UFH Performance

UFH is similar in use to a heat pump, it uses lower flow temperatures and can be slower to react to temperature changes depending on the thermal mass it is heating.

It is also more of a radiant heat rather than convection that you get from radiators (odd given their name!).

It needs to be on for several hours, but because it is a lower flow it doesn't necessarily mean that the heat source is running continually. There should be a mixing valve that regulates the flow into the system which needs to be set according to the type of floor, unless you have genuine weather compensation that regulates this temperature.

Also it should not be turned off but rather the temperature 'set back' to 3-4° C below the comfort temperature.

Thank you. I’m aware it’s different to radiators and am not expecting it to be instant.

Really I’m just keen to understand if the performance I’ve described is normal or not, so that I’m armed with information when confronting the builder. Does anyone have UFH in a similar sized room and if so how long does it take for yours to heat up? To be on for 6 or 7 hours seems excessive to me but I dont know if that’s normal

That will depend on the type of floor the heating is in, density of pipe work, heat loss of room, the mean water temperature of the system, the design temperature of the room, the floor covering above the underfloor.The temperature you let the room  cool down too has a major effect too. So many variables but I would suggest that it tends to be on for long periods way longer than radiators.

And as I said it being on ie not reached temperature doesn’t mean the boiler is on for the same period.

We've an ufh system although it's not long gone in. Wet system, Crete screed, ashp.50sqm.

It's set about 16c as we are working in the house, fitting out. Once it reached temp mid morning it stopped and temp was maintained until tonight.

The house is insulated to buggery. Walls floor ceiling, triple glazed. Outside 0-4c

It's been a journey. Trying to find the sweet spot has been interesting and surprising. Currently we are running on stats, previously we were on continuous flow. We are on a triple dip tariff

I can't say which is cheapest/efficient yet but our heat retention is suggesting constant running is not for us but honestly, it's a bunch of variables to consider so try not to obsess about specifics yet!

Have you got some independent thermometers to compare?

And how well insulated/draught proof are you?

We set ours to 18-19c and basically leave it on and it just keeps a steady temp everywhere. Although our house is modern and well insulated if that makes a difference? It sounds like you have an issue (system flush) if you can’t get to that temp

You can’t treat an underfloor system like radiators as it takes a long while to heat up - it’s not quick response. 

but it’s hard to advise without knowing more. Overlay systems are normally used under wood. If you’ve got tiles or stone would expect there to be a screed under it. Which means more thermal mass/slower response.  Helpful to know your level of insulation, and what the glow temperature is. 

in our 10 y/o very well insulated house our UFh runs from 0430-2200 every day (and is setback to 15c target overnight so will sometimes still be running). The flow temperature varies based on outside temp (the colder outside, the warmer the water) but at 0C external it’s still only about 28 flow. And gets as low as 21C flow (target is 20c) when it’s warmer out. (It only switches off completely when it’s above 13c outside).

id say OP isn’t  running  system nearly long enough. If the floor has a good amount of thermal mass and a low flow temp (which it should be for efficiency) it might never properly gets up to temperature 

I’d give the same advice I did to my sister when she was faffibg with her heating in a new house. Leave it ion for long periods. let the thermostats do the work, see how much it costs you. It’s not going to bankrupt you in a couple of months. If yob’ve got a smart meter you can track your spend with the energy monitor they gave you. If not you can take manual meter readings and work it out. 

I had to tweak the flow (balance) to get it hotter which meant the 3 hours it was on in the morning really made a difference and last the day - i could adjust it at the manifold and there was a flow return valve near the boiler - i used a sharpie to mark the original position so could always return to that worst case

Thank you for the opinions on this,

To answer the questions about how I have things:

Firstly, I'm not really sure how to judge whether the insulation is any good or not. I suspect it could be better, but it's hard for me to tell as the loft directly above is boarded. There are 2 large windows covered by blinds, one tiny window with no blind and some bi fold doors. I suspect the latter two without blinds aren't helping, but I also suspect it's not disastrous for the insulation. It's all double glazed.

I have checked with an independent thermometer and the Hive thermostat in the kitchen seems to be pretty much spot on.

It's a concrete floor in one part of the kitchen and a suspended floor in the other part, roughly a 50:50 split.

b33k34 - that's interesting to hear that you think I might not be running it long enough. It just seems excessive to me to have it active and the boiler running for 6 hours plus a day.

As mentioned above the plumber did say to me in his own place that he never turns it off, and just sets the temperature to 19. His theory was (as I think you're suggesting) that basically if you do it that way, then you never allow the slab underneath to cool down, which means it basically warms up quicker. Nice idea but it didn't work when I tried it. As mentioned when doing it that way then it's on for 7 plus hours per day.

Any more thoughts welcome!

Cheers

You may need to adjust the flow on your loops at the manifold. They should be at a set level usually at the indicator. You may not be getting enough flow in the loops that aren’t warming up.

from my own experience, small improvements in insulation and draughtproofing can have a significant effect. In my previous house we went all in on incremental gains and the difference was massive. So a bit more and better loft insulation, all draughts eliminated, better curtains etc. I expected modest improvements but it was night and day, and very noticeable. I would be suspicious of a boarded loft - you really want loft legs underneath the boards with additional insulation in the void they create.

An architect friend said that if you add up all the little gaps and cracks around doors and windows, skirting and floorboards, you'll end up with an area the size of an open window. It's a good thing to bare in mind when you are thinking about heat loss

Anyways

How new is the system? Has the screed dried fully? (apologies if that isnt what you have). That will suck heat to dry.

How many days have you been running it? Getting the slab warm wont be quick from cold

constant running vs on/off + heat loss is the question. It's not possible to answer that, only you can work out whats best. You might find you prefer one way even if it's slight less efficient.

With an ashp we are dipping into cheap tariff periods so finding out if it is better than constant running through expensive periods but not running the pump as hard, only topping up. As you can see, endless variables. If you have a boiler I assume flat rate?

Reaction times are way slower than radiator systems, thats a given, But they are impacted by slab depth. We actually reduced our depth to get slightly quicker reaction times and gambled slightly on heat retention being good - so far it looks like it has paid off. As I said we have super insulated - so much so that we are mildly concerned about overheating from our solar gain via french windows.

We have UFH in the main kitchen area (20m2) and we just use it like a normal wet heating system. On at 0400, off at 0600. On at 1600 and off at 1730. Temp set to 19C.

Insulation of the house is very average. The big difference between ours and yours is the floor covering. The pipes are set on top of 100 mil PIR with a 30 mil wet screed and then LVT.

korman - thank you, although unfortunately I think going with the loft legs is pretty much out of the question. We have a really low pitched roof for some reason, I already hit my heat too often and the loft legs would make it even worse.

Unfortunately I don't really know about the slab depth. In one half, it's a suspended floor, so I guess that's not really a slab. In the other area (half the room) there is screed but I'm not really sure how much. We had old tiles ripped up as part of a kitchen remodel, then a small layer added, then the overlay boards etc, then a layer of self levelling, then the ditra mat and finally the tiles. It's all been down over a year, but I've never been happy with it. I'm still in touch with the builders and their plumber, they've had a look and don't reckon there is anything wrong with it. Hence the idea of trying to understand what's a reasonable benchmark. We do just have a flat rate with the energy company.

Marko - thank you this is also helpful. Running for 4.5 hours is more in line with how much I had assumed we would run it for, although as you say the floor areas are different.

We have a really low pitched roof for some reason, I already hit my heat too often and the loft legs would make it even worse.

How much time do you spend in your attic to prioritise access over good insulation ? 

If it's that bad I'd just write it off as storage and then your only up for water/electric/roof issues so the banging your head goes away. 

You can't use underfloor heating like you would regular radiators run off of a boiler. The system is designed with a 'long and low' heat in mind. Gas boiler with radiators will heat and cool the air temp fast in your room, underfloor heating will very slowly heat a thermal mass (your concrete floor) and that will radiate heat in to the room, it heats up and cools slowly. Because the heating element, which is a great bit long pipe in your floor, is so big, it can be run at much lower tempuratures. Meaning the heating will be on for a long time but at a low tempurature. It is much more efficient in these systems to leave the heat on, the main energy required is in the initial heating of the therman mass, so on the other hand letting the slab cool down and heat up and cool down will cost you much more. 

Several people have said similar things but seems like you aren't listening and you just want someone to tell you you can run it like you would run older radiators heated from a gas boiler and just have it on for a couple hours now and then when you want it to be hot. Sure you can, but it will most likely be hot some hours after you wanted it hot. 

The best thing you can do to reduce the amount of time it is on now is reduce heat loss in your house through air tightness and insulation. 

Otherwise here are some things you can do to adjust how much energy your heating is using: 

- turn the thermostats in the rooms down

- Adjust the flow rate on your manifold for the floor heating

- If you have an adjustable pump it usually has different speeds and you can turn that down a level or two

- Turn the tempurature down in your hot water tank by a couple of degrees

- We don't know what gadgets or valves your floor heating has on it so perhaps there are other things. 

It will take some fiddling and monitoring with apps, smart meters or thermometers in the room etc to figure out what is most effective. 

Leave it on and let the thermostat do its thing. Try and get an idea if the boiler is short cycling. Is there much insulation under the floor. If there is hardly any under your suspended floor you will just be warming the air in the void below.

You can't use underfloor heating like you would regular radiators run off of a boiler.

Yes you can. That is exactly how we run ours. There is 100 mm of PIR below the pipes and 50 mm wet screed above, so not some huge mass that needs 'long and low' heat. Flow set at 40 C. Admittedly, when we turn it back on after the summer it does take a time operate just like a standard wet heating system. I assume that the screed retains some heat between cycles.

Room is at 19 C as I type this, outside 4 C. UFH has been on for 1.5 hours and now off. It will come back on at 04.30 for 2 hours.

N.B Standard oil fired heating, not Air source.

That is exactly how we run ours. There is 100 mm of PIR below the pipes and 50 mm wet screed above

Cool so the op just needs to rip out his floor and redo it and all will be good ? 

I have similar in my bathroom. Its in insulation directly below the tiles. Its plumbed into a trv hidden in the cistern cupboard. 

Takes about 10 minutes to warm up which is great in the morning when you step out the shower..... Cools down as quickly though but that's ok as I don't tend to sit in the bathroom for very long 

Cool so the op just needs to rip out his floor and redo it and all will be good ? 

You what? 

I'm assuming that the OP's UFH has been correctly installed. 100 mm PIR under the screed is the minimum needed to meet the building regs. It is a while since I did ours so things may have changed, but the wet screed on ours is only 30 mm (UFH pipes are 15 mm) and it still has enough thermal mass to keep the floor warm (well slightly above ambient) until the next cycle.  

Have we confirmed if the HIVE thermostat is set in a learning mode - i.e. is it set to be at a temperature at a set time, thereby adjusting the heating up period (which will be longer than the 3 hours you set), or is it set to "just turn on" at set time and that's it??  My system (Heatmiser), works with the modulating system in the boiler and the external thermostat to adjust the heat up time based on how cold it is ouseide and how long that room then take to reach the set temperature.  So, sometimes it will turn on for a realtively short period on warmer days, but when proper cold i comes on hours before hand.

All the other points around flow temps and flow rate are really important, especially around thresholds.

 Building regs is min 65mm screed from memory unless you used a poured screed

op states he has overlay UFH , so his slab could be anything . up to and including some of  the 90s we were pouring slabs with zero insulation...... 

insulation makes a hell of a difference - ive 180mm in my dining room extension slab  and the rest of the house is suspended concrete floor. you can tell when you walk from the kitchen to the dining room .... even without UFH 

@trail_rat

Good point. I just assumed overlayed over an existing insulated floor. If it is straight onto a basic concrete slab, then get it on 24/7.

@Bear. We have a 'wet screed' 30-40 mm from memory.

Does the '@' function work on the all new singing and dancing forum?

seems like you aren't listening and you just want someone to tell you you can run it like you would run older radiators

Incorrect. The very reason I posted the question is to listen to others, as I don't know much about the subject. I haven't once mentioned radiators and merely want to understand what the benchmark should be for my size room so that I can hold the builders to account. If the benchmark is that it should be on 10 hours a day that's fine, if it's that is should be on 4 hours a day then that's also fine, I just want to know if it's normal or not. Basically I want to understand if there is something potentially wrong with the UFH install.

trail rat - you could well be right about this. I guess insulation is supposed to be under the concrete slab, but as it's an old existing concrete slab then I have no idea what's under there.

ajc - I don't think the boiler is short cycling, but I can give that another go. Basically if I set the UFH to constant and 19 degrees, then the boiler is on constantly to get that temp for a decent chunk, e.g. 4-5 hours solid from memory. Once it gets there then the boiler stays off for quite a while. The reason for this is that the Hive stat stops at 19 degrees, but the room keeps heating a bit and peaks at 19.5 degress ish. There is then another good chunk of the boiler being off, e.g. 3-4 hours, then the boiler kicks in again once the stat drops below 19 and is then on again for another chunk.

I also forgot to say, the mixing valve at the manifold is set to be approx 50 degrees, but I don't want to go any higher than that.