2 zone heating radiators and underfloor heating

[DanDIY]

Hi. I have a new kitchen with wet underfloor heating. I have installed 2 zones, controlled by 2 port valves. The underfloor heating is a pretty simple install, just one zone with a mixer and pump. Its not wired in yet. I want to wire it up so that it it as efficient as possible.

As far as I can tell I have basically got an S plan system, but instead of one of the water circuits being a hot water cylinder, the second circuit is my UFH. I've a combi boiler, so the pump is integral to my boiler.

I've made the attached wiring plan and schematic.

The thing that is bugging me is that the mixer valve only takes water at about 30oC into the floor. The heating circuit going to the mixer valve is about 5m of 22mm pipe, so holds a fairly large "reservoir" of hot water. I would need to have the boiler output set fairly high so that my radiators are hot. So potentially this "reservoir of water would be circulating at 60-70C until the boilers internal stat cuts out.

Once the floor is heating up the mixer valve should only be taking a bit of water out of the central heating at 60-70C. The boiler can probably get this entire circuit of water hot in a couple of minutes. Once this pipework is hot, I don't really want the boiler running continuously if its not needed. Obviously if the return to the boiler reaches temp then the boiler will shut off, but there is a big differential between when the boiler return being hot will cause the boiler to shut off (I'd guess a few C below the set point) and the temperature the mixer valve needs the water in this loop of the heating to be experiencing (probably only 40C).

So - if anyone is following this. Can I / Should I add another stat at the mixer valve or boiler return, which will shut off the boiler if there is enough heat in the UFH "loop" of the central heating. This is the bit with the ?????'s in the plan above.
I feel that I should add this extra stat in order to cut the duty cycle of the boiler when heating UFH only. However, if I do, where do I position it. Is there a danger that the UFH mixer valve and pump will "suck" the hot water out of the loop immediately adjacent to the mixer, but not consume the hot water in the rest of the 5m of 22mm pipework. Do I need some kind of overrun pump in the central heating pipework to continue to circulate water in this loop of pipework.

I have searched around, and while I can see a lot of "off the shelf" controllers that seem to claim they will run things, they don't seem to be the most efficient method. I'm not sure uf I have over complicated this or under complicated it, by thinking too much.
Any advice gratefully received!

 

[DanDIY]

I think my first post must have been a bit confusing. I'll try and keep this brief.

I have now wired this up and it is all working in terms of the timer and thermostats working the zone valves and turning on the boiler.

I included a pipe stat next to my UFH mix valve. The intention was that this would cycle the boiler on/off as the boiler shouldn't need to be constantly pumping out water at 60-70C for the UFH once it is up and running. The best place for the pipe stat seems to be the hot input to the mix valve.

The problem I am getting is that when the pipe stat shuts the boiler down, the mix valve stops admitting warm water completely, pumping cold into the UFH pipework, even though there is still plenty of hot water in the pipe adjacent to the hot input of the mix valve. The pump for the UFH is still running.

I had assumed that the UFH pump running would be enough to "draw" the hot water through the mixer valve from the other side. This isn't working, the hot side of the mix valve is only admitting water if the boiler pump is on.

My question is - is this right. Does the mix valve need some pressure behind it, or should the pump be able to draw hot water through from the hot side, even if the boiler pump is off. I believe there are some non return valves in the mix valve, are these preventing the UFH pump from drawing the hot water through. Alternatively- could the mix valve be fouled up. The UFH has been in for about 3 winters and I've only just got round to wiring up all of the controls, so its only been on intermittently!

Any advice greatfully received, this is driving me mad now!

 

[Roger928]

Does the mix valve need some pressure behind it, or should the pump be able to draw hot water through from the hot side

A three port mixer will usually need positive pressure on the hot side.
You're drawing doesn't look right.

 

[DanDIY]

A three port mixer will usually need positive pressure on the hot side.
You're drawing doesn't look right.

Ok, thanks. Is that because of the non-return valves, or because of how the thermostatic capsule thingy inside the valve works.

I've re-drawn my schematic from scratch, rather than amending someone else's. Which bit doesn't look right?

 

[Roger928]

because of how the thermostatic capsule thingy inside the valve works

Quite a few thermostatic 3 port mixers have a restrictive valve seating arrangement on the hot port. Asymmetrical flow pattern I believe they call it.
Usual convention is primary pump and by pass valve before the mixer hot inlet.

 

[DanDIY]

I forgot to add, there is a bypass right near the boiler in case both of the zone valves are shut and a fault occurs causing the boiler to be on.

Am I being a tight arse by trying to make my boiler cycle on/off while ufh only is being called for?

Could I acheive this by keeping the pipe stat, but running an overrun feed to the primary pump (in the combi) to keep the pressure up on the mixer valve ?

Do I need a second bypass across the mixer valve, perhaps set at a lower pressure than the one near the boiler, so that it "lets by" fairly freely.....

 

[Roger928]

Am I being a tight arse by trying to make my boiler cycle on/off while ufh only is being called for?

Your boiler will do that anyway on the boiler stat.

 

[DanDIY]

Your boiler will do that anyway on the boiler stat.

Thanks for the reply. Will the boiler stat cut out even though the boiler return is cold (only at the temperature of the return from the ufh, so say 30 degrees). I thought it would only cut out when the returning water approaches that of the outgoing water.

If the boiler does cut out on its own stat (I'll have to monitor it) I'm back to the problem of the mixer valve suddenly going cold as soon as primary pump pressure is lost.....

 

[Roger928]

Will the boiler stat cut out even though the boiler return is cold

Wouldn't have thought so. But then its a combi you have which I know precious little about.