UFH Project - Part2 (AKA return of the Jedi...)

It's a self-acting 3-port mixing valve, very like a TRV. 2 inlets, one mixed outlet, the common outlet port is always connected to the pump suction port. The sensor bulb is on the end of a capilliary tube, so it senses the water flow temperature, not space/air temperature as a TRV would. It doesn't seem to have any electrical connection to the pump, si I'd guess that runs continuously, or whilst there is a demand from any one (electric?) room/zone thermostat.

It's manually adjusted so you need to be able to reach it to tweak it up or down as the weather changes.

There's probably a clamp-on manual reset thermostat on the flow header, probably to shut off the pump if it overheats. Excess temperature can cause huge damage to some floor finishes.

You should have got wiring/installation details from the supplier which would probably be better than my guessing.

Whatever suits. ISTR the recommendation is to start very low and creep it up gradually. Check the instruction on the Rehau site.

Oh, and BTW there should be negligible pressure difference between the flow and return connections (your red and blue lines).

You'd usually do this by connecting it to a local low loss header (aka cross-over header). This is a bit of straight pipe with an isolating valve at one end and a lock-shield regulating valve at the other, like a radiator. The tees for the F&R to the mixing valve ar along the middle bit of the pipe. I'd do it in 22mm. You need to restrict the flow with the regulating valve because the LLH has negligible resistance & will be a roaring short circuit if not regulated.

The water flows along the low-loss header at the same rate when the primary pump is on, whether the UFH/secondary pump is on or off. The UFH pump draws water from the LLH as the mixing valve requires it and returns an equal volume of water to the LLH. Primary flow must >

secondary flow or you get mixing by reverse flow in the LLH.

Connecting it like a radiator (flow to red, return to blue) is wrong, 'cos the primary pump would then exert a differential pressure across the mixing valve ports and interfere with its operation by the actuator.

You need the primary pump running all the time the UFH is in use. This is a bit of a bummer if you don't want the rest of your radiator CH on all the time but need your UFH on continuously; you then need zone valves.

You can only avoid the need for the primary pump if the mixing valve set is connected from/to a thermal store.

Not sure about your exact system, but this is how MINE is wired.

From the switched live to the whole system, I go through the timer and the master thermostat to run the motorised valve that controls the entire UFH.

This MV contacts run the subsidiary pump *and a relay*, whose contacts are between switched live and the boiler/pump. You need a relay so that e.g. calls for DHW do not start the UFH pump going. I.e. you cannot directly parallel the UFH pump and the boiler pump.

The auxiliary pump circulates water around the UFH as long as the stat calls for it. The mixer valve on mine is a 4 port wax capsule type thing, that senses the flow temp in the UFH and either lets in more hot water, or simply recirculates what is there already if its hot enough.

I have a second thermostat which sits between timer switched live output and a couple of units that clamp on the manifold to cut the kitchen circuits, as there is an Aga in there and extra heat is almost NEVER needed.

Somewhere I have a diagram of all this.

That there lies the problem, the whole lot is going under the living room floor, with a small service hatch under the carpet to get to it incase of servicing.

I explained this to the supplier. My understanding was that I set the 3 port mixer once, and then the wireless room thermostat and actuators (which fit on all 3 of the cold returns from the UFH) switch off the flow.

I will contact the system suppplier today, as see what they say.

Didn't understand a single bit of that :-)

Well I only read it because you said that! Seemed quite understandable in an english sense, however, I couldn't help thinking that a low loss header with the regulating valve stopped down to avoid the short circuit would necesssarily limit the flow to the UFH in an undesirable way. Is this not the case?

It shouldn't be, you want to limit the primary flow to the maximum that would be required by the UFH/secondary. Anything more than that would just pass straight through the low loss header and back to the boiler. Most of the time the UFH would operate at part-load and so the mixing valve would only be drawing a small portion of the water passing through the LLH.

You'd be better off setting the mixing valve up as an injection mixer, so that it only handles the smaller primary flow; you then get a smaller mixing valve. As described above, a mixing valve is handling all the UFH/secondary flow rate which is larger.

It all makes sense, honest guv.