JohnMo

The latest Viessmann ASHP do it slightly differently - on approach to defrost, they move an internal valve and charge a small buffer to about 50-60 degs and the whole defrost energy comes from that. Slightly more efficient to the normal.

Just the normal way, I just let the heat pump do its own thing - you don't need a buffer for defrost, you just need the minimum prescribed volume available, as detailed in the installer manual - you never did need a buffer, it just hang on from fixed duty heat pumps. Heat pump just moves the 4 port refrigerant valve to cooling mode, refrigerant does the defrost. Water circuit just provides the heat. It's only in defrost mode a couple of minutes. Attached from the other day when it was cold and defrosting. The attached is the temperature and heat flows. Green line is return temp (from heating system/volumiser), red line is flow temp, yellow is heat pump thermal output, blue area is electrical input. - no hat needed😀

Scottish building regs, just state a simple number, 0.5 ACH of whole house internal volume for MVHR (balanced inlet and outlet). Boost is above this rate. So no if buts or maybe, pretty simple, but leads to massive oversized ventilation on anything bigger than a shoe box. MVHR is also mandatory for any house that has an air test result of 3m³/m² @ 50Pa. The actual air test result is at 50Pa equal to gale force wind acting on all surfaces of the building - which can never happen, as there are always sheltered sides to a building. So actual air test result is factored down depending on your location etc. So in OP case 0.45 ACH at 50Pa is equivalent to 0.04 air infiltration, so pretty much meaningless in the calculation. Hence say that isn't the figure for air test result isn't the figure to add to spreadsheet.

Only thing I would add, the Jeremy spread sheet is designed for a high performance and pretty much airtight house. So if you are not that kind of house the calculation are pretty different and yes you do need to take into account natural ventilation as well as forced ventilation. So the build looks to be MBC which normally comes with a leakage rate below passivhaus levels. So at any point in the building life, the natural infiltration is next to nothing. So can be ignored to the gross heat loss pretty well sorted. But to do heat loss correctly you need to do the room by room approach Building regs drive you to 0.5 ACH throughput of the MVHR system. Passivhaus drive you to 0.3. Passivhaus is closer to where you need to be in winter so as not to over ventilate.

If you have just plastered that carries huge amounts of water. Plus if you don't have effective ventilation... Maybe get a dehumidifier until your ventilation is up and running (after decorating) Your vapour barrier is there to stop water vapour going into the structure and it rotting, why would you encourage water vapour to go there? NO. I would continue with the insulation and MVHR install.

We find exactly the same on a long thin (25m long) single storey. Open bedroom doors everywhere is the same temperature an hour later. Our last house 3 storey (built in 1830) bedrooms with doors open ended up same temperature as rest of house. We just never needed to have bedroom heating on.

But none of this detail is provided, so you are assuming, I was just asking a question. MBC build will need very little heat input (assuming it's not massive), so any heat added has very little place to go. So quite a bit of room temp equalisation will happen anyway, zones or no zones.

Its interesting when you read a Cool Energy ASHP datasheet, the broad range of outputs based on outside temp and how much things change. So pretty much the true reflection on how a heat pump operates. This is somewhat hidden by a lot of manufacturers which software gag the output. My heat pump max output varies by less than 0.5kW from + 20 outside to -15 at 30 Deg flow and it similar at 60 Deg flow. This is nothing to with physics, just software capping output as the outside air temp increases. But modulation is limited at 4.5kW (from 6kW nameplate) - pretty rubbish. It is the minimum modulation when you need it, that can kill overall efficiency. Stick in an over sized heat pump, you start to exaggerated any issues a correctly sized unit already has.

Whatever you do upstairs, do a floor buildup that eliminates any air gaps between heating pipes and finished floor. So people like @ProDave did a PUG mix, this acts like a screed so you get a solid heat emitter. You get lots on here that do different from that and then have huge issues. Are you likely to need cooling in the summer would be my question? My view why - would you zone?

If you have sized the heat source correctly, output at the sub zero temps should be at it close to full load. So it doesn't really matter how much it modulates at those temps. It really does matter at +7 and above degs outside. The heat pump naturally develops more heat output power, at minimum electrical input as outside temperature rises. So full load output of 10kW at -3 maybe closer to 12 to 14kW at 10 degs. So if you applied 4:1 to the 14kW it's a very different answer to 4:1 at 10kW (70% higher at temperature you want most modulation)

By people that want it to be true?

Its in the brochure, they state the 7 degs min out put for each model 4.4kW is 1.85kW 8kW is 2.1kW 10kW is 3.3kW etc They all do just about nameplate rating at -7 outside There is a huge technical book tha has all the details as well but I have deleted it https://solartradinguk.co.uk/wp-content/uploads/2025/01/Hi-Therma-Monobloc-brochure-2024-1.pdf

Just because you have asked for a particular temp doesn't mean you get a fixed out put of x degs. The heat pump like every heat source has a target hysteresis, so will overshoot each compressor cycle. The will be a bunch of other setting like cycle time protection that will supersede your ideal flow temp.

They nearly all do, you just need to search hard as they all call the files different names. Mitsubishi have a excellent data set, for actual UK models, how actuate a Czech data set is for the UK market is questionable. Especially as the data set has models not available in the UK.

I would look at Hitachi and HiSense also for their heat pumps. Hitachi and Hisense have a sharing agreement, looking at the datasheet outputs are idenical for each model. But Hisense comes with cooling out the box, Hitachi is an additional cost. But modulation is great. Neither are that expensive either. Sounds a lot. What is included in that cost

If get rid of all the small bore stuff, can you get rid of the second circulation pump?

Yep. Best gains are by getting decent modulation, lengthy cycle times and running the minimum flow temp possible. Everything is sh!te in sh!te out. If you don't have plenty of water volume and good flow rates your stuffed anyway, and stand a chance of getting a decent CoP.

A drawing of what you're doing and where you propose to locate the manifold and boiler/ASHP would actually help. We're all guessing otherwise.

Some info here also

This is the first topic I found on it - on another forum https://community.openenergymonitor.org/t/vaillant-arotherm-plus-cooling/26658/15

You have to buy a plug to make it cool - Valiant UK want to charge many hundreds of pounds for the part, but the same widget is also used on boilers (not for cooling) and it basically a resistor. There is thread on here that has all the parts numbers. The gas version is about £10. You plug it in and the heat pump that has all the cooling options show up on the controller - another UK ASHP con - premium ASHP you have to buy extras for, to make do a basic ASHP function.

Grant doesn't state anything or care, MCS don't care either. In the past under a different schem there was a requirement for cooling not to be possible - but that went away about 10 years ago. Most heat pumps will cool, its just a matter of finding what settings are hidden away. The new R290 Grant heat pumps, Grant state they will not cool, but there is a dip switch in the casing that can be set to allow automatic change over of heating or cooling, heating only or cooling only. Grant default is heating only. So exploration you may find it can cool. All ASHP have the ability to cool and they must move to that function (refrigerant 4 way valve) to do defrosts. You've contradicted yourself.

So obvious question, can you not locate the switch away from the steelwork? Wrap steel in 25mm PIR (or more if you have space) and seal all joints with aluminium tape - cold bridge gone. Or use a dry lining pattress box, instead of the normal ones for skimmed walls?

We started with One Insurance (think that's they are called) now with direct line. I found I answered the questions that were asked only. We are ICF, so our wall construction is concrete etc. Is it an open timber frame - or just a timber frame?

I wouldn't do that. Use a coupler in an emergency, but wouldn't plan to do it. Couldn't you decide where the manifold is going and make the connections to the manifold. Then pressurise the loop and add mechanical protection as needed.