@Oswiu

I assume your boiler is smarter than our old one, and has a display!  I’d encourage you to check it somehow versus the calibrated payment gas meter though still, just to be sure.  If you know the yearly gas useage in kWh, you can start with that.  Roughly, the peak daily load is 1% of the years gas use in the UK.  It’s down to the profile of heating degree days in your area, and it works for us, given you use hdd you could check that!  So if you used 10MWh of gas a year, assume 100kWh/day peak.  Then divide by 24 to give around 4kW peak load.  In practice you would want a heatpump more powerful than this, 5kW would just about do, 7 would be safer. 

For the radiator coupling - as above, turn all the heating on full.  When the rads have heated up, note the gas meter, start a stopwatch.  Wait exactly 15mins for good accuracy, note the meter again.  Multiply the difference between readings by 4 to give the kWh/h = power in kW.  Then you have everything you need!  Nb some meters need conversion from ft3 or m3 to kWh.

Posted by: @robl

@Oswiu

I assume your boiler is smarter than our old one, and has a display!  I’d encourage you to check it somehow versus the calibrated payment gas meter though still, just to be sure.  If you know the yearly gas useage in kWh, you can start with that.  Roughly, the peak daily load is 1% of the years gas use in the UK.  It’s down to the profile of heating degree days in your area, and it works for us, given you use hdd you could check that!  So if you used 10MWh of gas a year, assume 100kWh/day peak.  Then divide by 24 to give around 4kW peak load.  In practice you would want a heatpump more powerful than this, 5kW would just about do, 7 would be safer. 

For the radiator coupling - as above, turn all the heating on full.  When the rads have heated up, note the gas meter, start a stopwatch.  Wait exactly 15mins for good accuracy, note the meter again.  Multiply the difference between readings by 4 to give the kWh/h = power in kW.  Then you have everything you need!  Nb some meters need conversion from ft3 or m3 to kWh.

Unfortunately it does not! The data are from a Drayton Wiser system feeding into Home Assistant. I am not 100% sure what it means, but I think it related to the boiler actually heating as opposed to the heating being on, hence why I think it's a reasonable measure. I will do the calibration test as your describe, although waiting for a bit for the rads to warm up. I've just been all over the shop this weekend! 

I have seen that method of using current gas usage to estimate size, albeit slightly complicated by degree days. Tbf with all of these things I do tend to arrive at about 6kW, so they're pretty consistent, hence my question on if my 22mm piping would need to be upgraded. 

Posted by: @oswiu

Posted by: @robl

@Oswiu

I assume your boiler is smarter than our old one, and has a display!  I’d encourage you to check it somehow versus the calibrated payment gas meter though still, just to be sure.  If you know the yearly gas useage in kWh, you can start with that.  Roughly, the peak daily load is 1% of the years gas use in the UK.  It’s down to the profile of heating degree days in your area, and it works for us, given you use hdd you could check that!  So if you used 10MWh of gas a year, assume 100kWh/day peak.  Then divide by 24 to give around 4kW peak load.  In practice you would want a heatpump more powerful than this, 5kW would just about do, 7 would be safer. 

For the radiator coupling - as above, turn all the heating on full.  When the rads have heated up, note the gas meter, start a stopwatch.  Wait exactly 15mins for good accuracy, note the meter again.  Multiply the difference between readings by 4 to give the kWh/h = power in kW.  Then you have everything you need!  Nb some meters need conversion from ft3 or m3 to kWh.

Unfortunately it does not! The data are from a Drayton Wiser system feeding into Home Assistant. I am not 100% sure what it means, but I think it related to the boiler actually heating as opposed to the heating being on, hence why I think it's a reasonable measure. I will do the calibration test as your describe, although waiting for a bit for the rads to warm up. I've just been all over the shop this weekend! 

I have seen that method of using current gas usage to estimate size, albeit slightly complicated by degree days. Tbf with all of these things I do tend to arrive at about 6kW, so they're pretty consistent, hence my question on if my 22mm piping would need to be upgraded. 

@robi.  oh no we aren’t going to start over sizing heat pumps as we have done for boilers for years..

you missed a condition:

if the heat pump cam be down rated for heating but stays at the nominal vale for hot water over sizing matters less.

I agree with everything else though!

I went a bit overkill in this post, but TL;DR I decided we probably should upgrade the radiator pipes to the living room, although we don't need to, and that the piping around the house should be fine, especially with hydraulic separation.

Having come up short on answers to my piping worries, I bought @grahamh 's second book (50 things heating professionals need to know about Heatpumps) to try to get to the bottom of it. The book doesn't contain the required formulae, although it comments that they're "easy", but it does give a cheat sheet of sorts. At 1.5m/s flow rate and a DT of 5C, 10mm pipes should be able to carry 1.8kW, and 22mm pipes should be able to carry 10kW it says.

Not content with cheat sheet over formulae, I managed to find what I think is the actual formula, which is

Power = Flow rate x DT x Specific heat capacity of water

Which I could more simply label

P = FL x DT x Cp

So if I wanted to derive these powers myself, I'd simply need the components in the right units, plug them in to the equation et voila ! Unfortunately, the maximum flow rate doesn't align to this because m/s is not what we need which is kg/s in order to cancel everything out. We can very closely (to about 1%) approximate water's mass in kg as its volume in litres, so we need to find out how many litres are in 1.5m of pipe using the formula for the volume of a cylinder

V = pi x r^2 * l

Where l is the length of the pipe. I searched internal diameters of pipes, and found for 10mm pipe I could expect 8.6mm, and for 22mm, it's 20.2mm, so plugging these into the volumes, that's

V[1.5m of 10mm pipe] = 0.087 litres & V[1.5m of 22mm pipe] = 0.481 litres

So the max flow rates of these pipes are 0.087L/s and 0.481L/s.

Bringing it home in the formula above for power, I come to 1.82kW and 10.05kW. That's basically what I expected, so now I am content that I can use these formulae to demonstrate for or against needing to upgrade or not specific pipes to my prospective installer since 10kW is way over my house's expected peak heat loss, and 1.8kW is way over the expected heat loss to be covered by one single radiator.

Do my 22mm pipes meet minimum flow rates for heat pumps? Unfortunately heat pumps seem to state flow rate in yet another unit, litres per minute. My 22mm piping has a theoretical maximum flow rate of 28.9L/m, and some reference potential heat pumps have minimum flow rates of: Daikin Altherma 3: 22L/m; Vaillant Arotherm Plus: 9L/m; Samsung EHS Mono: 12L/m; Samsung EHS Mono HT Quiet: 7L/m; Mitsubishi Ecodan monobloc: 24L/m. I don't know what the answer is, or why they vary so much, but I guess I'd probably need hydraulic separation for the faster ones, remembering that these figures were their minimum rates.

I can also calculate that for the 1.5kW peak loss my living room, I'd need a flow rate through that radiator of 0.07L/s which with 10mm pipes is 1.2m/s. This is double the "speed" of any other radiator pipe, so I probably should upgrade this one to 15mm to make the place feel more "balanced", as otherwise I think we'd need to restrict the flow to every other radiator to get the right flow through the living room because of the pressures. That's bound to make the whole place noisier than it could be. I'm not sure on the pressures part, so I might revisit that in another enthralling post... Or maybe I should just drop the maths and let an installer install whatever they think is best. TBC.

By the way, I recommend @grahamh 's second book for people interested in the subject. I didn't read the first one since I assumed it'd mostly be covered in John Cantor's book Heat Pumps for the Home that I also read.

all heat pumps work at 5 degree delta t, so you can only squeeze 10kW down it. its q 68 in book 2 . 

@grahamh Hi, yes that's what I found. The above post was just me trying to arrive at those figures from first principles to form an understanding of how the different factors work together.

I think with that sorted this leaves my questions as

1. Will I need to upgrade the piping of the circuit? Very likely not
2. What can we do with that 1-1.5kW peak loss living room? I've considered filling in the vents and replacing the woodburner with a direct air version, which would lower air changes significantly. Nothing needs doing, but could upgrade rad piping for balancing
3. Will I need hydraulic separation in the form of a LLH or a buffer? The few installers I've had "estimates" from seem to think so. Still don't know
4. Is there anything to be done about the long pipe run? Can it go through the house maybe? Could put it underground, but the heat loss doesn't affect COP all that much anyway
5. Is a split or a monobloc system better for long pipe runs? I've heard splits are, but I don't understand why. Split is potentially better because it takes a lot of energy to pump water long distances
6. Could I ever get a good SCOP? No reason why not

So a fairly good resolution from my perspective, and if the stars align with regards to installers, pricing etc, then there's no reason not to press ahead.

all seems resonable to me, i would use allh because the pi runs are long, burying the pipe use rauvitherm pipe in pipe, its very expensive but brilliant or do a split. good scops are pretty easy to achieve. re installers my hot tip let them design it and choose the heat pump, dont force them into a unit they dont know. for me there is only one unit for your job ht quiet samsung, it is in my opinion the best unit on the market, im biased i have one at home. if you dont know an installer ask freedom (my old firm) or mr google is a good place to look 

@grahamh I had seen the new Samsung unit, and my closest installer does Samsung installs, albeit the 6th gen ones. I can't see that the Mono HT Quiet is MCS approved or SAP registered. Do you think that's coming? I don't think questions about whether its volume was under 0.6m3 were solved in the other thread. It's a good candidate for us because my wife thinks it's one of the more visually appealing ones.

it will be mcs soon, i would wait, its miles better than the gen 6, 

@grahamh Thank you, that's a very useful insight!

Posted by: @oswiu

It's a good candidate for us because my wife thinks it's one of the more visually appealing ones.

I have to say they’ve nailed the styling - it does look really, really good.

What do you think of the way the Red heat pumps look @oswiu?

@grahamh, when you get a moment, do you think you could post a few pictures of the Samsung in situ?