JohnMo

I wouldn't go that far, some do have a fixed flow rate, some don't. Don't confuse house temp and heat pump target temp. The heat pump has zero care about what happens in the house, it's just a heat generator. It looks after it's self. The target temp referred too is heat pump target flow temperature, the heat pump is always trying to get to that temperature, while also managing dT. It manages dT first to protect itself from damage of the refrigerator circuits. WC just takes reference to outside temperature only. Doesn't care about inside temperature, just simply follows a straight line curve based on outside temp. You adjust the curve so it coincides with a house temp you like. You have variations of the above, like load compensation, which does take references to house temp, these work well with radiators but not with UFH.

This is something heat geek put out there for pipe sizing, you will be looking at a dT of 5. So as you thought you need 28mm. The pipe your plumber is specifying is thick wall so closer to 22mm. I would have alarm bells ringing as he isn't doing the basics.

You can take a guess at it, but the only real way is Install a heat meter in the water pipes from or too the heat pump, this will have tappings in to the flow and return to measure water temperature. This then calculates kW and logs the data, to give kWh. You then need an electric meter or two, depending on how power is derived for the package, any external pumps and if you use an immersion. All these numbers are logged instantaneously and calculated to give kW and kWh. They will also give CoP and rolling SCoP in the software. SCoP, is function of running CoP and standby CoP. More time you spend on standby, the worse the SCoP. I can get a CoP of 6 when running at 7 degs outside, but spend quite awhile on standby also, so will end of with a daily CoP nearer 4. That's running full WC, but it's also a function of heat loss rate and heat pump over/under sizing. Lots of things at play. Really look to get the system performing, so it keeps the house warm first, log what's happening so you understand what the heat pump does and doesn't do. If a thermostat is switching on off in the house, your system needs to tweaked.

How big is your heat pump? Is the heat pump connected to a buffer, if so how far between the HP and buffer? If no buffer how far to cylinder and heating system? If UFH does include or not include a pump?

Our calculated heat demand triples from 10 Deg average temp outside and 0 average outside. By the time it's -5 the heat demand is around 4 times that used on a 10 deg average day. But in reality it's more than that. As background heating from fridge and general stuff proportionally gets way smaller. So at 10 degs if you needed 1kW of heating and general stuff in the house gave you 200W you only need to supply 800W of heating, on day where you need 4kW, the general stuff still gives you 200W, so you need to supply 3.8kW. So 4.75 the heating input is required. The more heat leaky the house, the steeper the heating curve. We are are on an average daily temp of -0.5, our coldest so far. But looks to dropping to -6 on Friday

If you just installed fan coils you could run a fixed flow temp of 30 to 35, depending size of fan coil.

Yes they can, because they are just flow temp rise influenced by outside temp. But unlikely a house with UFH would have the same curve as a house with radiators because design flow temps are unlikely to be the same.

That's how long is this piece of string question, depends on the workloads and that varies day to day from company to company and area to area. But 4 weeks sounds quite quick, I was waiting for 2 months for my first single phase, and that didn't work, nor the second or third.

Are you running constantly or cycling the heating on and off? The above would indicate on/off, based on Cosy time periods. Those low flow temps for a couple of hours at a time will do nothing heating wise via UFH. I have said a few time that Cosy is really a battery tariff, but can be used to reduce heating costs. Running UFH on off for short period needs high flow temps to work, not low ones more suited to continuous heating. You may need to think of Cosy differently - the cheap periods are just that, cheaper periods, the other periods except the high priced one are normal costs (not expensive). You setback the heating temp during the expensive evening time and all the rest of the time operate normal WC. 26 deg flow temps will never feel warm underfoot, nor will 30.

Just reread Meant to write And as way of example - mine does modulate the circulation pump and I cannot change that. Others will have external pump(s) or some like Grant have an internal pump that runs at a fixed speed, the speed is adjustable, but remains fixed after that.

Some will modulate flow don't. Basics are Target flow temp, dT, max and min flow temp (hysterisis). So compressor starts, at min flow temperature, once started tries to establish dT at minimal flow temp, as dT decreases, more flow temp is added, up to target flow temp. Once there different manufacturer do different things, but it's all about power input, to maintain dT and flow temp. WC is just a variable flow target temp based on outside temp.

You should be able to weld, but it is likely to look awful. No idea how you would go about it. Would be better to look a some butt joint seals.

You mentioned above when the bedroom radiator come on. Do you switch sections of the heating on and off? Do you have the same noise with everything on?

You do, but why would you? they will charge an arm and leg and give you a rubbish system. DIY it

Not true, just put the heat pump on your planning application with the location you will install. Then you don't need to use MCS at all. You end up express planning not permitted development. Move your utilities to octopus and they remove for free. People need to see and hear heat pumps. They just aren't noisy. Today 0 degs, pumping at 32 degs, was about 500mm before I could hear it. Do what you want, not some misinformed person next door. It's your money. After having a combi, then a combi with preheat and now UVC, UVC every time would be my choice. Heat how you feel it's right. But whatever you do install a heat pump cylinder. If you go gas PDHW so weather compensation and high flow temp. With a heat pump cylinder you can condense all the time.