Underfloor heating into existing Y plan heating.

25 Jan 2021
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United Kingdom
Hi, some years ago I had wet UFH installed in my conservatory. It was just chopped into the existing Y plan wiring and plumbing. The problem being that when UFH demands the CH/HW runs and vice versa. We have put up with it but now as we are spending more time at home it needs fixing.

The best solution would be changing it to an S plan but that would require major plumbing work. The overflow and cold feed are just before the pump in the upstairs airing cupboard and the boiler in the garage. So, as it stands, I can’t put a 2-port valve in line to the HW/CH as it would block the overflow.

The UFH has a 2-port valve on the return and a mixer and pump on the flow controlled by a separate room stat.

Am I right in thinking that the simplest solution would be to attach a relay to both the switched live from the CH/HW to the boiler and the switched live from the otherwise separate UFH stat/2 port valve/pump.

Any comments appreciated. Thanks
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The only solution is to change to S plan, one valve for HW, one for CH, one for UFH.
Y plan can't be used with multiple zones as one of the ports is always open due to the mechanical design of the valve.
Thanks for the speedy reply.
Surely the fact that 1 of the ports is left open shouldnt matter as the CH/HW pump ( which is in the upstairs cupboard just before the mid position valve ) won't be running if only UHF is demanded.
The UFH 2 port valve and pump are in the garage near the boiler.
The main pump will only run when there is demand from the HW cyl stat or CH room stat triggered by the orange from the mid position valve and lead on to the relay to trigger the boiler switched live.
I am sure some gravity flow will occur.
Perhaps I could put another 2 port valve on the return from the CH/HW so sealing that part of the system but still leaving the overflow open on the flow side.
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If you run the boiler without the pump, the boiler will overheat and shut off, with the possibility of damage.

The UFH pump is for that circuit only and is installed after the blending valve, circulating water through the floor pipes with the valve opening/closing to allow some flow from the primary circulation as heat is required. If there is no primary circulation, the UFH won't work and the boiler will overheat.

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