new underfloor heating

24 Nov 2008
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United Kingdom
Sorry if this is a bit long but it needs some describing. I recently had a simple extension and kitchen built across the back of my house and chose underfloor heating at the suggestion of the heating engineer. However, it is quite 'problematic' at times and I'd like to know if it can be sorted.

Basically, we had a new boiler fitted to our existing standard gas fired central heating system (radiators and water heating). The underfloor system was then connected into the existing radiator system with it's own pump and thermostat in the extension. This thermostat only controls the switching on/off of the pump while the existing thermostat in the lounge controls the switching on/off of the boiler.

What happens quite often is that the temperature of water in the pipes leading to the underfloor pump gets so hot (>50 degrees C) that the pump will not switch on anyway. It has it's own 'regulator' built in to stop the underfloor pipese getting too hot. This happens particularly when the water heating has been on too. To get the extension system pumping I have to turn off the boiler via the lounge thermostat (and turn water heating completely off) until the pipes leading to the underfloor pump have cooled down enough for the pump to switch on. I then turn up the lounge thermostat so that the boiler comes back on. It'll then run until the underfloor pipes get too hot again and I have to turn it off via the lounge thermo again.

I'm going to try changing the heating on/off times so that the heating comes on before the water heating does. I'm hoping the underfloor system will have a chance to start circulating before the water temperature gets too hot. Alteratively, do I need a separate boiler for the underfloor system ?
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Is there a pipe stat fitted perhaps switching out the UF pump when the temperature rises to high?
A mixing valve is the usual method with UF heating to blend the water down to a safe temperature.
Yes. As it was explained to me, there's a separate thermostat that makes sure it can never pump in water at over 50 degrees. I don't think there's any 'mixer valve' attached to keep the water at a safe temperature. Would this mix in cold water to keep it below 50 degrees ? Do you know what one might look like and where it might be situated. This seems like a better idea than the pump simply switching off. It doesn't sound too expensive to get installed.
normally on these systems they have trv style head to regulate the flow into the underfloor heating section. If so just turn this down to about 45 degrees and it should sort the problem out.
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The ones I usually see look like the blending valves used on washroom basins.

I don't think there's any 'mixer valve' attached to keep the water at a safe temperature.
There's your problem then.

Would this mix in cold water to keep it below 50 degrees ?
Not exactly - it would blend the flow water from the boiler with the return from the underfloor circuit.

Do you know what one might look like and where it might be situated.
It's commonly integrated into the manifold.
A pipe stat controlling the UF pump is not a conventional method though I don’t see why it wouldn’t work. You might have on/off cycling when the floor slab rises in temperature. A pipe stat with adjustable differential setting might reduce this cycling.
A lot of installers fit a high limit pipe stat set to about 15c higher than the design flow temperature in case of a malfunction and the design flow temperature being exceeded. A safety device in other words.

How is the UF loop connected to your existing radiator system?
My reckoning is that the connection is direct to the primary flow via closely spaced T’s. (But I could be wrong)
This means that when the normal room stat switches out the boiler , then no hot water from the primary circuit is available for the UF circuit.
Do you find the room will not heat up?
Ideally the UF circuit needs its own primary circuit independant of the radiators due to the heat up time lag.
Danfoss have a valve (return temperature limiter) which is connected off the rad circuit but having no experience of it I cannot say how well it performs. I think its only a recent controller launched into the UK market.
There is only one option - the u/f circuit needs to be on its own zone, or it won't work properly.

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