genou wrote:We have gas fired wet radiators driven by a Potterton Precision ( 30kW ), with the main circuit in 15mm with 10mm tails to the individual rads.
What I haven't got my head round is whether I can get enough flow through this pipe work, and what the relationship is between power output on a traditional on'/off system and power output on a trickle feed heat pump system.
Hmmm. 10mm. Penny pinching. Microbore was a thing once, years ago, then it wasn't. Too much small pipe makes the system noisy, and large rads simply cannot get enough flow to reach their rated output.
In a well-designed GFCH system the main circuit should be 28mm at the boiler where it is running HW and CH, then the CH should be at least 22mm with 15mm branches. No or very little 10mm. At least you have no 8mm.
Heat pumps typically need higher flow rates due to the lower temp on flow and return. You may get flow alarms on the ASHP if there isn't enough flow for it to deliver the required system output. Some ASHP installations even require a 28/35mm backbone as normal 22mm pipework means a significant rise in system velocity to meet demand and therefore create more system noise.
You will also need larger rads. Perhaps double the output. More wall space. This can be mitigated by using cast-iron rads, which have a higher output for a given wall space, and also suit a Period house.
As recommended here
https://www.isoenergy.co.uk/more-information/radiator-sizing-for-heat-pumpsBefore you go any further I would get a couple of specialist installers in and see whether they will guarantee the results with your existing pipework.
At the very very least, if you find the largest rads don't get hot enough you may need to remove some of the 10mm and go up to 15mm. That of course creates balance issues, but should not be insuperable.
As regards overall output, the system has to replace the heat lost by the house, so the overall requirement does not change. The reason the heat pump is often run 24/7 is that it does not have the extra oomph needed to raise the temperature rapidly, as you can do with a gas system.
Make sure your installer is well versed in setting up these systems to ensure you have a trouble free installation.
V8