JamesPa

Yes That the defining document (other than the legislation itself) is now mcs-020(a) published April 2024. This is solely a noise calculation and no longer specifies installation standards or requirement to use MCS. The title, quoted in article 6, is helpful in confirming this.

I was dismayed until I read article 6 and the document it references. Then I was delighted. What it apparently does is not what it actually does, at least as I read it. Please tell me if you think I'm wrong!

Not quite, it comes into force on 29th May https://www.legislation.gov.uk/uksi/2025/560/contents/made

The new pd rules (in England) remove the (admittedly arguable) need for installations to be carried out by an MCS installer. So I agree you could DIY or have your local friendly plumber/electrician do it to a sensible spec if you forgo the bus grant. The job probably amounts to (depending on your house) fit an electrical feed fit a uvc connect the heat pump to where the boiler was formerly connected swap out any necessary radiators None of this is anything that a reasonably competent plumber (plus electrician for the feed) can't do. Mine (which was MCS) took 6 long man days for the plumbing, and half a day for the dedicated feed. That said, unless you are being ripped off to the max, it's probably still just about cheaper to get the grant and pay the mcs overhead. However cheaper isn't necessarily better if it's the wrong system!

That is true but still there is a massive gap to be explained before fitting a 12kW becomes even faintly sensible. As I say above, MCS installers can be flexible, not on room temperature, but certainly on the assumptions made with loss calculations. That can make a massive difference, in my case a factor of 2! Obviously it's your choice but personally I wouldn't proceed until this is resolved. It's not cycling as such that kills performance, it's that compressors generally are at peak efficiency at about 40-50 pc of their max modulation frequency and the efficiency declines quite dramatically at lower frequencies. If this operating point corresponds to when you need relatively little heating energy, then it doesn't matter too much. However if you are permanently operating in the low efficiency part of the curve, then it can matter a lot. If you wanted to provide some more details perhaps I or someone else could suggest more specific actions. Interesting info would be House construction and floor area Rough location (or design oat) How do you/can you measure LPG consumption What is the heating pattern for the house that corresponds to the figures you have provided Also, if you have survey results and calculations they would help, you can of course redact anything sensitive. You don't have to do any of this of course, but I do fear that, if you proceed with 12kW, you will be posting again about a disappointing experience.

If the 80kWh figure is even faintly correct and assuming you heat your house, a 12kW heat pump is quite likely to be a disaster. It will be cycling all the time and running permanently at the least efficient part of its modulation curve, you will pay 30% more for your heating than you need to (possibly worse) and will quite possibly be less comfortable. Installers vary in their interpretation of the 'mcs calculations', the better ones recognise that they tend to overestimate and compensate accordingly. If you can afford to hang on, collect some more evidence and find some better installers. As a reference point I had two full 3 hour surveys done, one of which I paid £300 for. Both came to 16kW. I eventually had the 7kW Vaillant fitted, and it's an almost perfect match, perhaps 0.5kW over. This was fitted by one of two of the installers I found who used an ach of 0.5-1 instead of the standard 2-3 for my type of house and, unlike the survey monkeys, accounted for fabric upgrades that they couldn't see (but which I made a point of telling them about) and didn't double count room to room losses. I also produced smart meter readings to support their calculations If you can get your calculation down to 8kW then the 7kW Vaillant will do. Also worth considering is the 8kW R290 Mitsubishi which has 2 compressors, 6kW variable and 2kW fixed. It therefore has a wider modulation range than most. However your 80kWh figure suggests you need even less! Possibly the only exception to the above is that if your house is super low loss with a large concrete slab, then batch heating with an oversized heat pump is a possibility, effectively treating the slab as a storage heater. With the right tarrif this can be highly cost effective. Also, I would strongly advise against allowing any installer that wants to fit a secondary pump and buffer tank anywhere near your plumbing, unless there is something weird about your house (perhaps you could post some details of construction, floor area and rough location). A 2 port volumiser plumbed in the flow (or even in the return) is ok. Feel free to post any questions this gives rise to.

In my case because the Vaillant was one of a very few that was a good fit to the many constraints, because I mostly have radiators with only one fancoil (I now wish I'd specified a couple more fancoils) so cooling was a anyway secondary consideration (sofaik it doesn't work with standard radiators because all the cold water just collects at the bottom so even if you add a fan there is insufficient surface area to make much difference), because I knew that it was capable of cooling and that the coding plug was available in Germany for about gbp40, and finally because I can alternatively keep my house cool even in summer by the low tech approach of closing the curtains. I wouldn't have purchased one that was incapable of cooling unless there was no alternative.

I can confirm it worked on mine. There are a few obscure settings, most notably one about power company lock out, that need to be adjusted in the installer menu to get it working whether you have the part with the official part number or the one you refer to above. Openenergymonitor has a thread about this

At a guess because every serious EV manufacturer knows that range anxiety is becoming decreasingly important (and quite rapidly) as battery technology, charger availability and charger power (ie rate) evolves. So why invest in developing something that is going to be obsolete in a very few years and detracts from the mainstream offering?

Very much agreed. I have had PV for over a decade but only got an EV and heat pump last year. Both are absolutely unequivocally much better functionally, and cheaper to run (even without the PV), than the fossil fuel devices they replaced. Endless complaining about government whatever the colour (and frequently with no viable and coherent alternative suggestions), gets us nowhere, except possibly where the Americans have ended up. Actually doing something does!

Ive spoken with a couple of aborists about RPZs on several occasions. The things to avoid (they tell me) are : Compacting the soil over an extended area in the RPZ - because it means that the roots dont get moisture/cant grow (hence ground protection) Cutting tangential trenches through the RPZ - because you cut through lots of roots which, by and large, run radially. Radial trenches are not so bad Unbalancing the tree by cutting through lots of roots on one side only (which is not to say you should balance it by cutting through lots of roots on the other side!) Amending pipe runs if necessary to avoid these is good practice. Its possible to excavate under roots if there is no way of avoiding the RPZ Maybe that helps

I should add that the argument for radiant heating (made also by people who sell 'infrared wallpaper' and the like) is that it makes you feel warm (because your skin does warm up), even though the air temperature isnt warm - much like going outside on a sunny, cold day (we have had a few of those recently!). In a tent, or a building that has the the thermal characteristics of a tent (such as that described by @Iceverge above), your chances of heating the building are similar to your chances of heating a patio. So you might as well give up and instead heat the person. Obviously I'm being a bit extreme here, but underneath the deliberately extreme position is perhaps a grain of truth. The suggestion I am floating is that the strategy for heating a very lossy building may be different to that for the majority of our housing stock (which may be poor, but isn't generally that poor).

SOFAIK there is no good way to heat any building that has the thermal characteristics of a tent. There are only various flavours of bad ways! Like patio heaters, you are basically on to a loser!

Heating a building that has the same thermal characteristics as a tent does definitely require a different approach, for example an open fire so most of the effect is both instantaneous and radiant. Fortunately they represent a relatively small proportion of our housing stock!

There used to be such things as gas fridges for use on boats (maybe there still are), so I suppose one could make a gas powered cooling system. Given the US obsession with both aircon and fossil fuels, I'm surprised that it hasn't yet been mandated by executive order.

At the current time, like @JohnMo, its heated intermittently, because the ASHP cant possibly modulate down far enough and there is anyway solar gain during the day, so the heating switches itself off from about 9am till 10pm. The house stays at a fairly constant temperature because of thermal mass/solar gain. Unlike @johnmo I don't have a big concrete slab, its a 1930s house remember, but the heat capacity is nevertheless roughly 20kWh/K. The loss at -2C is about 7kW, so even at -2 it takes 3 hrs to drop a degree, at a more average 7C its nearer 6hrs to drop a degree, hence little saving in loss (and thus heating bills) from part-time heating Over the past 5 years I have gone successively from gas fired, intermittent heating throughout the season, trvs everywhere, flow temperature as installer left it (ie high, about 75C) gas fired, heating 24*7 in the height of the season, intermittently towards the ends of the season, trvs everywhere; flow temperature turned down to 50 (lowest possible on my boiler most of the season), 55 in the height of the season (necessary to get enough out of the rads) ASHP, set to heat 24*7, in practice heating 24*7 in the height and shoulder of the season, intermittently towards the ends of the season. Only 2 rads out of 16 with any form of thermostatic control otherwise open loop Each step has both increased comfort (because of reduced thermal gradients) and reduced my bills. I'm now looking at bills 25% lower than when I was doing the 'conventional' thing of gas boiler with bells and whistles controls and heating early morning and afternoon/evening only. The comparison is like for like, ie I have taken out the effect of price changes, and is actually an underestimate because I am not taking into account the fact that some of my space heating now comes from solar (unlike @JohnMo this isn't free because I get paid 15p/kWh for export, but it is less than my import rate) I would be dismissing this as chance were it not for the fact that there are good thermodynamic arguments why excessive zoning (whether in time or space) reduces the house energy demand by much less than the manufacturers of complex controls like us to believe. See Heatgeeks discussions on this from time to time. Of course the detail will vary from house to house.

The rules are, I believe, shortly to be changed to allow a second pump.

@Iceverge Your 40kW boiler will not deliver anything like 40kW because few houses have that much emitter capacity, which in practice is the limit in most cases. Unless you only heat the air and not the fabric (which is possible with an A2A heat pump used for short periods of time) or your house is very lossy, your house will continue to lose heat even if the heating is switched off, at a rate not much slower than if it is switched on. That heat still has to be replaced, so your part time heating doesn't actually save anything like as much heat as people suppose in the vast majority of use cases. I would therefore dispute your contention that ashps are unsuitable for houses only occupied part time but accept you may have to get a bit more efficiency to match running costs. That said with a couple of fancoils strategically placed you can likely get the best of both worlds.

Can you expand on this? I know they have some massive commercial systems that have 2 compressors but I was unaware of any small domestic units. The 8kW (NB not 8.5kW) r290 model has a 6kW inverter driven compressor and a 2kW fixed compressor. SOFAIK it's a first in the domestic market.

Presumably just as bad as 4 port?

They are not the same thing, a 2 port buffer would straddle flow and return and provide hydraulic separation, a volumiser will sit in either the supply or return pipes (not both) and provide zero hydraulic separation. Thanks for that clarification Many installers still seem to call 2 port devices in one of the supply pipes a buffer tank, but I'm happy to hear that this is incorrect terminology. I shall from now on say '2 port volumiser, sometimes incorrectly called a buffer!'

Not when I put the ashp in. It's a 1930s house with two modest 1980s extensions. About 80pc of the otherwise solid walls have iwi. The upvc double glazed windows are from the 1980s, glass replaced with low e argon filled, frames original. There are in addition three crittal metal framed windows with perspex sheets to give a bit of secondary glazing. There is no underfloor insulation anywhere, but the loft has 30cm. The insulation upgrades have been done piecemeal over 20 years as rooms have been redecorated. The ashp install cost 6k after grant replacing an aging boiler which was nearing end of life. The dhw system, previously indirect with very noisy pumps, is revolutionised and the heating much more comfortable. Running cost 20pc less for greater comfort. It is definitely an upgrade. Does that answer your question?

To be clear I'm referring to a 4 port buffer tank, or indeed a 4 port llh or phe in a domestic situation other than a mansion. In principle they aren't an issue. In practice they are almost invariably, it seems, badly specified and/or badly controlled and so cause mixing between flow and return. This means you have to increase the flow temperature for any given emitter temperature, reducing COP significantly. In extreme cases the mismatch in pump speeds either side can deprive the heating system of energy and compromise defrost. They are mostly installed principally to protect installers from call outs, not for any legitimate system reason. Finally they make fault diagnosis more difficult and mask issues that should be corrected. Unfortunately many installers still use them but interestingly will not defend their position when asked to on the Renewableheating hub podcasts about buffers. A 2 port buffer tank, also called a volumiser, will still reduce cycling and suffers from none of the above problems.

Im not sure what you are saying. My house is 1930 originally solid brick. About 80% of the walls do now have typically 50mm of insulation. The floors are uninsulated, the loft has 300mm. Double glazing is from the 80s with the glass only replaced with low e argon filled. Certainly not a passiv haus, definitely not airtight, and no MVHR. My ASHP is 20% cheaper to run than gas because, I think, it has no buffer tank, is right sized, I operate it open loop on weather compensation without external controls. In other words its kept simple as the designers of the unit intended. All I am saying is that I don't see any good reason why almost all retrofits cant be like this as it involves nothing clever at all.

I completely agree with that sentiment. Selling them as cost saving is mis selling because its dependent on electricity tarifs etc. They should be sold IMHO as approximately cost neutral with better comfort and environmental credentials, and should achieve cost neutral or better - ie its a genuine heating upgrade. Its still a better deal than a new kitchen! Currently the impression many seem to have is that a heat pump will cost more to run and give lower levels of comfort, which should never be true (but, sadly it appears, sometimes is true). I do take the point that DHW may be significant, but even this can achieve a good COP if the heat pump progressively ramps up its flow temperature (which mine certainly does) rather than whacking it straight up to 55+. Very low loss houses, where the DHW might actually dominate the heating, are obviously a special case, but a negligible proportion of our housing stock.