Do retrofit ASHPs need larger pipes to the rads?

[PeterW]

Depends on tank size tbh but most internal coils are 22 or 25mm so 28mm up to the tank gets an increased flow through the coil and that’s about it. 
 

I always site the valves in one place and then spur off to the different circuits, so would be 28mm flow and return to one location and then 22mm everywhere else so that allows multiple flows (ie tank and heating at the same time)

 

Buffer should be used for high temperature feeds to UFH to stop short cycling, you can drop the ASHP temp to closer to the floor flow temp and not buffer it but it can cause problems with larger ASHP that can’t modulate down.  

[Adsibob]

18 minutes ago, PeterW said:

Buffer should be used for high temperature feeds to UFH to stop short cycling, you can drop the ASHP temp to closer to the floor flow temp and not buffer it but it can cause problems with larger ASHP that can’t modulate down.

So maybe this is the issue for me. I don’t really have space for a large buffer tank. I am getting around the short cycling issue with my gas setup by going for a boiler that can modulate down to one seventeenth of its maximal output, so about 1.84kw, and running a towel rad “to infinity”, as @Nickfromwales put it, whenever the UFH is on, as was suggested on this thread.
Would the same workaround work with ASHP? I am future proofing here, so Can assume someimprovement in thetechnology. In the same way that the modulation of gas boilers has got broader in range as technology has advanced, I would expect that over the next 12-15 years, ASHP will develop much broader modulation as well.

 

 

 

[TonyT]

MCS ASHP guidelines advises not to use microbore, imagine this new scheme will follow MCS existing paperwork?

[Nickfromwales]

9 hours ago, TonyT said:

MCS ASHP guidelines advises not to use microbore, imagine this new scheme will follow MCS existing paperwork?

It wouldn’t matter if larger rads were fitted to the microbore if they were pumped from a smaller buffer or low loss header for hydraulic separation. Just about anything can be mitigated with a decent installer. 

[J1mbo]

The higher flow rates requirements comes from the smaller dT across the radiators that is sought - gas/oil systems typically specify 20°K across the rads, whereas heat pump wants 5. All else being equal, the flow rate is quadrupled. In round numbers, 1m3 per hour for each 6kW of output. Typically we want the water velocity less than 1m per second in the pipes, so more about 6kW then best to have 28mm flow from the heat pump and branch to 2x 22mm circuits to the radiators (e.g. upstairs/downstairs might be convenient).