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We recently had some work done, a new open plan room, a new boiler and underfloor heating (UFH) put in. We had some confusion, a lot of which comes from not having a basic understanding of how the Boiler and UFH actualy work, but essentially I want to understand the best way in order to set up our Nest Smart thermostats.

I'll explain the setup first, the original House is all via a combi boiler and traditional radiators, we just had a 50sqm extension to add a new room that has wet underfloor heating and is well insulated and is tiled. (i.e this room is nice and cosy). The room has 3 zones, but for all intent and purposes, these zones might as well act as 1 for us, the only reason we have 3 is because its a large room and would heat up quicker. The new UFH goes back to a manifold which is feeding off the Combi Boiler and has its own pump which as I understand mixes in hot water to the cool water returned from the radiators. (for reference the UFH manifold valves only have dust caps at the moment).

We got our electrician, to set up 1 Nest Thermostat in this new room. (he was new to it all as well but we worked our way through it). The way he has set it up is to have the single thermostat hooked up to the Combi Boiler and also the power for the Pump. If my understanding is correct, when there is a call for heat the Pump turns on/off, and the temperature controls the temp at the boiler. That more or less works for us, I don't need fine control of the UFH temperature, just an on/off. But the issue I have with doing it like this , with 1 thermostat is that this new room gets very warm and stays very warm for several hours, so the smart thermostat realizes this and switches off any call for heat for a good few hours - the problem is the rest of the house gets very cold at times.

I think it work work better with 2 thermostats, 1 for the combi boiler and the other for the UFH pump, so that the UFH can be switched off earlier as its not needed to be on as long. But I dont know if the thermostat is smart enough to switch on the boiler too when the UFH is calling for heat as it is totally dependent on that being on. The other issue is . maybe I am over thinking it, does switching the UFH off earlier , save us any money, maybe 1 thermostat in a different room in the old house works better. If I wanted finer control over the UFH temperature, are their specific valves that go where those dust caps are that are for this, or is all they do is turn on and off? as you can see I have some confusion! so any help working out what I need to do and what are the right questions I need to be focused on.

  • I'm a bit confused as to what you're asking. It sounds like you're on the right track to have a separate thermostat for the underfloor heating. – Tester101 Mar 7 '16 at 15:54
  • Sorry, your right there wasn't a clear question at the end of that. I am asking 1) Is using 2 thermostats, with 1 connected to the pump of the underfloor heating the right way to go about this? all documentation, points to connecting to the valves, but does this achieve the same result? 2) is there an advantage of using the valves? i.e control the temperature? if so, what sort of valve do I need. 3) How smart are the Nest thermostats, will the combi boiler switch on, but only activating the under floor heating thermostat in my example. – Ray_Hack Mar 7 '16 at 16:24
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You require your heating engineer to fit 2 zone valves below the combi boiler. One controls the underfloor heating, the other controls the existing house heating.

Option 1 - You would need a 2nd nest, or

Option 2 - A 3rd gen nest can be used to control the house heating zone valve and you use the hot water side of the nest to control the underfloor htg zone valve.

As the nest can only read one space for temperature you would need to fit a room stat to control the other space Sound tricky but it works

  • option 2 is terrible as will keep on the UFH even if the room is hot enough – DDS Mar 15 '18 at 16:14
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System badly done: you needed two thermostats (and 2 zone valves), one connected to the UFH and controlled by the nest, another connected to the 'rest of the house', piping should be divided before the branch that goes to the new room or a t.stat will always have control over the other. All two should (trough a couple of relays) be able to turn the combi on/off.

If you don't care to heat the 'new room' independently from rest of house, you can wire the 'whole house' stat to control directly the boiler, and let the nest control only the circulator of UFH.

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