UFH double thermostatic control

11 Feb 2009
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United Kingdom
There is a vaillant system boiler with unvented hot water cylinder, with a number of radiators fed from this. My G3 Gas Safe installer is currently installing this.

In two rooms on the ground floor there are two loops (one loop per room) for UFH.

The UFH is on its own circuit from the boiler with a 2 port valve, actuators sit on the manifold controlled by thermostats in the 2 UFH's rooms.

The house heating system will be time controlled from one location.

Can you control UFH with two thermostats, a main one that controls the overall buildings temperature (and main system), ie one that controls the highest temperature of the system, and then individual thermostats in rooms where the UFH is - that can only reduce the heat (not increase it) - if the occupier requires. Sorry if that sounds complicated.

I am considering Vaillants 'V' smart controller, but still getting up to speed where this all fits together.

Any thoughts appreciated.
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I think removing the 2 port valve and connecting the UFH to the main central heating flow and returns would provide the above.

Also removing the 2 port valve requires just a 2 channel controller - which the V Smart stat can provide.
You may need to rethink your system philosophy.
UFH is a slow-response system, depending on the mat size & flow rate it may or may not have the capacity to reach and maintain comfortable room temperature on its own. If the UFH has made the room too warm it'll take ages to cool down (because the UFH is heating floor mass rather than air).
If you have rads and UFH in the rooms then better to set the UFH temperature low (say 17 or 18 using a floor probe thermostat) and rely on the rads to add a quick burst of heat if needed.
Thanks, some studying of how the two systems heat a room sounds in order.

If the UFH has made the room too warm it'll take ages to cool down (because the UFH is heating floor mass rather than air).

Will not the UFH manifolds mixing valve, and individual room thermostats (in the rooms with UFH) connected to the actuators, assist in controlling the amount of heat that gets through to the floors.

If hot water flowed constantly through the manifold would not the room thermostats and actuators control the room temp regardless.
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Yes the control system on the UFH will regulate the energy input. But consider the amount of energy required to raise the temperature of a block of concrete by 10 degrees. The UFH will deliver that energy and control that energy delivery to maintain the temperature of that block of concrete. The concrete sheds its thermal energy into the room above quite slowly- if you switch the UFH off the concrete will probably still be warm in 2 hours time
EDIT Google specific heat capacity, that'll give you some of the physics behind the different systems
So its acting like an old fashioned storage heater ... I can see that controlling with a simple on/off thermo stat is going to be difficult.

I see terms like 'adaptive' and 'optimum start' etc quoted for UFH controllers, as well as temperature probes that from what I can gauge go into the floor slab,do these help as much as the literature say they do ?

We have reconfigured the system, so the UFH is on its own circuit now - the rest of the heating is now on a totally separate controlller and boiler.

I am trying to specify Vaillant Controls that will allow overall control of the two rooms upper temp and time, yet still provide individual control in each room so occupiers can turn down their individual thermostat if needs be, bit of a brain teaser ... if any one has any ideas.
Heatmiser sell a PIN lockable thermostat, and that has an 'Up Down Temperature Limit' setting - which only allows the temp to be altered withn a specific range +/- a setable value. These could be used to control the Actuators on the UFH manifold.

It has been suggested to run two systems in parallel with timers set identically ie the UFH contollers (heatmiser controls) being one and the V smart the other, does that mean two wiring centres controlling the same 2 port valve ... ie if the V Smart is not calling for heat (valve closed), could the UFH's individual thermostats for the UFH'd rooms open the 2 port valve themselves, would the V Smart simply ignore that the Valve had opened ?
I would want to avoid the UFH system calling for heat, but the V Smart's thermostat deciding that no heat was required.
Could the V Smarts thermostat go on one of the UFH'd rooms, and the parallel system's controller go in the other. ?

My brain .....
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All the talk of adaptive control is to compensate for the delay between UFH drawing heat and any of that heat becoming apparent to the room users. Yes similar principle to night store heaters but hopefully not quite as rubbish.
Yes your UFH thermostat should be monitoring the floor temperature- you'll discover over time what set point to have that stat on to give you a comfortable environment.
Having local 'i'm too hot' controls linked to the UFH will be a waste of time due to latency issues already detailed- the room user will open doors or windows. Remember the only control over the heat disappation from the floor is the temperature difference between floor and rest of room.
If you really want to pursue this idea
you're almost into PLC territory here. So its either Arduino time or a lot of relays (which will allow 2 control systems to operate one valve).
The problem you have is with system logic.
If room temp is below local stat setting and main stat setting then call for heat.
If room temp is above local stat then cease call for heat.
So what is the point of the main stat?
If you want to do this then stop looking at particular controllers and start creating a logic map for the system, detailing what each sensor is (thermostat, time controller) and what should happen when each sensor is in each of its available states. This will have to be a matrix since outcomes will depend on states of 1 or more sensor.
Once you've got that mapped out you'll be able to determine the feasability of your scheme and then go shopping for hardware to implement it.
Plan B is set the UFH to achieve a room temperature of 14 or 15 and use WiFi or Bluetooth TRVs to give short term boosts to room temp.
Sorting out the system logic sounds good, I tried that at the weekend on peices of paper drawing out the circuits, but I just ran aground, are there any templates to help with this sort of stuff.

The rooms will mostly be vacant during work days, but need to be up to temperature by 6.30pm.

I read lots of threads where people are having issues controlling UFH, effectively and efficiently, too hot floors, rooms never warm enough, or large heating bills.

It seems a quicker responding heating system would be more efficient, and effective in our circumstances, so it has been decided to install additional radiators in both rooms, as the piping is already there, its felt that using the UFH as the primary source of heating would be expensive in our circumstances, and also would not take advantage of the Vaillant Load Compensation we have been sold on.

However, we still feel the UFH could still provide some heat, as its installed and mostly ready to go.

UFH and the radiatiors will be on there own separate timed zone, so we are looking to time it to come on and off, before Radiators come on - but only at a very gentle heat, so that rooms are not heated so much by the UFH heating that it is umore than say 13/14 deg., and so remove the need for occupiers needing to control it or reduce its temperature. The Radiators can be contolled by TRV's, but the UFH will still need some control to know when its up to temp and to turn off ... still trying to peace it all together, but think this might be a better solution.
Heatmiser floor thermostats ... do they go into the screed, presume a (very carefully) drilled hole could be made for it to fit down. In view that radiatots would be in the room, could you control the UFH with the floor probes only ?
Am interested to look at the PLC route, relays were suggested by speaking with this company:
Opentherm looks interesting, and a VR33, and a way to claw back any lost Vaillant Load Compensation if using 3rd party Thermostats.
It seems engaging companies to provide a solution can be bit fraught, without a clear 'logic' plan, people go off in the wrong tack, and you never seem to get an answer on how to move forward.

Guess one needs to dive into lokking at Programmable logic Controllers.

Its time though to learn all this interesting stuff ... but which is never in surplus.
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Unless you're planning a PhD in heating control you probably want to skip the PLC system. In terms of comfort v cost I'm not sure there would be any payback in a domestic setting from such a beast.
Yes your UFH thermostat should be monitoring the temperature of the concrete slab, if it monitors room temperature you run the risk of (when room is cold but slab is hot) throwing energy round the mat without getting any benefit from it
No idea what this load compensation thing is you've become hooked on. Commercial premises sometimes have optimisers installed- these use temperature sensors (not thermostats) to monitor indoor & outdoor temperature and then vary system start time to deliver a predetermined temperature at opening time.
How efficient your UFH is will depend on how efficient your boiler is when operating at the low load/temperature of the UFH system. You may find it works most effectively only drawing heat when the radiators are on (so the rads give you a quick boost, the ufh maintains room temp rather than bringing it up to temp).
I was recently speak to an UFH company, and they were saying my floor will initially take around 8 hours to come to temperature and after that, just leave it on.
With a room stat, it will come and go as it pleases.

I'm waiting for the electrician to wire up the thermostat(s) as the house install, combi boiler doesn't have one yet either.
From memory, we have two, two way valves, one of the UFH room and one on the house side, plumber said have two thermostats, that way either can fire up the heating as needed. Previously we had just one on the UFH circuit, but he thought the two would be more controllable.

I was going to have a look, but it looks a minefield, even though the plumber says it's straightforward.

The UFH room is a new install. House boiler been running for a months now.
Think we are on S Plan, but don't quote me on that.
We can run opentherm, but not sure if it's worth it, I haven't got any thermostats yet.
OK so I will have radiators in the rooms for a more reactive source of heating, controlled by one Vaillant Vsmart controlled vaillant combi boiler, the thermostat will be in the common hall area.
I have an additional Vaillant system boiler controlled by another Vsmart, somehow I need to configure that to control the underfloor heating in both rooms, and if possible add additional 3rd party Thermostats to each room to then control the actuators, when the boiler is supplying heat. These thernostats will also allow a screed probe to act a high limit stat, to safe guard against overheating.

I spoke to Honeywell who suggested to make all this work I need an Opentherm to VBus translator, from my reading the VR33 does that job, but whether it will work with the VR66 Vaillant control centre I do not know, would the 3rd party thermostats and 3rd party blending valve and pump on the manifold of the UFH work with the Vsmart - or would the Vsmart get excluded from the heating side (ie controlling just HW), would the Vsmarts 'air' wall thermostat have any use then ?.
Forum threads on the TADO website suggest that with reverse programming companies developing controls could put such a product on the market - but whether they have or not yet requires me to make some more phone calls, I can't find anything online, apart from lots of long forum threads, with folk playing around with all this stuff and slightly older boilers, sadly though none with a Vsmart.


other forum threads on this subject:


With the Vsmart System Pack came a VR10 probe, I am disappointed to be told this is designed to go only into a Vaillant Cylinder ... so no 'advanced load compensation'on the hot water side then.
I must admit I have been down this road before with very high end imaging equipment, the manufacturers make it as hard (and often impossible) for you to break out of their camp if you need to consider 3rd paty solutions to work with their kit, or need spares, or need servicing ... OK maybe this is just a domestic boiler, and not as onerous, but I'm frustrated to find out the VSmart will not interface with OpenTherm.
Why do some manufacturers offer OT control, yet a limited number (like Vaiilant) do not)
I think you're going to struggle to get what you want using domestic controllers. You'll have a much better chance of success with an Arduino type of PLC with temperature probes rather than thermostats (so your logic program has access to actual temperatures rather than the binary state delivered by a thermostat). That'll give you data about heat loss rates, add an outdoor probe & you can have full optimisation

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