Luke warm radiators - on Rad + UFH System

21 Jan 2021
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United Kingdom
Our house has a heating system identical to the one pictured. System boiler with unvented HW cylinder, upstairs radiators (12 of them) and about 100 square meters of UFH. All separated by individual 2 port valves.


We've recently been renovating the far end of the house (and far end of the radiator loop) that we've not used since moving in, and I've fitted new column radiators, and they're just not getting warm.
I've done the following:
- Balanced the system multiple times - if I turn off all other radiators, the end ones get hot
- Run the balanced system without the UFH running at the same time - all radiators get hot

So I assume the UFH is "stealing" a majority of the flow so that it can't get to the end rads. I can solve this by running the UFH before the rads, so that it builds up thermal mass in the slab, so it's not a massive issue.

But would it be worth putting another pump after the valve on the radiator loop? I'll be draining the system soon to change another rad, and there is plenty of room in the airing cupboard.

Edit - The boiler is a 32kw one, which should have enough power for this system right?
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100,000 btu boiler, pretty big.

From the rad sizes, calculate the heat requirement.
Add the ufh.
Add whatever for pipe losses depending their size and insulated or not..
Add 15000 btu for hot water
Add 20% for margin
Is the boiler still big enough?

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