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