters bigger = good, you can run at a lower flow temperature = higher COP

Not sure why it takes hours to get to 45 if you 'put in' 45 (where are you putting in 45?).  How long the flow take to get to temperature when you are doing DHW?

I have a faint hunch that, because you have large emitters, you are running at a low flow temperature, which of course means more water is needed to supply the energy for defrost.  Linked to this I suspect that for some reason to with the balancing, although primary and secondary are connected, defrost water is effectively being drawn from the primary only or principally from the primary

32mm pex has wall thickness 3mm so has an ID of 26mm.  16m contains only 7l water, no way thats going to be enough at 35C

However the 'hours to get to 45 when set to 45' needs teasing out.

I mean, if I put in 45 on fixed flow as the flow temp, open all Zones and leave it,  it will never get there, spends hours ( I’m mainly UFH in screed) and it won’t ever reach a temp higher than 38. But, what temp would I expect a 11.2kw heat pump to reach on a 16kw emitter surface at dt 25. I think 38 might be the highest it can get to and needs. Or am thinking about this wrong. 

I hear what you’re saying about system volume. I think they may have just needed a volumiser on the return and not the LLH they put in. But who am I….. ahhhh this is driving me nuts 

Posted by: @davidnolan22

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I mean, if I put in 45 on fixed flow as the flow temp, open all Zones and leave it,  it will never get there, spends hours ( I’m mainly UFH in screed) and it won’t ever reach a temp higher than 38. But, what temp would I expect a 11.2kw heat pump to reach on a 16kw emitter surface at dt 25. I think 38 might be the highest it can get to and needs. Or am thinking about this wrong. 

Fundamentally emitter output increases with flow temperature.  Once the system has got to the point where flow temperature is such that the emitters are emitting 11.2kW, the flow temperature can go no higher because the return temperature is such that 11.2kW is needed to get it back up again.  

So if your emitters are 'oversized' and your heat pump 'right sized' then you will end up 'capping' your flow temperature.

The fact they cant go higher doesn't matter, so long as your house is warm, which you say it isnt.  

If some bits of the house are warmer than others it might be a balance problem, but balancing when you have so many pumps sounds like a nightmare.

Of course another possibility is the 11.2kW never reaches the emitters, unfortunately we cant tell easily because of the LLH.  Whats the deltaT when its 'maxed out'?  Your primary flow rate 28l/min will move 9.8kW at DT5 (Im assuming system is filled with water not glycol, if its glycol then the amount moved is less).  So you should be seeing DT6 or thereabouts when maxed out.

I trust your UFH screed has plenty of insulation underneath it, otherwise some of the energy delivered from the heat pump is lost downwards.

260sqm isnt enormous, my house is 200 sq m and runs on a single radiator zone directly from the pump inside the (Vaillant) heat pump casing.  Would it be practical to bypass the LLH and all but one of the pumps (ideally switchably so you could 'try' it.  Then you could guarantee that all the heat produced by the heat pump was ending up in the house at maximum efficiency, and that you had plenty of water for defrost.  Or do you know for certain that all the pumps are necessary to overcome flow resistance (seems unlikely!)- I guess what I'm really asking is - was there a good reason to make this installation so complex?

From what you have said so far there is no obvious cause, except possibly that the heat pump is pretty much right-sized but the losses in the LLH combined with balance issues prevent the heat getting to where it is needed, while the system separation reduces the volume of water for defrost, the effect of which is exacerbated by large emitters even though this is normally good news

That’s a really thoughtful reply. Thank you. It’s a great help. 

lots to ponder. My view has been consistently that i don’t need much more than 35 to 38 degrees at -3, and that’s the most it will ever get to. all the pumps and balancing has been tough and the defrost were definitely a problem. 

I think the pump outside can keep me warm, and, I’ll get to a point where it will. But, it should not be this hard.  The new header has not current helped. But I’ll try to get to learn it better in the next few days. 

How were you running it before the cold snap.  If it was setback overnight your screed would cool down and depending on how thick it is could take an enormous quantity of heat.  The heat pump will be able to get to a higher temp but as the return is cool it will be running flat out so the heat is going somewhere.  That’s either into tonnes of concrete or to the outside if it’s badly insulated which doesn’t sound like the case