Nickfromwales

The 6 will suffice if it's given access to electricity on demand. I would go to a 9 if only if you have a bath. I fitted a 6 in Somerset and it's happily servicing a bathroom, kitchen sink, and utility sink in a household of 3 occupants. Great! No problem with adding an induction hob then ?

So as for an electrical design, you're already at ~20kW before sockets / lighting / kitchen etc Is this on it's own electricity supply?

My replies relate to the OP No gas. A gas combi is deliciously simple and quick, and I'd still recommend one today if I thought it were the CORRECT decision. Cost / economy / running costs and practicality will be all the things the person PAYING for it will be asking about. Design of retrofit is easy, and often free, just the design will need the property garnished with additional floor insulation / wall insulation / roof or attic insulation, and possibly upgrades to the windows and doors to meet the lower heat output per m2 that UFH can provide. It is not a good choice for retrofit AFAIC, unless it is highly insulated and "energy efficient" in it's current state.

Putting convection heating into a retro fit is in no way whatsoever a bodge my old china mug If the occupants are in a tiny 2 bed terrace, there's a bloody good reason for it! UFH and an ASHP really need to be designed in at the outset. Lipstick on a pig and all that eh?

Fortunately I spent about 13 years fitting new heating systems into exactly these types of properties We had to get very inventive of how to get heat into these and often the only way to get sufficient heat output was to fit two smaller radiators in the same room wherever there was room to do so. UFH in a retro fit is bloody hard to get right, even harder to insulate against the losses from, and are utterly impractical in small rooms with lots of furniture. The capital cost and impact to living pattern vs heating use is also a huge PITA. Horses for courses

They can go behind furniture without issue. They heat by convection rather than radiation, so no problem not 'seeing' them . Fit multiple smaller ones to get the required heat and fill any available 'gaps' to suit the room layout. Number one rule; DON'T put them on outside walls wherever possible.

Why make it so complicated? Willis heaters go in, paralleled up in qty to match max heat demand + 50%, on a recirculating loop ( or through a small low loss header for a very small dwelling ), and just let the dwelling suckle heat from that loop as it wants. Call for heat kicks in the heaters, and they go off or into 'satisfied state' until the recirculated water temperature drop low enough for the willis stat to kick back in. Done a few like this recently where there was no gas, no room ( or desire ) for an ASHP, and low / infrequent demand. Works really well. Keep it simple here Heating; Electric panel radiators are cheap as chips, and will heat the place almost instantly / on demand, and are very well suited if infrequent use is typical. Something as small as ~2kW units ( x2 for even spread of heat if more of a spread of heat is necessary ) which will kick in / out thermostatically as required. I'd buy the ones that have separate wall mounted wifi room stats / timers for ease of control & to prevent overheating the space aka economic running. These can run off 13a plugs with no special needs for a big electricity supply. Forget UFH unless you go for an ASHP, but that would seriously have to be justified by the usage of the dwelling mandating such a complex and costly setup? What is it's intended long-term use? Hot water; Fit a size 6 Uniq ( Sunamp ) and you'll only need a 3kW electric supply to get high pressure DHW on a 22mm supply That will tee off to do basin / sink / shower from one source and no need for a discharge pipe and G3 annual inspection. Note: the Sunamp isn't a hot water tank it's an instant water heater / thermal store, so no actual 'stored water' to speak of. Next to no losses in comparison to an UVC, and a far, far more simplified installation with prob half the space or less taken up. It'll go into a kitchen unit as its only ~580mm deep x ~370mm wide x ~650mm high. Near zero maintenance too, as its only requires a single tiny expansion vessel that can be checked in minutes ( by anyone who's competent to do so ). Electric boilers, and even worse electric combi's are very electricity hungry. If you had an electric boiler and a big electric shower you'd need a big electricity supply to match. Fit a big electric shower and it'll also be massively dependant on cold mains pressure / flow rate. It will suffer to a point that it's useless if anything is running off the same cold mains supply ( in the main building ) at the same time, and you'll end up running it on it's lowest setting most of the time just to get a decent stable temperature out of it. God help you if someone goes to use the kitchen sink whilst you're in the shower!!! The only way to combat that issue is with a 100-200L cold mains accumulator, which is a BIG vessel to try and integrate into somewhere where space is already tight.

Secondary heat Ex is pumped, so no pump = no hot water too

Get on with the..........

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May just benefit from balancing out the rads a bit more. Tweak back the ones that get hot first, but other than that I’d just up the flow temp a couple of degrees to compensate. Retro fitting an ASHP is quite a tricky patient.

Do the rads and UFH share the same tapping off the buffer and the same pump? I know there will be a pump on the UFH manifold

Does the ASHP drive directly into the heating, or is the heating pumped from a buffer or low loss header?

I don't think it's a good idea to fill a narrow trench with all the above tbh. Maybe a much wider single trench but not 'squeezed' in like sardines. If you ever needed to replace a section of the drain pipe then you'd have to excavate a long enough trench to lift the cables / pipes out of the way to facilitate further excavations to suit. Typically these services go down and are then never seen again, so down to paranoia to some degree, practicality to another. If, for arguments sake, you were a developer, perhaps you wouldn't really give a hoot, but if you owned this for the rest of your days then maybe worth a second thought..... A machine driver can blast through ground works like my kids go through food and clothes, so personally I'd just stump up the extra for the fines ( sand / dust / pea gravel etc ) and set the services into one 'side', and the drainage into the other. Segregation is nonsense, given what I've uncovered over the years!!!

It's flow rate. Are the valves new, and are you testing this with all the TRV's fully open ( assuming you have TRV's as new rads have been fitted )? Can't you chop out the micro-bore and extend the 15mm pipe to the rads? Quickest way to eliminate sludge is to turn all rads off bar one and then switch the heating on from cold. If that rad warms through evenly and quickly then the rad is fine. How old is the pump? Is this open pipe aka gravity, or sealed and pressurised?

Do you need your C&G’s before they’ll let you attain G3? I know it gets done with the fresh faces as part of their ACS, but I’m not sure if you can walk in “of the street” and do your G3...

Too clunky and noisy to have inboard on a TF house though, hence me chasing solid state.

....sorry Terry, you're not my type Just a heads up on how a simple man like myself would switch one of these on / off from a 230v signal would help. I assume the 230v would fire the coil in a zero volt relay and I'd have to generate the 5v from an 3rd party adaptor / transformer and send that through the zero-volt side of the relay? Simple on / off operation will suffice, mainly because I try and set things up so some poor fool would be able to reasonably quickly identify what's been done and how it works ( in the event of a breakdown and I'm not of this world anymore ) for fault-finding / ongoing maintenance & repair by A.N. Other. I strayed into the Boffins corner once, but was chased back into the woods by a headless horse-man.

I turn things up roughly 0.5o per day, allowing the full 24hrs for things to acclimatise at their own pace. Takes a good week or so to get things to 'simmer down'. Having the Ivar type blending set, a-la @JSHarris's type of set, is of huge importance as if you try to do this type of heating management with a normal thermo-mechanical type of blender ( aka TMV ) you''ll never reliably get down to a flow temp accurate enough to stop the above-mentioned overshoot. The room stat ( correct choice of ) is of equal importance. If you don't get those two elements right you'll not stand a chance, but if you get it right the curve for the heating hysteresis will be as close to a flat line ( as possible ) eg a near equilibrium as far as achieving a comfortable thermal constant. PH is nuts, and very difficult to convey to those who have had no direct experience of 'it'. With a heated slab and the house at equilibrium you can open all the door and windows for 10 minutes when it's -5 outside, blow all the heat out of the house until you're freezing your knackers off, and then just shut the doors and windows back up again. Within 3-5 minutes the house is back to how it was. For those who don't want to fit UFH, in conjunction with a decent slab, good luck with that!!

Do you have a link to these please, and did you have to fit a heat sink to them?

For the amount of heat you’d need from them, if used in conjunction with wet UFH in the main slab, I’d stick to the sheer simplicity of electric towel rads + electric UFH. If you’re relying on these rads as your main auxiliary heat source, eg no UFH, then prob best to feed them from a HP as you won’t be able to load shift. Would be disproportionately expensive as a heating design though, imo.

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F264509828782 Good / bad ?

I would still fit a heat pump just for cooling. Just great for killing two birds with one stone. If you’re not in ‘need’ of cooling it is very hard to justify the capital expense, ( at a time when every penny counts ), and more of a concern when you factor in it’s inevitable maintenance, repair, and ultimately it’s replacement.

KISS at its absolute finest.