I’ve got a mid-’90’s house with the conventional gas CH with unvented tank in an airing cupboard upstairs which also contains the circulation pump and 3-way motorised valve. The boiler is downstairs with a timer, and a thermostat in the hall. I have wet underfloor heating in my extension to the rear of the house, so there is manifold running off the boiler with its own circulation pump and a separate thermostat in the extension for the UFH. There is no timer on the UFH system, it’s just on 24/7.

My understanding is that the UFH is completely segregated and independant from the main CH system.

I’ve noticed over the past few weeks that if I wake at night often I can hear the main CH system circulation pump in the upstairs airing cupboard whirring away. The rads are not on, the timer has not commanded either the main CH to come on or the hot water cylinder to be heated, so can’t understand why the CH circulation pump should be on.

I investigated further last night and found the boiler was on so turned the UFH stat down, and low and behold the boiler turned off and the circulation pump turned off too.

Am I right in thinking that if my UFH was on there is no need for the main CH system circulation pump to be on too as the UFH system has its own pump? I can’t think of a reason why the main CH system pump should also be on?

I still use the plumber who installed it but want to establish my own understanding/opinion before calling him in (assuming there is a problem).

Thanks all.

I’m not a plumber but I am a fully qualified exorcist*. Was the extension built over an Indian Burial Ground?

* I’ve had some business cards printed

No, if the main circulation pump was off, the UFH would have no way of getting any heat. The UFH pump only circulates water from the manifold around the UFH, you still need to get the water from the boiler to the manifold. If the thermostat is on, it should signal the boiler to run and the pumps to start. It sounds like it is working correctly

Not a plumber… the firebox in the boiler will only heat one circuit, so there must be a heat exchanger somewhere to transfer the heat from that primary circuit to the UFH circuit. Is that in the manifold? Wherever it is, it wouldn’t be a surprise if the pump has to run to circulate water in the primary circuit.

Not sure about Indian burial ground, but near me there was a very large Saxon settlement which was taken over by Vikings and then the Romans, so the whole area has alot of bloody ancient history. Wouldn’t surprise me if the house is on layers of dead and buried people. Not sure the ghosts of Saxons, Vikings or Romans would have much knowledge of modern day CH systems…. maybe the Romans might as they invented CH.

However the neighbours cats love using my back garden as their litter tray and I can’t imagine cats with their sixth sense would feel comfortable crapping on ghosts. They don’t even get spooked when I knock on the window as they’re curling one out. They just look over at me nonchalantly and carry on. Little bastards!

No thats not right.
The switched live to the UFH has been wired back from the timer permanent live. That is then switching back through the timer and running the main pump.
Should be in the ‘Heating on’ terminal.
Easy enough fix. Just one wire out of place.
Take a snap with timer front removed and I will tell you what wire to move.

Not a plumber… the firebox in the boiler will only heat one circuit, so there must be a heat exchanger somewhere to transfer the heat from that primary circuit to the UFH circuit. Is that in the manifold? Wherever it is, it wouldn’t be a surprise if the pump has to run to circulate water in the primary circuit.

There is a motorised valve on the manifold too, so I would imagine when the UFH is commanded to come on by the stat the motorised valve actuates to open so the water can flow from the boiler. If the UFH is not on but the main CH is the valve would be closed so the circulating water doesn’t go from the boiler through the manifold and into the UFH circuit.

I was sort of assuming the water in the UFH circuit was closed and just continually circulated through the boiler then the UFH circuit, but I guess if it is connected to the rest of the system then maybe the upstairs pump will need to circulate water around the whole system. Clearly the boiler firebox is heating water for both the UFH and CH if both systems were on at the same time. The boiler is sized to run both simultaniously.

Thanks MellowYellow. I’ll try to get a snap later.

There are two ways for underfloor heating to be taken off the ch system one is off the primary’s that’s sounds like what you have
The other is from the rad circuit when the rad gets hot there is a switch that starts a pump and heats the underfloor
both need there own pump.
The boiler will need to run and the boiler pump for the underfloor to work if fitted corectly the underfloor pump is only meant to push water through the underfloor cuircut and not pull from the boiler.
The odd thing is you say you have a mid position valve so when the boiler is working that valve will always be open to either hot water or heating ? Have I got that right ?

Also you say that there is only a stat ? A stat and timer would be much better

It’s not possible to answer your question definitively without knowing how your system is plumbed and wired. There are many variables and as you have admitted and shown by your misunderstanding of how UFH works, your knowledge is limited. Therefore, it’s quite possible there are components to your system you are unaware of or don’t recognise correctly.
However, If both pumps are running when rads and water are off and UFH is on. Check that there is no heat into the rads, check that the cylinder coil is not circulating and if this is the case, It’s most likely designed to operate as is.