@cathoderay are you running on weather compensation? You might want to consider bumping the settings up and see how that affects things

@cathoderay 

What I was getting at is that if your ASHP were performing according to the Midea specs then it would be close to meeting your requirements. Not quite there but close. It might be running flat out and so be less efficient but you'd expect it to be nearly coping and your rooms to be nearly at their specified temps.  As it is it's a long way off. 

In terms of ASHP sizing, I've heard of the design plus 50% that Derek mentions (this is what my supplier used and I'm happy with it. I've never seen my ASHP heating at more than 60% and it's mostly 30-40%) but I've also heard of design plus only 10% to allow for defrosts. See replies to my questions below.  

https://renewableheatinghub.co.uk/when-is-a-kw-not-a-kw

 Keep us updated anyway.

Kev

@batalto - I'm currently on the standard Freedom weather com curve, 55@-2/37@15. Bumping up the top end will do even nastier things to my COPs and is going somewhat off-piste ie beyond recommended upper limits.

@kev-m - thanks for the link, I hadn't seen that rather recent (Dec last year - this problem should have been dealt with years ago) article but it does explain the situation well, and the confusion that results. My problem has been I have learnt about this need to over size after the event, up to and including the installation I was still in the 14kW is more than 12kW (17% more to be precise), should be OK even if performance falls off a bit at lower ambients, and anyway the FHP calculator says its OK so it must be OK.

As @grahamh says, it is absurd manufacturers can characterise a unit on a nominal output in near ideal conditions (while at the same time obfuscating real output when it really matters), it is bound to mislead people, and it is bound to end in tears for some. As I have said before, I think MCS are partly to blame, because of their exclusive focus on COPs in their unit certification. All they have to do is add the capacity (output) data to the tables already present on their certification pages, and highlight the values that matter, the cold ambient temp outputs. EN14511 or whatever it is that allows manufacturers (and suppliers) to mislead about output should be torn up and replaced with something fit for purpose. A heat pump that a manufacturer describes as being a 12kW model should be one that produces 12kW at -2 outside and a set flow temp (say 45 degrees).

I agree my setup is a long way off, I think I probably have two problems, undersized heat pump and inefficient PHE. The emitters are adequately sized, as long as the water getting to the rads is hot enough (mean rad temp ~41 degrees, based on a secondary circuit flow/return of 45/37 at -2 outside), but it isn't.

Current room temps, coming up to 1300 hours: bedroom 17 (vs 19 design), bathroom 18.5 (vs 22) and kitchen 16.2 (vs 19). Basically, no change from first thing this morning, despite the heating being on full all the time.        

@cathoderay if you really are struggling then it's time to investigate deeper. Step one, go flip the dip switches. No one would ever know and you'll get a power boost - see how you get on with that.

1800 hours: bedroom 17.5 (vs 19) bathroom 20.0 (vs 22) kitchen 17.4 (vs 19). All have benefited from recent solar gain, especially bathroom and kitchen, but all are still below design temps (in brackets). House feels chilly, and I have taken to wearing two fleeces. The building has a high thermal mass, and I think it cooled down overnight last night, and is struggling to get back up to temperature. It will be interesting so see what the temps are tomorrow morning after what is forecast to be another chilly night.

@batalto - no one will ever know - until something goes wrong! I have no doubt that @grahamh (the font of dip switch knowledge) knows what he is talking about, but that doesn't make it an official recommendation from Midea/Freedom, and at this stage I am wary of making my own adjustments without at least talking to my installer (still yet to return from holiday). What I can do is document the actual performance, as I am doing here.   

Hi @cathoderay

It would appear that the PHE is possibly undersized and I am not convinced that it was the correct solution in the first place. To check the size of the PHE have a look at the supplied documentation and see if there is a copy of the Bill of Materials (BOM), which should detail the make and model of PHE. If a BOM has not been supplied then contact your installer and/or Freedom and request one.

I agree with your assessment that your heat pump was producing approximately 6kW of heat energy when you made your measurements, but from my understanding a heat pump will only supply the demand, and will pull back once the demand has been met. As Kev has suggested, the problem may not be with the heat pump, but with the rest of the system and weaknesses in its design.

My understanding is that a PHE is installed within a domestic heating system, so that the toxic version of anti-freeze can be used in preference to the non toxic version. I don't know the relevant costs, but I suspect that the non toxic is more expensive. You could ask Freedom why a PHE was specified in the first place.

I would suggest that you obtain one or more thermometers, that can be attached to the pipework giving good thermal contact. Monitor the temperature of the water entering the PHE from your heat pump, and the temperature of the water leaving the PHE. Try recording the temperatures every minute over a suitable period of time. Also note how your system is operating.

A further test would be to open all the radiator valves fully, whilst noting their approximate setting, this also includes the lock shield valves. Then with the system as stable as possible, measure the temperature of each radiator, and also repeat the test on the PHE.

When you have some results then post them for analysis.

Hi @cathoderay

Having looked in detail at the manufacturers comprehensive data for various models of ASHP, it would appear the the Midea range are not as good at maintaining their output capacity at lower outdoor air temperatures, when compared to other manufacturers. If specifying a Midea heat pump, it would appear that it is even more important to oversize, to ensure that it can cope at low ambient air temperatures.

Hi @cathoderay

Your installer is obviously rich if he can afford to take extended holidays. I wonder if that is thanks to RHI?

@derek-m - I don't know the model of the PHE, only its's make (Secespol) which is on the foam insulating jacket, which can't be removed without damage because the PHE is wedged into a very tight gap between flooring boards and the DHW cylinder. There was no documentation at all that I saw that came with the PHE. My installer said they were a generic one, sized to match any domestic ASHP - presumably to keep things simple. I don't have a BOM, just a basic quotation.

Freedom have refused to talk to me, saying they are business to business only, from which I have to infer they can't be bothered to deal with mere punters, even ones who have taken the trouble to find out about ASHPs. I will have to wait until my installer gets back and then talk to him.

The need for a PHE is unclear. It (or a buffer) is present in both the two Freedom/Midea installation manuals I have seen, but the earlier version made a point of saying its use was mandatory (no PHE/buffer, no warranty) whereas the later one does not appear to say this. @grahamh may know more details, I believe he wrote at least some of the Midea manuals.

I can see some advantages to a PHE, separating the relatively dirty and more likely to need tinkering (bleeding, draining water off to clean the magnetic filter etc and then topping up again) with rad circuit from the ASHP circuit. The AF that went into my system was pink, not sure whether that tells us anything. I can clearly see having toxic AF in any domestic circuit could cause problems, but even with a PHE, the primary circuit is still going through the DHW cylinder coil, and surely that is where cross-contamination is likely to occur, possibly unseen?

I was going to get some clip on pipe thermometers, until I saw their quoted accuracy ranges, most are +/- 10 degrees, the pricier ones +/- 5 degrees, and so not really much use, if one had a plus 10 error and the other a minus 10 error then two pipes at actually the same temp would show as being 20 degrees different! I think I am more likely to get more accurate readings using my IR thermometer either on pipework with the black insulating tape emissivity fix or on the rad immediately adjacent to the entry and exit ports. As it is the same instrument taking both readings, for differential readings, any fixed error should cancel out, eg readings of 30/25 and 35/30 show the same drop, 5 degrees, even if one (or both) absolute values are wrong.

I will set aside some time, hopefully tomorrow am, to monitor the PHE input and output temps. What sort of interval and time frame did you have in mind? Every 5 mins over 30 mins? Every 10 mins over 60 mins?

On your second comment, "Having looked in detail at the manufacturers comprehensive data for various models of ASHP, it would appear the the Midea range are not as good at maintaining their output capacity at lower outdoor air temperatures, when compared to other manufacturers. If specifying a Midea heat pump, it would appear that it is even more important to oversize, to ensure that it can cope at low ambient air temperatures" - very interesting, and Freedom should know this, and make sure installers are aware of it, both generally, and through there Heat Pump Calculator.

Here again is my plot of the Midea data, first included in my original post in this thread, for ease of reference here again. It does show a marked drop in performance from 0 degrees ambient downwards for all flow temps, perhaps the effect of defrost cycles kicking in): 

My system was designed assuming a delta t of around 30 (rooms around 20, rads around 50, driven by an ASHP set to 55 degrees at -2 outside, ie Freedom's standard weather comp curve), the relatively high flow temp a compromise to avoid insanely large rads which would be awkward or impossible to fit in a small cottage with limited wall space. But the rads didn't get anywhere near the design temp this morning: at say zero outside, Freedom weather comp should give a LWT of around 52-53 degrees (it is almost one degree drop in LWT for each degree increase in ambient), but the average rad temp (not factoring in their different sizes) was around 30 degrees, so a delta t closer to 10 than the 25-30 or so degrees it should be. As discussed earlier, the primary circuit was closer to the correct LWT, but still cooler than it should be, and downstream of the PHE, things were far worse. It does all seem to add up to a combined problem, a heat pump with inadequate LWTs at low ambients, and a PHE that isn't working as it should.

Whoever would imagine that some installers get rich on the back of government grants! My installer (unlike some of the others) comes across a a genuinely straightforward and decent guy, not 'on the make' and keen to help, and his extended holiday is to see a grandchild for the first time, having been isolated from them by covid for the last two years, so I think we can allow him that!        

My installer seemed to have clocked that the 16kw really was only 13.5kw when compared to other brands (I havent looked at the data Midea provide too closely but I assume he was right given the above discussions), he told me this after I questioned why spec 2 x 16kw when others had quoted a 12kw and 16kw from other brands, he said our house was borderline on the 13.5kw and therefore the reason for 2 giving us plenty of redundancy and given the one driving the upstairs radiators are likely to call less for heat it with it rising from downstairs, it can go off and do the DHW with little effect on upstairs room temps. Still waiting for UFH to be fitted, but overall the upstairs holds the temps pretty constant using Temp Comp of 50 @ -7 and 25 @ 13. I havent seen the flow temp lower than 30 degrees even when 20 outside so guess thats as low as it can get.

Think the K2 rads have 28mm send and return with 15mm tails to the rads, The installer was keen to have the pipes as large as possible. Will only really know how its all going to work out next winter with the UFH installed and 2nd pump running, but the upstairs is so much more comfortable now in every room compared to the previous storage heaters,  and as of today we'd have need to have turned off them off given our E7 night time rate going from 7p to 17p.

Think I'd be checking rad pipes and flow rates etc.

@mookyfoo - two 16kW heat pumps is certainly plenty of redundancy in a house with a heat loss of around 13.5kW! If two are really necessary, one would have thought a 16kW one for the heating plus a (much) smaller one for the DHW might suffice. Two 16kW units = ££££, possibly even £££££ with installation costs added, and one has to wonder... It will also mean more expensive annual maintenance bills.

We haven't so far as I know discussed here on the forum the energy price cap rise that came into force yesterday, in many ways it is the big thing, and then it is set to rise again in October. I am extremely lucky, I got onto a fixed rate just before the $hit hit the fan, but I dread the shock I will get when my fix comes to an end.

I spent the hour between 0800 and 0900 this morning monitoring various temps and here are the results (sorry, in screen grab form, I can cut an paste the actual numbers if anyone wants them): 

It looks like there was a defrost cycle around 0820. Apart from that:

(1) bottom line first: none of the rooms that I measured got to design temp, being variously 1.5 (bedroom) and 3 (bath/kitchen) degrees below design temp, at an outside ambient of around 3 to 4 degrees  

(2) during this period, the PHE seemed to be working as it should, the primary in/secondary out delta t values are small. I am sure this is not always the case, however the above data is what the readings show, so that is what we have. I may try again later, just monitoring the LWT and PHE pipe temps. This coming night is also forecast to be cold, so maybe do some more measurements tomorrow morning

(3) the room delta t values are too small, or non-existent (bedroom) - this is despite the rads being nominally balanced. The two bedroom rads on the same branch do this, they go hot one day and cold the next for no obvious reason. All rads have been bled, mostly water straight away

(4) there is a notable drop between the LWT and the primary pipe going into the PHE, sometimes as much as 10 degrees. All pipework in the primary circuit is lagged

(5) there is another drop, between the PHE secondary out and (all) the rads (which based on yesterdays measurements, averaged 30 degrees. This morning, the kitchen rad was the most representative, the bedroom rad is sulking, and the bathroom one tends to get special treatment, in an attempt to get the bathroom up to temp

Based only on the above, the PHE doesn't appear to be the main problem. Instead, the heat pump appears to be inadequate (tolerable LWTs, but maybe it just isn't pumping enough heat (ie water) round the system?), and there are unexplained losses between the LWT and PHE and then again between the PHE and the rads.

Bottom line remains: rooms 1.5 to 3 degrees below design temps when it is 3-4 degrees outside.

@cathoderay honestly, just bump up they power. If there is ever a problem you can just swap it back. However I doubt anyone would even check and if they did you can say that's how it was installed in the first place - who would know or even remember. I doubt they wrote it down or photographed the board