UFH control for heatpump system

A question for those who know more about UFH control than me…!

Our ASHP system (Vaillant Arotherm+ 7kW) is rads everywhere except our large living/kitchen/dining area which will have UFH. Rads are not zoned and TRVs will be set to cap excessive temp so the system is as ‘open’ as possible all the time.

The question is how to control the UFH? It is in one (large) room, ideally it would work just like a radiator with a TRV so if the room temp goes a bit above the set temp it would reduce/stop flow.

But how do I set up the UFH like this? Can I use a simple on/off stat in the UFH room and a 2-port valve somehow? It really doesn’t need anything too complicated. The manifold is planned to be a simple low-temp one from JK (no pump on the manifold).

Any pointers gratefully received.

Ordinarily UFH systems have a pump on the manifold, or it can be remote from the manifold, and a thermal actuator on each UFH loop. Each room then has a thermostat that controls the thermal actuator for the loops in that room. The control box for the thermal actuators also controls the pump / boiler on / off.
The way your question is written suggests that the system isn’t yet installed as in ‘will have’, so hopefully you have some options.
UFH heating systems are not normally turned off but rather they have ‘normal’ and ‘set back’ temperatures, the set back temperature being 2 - 3°C below normal temperature. Set back temperature is used at times, say overnight, when a lower room temperature is desired.
You could, as you suggest, use a basic thermostat and a two port valve to close off the UFH system provided any by-pass (heat dump) arrangements for the ASHP are met. You could also use a mixing valve to control the flow of hot water to the UFH circuit, subject to any pumping arrangements. There are mixing valve available that will take a standard TRV head with a remote sensor to monitor the manifold temperature. It would therefore be possible, in theory, to replace the TRV head with, say, a Drayton Wiser wireless trv linked to a Drayton Wiser room thermostat. Such a system would also give you control of the Wiser elements from your phone. Such a system is perhaps more complex than you want and costly!.
It is also possible system has the UFH mixing integrated into the ASHP.
If I were the installer I would be very concerned about the lack of control on your system.
I have tried to provide you with a helpful generalized answer as I don’t know the property in question. If it is new build or similar you really should involve a competent heating engineer and ensure your system meets current building regulations. Furthermore, if it is a house then I would definitely zone the radiators because you say the ASHP is 7kW, that suggests the house is very well insulated. In that case it is unlikely the upper rooms will require any significant heat.

Thanks for the reply Bob, that’s really useful general info.

Note that the UFH manifold has no pump or mixer on the manifold itself as it’s a low temp system (45C max). It’s a https://www.jk-gb.com ‘JK-LT’ manifold. It will rely on the main system pump. So I agree I do need some kind of control element but it doesn’t have to be per-loop as the UFH is in one room only. Hence my comment about treating the entire thing as more like a big rad with a single TRV.

There is a central thermostat in the hall (middle of the house) which will have a setback temp and is what controls the ASHP (calls for heat).

I’m looking to run the system as ‘open’ as possible to provide best ASHP efficiency. In general low temp heat pumps work best with minimal control and zoning if the heat requirements of each room are well matched to the emitters - you then really just need to set your TRVs slightly higher than thermostat temp to turn off the rad if the room gets too hot e.g. due to solar gain.

Or so all the theory goes…

Yes the system isn’t fully installed yet so we can change things - just climbing the learning curve on what’s possible, appreciate the guidance.

45°C for UFH is pretty hot (mine runs at 30°C and I’m hoping to use a Shelly TRV to bring that down). It is running in a thicker than usual concrete slab for greater thermal mass though.

My manifold has a TRV that mixes the incoming flow and the water in the pipes to maintain that flow temperature. The pump just circulates the UFH water (from a thermal store). Each loop has a small actuator on the manifold; I could operate them independently, but it is a single zone so all open at the same time.

I’ve not got any other thoughts, other than I think the floor temp could well be too hot and you may get massive overheating & hysteresis if you have any sort of thermal mass in the floor. Is it going to be carpeted? I’d also worry you are going to struggle to get a good delta to get a good COP.

If there are no actuators on the loops, and there is no ‘mixing’ in the mainfold, then your only option is a simple motorised valve controlled in some way. Again, remember that at 45°C flow, the room will continue to heat after the flow stops.

Your supplier has a fancy website. What do they say?

Hi Brian thanks for the reply.

The way the system works is grooves are cut into the existing screed then levelling compound and tiles on top. So the pipes are fairly close to the surface and therefore fast reacting.

I will give JK a call and discuss but as you say perhaps 45C is a bit high (the system is designed for 40C max but it would be foolish not to provision for slightly higher in case our heat loss calcs are a bit off).

As the system will be running at pretty low temp most of the time (due to weather comp) I was hoping to avoid the extra complexity and energy use of a separate pump and blend valve.

Hi James, I am a professional heating engineer holding national qualifications in UFH, ASHP’s, GSHP’s and solar as well as being Gas Safe Registered. So, I do know more than a little about heating systems. The system Brian describes is much more typical of most common UFH systems.
As you mention ‘cutting grooves’ in the existing screed I take it the job is retrofit. If it is and you are not committed to JK I would strongly advise you to take a look at Nu-Heat Lo-Pro system. Nu-Heat are probably the premier UK company for UFH linked to ASHP’s. They will do a system design for free too.
As Brian notes if you are planning on different floor coevering then you are going to have problems, for example the maximum floor temperatures for engineered wood floors are 27 - 29 °C AND you MUST have a thermostat embedded in the floor. If all floors are hard tiled then floor temperature is not such an issue.
Modern heating pumps and modulating valves use only a few watts of power so annual cost is minimal.
To me lack of controls on the system equates to a less efficient system.

My concern is that it will actually be uncomfortable. Your feet will be too hot. Think of the temperature gradient. It is very steep from a floor temp of 45 to a comfort temp of 20.

If tiles are your finished surface, then more so.

Thanks for all the replies. I’ve also managed to do a bit more research myself now.

Note that the system will be using weather compensation - so flow temp is adjusted depending on outside temp. It’s designed for 40C max flow temp at -3C outside temp (but worth making sure we have a bit of margin to run slightly hotter in case thermal loss calcs are a little off hence 45C abs max but hopefully that’s never needed). So most of the time it will be running much cooler than 40C.

As it’s an ASHP system it will be run contiuously (with possible 1 or 2C set back at night).

I’ve found the thermal output curves for my floor type (porcelain tiles), and it should be OK at 45C flow to keep at or below 29C floor temp. My detailed JK quote confirms this too (now I’ve read it thoroughly).

Note that the pipework pattern is ‘bifilar’ (snail pattern) so you get flow and return running adjacent which keeps the floor temp more even, even at lower flow rate / wider DT. If you go with the serpentine patterns (NuHeat seem to favour these) then yes you will need to have a very low DT to stop the temp across the floor changing too much from the ‘flow’ end to the ‘retun’ end and hence you’ll need a high flow rate.

So my plan is to set the flow rate (using flow adjusters on manifold) based on the worst case output required and leave it at that, the weather comp will largely take care of the rest. DT across the UFH looks like it will be about 10C in this case from my calcs. Flow (and hence thermal output) can obviously be tweaked over time. I’ll have a zone valve that will shut off if the room gets excessively hot (or flow temp goes above 45C - i.e. something has gone wrong) but that’s it.

I realise this is somewhat counter to traditional thinking but from talking to varoious heat pump experts it seems heat pump efficiency does suffer with zoning which can cause inefficent cycling. We always ran our house from a single thermostat at a continuous set temp before we started our renovation, and it worked well, with a couple of degrees set back at night.

It’ll be a few months before this is all fitted and up and running but the system will be heavily instrumented and I’m hoping to write it all up and give performance notes / graphs so will report back!

Very worst case I can retrofit a more complex blend / pump and control system if needed (will leave space/options to do that) but I’m pretty confident now that this method will work.

Note HeatGeek have done a nice vid on this: Under Floor Heating designing, spacing and balancing | Toolbox Talks - YouTube

Always the best way :wink:

One other thought, fit lots of ‘pockets’ in your pipework for temperature sensors :slight_smile: I also wish I’d fitted a flow meter on my DHW so I know when HW is being drawn.

“fit lots of ‘pockets’ in your pipework for temperature sensors”

  • Good thought, though having played about with our old system using temp probes with a bit of thermal compound on copper pipe and insulating over the top works well too.

“wish I’d fitted a flow meter on my DHW so I know when HW is being drawn”

  • If you don’t need to know the flow, again a temp probe on the pipe will show a spike when hot water is drawn.

That is what I have, but I still wish I’d fitted pockets.

Sadly, it doesn’t particularly when there is a second draw as the pipe is already hot. Yes you could look at the temp change rate, but it doesn’t cut it.

In particular, I want to know when a shower is being run. This is usually a longer draw than filling a sink and that is not detectable by temperature change.

YMMV

Do you have any recommendations for where to get pockets (and requried fittings)?

Similarly any recommendations for a suitable flow meter?

sensor pockets 50mm - Google Search - need a pipe fitting with a 1/2in female. I’d use a Tee and put the 1/2"" fitting into that so the pocket didn’t go into the passing pipe.

This is one thing I have really struggled to find. There are a few suppliers about and I did get a quote from these guys SIKA Vortex – Pressure, Temperature and Flow Instrumentation; Star Instruments a lot are out of stock (all going in HPs).

[edit]
Not tried it, but if using plastic; https://www.screwfix.com/p/hep2o-brass-push-fit-adapting-female-coupler-15mm-x/799fj & Sensor Pocket 50mm long 1/2inch fitting | eBay might work into a plastic tee.

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For an extremely comprehensive range of plumbing fittings ‘BES plumbing supplies’ is a large national distributor who do mail order. It is fair to say if they don’t have a particular fitting then one is likely to struggle to find it. There are both 15mm compression and end feed tees that have 1/2" BSP threaded branch, or end. John Guest do 1/2" BSPF to 15 mm parallel fittings as well.

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Brian, an off the wall idea but I know it works providing you dabble in a bit of electronics! You could try putting a small microphone on the shower supply pipe and sense the change in sound when the water flows. The application I know it works in was when a grinding wheel touches a metal block on a surface grinder.

Interesting Idea - I like parallel thinking!

you mention there are rads in the house too. How many up + down stairs?
What temperature do you run those on now?
Is the heat provided by the ASHP already? Or is that to be installed when the UFH is done?

Thanks for the recommendationd Brian and Bob.

The system is completely new (whole house renovation) - rads upstairs and in half of the ground floor, UFH only in the main living/kitchen/dining area. Heat loss calcs and appropriate pipe sizing/flow calcs all done and double checked so reasonably confident it’ll work well (fingers crossed).

ASHP is to be installed once rads and UFH pipework is in and flushed.

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