Rads upstairs cold when underfloor heating on downstairs

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I have an unvented oil-fired system. Downstairs is underfloor, upstairs is wet rads. The problem is that when I have both downstairs and upstairs heating on, the rads are cold upstairs. When I turn the underfloor off, the rads upstairs get hot.
 
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I have an unvented oil-fired system. Downstairs is underfloor, upstairs is wet rads. The problem is that when I have both downstairs and upstairs heating on, the rads are cold upstairs. When I turn the underfloor off, the rads upstairs get hot.
This is where a diagram of the system would paint a thousand words.

Is this a new install, or one that's been working for a while ? If it's been in for a while, has it always been like this, or just started ? When you say the rads are cold, do you mean completely cold (no heat at all), or they only get lukewarm ? Does it still have the problem if you (temporarily) turn off most of the UFH loops ?

I can think of a few possible reasons :
- There's a lack of flow through the boiler, and when the UFH is on it's drawing more flow than is available and so starving the rads of flow.
- The rads are supplied through a motorised valve, and there's a wiring issue so that the valve shuts when the UFH is on.
- There's a 3-way motorised valve, and it's missing the upstairs signal and so winding to UFH only when both are on.
 
It's been working fine for 7 years.
The rads are completely cold apart from one or two that are lukewarm.
I haven't tried tempoarily turning off the loops (not sure how to do this tbh)

There is a motorised valve through which the rads are supplied and it only shuts when the timer 'tells' it to.

Hope this info helps
 
It's been working fine for 7 years.
So that rules out a design problem.
The rads are completely cold apart from one or two that are lukewarm.
Sounds like there is still a path available, just no flow.
I haven't tried tempoarily turning off the loops (not sure how to do this tbh)
I've only ever seen a couple, but some manifolds have a small ball valve on each branch and if you turn off one valve on each loop it will restrict the flow through the UFH system as a whole.

Another though is a mixer valve failure on the UFH system. If that failed in the fully hot position then it would taker a large flow rate from the system - as well as overheating the floor and possibly causing damage. There is probably a thermometer on the manifold which would allow you to check that.

Or the boiler O/P is low, it can't maintain temperature, and so the UFH is compensating by taking more flow.

Or the main circulator pump is failing and reducing the available flow rate.
 
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Thanks. Doesn't sound like the easy solution I was hoping for.

Thinking about it, the hot water isn't as hot as it used to be and the radiators aren't hot hot, only very warm when I switch off the UFH. They used to be very hot, even when they were on with UFH and hot water. Sounds like it could point to a boiler problem?
 
Thinking about it, the hot water isn't as hot as it used to be and the radiators aren't hot hot, only very warm when I switch off the UFH. They used to be very hot, even when they were on with UFH and hot water. Sounds like it could point to a boiler problem?
Those new symptoms do suggest that.
 
Sounds more like a circulatng pump problem. Is there a separate pump on the UFH?
 
Yes there is a pump for the UFH and a separate pump for the upstairs rads. Both pumps spinning.
 
The upstairs rads have been on all day on their own i.e. no UFH or hot water and some of them are luke warm at best.
 
Do you have thermometers on the UFH? If so what is the temperature? How hot is the boiler flow pipe?
Does your boiler heat a thermal store, or does the boiler feed the radiators direct?
Is the cylinder on the same circuit as the upstairs rads? If so, is there a 3 port valve controlling the flow?
Is the hot water hot?
 
Don't have thermometers on UFH, just a thermostat that's set to 40 degrees.
Boiler does have a thermal store
Cylinder on separate cicuit to upstairs rads
Hot water is hot
 
New pump required on rads by the sound of it.

Nb: this simonh2 fellow sounds a bit inexperienced, to put it politely.
 
But why would rads be stone cold when UFH is on but some of them get hot when UFH is off if it was a pump issue?

I am completely ignorant by the way so forgive me if it's a stupid question
 
But why would rads be stone cold when UFH is on but some of them get hot when UFH is off if it was a pump issue?

I am completely ignorant by the way so forgive me if it's a stupid question
The only stupid questions are the ones you don't ask - and so don't find the answers to.

If the flow rate from the main circulator pump is low, then the UFH pump will take all the flow and none will go to the rads. Similarly, if the boiler output is low (not hot) then the UFH thermostatic mixing valve (TMV) will open up so that the UFH takes a lot of flow from the boiler rather than circulating mostly around it's own loop - and so taking all the flow from the main pump.

A third option is that the UFH TMV is faulty and stuck in the full heat position - that would also take excess flow from the boiler, but would overheat the UFH system (possibly causing damage) and make (in this case) the downstairs rooms quite warm.

Whatever the cause, there isn't enough flow through the boiler to satisfy the UFH and the rads. Since it's been working OK until recently, then the assumption is that the system design is OK and something is faulty. Either the main circulator pump is not performing, or there is a blockage (either of which would restrict the flow rate available), or the boiler output is down and that's resulting in the UFH taking a higher flow rate to compensate.


Just in case you're not aware, UFH typically has it's own circulation - with a pump and TMV, plus manifolds to distribute the water to different loops, and collect the return flows (my brother's new setup has 16 way manifolds !) The TMV sets the temperature in the loops, and the water mostly circulates round and round in the UFH system - with enough hot water mixed in from the boiler circuit to maintain temperature. That way, you get a relatively uniform heat across the whole floor - rather than a hot spot at one end of the pipe, getting progressively colder towards the other. The temperature is much lower than is normally used in radiators.
 

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