UFH Cant get up to temp

[Edward]

Hi. I have UFH in my kitchen and it wont get to target temp of 18 degrees, max is approx 16.5 degrees according to the thermostat.

 

The set up is as follows: Gas boiler, Grundfos pump running from boiler to manifold about 10m away, 3 circuits off manifold (1 zone so no actuators), another pump pushing water through the system. Mixer value. High temp thermostat connected. The variables i can have had changed are the pump speeds, the boiler flow temp (reduced this slightly to ensure condensing occurs), the mixer temp and the flow rates which were set at 5l/m.

 

I first noticed the UFH didn't get up to temp when i turned the boiler flow down from about 65 to 57 degrees - which is as far as im aware still plenty hot enough to run UFH. I reduced the flow meters to about 1.5l/m as my understanding is a lower flow rate allows for a larger dT thus more heat output. However this was causing the high temp thermostat to kick in and shutting the pump off.

 

I have now increased the mixer temp to 55 degrees (not an actual measurement, just a number on a dial) and allowed the flow rate back to 5l/m. This appears to prevent the high temp thermostat from kicking in. However the room does not get above 16.5 degrees. I am beginning to think external environmental factor are the influence as the weather is much colder and the insulation underneath the (suspended) floor probably needs inspecting.

 

My question is what speeds should the pumps be set at, whats the optimum flow rate to allow for a sufficient dT? What influence does my mixer valve have on heat output? Any other suggestions would be welcome.

 

[Radian]

Two things: You might still be able to get boiler flow down a bit, try 60^oC (every little helps) the other factor is time - you don't say how long the UFH is programmed to be on at the 18oC set point. If this is less than 24/7 you've still got a control to tweak. If it is 24/7 you've got a big heat leak there and either give in and use additional space heaters during exceptional weather - or revisit your floor makeup as you said.

[ProDave]

Kitchens will be harder to heat with UFH than most other rooms because when you take out the area under the kitchen units and under the fridge, you could well be down to not much more than half the floor area being heated.  We notice the same in our utility room that it is the slowest room to heat up, but it does not bother us as it is never "cold"

[JohnMo]

Couple of things

Bigger DT means lower mean temperature, which means lower output.

 

Mixer like the flow temp 10-15 hotter upstream than downstream of the mixer.

 

Boiler condensing is a function of return temp not flow temp, if your return temp is below 53 deg your are condensing.

 

You say it worked, then you changed stuff, put it back the way it was.  If it reaches temp, all good.  Then change things one at a time.

 

If you reduce flow temp, to get the same heat out the floor you DT needs to reduce.  If you decrease boiler flow your mixer may need adjusted to keep.the same floor flow temp.

 

[Nickfromwales]

Are the pipes in aluminium spreader plates / screed / other? Can you give us more info please?

[Edward]

11 minutes ago, Nickfromwales said:

Are the pipes in aluminium spreader plates / screed / other? Can you give us more info please?

Foil backed polystyrene, I think about 50mm deep, sitting between suspended timber joists with 150mm insulation underneath. 
 

Is it an issue the boiler and pumps will be running between 4.30am and 8pm non stop whilst the system tries to reach temp?

[Nickfromwales]

21 hours ago, Edward said:

Foil backed polystyrene, I think about 50mm deep, sitting between suspended timber joists with 150mm insulation underneath. 
 

Is it an issue the boiler and pumps will be running between 4.30am and 8pm non stop whilst the system tries to reach temp?

Thanks.

Do you know if the upper side of the insulated foil panels are in direct contact ( under slight compression ) with or are bonded to the underside of the wooden deck?

If not, the heat transfer characteristics of this build-up will be poor. 

[Edward]

On 11/12/2022 at 13:35, Nickfromwales said:

Thanks.

Do you know if the upper side of the insulated foil panels are in direct contact ( under slight compression ) with or are bonded to the underside of the wooden deck?

If not, the heat transfer characteristics of this build-up will be poor. 

I dont know in all honesty. We didnt want to raise the FFL when installing the UFH so the plumber had to batton the side of the joists to put the foil panels in-between said joists. There is a chance your statement could be correct however. 

 

I seem to have fixed the issue somewhat by increasing the flow through the pipes to its max. I think this prevents the high temp thermostat from being triggered. Ive decreased the mixing to compensate for the lower flow temp from my boiler. All the literature i can find says the flow should be around 1.5 - 2 l/m but this just doesn't seem to work on m setup 

[Temp]

Can you measure the actual floor temperature?

[Radian]

On 10/12/2022 at 15:45, Edward said:

Is it an issue the boiler and pumps will be running between 4.30am and 8pm non stop whilst the system tries to reach temp?

Do you mean 'is this a bad thing?' In this cold weather it would not be surprising if it took 6 hours to reach setpoint. What your boiler is actually doing during this time might be an issue though. If this is the only heating to come on overnight the boiler might short-cycle too much if it can't modulate down low enough to keep the return temperature rising too far. You need numbers for all the parameters involved: temperature either side of mixer, boiler flow and return. for the required flow rate divide the total loop length by 40 to get litres per minute.