JamesPa

I think so if it makes a material difference. The key points of MIS 3005 D are that it requires a heat loss calculation to be done with specified minimum interior temperatures, exterior temperatures dependent on location, and 'otherwise complying with BSEN12831-1:2017' Without referring to the latter I don't know what it says, but it would be surprising if it doesn't allow some latitude for accounting for the actual building. Unfortunately its £460 to buy. I doubt many if any installers actually have a copy so they will most likely be relying on interpretations/software produced by others. Thus they may be (and in my experience are) open to persuasion or may dig their heels in, depending on how confident they are in their own abilities/judgement. In the end they know that the MCS rules, followed to the letter, are their protection because following makes them essentially bomb proof. However the better ones also know that pleasing the customer and doing the right thing, with an appropriate interpretation of the MCS rules, is a better way, in most cases, to gain a good reputation.

I don't have MVHR, I have a 1930s house with (originally) solid walls, that has been subjected to various fabric upgrades at various times including double glazed windows that have no trickle vents and solid flooring downstairs. MCS ACH = 1.5-2.5 based on age. I ended up with two installers that provided satisfactory quotes. One assumed ACH = 0.5, the other a bit higher (0.8 I think). 0.5 gives a total loss that is consistent with the measured loss (based on analysis of half hourly gas consumption over 2 years), 0.8 ACH works out a bit higher than the measured heat loss, but still resulting in the same design choice of Heat Pump. I understand that decent MVHR has a recovery rate of 80% so I cant see why one would assume an ACH above (say) 0.25 unless its expressly mandated, specifically for heat loss calculations (which is what they are supposed to do) in the British Standard (to which I don't have access) referenced in MIS 3005-D.

From my personal experience it seems that the MCS 'rules' are interpreted with differing degrees of latitude by different installers. Sizing my own installation turned on: allowing for fabric upgrades which I know have been done (because I did them) but were not visible - some installers would and some point blank refused. This made a difference of 4kW assuming an ACH value substantially lower than the default - again some installers would and some point blank refused - this made a difference, on top of the above, of 3.5kW If your installer wont factor in something that matters, my advice is to find an installer that will. The actual text of MIS3005-D, the governing document - which is worth reading, is very much open to flexible interpretation. Following it to the letter and refusing any interpretation that the 'letter' leaves open probably gives maximum protection for the installer, which may not what you, as the customer, wants!

fair point, particularly given my experience referred to above with my gas boiler, albeit down to my stupidity!

Two because the two circuits are totally separate. The reason for the one on the primary circuit is to prevent overpressure due for example to forgetting to turn off the filling loop properly. I actually did that with my old boiler (where the PRV was) isolated. It blew a radiator quite spectacularly, fortunately it didn't blow anything under the floor otherwise it would have been an expensive mistake. If your mains pressure is low then its perhaps less important although a fault in the immersion could conceivably cause boiling in the primary (through heat transfer to coil) so there is (just) a way to get overpressure even with low mains water pressure, albeit very unlikely. If there is one in the heat pump then I cant see you need another.

Indeed. The Ideal R32 range looks suspiciously like Mitsubishi pumps to me, albeit with Ideals own control system. The physical dimensions are a perfect match.

I thought that their R290 products are rebadged Midea, at least thats what the people at Adia told me. I (and they) could be wrong of course! Otherwise agree with everything you say above, check EON tariffs as well as Octopus if you have an EV (and possibly if you dont).

Interesting discussion. Having recently got my heat pump installed, and possessing an EV charged from a 13A socket only, I am in the process of moving from Octopus to EON Next which gives 7 hrs at 6.7p/kWh without any requirement to have a dedicated charger. I don't do that many miles but the negative comments about OVO influenced me away from them and my calculations suggest EON works out cheaper than Cosy or OVO, in the latter case at least in part because of their poor export tarrif. Its also a lot less faff having a block of cheap electricity rather than 3 blocks. If anyone has any tips for 'getting on with' EON I would be grateful, otherwise I will report back in a few weeks.

My 15 year old Worcester Bosch (presumably now in the skip) actually had a WC add on available (I don't and didn't have it). Totally analogue so far as I can tell from the limited information now available!

There are clearly some 'perception' factors at work (we experience the same) which control systems ignore. I do wonder if humidity is one factor at work. If we do some washing and dry it indoors, even though we put the dehumidifier on and the measured house temperature is the same, it feels colder!

Noted. I think its WC that is the game changer in terms of stability, a technology thats been around for decades and is mandatory in some countries. It seems illogical to some, given that its room temperature that matters, but of course its perfectly logical because it means that the control system is ahead of the game rather than always playing catch up and the emitter temperature and hence output is more closely matched to the demand. Its something of an indictment of our heating industry that the technology was shunned in the UK until heat pumps forced it to happen!

For what it's worth my ashp was finally fitted a fortnight ago following a two year long and very painful planning saga. Installer set it up in a mode combining wc and control based on room temperature, all using the heat pumps own system controller with no external controls. After the first week I disabled the room temperature element of the control, and now have it set up on wc only, trvs fully open with heads unscrewed so they can have no effect. The result so far, in my house and with my pump, is a more stable house temperature than with the mode in which the installer left it, and much, much more stable than was ever achieved with my gas boiler operating at fixed flow temp with a thermostat and trvs. I have rads, not ufh with a massive slab BTW. Obviously this is one case only, however its doing what the theory says (and what should be the most cost effective), so it's not really a surprise. I would suggest that those who are sceptical of wc might wish actually to experience it if they haven't already!

Thanks. I was coming to that conclusion myself (and its the general approach I have been preaching for some time - get the WC curve right before you anything more sophisticated). My installer is back today to fit two rads that weren't available when he did the main job last week. I don't want to touch too much while he is still around, but once he is gone I consider it to be open season! That said, I can see why he has set it up the way he has. Most wont bother with or understand the tweaking of WC curves and the way he has set it up will get somewhere close eventually I think, without human intervention.

Reading the expanded mode description I think the controller may ignore energy integral if this mode is active, relying on oat to restart. My installer has also enabled adaptive WC (which requires expanded mode). I guess his thinking is that this will eventually sort out the WC curve, which is probably true!

My boiler, now siting on the driveway waiting to be removed, had a pump anti-seize function doing exactly this. I imagine heat pumps do also. Mine seems to be varying by about 0.2bar around 1.7bar One thing I have noticed is very long cycle times in the mild weather - too long in fact - the heat pump can be off for more than an hour and the house cools by 0.5C in that time which is noticeable. I thought this was governed by the 'energy integral', but it seems that the room temperature mode (inactive/active/expanded) may be dominating the cycling as the compressor doesn't switch on even when the energy integral is as low as -150 degree minutes (I have a suspicion that it comes back on at -180 degree minutes but cant prove it).