PhilT

They are probably too busy managing their tariff schemes, "saving sessions" and the traffic from Bulb (hope they haven't wasted their time!)

This is all fine in theory and the process my son went through was very similar, with the initial design coming up with a 5kW Midea as the recommended ASHP for his 150sqm passivhaus. Practice and the experience of the last 2 weeks paint a more cautionary picture. There are a number of factors to consider outside of the theoretical calcs. An almost new supposedly over-sized heat pump such as my own was defrosting as often as twice an hour recently during the most humid freezing dark. That's 6 minutes defrost cycle plus 14 minutes to return to flow temperature, 40 minutes out of the hour at below set power level. I suggest that would not be factored into any theoretical calcs. Heat pumps deteriorate with age, even when regularly well maintained. You are designing for the lifetime of the heat pump, not just when it is brand new. There may be exceptional circumstances where you need higher room temps, for example a family I know all went down with seasonal flu during this recent cold spell and needed at least 22degC to feel warm enough to help them get better. You need this kind of flexibility that traditionally has been the realm of the classically over-sized gas boiler. Modern heat pumps have sophisticated variable speed scroll compressors that are much more tolerant to cycling than their reciprocating ancestors, so theoretical over-sizing, and about £400, is, in my view, a small price to pay for all that enormously valuable headroom.

Absolutely good and fair information on which to base a sound decision - what this forum is all about. I would only emphasize the choice between having the ability to cover temperatures down to -10degC, as I have done here in sunny Aylesbury (unless you are near SteamyTea) and covering -3degC design temp but being prepared to either go cold on rare occasions or have some kind of heating backup plan.

Just to follow that up with a bit of hard data, here's my MCS heat loss design calc:- Indoor average design temp. 19°C House area 137 sq metres Peak power/heat loss 8.26kW At design temperature -2.4°C And here is how my 11kW HP has been coping on the 15th Dec, operating in WC mode with a degree of manual intervention

Take heed of the recent tales of woe!!! For the sake of a few hundred quid, if you want to be able to maintain that design room temperature, ALWAYS, you will definitely need to go larger. My son has just moved into a new passivhaus with identical floor area to yours and has gone with a 7kW HP which has provided an extremely reassuring bit of headroom during these exceptionally cold last few days (-10degC on a couple of nights). Alternatively you go with the smaller unit to cover 90% of the year and have a few electric radiators for the freak cold days!

OMG I really hopes hope it's not a lot, but I know what you mean. One of the main learnings for me is just how much the defrost and ramping back up compromises the nominal power of the unit. Mine takes 20 minutes to get back up to the flow temperature at which it cut off for a defrost cycle. Nine defrosts so far today between 5am and 4pm. To a lesser extent, but same principle, the extent to which hot water breaks need to be taken into account when doing calcs purely for heating.

That's impressive, first time I've seen Enerphit, very interesting. The sub zero freeze didn't start here till Thu 8th - would be interesting to see your daily average since then. My son has just moved into a new Passivhaus with similar floor area to yours - would be interesting to compare

It's been highly variable over the last week, from twice an hour to nothing all day. As a new user it's been very interesting to see how the new HP performs, and hear other's experiences, during these prolonged freezing periods, with plenty of useful learnings along the way, this forum being a great help. Thanks to all contributors

Not bad for -3degC outside in a 190sqm property at 20degC inside. My old self build of 130sqm has about the same power draw so you are doing around 50% better than me! You'll get a better result by setting back a couple of degrees at night then running a bit higher during the day when outside temps are maybe 6degC warmer at the moment and the heat pump runs more efficiently with little or no defrosting. I go 18 night, 20 day

From your photos it looks like it's in a defrost cycle, are you able to watch it every time the power drops (lots of steam etc.)? Mine shows a similar flow temperature drop on defrost, but occuring no more than twice an hour in the small frosty hours at -5degC. If yours is defrosting that often there may be something wrong with it.

It's difficult to tell exactly where the problem is right now. Get an EPC assessment, they don't cost much, or very diligently work out your heat loss using one of the many online tools - others on this forum can recommend the best ones. That will tell you whether or not your HP power rating is adequate. Is it in good working order/have you had it checked/serviced? If it's working well and you can't improve insulation/air tightness any further, have you been warm enough for 95+% of the year? If so you may be better off just using some form of supplementary heating eg electric radiators to get you through the extra cold times.

Not sure I follow. Do you mean the drop you see on the MELcloud graph? This always happens when the HP pauses for a mode change, heating to hot water then back again. If you try pump on speed 1 throughout it should still drop, maybe to a lesser extent

It's the defrost cycle, lasts around 6 minutes

that's really good. Do you have a heat pump? How do get it so high in the summer?

The app does all that for me. Have been matching it to heat pump draw for the last few days sunshine.

I have a similar size house (self build 1992) with 11kW HP. It's been as low as -5degC the last few nights. Defrost cycle 5min once or twice an hour on average during darkest coldest times, less during late morning/afternoon. My EPC heat loss is 8kW at -2degC. Do you know yours from your EPC cert?

Wallbox offer a bi-directional charger and Solaredge say they will have one next year. I won't be getting batteries as I hope to get an EV and use it's battery via a bi-directional charger for V2H/V2G when available https://www.smappee.com/blog/v2g-v2h/?gclid=Cj0KCQiAnNacBhDvARIsABnDa6-PTUyTj50TGYtZst-uK5hTieeuFdNlp54XJMfpMhjMMdLk1fNrt4oaAvMdEALw_wcB

As a new user just started on Tue 6th would be interested to know - how much % of total generation have you guys been able to use this year?

sorry daunker who made the original post

This is the Vaillant Arotherm Plus WC - a choice of 14 curves of which Marvin's looks like it's been set to the 0.6 curve

Do you have the MELcloud app or any way of viewing and selecting the options? There are three - "Room", "Flow" and "Curve". "Room" is full auto adaptation which uses both room temp and outside temp to "learn" how to manage power most efficiently to achieve and maintain target room temp, and takes some time, but is unfortunately not available without a Mitsubishi remote room controller, and Mitsubishi advise against using it for UFH anyway. "Flow" is user selected flow temp "Curve" (Weather Comp) is great for UFH if you put the work in to get it well balanced as JohnMo mentioned. I have radiators throughout and have been trying "Room" for a few weeks. I found it did work well mostly, but not quite as well as WC with manual intervention (the Mitsubishi central controller allows +/-6degC) to lower the curve when room temp is reached. Do you know how to select and manage your "Curve" setting? It doesn't appear to be available on the MELcloud app, only on the central FTC6 controller.

Thanks George. No charger yet - waiting for bi-directional to be available at a reasonable price (could be a while!). Hopefully it shouldn't matter as a charger would work directly from the mains, indirectly supplemented by the sum of my 2 banks of 8 panels into 2 inverters. My concern would be if Solaredge made their own bi-directional charger offering, which they suggest possibly 2023, conditional on having only one inverter covering all 16 panels in order to make use of the full 16 panel output AND the full functionality of a bi-directional charging system for which, no doubt, they would have a very clever monitoring software application like they do for the PVs. Anyway, at last I got connected today and with a nice bit of pure blue sky sunshine for a couple of hours around mid-day my 16 panels maxed out at around 1.6kW per inverter. It's a good feeling when it finally happens!

None, and personally I doubt that any current brand/model should be avoided on the basis of standby power consumption. For me the most important takeaway is to be very careful with one's own attempts to measure standby consumption. Things may not be what they seem! Look up the manufacturer's data sheets, they won't lie.

The friend who recommended a heat pump to me has his located a few feet from the neighbour's back door and it's just not a problem. The roof idea is a good one, as is any elevated postion, "Because cold air sinks relative to warm air, and because the ground radiates heat very efficiently during calm, clear nights, the temperature at or near ground level can often be several degrees cooler than the temperature at the 2-meter thermometer height" (weather .gov.uk)

My new installation went in on Friday. I was told it would be one 6kW inverter connected to 16 panels, but they actually installed 2 inverters connected separately to 8 panels each. Will this compromise my ability in the future to have only one charger (or only one of any device) being able to make use of the full power from all 16 panels?