Underfloor Heating not warming room

[martin69]

I have a Polypipe wet UFH system which heats a large new extension (5 circuits off manifold) and an old study where the floor was relaid (2 circuits off manifold) with 100mm of insulation over concrete and under the screed. It works well in the extension, but will not heat the study.

The boiler's been running 24/7 for days and the room temperature refuses to lift above 15 degrees, in fact it's now fallen below that. It's about 4m x 2.2m, with 2 circuits of pipe, the temp on the mixing valve is set to 50c and the pump to 2. Flow rate is just below 2. The room has carpet and underlay on the floor (I checked and was sold special (more expensive!) stuff suitable for UFH). Admittedly it's an old room with solid walls and 2 windows, one of which is single glazed and bearing the brunt of the east wind.

The carpet feels warm to touch, but tbh there is very little difference in temperature between the flow and return pipes on the boiler side of the manifold, while the excess heat is gently warming the upstairs rads through a reverse flow on the return circuit.

I can't find any recommendations for the correct flow rate, but am wondering whether to try reducing it so the floor can absorb more of the heat - or is there a limit to how much it can hold and dissipate?

Any suggestions gratefully received thanks.

 

[ironsidebod]

When was the flow rate set? It's normal to have to adjust the flow when you go through the first cold period with a new system you would often need to increase it.

Have you measured the temp difference in flow and return or just feeling it? It should be around 8 deg difference. Might be your flow is too high and needs reducing a bit if F & R are too close. The difficulty with UFH is you have to tweak it and wait several hours to see if there is a difference, and keep fingers crossed the weather stays the same!! Obviously not a problem at the moment.

 

[martin69]

Thanks for the thoughts. Flow rate was set some time ago. I shut it back to around 1 last night, and there is now an appreciable difference between flow and return pipes at the manifold - I don't have the equipment to measure it. However the room is colder, so guess I may have overdone it!

 

[ironsidebod]

Yes quite possibly , UFH is sensitive to small flow changes, try 1 3/4 for a bit and if it's warmer try 1.5 and go from there.

 

[John D v2.0]

Reducing the flow is a waste of time and the room will get colder. Your problem is the heat loss to outside is greater than the heat transfer from floor to room.
You either need to increase the room insulation, decrease the carpet insulation value, or fit secondary heating.
Or get a warm jumper!

 

[ironsidebod]

If the flow is too fast no heat is transferred to the room! It's a case of finding the point at which the room warms and then fine tuning to cope with the heat loss.
That said the OP does describe a room that will lose heat rapidly. No harm in trying to find a good flow rate.

OP: Get an infrared thermometer off ebay so you can test the floor temp, you'll be able to see what flow rate is giving you the highest temp. You can then deal with the heat loss in the room

 

[John D v2.0]

If the flow is too fast no heat is transferred to the room!

Actually the opposite.
If the flow is faster the outside of the pipe will be hotter because it didn't have as much time to cool down in the ufh pipe.
A hot surface emits heat faster than a cooler one.

 

[Dan Robinson]

Heatloss is to great. Chances are the system is inadequate for the situation.

 

[ironsidebod]

Actually the opposite.
If the flow is faster the outside of the pipe will be hotter because it didn't have as much time to cool down in the ufh pipe.
A hot surface emits heat faster than a cooler one.

Fair enough, our system was screwed up when we moved in. One of the smaller rooms definitely heats slower with more flow, took some balancing, the larger rooms absolutely need more flow. Just going on my experience of that.

 

[Dan Robinson]

Your experience is totally misunderstanding the reality.

 

[John D v2.0]

It's just about possible if you stretch imagining a little, but depends on the feedback loops involved.
EG If that rad has the lowest resistance and is throttled back a lot, there will be enough head to take the water the long way round the system therefor keeping the boiler firing on high and a good flow temp. If the rad is opened all the way, maybe it will just short cycle and the boiler will cut out and go into anti cycling leaving no hot water into the rad for a certain time period.

 

[ironsidebod]

Your experience is totally misunderstanding the reality.

Fair enough. I'm clearly not a heating engineer. But having to sort out an UFH system with none of the original paperwork/design flow etc has been interesting. It seemed like the smallest room that has only one loop fails to warm as quickly with a flow of 2 than at 1.25, if you're saying that's impossible, great, you know far more than I do but why would it seem that way?

 

[John D v2.0]

With respect, Dan is just stating the physics. It's hard for anyone on here to speculate on why you're seeing something that appears to defy. Scientists spend a lifetime doing experiments to narrow these things down.

 

[ironsidebod]

With respect, Dan is just stating the physics. It's hard for anyone on here to speculate on why you're seeing something that appears to defy. Scientists spend a lifetime doing experiments to narrow these things down.

Well yes I appreciate that, I get what's being said, I suppose it was more of a rhetorical question than anything