What is the most efficient way to run wet underfloor heating?

Jenny7026

We bought a house last year, which has:
Downstairs: wet underfloor heating, with analogue, non-programmable thermostats on the walls of the three rooms.
Upstairs: radiators with thermostatic radiator valves.
The whole system is run off a combi boiler, and a single programmable timer unit for absolutely everything - either the whole house is on, or the whole house is off, subject of course to what the TRVs and thermostats are doing. When the house was originally renovated the upstairs radiators actually had programmable TRVs on every radiator, but the next owners took them off and put standard TRVs on instead.

We have never had wet underfloor heating before, and additionally the last house we owned had multiple heating zones with digital thermostats, so we only ran certain rooms at certain times and had very low energy bills. My partner works from home downstairs so uses the UFH during the day, and then we mainly need heat from the radiators around bedtime. The issue we have is the UFH is slow to heat up, so the upstairs radiators have to run for a lot longer than they are needed. For example, we either have to run the bedroom at 19 degrees for the whole time the UFH is heating (waste of money since we don't need it at 19 degrees unless we're actually in there) or we have to remember to come upstairs and turn the radiator up at the point where we actually want the extra heat, and turn it back down again afterwards.

I have read things saying UFH should be on all the time, just with the thermostats to regulate it, and other things saying it only needs to run for a few hours, consistently (avoiding it getting really cold). Which is true? And if the former is true, is there anything much I can do to avoid wasting energy on the upstairs radiators for all that time? We have a single remaining programmable TRV that we could put in the bedroom if it would help, and we would also potentially consider bigger changes to the thermostats and timer if it would help in the long term. Some days our energy use has already been ridiculously high and I want to get this sorted before we lose our fixed tariff and everything gets even more expensive!

getmore4less

Programmable thermostats on the radiators.

Another strategy might be turn upstairs off.
Keep the doors to all rooms shut.
Open the doors in the evening to let the downstairs heat go upstairs.

Then the UFH will restore down stairs overnight.

NSG666

It might be worthwhile getting a heating engineer to look at your system as a better understanding of the existing pipework layout and access might give other options to the programmable thermostats e.g. putting zone valves into the existing set up and changing the controls.

If you phone an engineer to get your boiler serviced and advise on the system i means the engineer is getting paid for the visit and more likely to attend and spend a bit of time looking at it for you. 

Jenny7026

Thank you, these are really helpful suggestions! We will definitely put the programmable TRV in the bedroom in the meantime as that's the only room upstairs we really care about getting to a higher night temperature, everything else can run low if need be. I hadn't considered a heating engineer looking at it but that's a really good idea as we have zero documentation for this system from the previous owners (we didn't even know for sure it was wet until we finally found the manifold in a hidden cupboard). We know the manufacturer's name but their website is pretty sparse.

So it sounds like we've got options for the upstairs - for the downstairs, does anyone know about the question of how best to run wet UFH efficiently? Ie on all the time, set times, etc? At the moment we seem to be playing catchup a lot (letting it get too cold before we turn it back on, so it ends up running for longer).

Bendy_House

Hi Jenny.

I'm afraid I know little about UFH, so can't advise on the best way to 'run' it. That info must be 'out there', tho'!*

You found the manifold? Good. In theory, the boiler 'flow' should be splitting somewhere around there to supply the UFH, and the upstairs. If you (your plumber...) can locate this 'tee' point, a zone valve can be fitted to control all of the upstairs in one go. This zone valve will be controlled by a Prog Stat mounted, say, in the main bedroom upstairs, and all the other bedrooms should have normal TRVs to set whatever temp they individually should be at.

That is probably the simplest and cheapest way to sort out the upstairs. The ProgStat will get it warm in t'morn, and before bed, and maintain whatever other temp you want in between times - say a min of 16oC overnight, but 14oC is fine for during the day (ie, it'll rarely come on at all then). A 'Smart' ProgStat (wiser, Hive, Nest, etc) will also have App control, so you can override this at will.

The UFH (that's great, by the way - by far the best form of heating) is a different kettle of worms. I'm sure you understand that it's relatively slow in responding to demand, as the whole floor is a kind of heat-sink. There's also, I understand, two types of control; one that has its sensors embedded in the floor (which seems nuts to me), and the other type which is 'normal' - ie senses the actual room temp.

The GREAT news is that your UFH is already clearly zoned, with each of the three main rooms having its own analogue 'stat. Most likely, these will be 'normal' stats that sense the room temp**, so the obvious upgrade here is dead easy - simply replace them with Smart Prog Stats. If one room is clearly more significant than the others, then a cheaper and less confusing option could be to replace two of them (the rooms that are of secondary importance) with just Daft Prog Stats, so you program them and let them get on with controlling these two rooms. Ie, you'll have no App override, but just 'manual'. But that's your call.

Then the main room can be 'Smart Prog Stat controlled. If you want all three rooms with complete App/remote control too, then you'll effectively need three 'stats, although I think they can share the same internet 'hub'. Most systems, I think, will do this.

(*An 'obvious' first suggestion - which probably means it's wrong - would be to take the heat lag into account in your timer settings. Ie, when you get your ProgStats fitted, have them turning down the heat from 'comfort' to 'overnight' (whatever actual temps you like) a good, say, hour before bedtime, because it'll likely take this long to respond. And coming back on a good hour earlier in the morn before you get up. See how that goes, and alter that 'lag' time accordingly.)

(** If removed from the wall, any wires going to an underfloor sensor will be obvious. Chances are there won't be any.)

knightstyle

With under floor wet heating I found it important to have the water temp in the pipes much lower than the water temp used for the radiators.
There was a thermostatic valve near the manifold and set to 45 was about right, boiler temp was 65 I think.

Bendy_House

knightstyle said:

With under floor wet heating I found it important to have the water temp in the pipes much lower than the water temp used for the radiators.
There was a thermostatic valve near the manifold and set to 45 was about right, boiler temp was 65 I think.

I think even much less than that - around the mid-20's degrees? Anything hotter than this will cause all sorts of bother.

Rodders53

Bendy_House said:

I think even much less than that - around the mid-20's degrees? Anything hotter than this will cause all sorts of bother.

Think ain't good enough!  Only excessively high temps are likely to cause issues - very hot floor surfaces being one!

The floor surface will need to be that high to warm the room air!  But there are many variables in floor construction and ufh pipe spacing to make it no one number fits all situations or installations.

My screeded floor ufh mixer valve is set around 40 C, but we still have cold spots on some of the floor tiles where the pipes 'are not'.  Poorly designed/installed originally, sadly.

Heat pump designs with 15mm pipes at 150 spacing could likely run at circa 35 C, Google suggests.

Wooden floor overlays may need higher temps.

Then the loop flow rates come into the equation.

Bendy_House

Thanks - yes, the running temps are much higher that what I said.

Jenny7026

Thanks very much for everyone's help! I think we will need to think about whether heating zones might save us money. We have put the programmable TRV on and set the timer (currently trying 3h morning, 2h night to see how that goes) so fingers crossed that helps in the meantime.

Bendy_House

Jenny7026 said:

Thanks very much for everyone's help! I think we will need to think about whether heating zones might save us money. We have put the programmable TRV on and set the timer (currently trying 3h morning, 2h night to see how that goes) so fingers crossed that helps in the meantime.

Is this ONE prog TRV? And presumably in the main bedroom? Are the other bedrooms being used, or are the TRVs there turned 'off' - ie just to 'background', say '1'?

3 hours in the morning seems a lot! Unless you crawl out of bed very slowly

I'd have thought that, say, an hour would cover this for most folk, with rads coming on a half-hour before waking up time, and off again before folk are finished in the bedroom - the room should stay warm for a good half hour after the rads been turned off. Ie - I would time the rads to go off a half-hour before the time folk will be actually leaving the bedroom.

Ditto in the evenings - how long does the bedroom take to get to comfy temp? (And 'comfy temp' for a bedroom is usually only around 18oC or a little more)? Again, I'd have thought that, if 'bed' time was, say, 11pm, then having the rad come on at 10pm should get the room up to 18oC+, and it could be turned off at 11pm 'cos folks should be under the duvet keeping warm...