JohnMo

I came to similar conclusion with our summer house. It runs off the same heating system as the house (UFH). I dumped the UFH in the summer house in the end, and used a fan coil radiator instead. To me you need two different flow temps one for upstairs and a lower one for downstairs. To achieve this you need a mixer downstairs. Two ways to do this An electronic mixer and treat the system upstairs as you would radiators. Your ASHP should be able to control the mixer - you would need to double check. Or you add a mechanical mixer and pump to downstairs UFH and run a fixed flow temperature. Running WC upstairs should give a degree of temperature modulation. If you go this route go for an IVAR mixer set. Like this - the option with the BritTherm is a good option, I used that same pump for a couple of years no issue, nice quite and pulls low watts. https://www.heatingcontrolsonline.co.uk/ivar-thermostatic-mixing-valve-pump-set-p-1073.html Or https://www.outsourcedenergy.co.uk/product/manifold-mixer-station-wpb/ You want the one that adjusts from 20 to 60

Is this a new install? If so, tell them the UFH not working and needs to be fixed.

To be quite frank, that's good. Lots of zone control can result in boiler running for very short cycles, bad for boiler life and has increased gas consumption. I would start a new thread asking how best to insulate the floor from below. Because you want to stop heat being leached away, but also make draft free.

Didn't at the time, all I knew was the heat loss of the house was supposed be x, but I was actually using twice that. From there it was a voyage of discovery and learning. Plumbers weren't interested, it just the way it is! I soon realised the boiler as in cycle on off mode continually.

Do they leak? if so maybe need to be changed Where is the condensation - if on the glass, it would say your rooms are very cold or the ventilation isn't correct, photos help. Do your windows have trickle vents are they open of closed? You say room temp, is that a single room or all rooms? A thermal camera, make the job quick, a candle flickering if held near a source of draft is a low cost options

But how deep? Replace seals, either replace with new seals from manufacturer or replacements from Screwfix etc Don't you will cause way more issues with damp Good guess will be drafts, you really need to understand where they come from and fix them, most is more time than cost. Draw curtains at night. Also how do you operate your heating system, tell us time schedules, room thermostats or TRV - as much info as you can. At lot of uncomfort comes from the way people operate the heating. What boiler do you have system (with cylinder) or combi (no cylinder)?

Sorry not exactly new - European manufactures have been designing boiler for low and slow weather compensated, room compensated heating systems for decades. UK manufactures offer drummed down technically poor boilers, manufacturing and installing, with the basis of design being S and Y plan heating systems, they should have been banned the day condensing boilers were mandated, a few decades ago.

Im running 300mm spacing without issue, wouldn’t fret too much. I have 1x 100m pipe doing main bathroom, then it does hallway by front door and then kitchen diner

I'm a bit confused. Is this a new build a renovation or something else? A drawing would help showing what was planned and what you got. What is the wall construction? The doors and windows have to straddle wall insulation, if it's doing that, the detailing on the floor is rubbish. If windows and door are not a straddling the wall insulation, get the installer back to fit them correctly. Does you floor match what the walls are doing with respect the cavity?, if so the door looks from the photo to straddle the cavity. Was that work agreed to was that shown on a drawing they worked to etc - if shown on drawings and they agreed it was in there scope, get them back to do the job correctly.

Have a look for a Heat Geek installer in your area. They have been trained properly on low temperature heating systems etc. if you go on to the heat geek website there's a page for finding local installers

It is easy you don't need anything fancy. Start of trial take gas meter reading, getting a book or your phone, sit by the boiler for an hour and see what it's doing. Measure gas usage over 24 hrs. Note down outside temperature. If you have a smart meter, it will tell you usage every 30 mins anyway. Start trial exactly as you are set up now. Run first phase over a week to get some decent readings. If boiler is cycling a lot that is the first thing to fix. Time the whole ground floor to heat at the same time as the garden room UFH at a normal temperature (20 Deg for example). Again monitor over a week what happens. While you are doing the trial, test with your hand a radiator when room is up to temp are your trvs closing off the radiator or does it just run a little cooler. So basically are your b.vakves on off or modulating? Then look at the numbers, are you using more or less gas is the boiler cycling as much etc. Then start to reduce boiler flow temperature again see what happens in baby steps. I spent months understanding what my boiler was doing, went from thermostat micro managed room climates, to a fully open system, reducing gas consumption by over 50%, mostly due to boiler short cycling.

A big Viessmann combi A Vitodens 200-W has a 17:1 turndown, so you could install a 30+kW for great DHW performance and still it would tick away down to below 2kW when it's mild. Set the boiler to run WC mode and it would fire up alter flow temp automatically and boiler would run for long periods at best efficiency. Comes with up to 10 years warranty Atag make a good boiler, turndown isn't as good as Viessmann, but a solid machine.

Is this a new build or retrofit? What's below the UFH pipes?

So your boiler has a minimum output of a out 10kW, which is pretty huge. So you really need most of your radiators on, while you try to run the UFH. Bringing down your flow temp will help, as it will slow down the system and reduce radiator output, so thermostats cycle less. Tell us more about what is controlling your radiators?

If you count is 1 full cycle in under 10 mins, you have issue to fix. Your boiler should start, modulate down and run for a prolonged period.