JohnMo

That is sort of what I was saying, could you turn down the temp, run at lower temp and get a better COP? Run all the time while there is a demand for heat from the house?

But remember the buffer is there to stop short cycling. Is it better for the boiler/ASHP, to run long and low for a longer period while there is heat demand?

Started as my own design, using what the boiler manufacturers manual said was a sound design, based on weather compensation and huba high degree of boiler modulation. But well insulated and airtight, throws conventional design, or the norms of, out the window. It also confuses the normal plumber. Our old house, not well insulated and not airtight (fairly normal), had UFH you could set the water temperature within a range of 10 degrees and just made no noticeable difference. New house +/- 1 or 2 degrees makes a noticeable difference, of either under or over swing in temperature.

Sounds a good idea, however no brand or suppliers names. Lots of people use Wunda trade, but they would only supply to my address for huge postage costs, so went elsewhere.

My UFH heating started without blending valve, and without pump on the UFH manifold, had issues getting to work consistently well. So added pump and mixer at the UFH manifold and it then worked well all the time. Having the blending valve at each manifold allows you to fine tune the flow temperature to the least temperature overshoot. For what it saves you now to do without blending valves, I would just install for the added ease of tuning.

Don't disagree that it need blending. With a 150 l cylinder the gas boiler has plenty to work on, so don't understand why it would short cycle, unless it has a super small coil. Mine does not short cycle, set up exactly the same as far as cylinder and boiler. Are you saying every uvc or thermal store on gas boiler short cycles? Hydraulically it's the same. Easy way to sort out, is for Chantal, to keep an eye on the CH flow temp at the boiler. Flow temp should be a steady 60 degree over about 5 to 10 mins. If it is, it's not short cycling. If it does go up and down the a replumb may be needed.

Not sure I really agree. There is no short cycling, the boiler is running constantly, as stated in the original post. Joe90 had a mixer valve, this system does not. Even when he replumbed he still had issues, because of issues with valves in the manifold.

So my setup is similar to yours. My insulation levels are very high, and the UFH is in thick concrete so mine will respond differently to others. This is how mine is connected. Boiler to coil in cylinder. Cylinder contents connected to UFH. But my pump is at the UFH manifold and has temperature mixing valve also. Controls. Thermostat, wired to a two port valve on the UFH supply from the cylinder. The micro-switch contacts in the two port valve, give a volt free call for heat to boiler, this signal goes through the cylinder thermostat. My settings are, boiler fires at 50 degrees, the cylinder thermostat is set at 40 degrees, UFH manifold regulating valve set at 34 degrees. Thermostat is half way up cylinder, so expect top of cylinder is close to 50 degrees. Operation Thermostat calls for heat, two port valve opens, if cylinder is below 40 degs, boiler fires, if at 40 degrees boiler does not fire. Boiler runs until cylinder thermostat gets to 40 degs. If thermostat is still calling for heat and cylinder goes below 37 degrees, boiler fires. I run the UFH pump 24/7. Setting up temperatures. Set thermostat at desired temp. The flow temp through the UFH is determined by the following. Doesn't get to temp, flow temp increase. Temperature overshoots, turndown flow temperature. Leave 24 hours between fiddling with temperature.

Not sure what you mean by " despite 60% water being added to it" So, boiler is trying to heat water in cylinder to 60 degrees, but when it gets to 40 deg you are pumping the water away to the UFH. The pump from your boiler and the UFH are possibly the same size. So the heat given to the cylinder is being extracted, by the UFH pump, as quick as it can be delivered. You are possibly lucky the system is not heating up, as you could be delivering 60 deg water into your floor, which would not be good. More once you clarify the point asked for above.

Just looked at prices - less than £7.50 m2 including vat. Prices may gone up but still not that expensive.

We used roof maker, passivhaus certified fixed units.

Hi, do plenty of shopping around, as prices for the same item varies tremendously. UFH is something you can design (using free software) and install yourself. ASHP various people on here have bought for an ASHP for a good price and local plumber and electrician installed, for many thousands less than an grant certified installer. MVHR install plan well ahead for the installation, Posi joists are your friend. Do not go flexible pipe or hard plastic pipe with tight bends. Either metal duct or semi flex with inlet and outlet plenums (easy to install and quite), use 90mm duct to velocity of air low. Above all, enjoy the journey.

Are you sure you are not building a shed or summer house?

It is a bit expensive, sunnyboy do one, not sure of the price, but not cheap I suspect.

Think this one let's set export limits. https://www.selectsolar.co.uk/prod/1597/solis-6-0kw-4g-dual-mppt-single-phase-with-dc

Our gas pipe runs through insulated Posi rafters and is run in Copper (22mm). New gas boiler circa £500 to £1k, make the heating system low temperature, to future proof. You may need a buffer tank. Combi or system boiler up to you. If you do use a combi, get one that take preheated water such as an Atag, then you can have a cylinder upstream of the DHW that allows to use to solar preheated water later if required.

And piping from water main to house is usually plastic and the water main is either plastic or lined in plastic - plastic everywhere

The 3 column one is 2853 BTU. https://www.homesupply.co.uk/Eastbrook-Rivassa-600-x-608-Gloss-White-3-Column-Horizontal-Radiator?gclid=Cj0KCQiAip-PBhDVARIsAPP2xc182-ebOJ-_LCV6gYPnx90wMzdnD3iF7siji72roHjODDIc6FgneJ8aAjcBEALw_wcB That would be better suited. If you are going ASHP, the bigger the better really, even go the next size up if you have room

I used GRC Aquatech in Aberdeenshire

Used PERT-AL-PERT pipe which can be used for the UFH, general plumbing throughout the house including supply water to drinking taps.

Did a simple Google search for the number of columns and overall size found this, different colour, but same size and number columns, which gives a T50 rating. Will be pretty similar if not the same rating https://www.homesupply.co.uk/Eastbrook-Rivassa-600-x-608-Matt-Anthracite-2-Column-Horizontal-Radiator

Where are you in the country?

We mounted the boiler on a large piece of 18mm ply, on an external insulated wall. Plumbed in plastic. Except close to boiler. Floor in Caberdeck chipboard 22mm thick, glued with D4 expanding wood glue used at all joists and all joints. Expansion gap at perimeter.

Daft question, why do your heat loss calcs have zero for all ventilation losses. The MVHR is not 100% efficient, so there will be some losses, which will add to the heating kW required? Our ventilation looks to add 11% to the heating loads at 0.5 ACH.

Not sure of you ground floor make up. But could you bury these pipes in your floor insulation, use that as your insulation, put them in flex conduit so they are replacable to comply with building regs.