Reverse flow on one circuit (UFH)

28 Dec 2021
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United Kingdom
Hi all,

One of the circuits in my heating system seems to be reversed and can’t work out for the life of me why it might be. Any guidance would be much appreciated.

I have an Ideal s24 system boiler in the garage that heats a UFH circuit downstairs, radiators upstairs, and a megaflow unvented cylinder upstairs.

The flow rises from the boiler up to airing cupboard. In the cupboard, the flow splits into the three circuits, each controlled with a motorised valve.

When either / both the rads or / and the hw are called, all seems to be fine. But when the UFH is called, the flow reverses (ie flow goes cold and return goes hot). The boiler seems to cope but is definitely not happy.

Any ideas what could be causing this?Might a non-return valve / check valve sort it out?

The first plumber thought the boiler pump was broken so replaced that; the second plumber admitted he was stumped but thought the mixer valve on the UFH might be broken; the third plumber thought the pipes might be crossed between the airing cupboard and the UFH mixer so switched them round but it’s still doing the same thing.


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If you put it on rads only when they're hot put it on uf as well check the flow is correct by feeling the pipes. Then turn the rads off.

Check the temp of the pipes on the manifold and boiler...... I have a feeling that the boilers wired up so that the pump doesn't come on but only the uf pump when it's on uf only. You said system not heat only so I'm assuming the boiler also has a pump.

It's a guess really you need to know exactly what's happening to diagnose the fault
Feel the flow and return on the boiler when only the uf is on to see if the circuit is going in the right direction.

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