JohnMo

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.

I would be more concerned that you actually discovered the steel and you can put your hand on it. It should be fully insulated not hidden behind the plasterboard uninsulated.

How big are you planning - floor area and number if storeys, on how big a plot?

But do you really get that many days at -3, compared to days at 7 to 10 degs.

Maybe, but most houses on here are outside their comfort zone, ventilation heat loss just too small, to allow s tick the normal tick box. Hence plenty are specified way over sized.

Use a normal domestic one, your just heating a buffer as far as it's concerned. Depending on hot tub size a big one. To keep the ASHP inhibited and pressurised you may need to plate load the hot tub, but size the Plate Heat Exchanger (PHE) correctly, you have a 2 Deg approach temp. Then you have no issues with DHW heating, it just switches the diverter valve as normal. Add a second zone for cooling, so it bypasses the hot tub or have the hot tub cooler in summer and as cooling buffer. Heat pump doesn't care if it doing heating or cooling it just moving the refrigerant 4 port valve to a different position (as long manufacturer hasn't disabled the cooling function). Most heat pumps have the functionality to have different zones as cooling only, heat only or both. And the functions can be independently set for each zone.