UH - Thermostat won't reach set room temperature in colder weather.

[nathan656]

Hello all,
 

First-time poster here, we have a wet underfloor heating system on the ground floor with laminate on top of screed. We have a Vaillant condensing boiler.

We prefer our house warm, leaving the thermostat at constant 22.5 degrees Celsius throughout winter. We're in London and the UH performs well in moderate temperatures. However, when it drops below 5 degrees Celsius outside, the system will run continuously, with the thermostat reading room temperature of 21.5 Celsius without ever reaching the set temperature.

I've ensured all settings at the manifold are correct, with a flow temperature of 58 degrees Celsius going into the loops (yes I know that is high), max power setting on the pump, and flow rates slightly over 2 litres per minute. We have a Heatmiser thermostat, and using the app I can see continuous running between 11pm-4am in colder weather. The thermostat is located in the downstairs hallway due to an open plan layout, and moving it would be challenging as it is hardwired.

I tried lowering the set temperature at night, which stopped the continuous running, but then it would take around 6 hours of continuous running to get back to 22.5 in the morning, so nothing gained. In summary, during temperature drops, the UH lacks power to meet demand, causing extended warm-up times.

Could the proximity of the thermostat to front door be causing a false reading, or perhaps I need to increase the already high flow temperature? Any insights would be greatly appreciated.
 

Edited February 26, 2024 by nathan656

[dpmiller]

what is the return temperature from the loops?

[OwenF]

A few points:

Try a stand-alone thermostat somewhere away from the front door - is that getting close to your target 22.5?

Regardless of what the thermostat says, are you comfortable? 
 

UFH that has been thrown in tends to have wide pipe spacings. It seems to be a common occurrence that they just cannot produce the watt output to meet the heat loss. Not much can be done about that without ripping out.

 

 

 

[John Carroll]

26 minutes ago, dpmiller said:

what is the return temperature from the loops?

 

That's important but one might expect ~ a 10C/12C dT so returns around 48C/46C.

Each loop will then, at a 2LPM flowrate will emit 2*60*11/860, 1.53kw, gives some idea of what you might expect depending on number of loops.

[JohnMo]

7 hours ago, nathan656 said:

max power setting on the pump

Your pump should set to match the flow rates required. Turn it down until you see the flow meters drop, then set it one speed higher.

 

2l/min isn't a high flow rate, tweek the flow rates up where the area served by the loop is too cool.

 

Thermostat should be away from any outside influences so by the front door may not be the best place.

7 hours ago, nathan656 said:

can see continuous running between 11pm-4am in colder weather.

Don't see that as an issue, if you have a modulating boiler the boiler modulates to match demand, if the boiler was perfectly sized and flow temp was set correctly the boiler would/should run all the time.

[ProDave]

If I am reading this right, the ONLY room thermostat is in the hall which is often a cold a draughty place.

 

What is the room temperature in the living rooms?  I take it there are no individual room thermostats as is sometimes common with UFH.

 

My point being if the rooms are warm enough, and it is just the silly placing of the only thermostat that is the problem then you need to move the thermostat.  You can get wireless thermostats if re routing the wiring is too disruptive.

[nathan656]

4 hours ago, OwenF said:

UFH that has been thrown in tends to have wide pipe spacings. It seems to be a common occurrence that they just cannot produce the watt output to meet the heat loss. Not much can be done about that without ripping out.

 

See attached pics of our pipe.   Is this how it's supposed to look?

 

4 hours ago, dpmiller said:

what is the return temperature from the loops?

 

I don't know as there is no temperature gauge for the return, but they don't feel hot to the touch.

 

2 hours ago, ProDave said:

If I am reading this right, the ONLY room thermostat is in the hall which is often a cold a draughty place.

 

What is the room temperature in the living rooms?  I take it there are no individual room thermostats as is sometimes common with UFH.

 

My point being if the rooms are warm enough, and it is just the silly placing of the only thermostat that is the problem then you need to move the thermostat.  You can get wireless thermostats if re routing the wiring is too disruptive.

 

Correct, three UH zones linked to ONE thermostat which is in the hallway.   Can you recommend any smart wireless thermostats?

[JohnMo]

15 minutes ago, nathan656 said:

Can you recommend any smart wireless thermostats

Not sure why you need smart for UFH, but the Computherm RF range are good.  I have Computherm Q20RF in my hall (£60 from Amazon). Nice solid feel, easy enough to setup, can select a hysteresis of 0.1, which is great for UFH.

[SteamyTea]

You make no mention of the boiler size, or the expected heat loss of the building.

 

[Lofty718]

How much insulation is under there? It looks like 40mm

Edited February 27, 2024 by Lofty718

[nathan656]

On 27/02/2024 at 20:33, Lofty718 said:

How much insulation is under there? It looks like 40mm

I'm not a builder but yes I think it's 40mm

 

On 26/02/2024 at 12:08, SteamyTea said:

You make no mention of the boiler size, or the expected heat loss of the building.

 

The house is semi detached 4 bedroom, new insulation on first floor joists and roof.  Wouldn't expect a more than normal amount of heat loss.   Boiler is a Vaillant ecoTEC plus 630. 

 

On 26/02/2024 at 07:10, OwenF said:

A few points:

Try a stand-alone thermostat somewhere away from the front door - is that getting close to your target 22.5?

Regardless of what the thermostat says, are you comfortable? 
 

UFH that has been thrown in tends to have wide pipe spacings. It seems to be a common occurrence that they just cannot produce the watt output to meet the heat loss. Not much can be done about that without ripping out.

 

 

 

Interestingly, the stand alone thermometer in the dining room is getting even lower readings.  The house is not cold, but not the cosy temperature we would like.   Looking at the pictures of our UH, would this be called wide pipe spacing ?
 

Edited March 1, 2024 by nathan656

[Andehh]

You might be hitting your limits here, 40mm isn't much insulation, and that is cheap stuff laid there. UFH pipes are pretty crudely laid.

 

22 degrees in a hallway would make me believe rest of house is probably warmer then that, esp upstairs.

 

Might need to run heating a couple hours longer during very cold periods

 

Replace laminate for tiles might get you an extra degree or two more.

 

Are you able to upgrade insulation anywhere else? Loft? Draft proving etc?

 

At the end of the day heat in vs heat out is a limit every house has at some eventual point, less insulation lowers that point.

Edited March 1, 2024 by Andehh

[Dave Jones]

fit some radiators.

[TonyT]

rough looking install unfortunately

 

maybe some picture of the manifold set up might help

[Temp]

Think I would try turning up the flow rate.

 

 

[Spinny]

Heatmiser do a wireless thermostat - the Neo air that is battery powered and wireless and connects to the Heatmiser hub. You could swap that in to control that zone.

 

Another option could be the remote Heatmiser wireless air sensor but I see some reviews say it may not be very accurate.

 

You could also try taking the floor temperature using an infrared gun thermometer - lots available on Amazon. There are also thermal cameras including ones that plug into mobile phones depending how much you want to spend. (Not going to solve anything directly just give you more data.)

 

Another idea might be some kind of space heater that you could plug in and get Smart Control over that you then run automatically as a top up when the outside temp is very low.