Posted by: @robs

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The specs of the 14kW Mitsubishi give a minimum output at 7C OAT of 4.2kW with a 35C flow temp, 3.7kW with a 40C flow temp, and 3.2kW with 45C flow. At higher OATs it's even worse, for example 6.2kW minimum at 12C OAT and 35C flow temp.

Thanks for that. Where did you find this? I've been trying to get the same info for the 8.5 R32 EcoDan to compare but Google hasn't been any help.

Posted by: @jamespa

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Further to the above here is a template for what you might say if you wish to decline sign off.  ? indicates info from melcloud or elsewhere, or references to legislation ( I am happy to find these tomorrow).

That is brilliant! Thanks. I've got to dash out but I'll have a proper read through when I get back later but it looks great. Thanks again!

@sandman1600 

Lots of graphs there.  In the middle of the night its close to 0 degrees and it starts to run more stable, still the odd wobble but no great concerns.  Your DTs are 3, you flow rate is 16-17l.  assuming you have glycol, that means your KW its needing is about 3.5 to 4.  Does that keep you warm?  If so you are 3-4 degrees above design temp (if its -3, depends where you live) and your needing not much power to keep the house warm. But this is NOT a heat loss estimate.  

In the day its much warmer and it cycles a lot, but its fairly normal to cycle 1-2 an hour in warm weather and for it to turn off for a bit.

James giving really good advice. My point on the flow to the unit is that the minimum for the the 14kw is 18l/min.  I can't imagine it's happy with that little flow and how that affects the heat exchange, but I'm no expert in that, and don't turn the flow up through the unit as you'll get a bigger problem. But its running just below the minimum flow and still has narrow DTs, to me it means that its struggling to operate to such a small load.

My 11.2 has 28 l/m, runs DTs normal 4-5, but when its cold and I really need it, they will stretch to 7 or even 8.  

One thing that might help some of the wobbles is to turn on the quiet mode all the time and set it to one bar. That restricts to compressor to 50% max output and might help some on the wobbles, but won't change the cycling much. 

Posted by: @sandman1600

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Thanks for that. Where did you find this? I've been trying to get the same info for the 8.5 R32 EcoDan to compare but Google hasn't been any help.

It's from: https://library.mitsubishielectric.co.uk/pdf/book/Ecodan_ATW_Databook_Vol.6_.0_.pdf#page-1

Suggest you use the menu (top right) and download the PDF - it's a big document as it has all of Mitsubishi's air to water heat pumps. The image I posted was from page 92.

The 8.5kW can modulate down more than the 14kW, 3.2kW vs 4.2kW at 7C OAT and 35C flow. But the newer 8kW R290 heat pumps (PUZ-WZ80VAA) are even better and can modulate down to 1.8kW at 7C and 35C. 

@robs 

And its also go an external sensor that you can place away from the back of the machine that makes weather comp much better. 

@davidnolan22

Thanks and apologies for the graph dump. I was hoping a bigger picture might be useful. Temperature overnight was around 0, maybe -1 degree. The temperatures we're getting are fine. Maybe a little cooler than I'd like in some rooms but nothing dramatic and definitely not cold.

So 3 is a bad DT?  

I noticed the flow rate had increased to 18l occasionally but is also currently showing as zero for long periods. Is this when it's off? I've attached the updated images below.

So if we're only using 3.5 to 4 kWh, then an even smaller pump would work. Would an 8.5 model be worth considering?

@robs Thats's great thanks. Has a lot of useful info.

Trying to figure out the best option if we get an opportunity to swap it. The new 8kW model sounds interesting. We were warned off R290 coolant for some reason... fire/insurance risk maybe? I can't remember why.

@sandman1600 

it’s showing 0 when it’s off.

there is nothing wrong with a DT of 3, but it’s about energy. The energy to the house is carried in the water and transmitted to the house via your emitters (UFh). If it comes into the house at 38 and leaves at 35, this is a DT of 3, and that tells you the energy transmitted. The heat pump then tops that 35 back to 38 via the heat exchange in the unit and the cycle continues.

nothing wrong with DT3, but tells me it’s not working very hard. If you have a flow of 40 litres a minute coming into the house (which is more typical if you think the heat loss is actually 14) then a DT of 3 would be 8 to 8.5kw depending on how much glycol in your system. But you have a flow of 18, which is the minimum the 14 unit can run, and that is 3.5 to 4kw of heat with DT of 3.

so, there is plenty of ways of getting 3 to 4 kw of heat onto a house, but the question for your installer might be: have they found the best one.

The mitshi machine I think is generaly a fixed flow rate pump on the primary circuit. So your installer has set that flow rate. That indicates they may know that you won’t need the full power of the 14 often or at all.

The other thing to check is the house flows. Are to running it fully open, and plenty of flows going through each loop on the manifold?  Sometimes people run zonne and close loops, and this can further restrict the available surface area to get rid of the heat

normally it’s fine to close the odd loop a bit, but I think your total system volume and emitter power might be on the low side for your unit. 

I would also like to add: I am NOT a heating engineer and have never installed a heat pump. I have one that has been a tricky little bugger, but please take my advice in the knowledge of that.  Advice from James and others over various web sites has been very helpful. 

@sandman1600 

it’s a bit of propane, I really would lose sleep over it 

@davidnolan22 Thanks yet again. I appreciate that its not your profession but its a lot more than my knowledge so its helpful to figure out what's going on.

Thanks to @jamespa I've been looking at the various bits of legislation and even their documentation and RECC that details what to do if things aren't performing as described, so I'm really hoping we can agree an exchange of the heat pump for something more suitable.

I'm pretty sure the flows are all fully open but I will double check.  

Posted by: @sandman1600

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Trying to figure out the best option if we get an opportunity to swap it. The new 8kW model sounds interesting. We were warned off R290 coolant for some reason... fire/insurance risk maybe? I can't remember why.

If you think your heat loss is between 4.5 and 8.5, the r290 is probably a goodun because of its.unusually large modulation range, due to having two compressors.  R290 will go up to 70 which also means you can do the legionella cycle on the heat pump itself provided the controller allows.  It's quite good looking as well.  It was my close second choice after Vaillant but then ruled out for (rather bizarre) planning reasons.  Had I known about it a year earlier it might well have been my first choice.

However your installer may not give you the choice!

Most of the third party bodies are by reputation pretty useless.  Best try to sort it with the installer (and poss whoever the funding body is).  If you can somehow arrange for them not to be paid they will do something for sure.  Make sure you have the evidence, cop of 2 violates all the rules (I will post the legal references tomorrow if you don't find them) so they will hopefully struggle to argue against provided your ducks are in a row!  If melcloud separates heating cop from dhw cop make sure it's heating that you quote otherwise they will say it's down to dhw.