Mixing 10mm and 15mm with radiators

[RFw3Duwrd7Yu]

I'm replacing boiler with an air source heat pump, and adding / replacing about half of the radiators in the house. The feed/return loop plumbing is all 22mm, but while old radiators are 10mm plumbing mostly off a couple brass manifolds, new rads are 15mm plumbing. Will mixing piping in this way be a problem for my system? Hoping to avoid swapping out all the old 10mm copper unless I need to...

 

[RandomGrinch]

From what I can make out (and I may be very, very wrong!), the bigger the pipe work, the better, given the lower temperatures involved.
See section 1:5
https://mcscertified.com/wp-content/uploads/2020/07/Heat-Pump-Guide.pdf

Edit: p22 states:
Please note: Mini-bore pipework (6, 8 & 10mm diameter) is generally not suitable for heat pump installations.

 

[RFw3Duwrd7Yu]

Aye, I'd definitely taken note of that - and that's why all the new rads are going in with 15mm. What I'm wondering here is whether I can get by without causing some kind of technical fault by using existing pipework (which will probably be replaced later in the future for the sake of low temp flow).

 

[Agile]

I do not think that you will create any technical fault.

But just that the balancing will need to be done carefully.

And depending on the 10mm runs you might never get quite enough heat into some of those rads.

 

[Madrab]

I would avoid 10mm.

Heat pumps typically need higher flow rates due to the lower delta's on the flow and return. It could create flow alarms on the ASHP if there isn't enough flow to obtain suitable throughput 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.

That aside, when you mix pipe sizes then balancing becomes key as the lower resistance through the 15mm pipe will mean those rads will naturally obtain more flow.
That and old 10mm pipe and manifolds could be furred up somewhat unless the system water was pristine and kept that way throughout it's life up till now.

I would ensure that your installer is well versed in setting these systems up to ensure you have a trouble free installation