@sparkymark, hello and welcome to the forum. Are you asking if an ASHP system needs larger bore pipes for its heating/HW circuits than a gas system? If so the answer is in theory maybe but in practice not really.  Our new ASHP system has standard 15mm pipework to the radiators and some 22mm around the ASHP and the tank and for the main HW feeds.  That's the same as I remember our gas systems.  From what I have read that's the same for others.  Some sources advise against microbore but then again some have used it. 

You might need an extra pump or two to circulate the extra water because the radiators will be typically bigger, especially if you have a big/complicated house.  It's more important to get the design of the system right with an ASHP.  Gas/oil will be a lot more forgiving of a poorly designed set up.

Hi Sparkymark,

Welcome to the forum.

As Kev stated, in most instances the existing pipework is adequate for use with an ASHP. The pipework is merely the means of transporting the heat energy from the heat source (Boiler, ASHP etc.) to the heat emitters (Radiators or UFH), and provided there is sufficient flowrate at the lower temperature preferred for ASHP's, then there should not be a problem.

In situations where there may be a problem, it could be possible to resolve the issue by installing a larger pump rather than replacing the pipework.

Welcome to the forums @sparkymark – what Kev and Derek said is spot on. From our experience, the bulk of our property has 15mm pipework – for some reason, when the previous owners ran the central heating (old oil boiler) to the northern end of the house, they reduced the pipework to 10mm. The issue we ran into when we switched over to an air source heat pump was that there was insufficient hot water getting through to that part of the house. The 15mm circuit was great. This was remedied by increasing the size of the distribution pump and increasing the size of the rads. This work was carried out in April, so we'll only really find out whether it's 100% worked when this winter rolls around. 

Hi SparkyMark - when "they" talk about converting the gas grid to H2, this will only be at 20%, so you will still be burning 80% methane.  So it helps a bit - like a hybrid car.  But the answer has to be to stop burning stuff, so electric all the way.  Above 20% and there is not enough energy (specific heat capacity) in the H2#, compared to CH4.  So you have pump it faster, that leads to all sorts of losses.

Don't even get me started on H2 for cars 🤔 Up to 70% losses compared to using electric in BEVs

I will get off my soap box now

@julianc H2 is much more convenient for the consumer. Stores far more energy per litre, quick to refuel and retains the existing infrastructure of petrol stations. I would personally be surprised if H2 didn't make some kind of inroads given the intrinsic issues of battery's for range and heavy loads - you simply cannot run heavy equipment on batteries for extended periods.

Current (best) battery energy density 900wh/litre

Diesel energy density: 9,700 wh/litre

Diesel is nearly 11x more energy dense and takes seconds to replace the capacity compared to hours for batteries. This is why it is hard to transition away from fossil fuels. H2 makes a good case in that sense. 

I totally agree fir large loads. Maybe lorries. And energy intense industry like steel 

But for cars, I still think BEVs are the answer. Charge them overnight (where possible). New cars coming out like Tesla and Hyundai Ioniq 5 go to 80% charge in 18 minutes. And with 280 miles of range. 

Vehicle to Grid will allow balance of the electric demand. 

And there are huge losses in H2 production compared to using electric directly. Plus H2 is much more expensive. I think BEVs will win the economic argument. 

We do need more ultra fast chargers. 

@batalto, good, valid points, but the reality is the world is taking strain with all the CO2 that's being released. At some stage (if it's not already too late) we have to stop burning fuels or our living environment is going to become horribly unstable and difficult to live in.

Hi Mark, if you already have a plumbing background I can recommend the training videos from Freedom Heatpumps:
https://www.youtube.com/c/SamsungEHSHeatpumps/playlists

Most of what they describe are Monobloc heatpumps, which means there's a single outdoor unit with electricity goes in and water flow/return water pipes come out - much the same as an oil boiler. If you look at some of the system diagrams you can see they're not a million miles away from that.  The main thing is that the output water temperature is lower, which means heat emitters need to be larger - larger radiators, larger coils in the cylinder - and also the HP can be a bit fussier about flow/return rate (Freedom put in a plate heat exchanger so the HP only has a short pipe run to the HX, rather than running through all the rads directly. Other installers don't do that). The lower you can keep the output flow temperature, the more efficient the HP is. Also they don't work as efficiently in really cold weather (sub zero for long continuous periods, which is relatively unusual in the UK).

Because of the lower temperature you need to do a proper heat loss calculation, rather than just guessing as many gas installers do. Larger ASHP are a lot more expensive, and cycling them drops the efficiency, so it's important they're sized right. Freedom have a service to do that for you, or there's the MCS spreadsheet which isn't too hard to use.  You just need to measure floor area, wall heights, windows and radiators, and then say what everything is made of (cavity wall or solid, how much loft insulation, etc).

Basically gas boilers give more margin for error, whereas heat pumps are a bit fussier and need a little more attention to detail to get things right. But at the end of the day they aren't massively more complicated than today's gas boiler systems and smart controls.

(this is not an ad for Freedom, BTW, I just found their material very handy in demystifying how things work)