Combi with UFH / 2 valves but 1 Zone control

[masamune]

This issue dates back to our extension and cowboy-ish builder.

In short, they made a monkey out of the polypipe UFH kit. Pump was missing, stat was a timer, and plumping was incomplete etc. We were nearly left with a non-operational heating when he finally got a 3rd party heating engineer to rewire things and fit a thermostat.

Although we got an operational system, instead of a 2-zone split between heating/UFH we have 1-zone control, heating both the house rads and wet UFH screed.

System:

• Vitodens 100W combi
• Polypipe manifold/ 1-zone valve
• Polypipe Single Zone Unit PB1ZM
  
• Honeywell wireless stat
• Honeywell valve

They wired the honeywell valve (ad-hoc) to open at the same time as the polypipe valve, so all UFH/rads run together. The plumbing/valves are in place for this to split easily. I will post another pic with the wiring L/N/E/L1 in view.

I want to fit another (wired) stat in the kitchen for independently controlling the UFH and let it run much longer as required to maintain screed temperature correctly.

How should I go about this?
The master unit is 1-zone, my understanding is that I need to replace with 2-zone so that it can take in 2 stats and control 2-valves separately, while boiler takes signal from the unit.

Alternatively, could I not just rewire one valve to signal from the new stat? both stats would need to signal into the 1-zone unit, so if one stat is on at any time the boiler keeps running. Not 100% this will work...

 

[Madrab]

You can use a polypipe slave unit to give you mulitiple zones but that then controls actuators that are fitted to the UFH manifold. That's if you want to multizone a full UFH system

If you have 2 2 port valves just now, one on the UFH side and one or the CH (Rad) side then all you need to do is wire each 2 port to it's own stat and then wire them correctly, in the wiring centre, so that each 2 port sends power to the boiler (and the pump on the UFH) when it's stat activates it.

 

[masamune]

Thanks, I will draw a layout of the wiring to make sure things are done properly. I also have the wiring diagrams from the valves/master unit.

Any particular wired stats you'd recommend? (I say wired because the unit will go directly on the wall behind and it's easy to drill). I am looking at polypipe among others, possibly to match the unit of the UFH master and rewire the honeywell stat to CH only. I guess it could be any reliable stat with daily programming?

Just to add another observation. Before the 2-zone valves configuration, the boiler's internal pump originally kept circulating hot water for several minutes after cutoff, to cool down I presume. Now, once the stats signal off the valves also close and the boiler can only circulate in a small internal loop, from what I have been told. Does this have any adverse effect to the boiler's life?

 

[Madrab]

The stat would wish is really down to you. If you are using a polypipe system then using a polypipe stat isn't a bad idea. Honeywell's are always good quality and relatively easy to use as well.

Ideally you would have an autobypass fitted to ensure min flow through the boiler on overrun for heat dissipation, if the pump is pushing against a closed system when the valves shut.

 

[masamune]

Thanks again, I had a look at the manual and there is indeed an internal bypass,

"The boiler incorporates an internal bypass
to ensure adequate water flow"

https://www.manualslib.com/manual/1236682/Viessmann-Vitodens-100-W.html?page=17#manual

 

[Madrab]

Indeed but there's nothing wrong with adding an external autobypass to a S Plan system like this just to ensure adequate flow though the system.

 

[masamune]

This all worked just fine.

I looked at the wiring diagrams, separated the Honeywell stat/valve on their own and just kept a connection to the boiler S/L.
Then used the Polypipe 1-Zone controller to wire a Heatmiser stat, so I now have a proper 2-zone system controlling UFH and CH separately.