Underfloor heating issue

Calidad

Hi,

In a new build with a wet UFH system installed on the ground floor. Have never paid too much attention to it (it barely been on until now) since we moved in during the Spring. We have three zones on the ground floor, and according to the thermostats the temperature rarely drops below 19 degrees even when the UFH is off. 

However, when we turn it on it quite quickly climbs to 21 degrees, but if we have say set it to 23 degrees it climbs to about 22 degrees then starts to stop in temperature even though the UFH is still on. This is happening in all three zones. 

The pipe in the UFH heating is hot and the actuators seem to engage and you can hear the system “working” but we can’t figure out why it gets to about 22 degrees and then just drops back down to about 20.5 degrees. The house is well insulated and it shouldn’t struggle to get to 23 degrees, the upstairs with radiators hit that temperature very quickly. 

We have a gas boiler (system). 

We also never feel the UFH underfoot. 

Any ideas? I guess we’ll go back to the developers, but wondering if we’re missing something obvious. 

ritz55

Just had the same on a new renovation. First time with UFH. Builder told us to stick it at 35 degrees for 24 hours and after that we could feel the floor getting hot and the house being super hot. After that we turned it back down but when the heating comes on even if we let the stat to 23 degrees we can feel the heat now. 

Maybe the first time is to get the concrete slab warm enough so it can retain the heat. 

ThisIsWeird

I wonder if the room stats used in your modern system will have two sensors, one 'air' and one 'floor'? Is it digital? Does it mention this on the display?
If it has, then perhaps the two are working together to get to the requested temp in the best way possible, with the floor one, say, throttling back the UFH so it doesn't overshoot, something like that?
Don't forget, any change in temp you make will take hours to manifest itself.
Mil has 20-year old UFH with only air-sensing analogue room stats. When it's set to 21oC, the floor can be felt if bare foot. Below this temp, the floor isn't necessarily 'warm', but it's obviously not cool or cold either, as an unheated floor would be - it feels mild. Very pleasant.
Bear in mind that if mil slammed her room stat up to, say, 23o, an air-sensing stat would only turn the heating off when it reached that room temp - by which point the floor slab will be scorching, and will still be giving out a large amount of heat for hours afterwards. So the room's temp would soar way above 23o in her case. 
It may be that your system is more sophisticated, and is raising the temp incrementally, monitoring both the pipe and air temps to ensure no overswing.
What actual temp do you want? Set that temp, and give it a day.

Calidad

We can’t seem to get it up to 23 degrees. 

We turned it on at about 4PM today when it was already above 20 degrees. It quickly went up past 21 - I think has high as about 22.4 then started to drop back down to between 20-21 I then turned it off. 

Hour later it’s gone back up to 21.7 without being on. I really can’t figure it out. I appreciate it’s a gradual heat, but what’s strange is the temperature dropping while it’s on. 

We have different temperature settings for different times of the day, I guess we could set a constant (say 23) to see if it reaches that. 

We only seem to have a setting that states room temperature on the thermostat, which are digital. 

ThisIsWeird

Try setting the required zone to 23o (that's pretty high - are you sure you want it there?), and don't touch it for a day. 24 hours.

Could we have a close-up pic of your digital stat, please?

daveyjp

What flow temeprature is the boiler set to?

Calidad

Was just setting it to 23 as wanted to see how quickly it could heat up - wouldn’t be looking to keep it there. 

ThisIsWeird

Calidad said:

Was just setting it to 23 as wanted to see how quickly it could heat up - wouldn’t be looking to keep it there. 

Thanks.

Ok, that 'stat doesn't suggest it also has floor temp sensing, only 'room', so I'm going to assume it doesn't have a floor sensor - but I don't know, it might have. But, if it is only 'air', then it's mangled my earlier assumption that it could be the system's way of getting up to temperature deliberately 'slowly' and controlled, so as not to overshoot.

The only way to test if it'll get up to a higher temp - 22 or 23o - is to set it there - and wait. You need to give it a good half-day or so, or else it's pointless.

Was amused/annoyed by mil's 'control' of her UFH when she first moved into her bungie a few years back. She just would not listen to our exhortations to not keep fiddling with the 'stat, and to also not expect to be able to turn it down at nighttime and back up for brekkie; she understood how it worked, and that it would take around 6 hours to respond, but she just couldn't help herself... :-)

Took - literally - months. But it's now the best thing since sliced whatsit.

Try it, and give it time.

Bottom line, if this is a conventional UFH system with the pipe embedded in a screed layer, then it will take many hours to respond to a changing temp request, so 'turning it up and down' just ain't going to work. However, it may be dry-laid - whatever it's called - with the pipework laying immediately below the floor layer, in which case heat transfer would be faster. "We have different temperature settings for different times of the day"; you need to take the time lag into account with these timings. Quite literally, if it takes 3 hours to respond, you need to factor that in! Or 5, 6 whatevs. 

If you have it at, say, 18oC, and then turn it right up to 22, then it isn't going to start getting the room up to that new temp until the heat has percolated its way up from the embedded pipes, which typically takes a few hours. If you turn it back down after, say, 2 hours, frustrated that the room temp hasn't increased, then - guess what? - in a couple of hours the floor will be warmer, from that earlier called-for heat. You'll be like Nan - chasing her tail trying to get the room to do what she wanted. "I guess we could set a constant (say 23) to see if it reaches that." That's worth a try, but you need to give it time.

Ask the builder - they must know what the response time is, and how best have different temps at different times. If mil wanted her house cooler overnight, then she'd have to trun it down a good 4-6 hours before bedtime - ie late afa'/early eve - and then back up in the middle of the night! 

If you try the all-day 23o test, and it just doesn't get up there, then Daveyjp's call sounds good - the boiler flow temp/manifold blend might be set too low. Bear in mind that the lower the flow temp, the more efficient the boiler runs, so it may have been deliberately set that way.

ComicGeek

Check to see if you have a blending valve on the underfloor heating manifold. Ideally this would be set as low as possible while still meeting the temperature in the house - which it sounds like it's doing well, as it's reaching 21 degrees.

A really good quality new build could have the underfloor heating manifold set at 35 degrees flow - by the time the water has been pumped around the floor, the floor surface temperature may only be 22-24 degrees, so you won't notice that it's hot when walking on. That is a sign of a well designed and well installed system, not a fault. Nice even temperatures throughout. 

You only need to change it if it doesn't get to 21 degrees on the coldest days, but even then you can turn it down when it's milder.

There will be a deadband in the controls, so that the heating turns off before it exceeds the target temperature (to stop it overheating), and then delays the heating coming back on until the temperature has dropped. Stops it turning on and off too frequently. You can change the deadband in the thermostat, but I don't think you can reduce it less than 2 degrees. So if you set the target temp to 23 degrees, it will heat up to 23 and then turn off - the heating won't turn back on until it drops to 21, which seems to be what is happening.

ComicGeek

Also don't just base it on the thermostat reading, that's only picking up air temperature which isn't the best way of measuring comfort. If you're not thinking about being too hot or too cold then it's the perfect temperature, regardless of what number is showing on the thermostat.

FreeBear

ThisIsWeird said: Was amused/annoyed by mil's 'control' of her UFH when she first moved into her bungie a few years back. She just would not listen to our exhortations to not keep fiddling with the 'stat, and to also not expect to be able to turn it down at nighttime and back up for brekkie; she understood how it worked, and that it would take around 6 hours to respond, but she just couldn't help herself... :-)

Left the old thermostat on the wall when I had a new heating system. I'll tell the lodger she can turn the dial to her hearts content. Meanwhile, actual temperature settings are determined by the home automation system.