Underfloor heating, radiators and Nest

[patstew]

I currently have a radiator central heating system, controlled by a Nest with OpenTherm (combi boiler, no zones/valves). I'm now getting an extension built which will be one big room from the new structure and the existing kitchen and dining room. I'd like to install underfloor heating in this new room. As I understand it, the done thing in this scenario is to move the existing radiator circuit onto a zone valve, then have a separate thermostat and zone to control the underfloor heating. However, I quite like the Nest/OpenTherm that puts out a continuous low level of heat through the radiators, and I think I'd have to get rid of it and go back to On/Off relay control of the boiler if I did that? Or, since my existing radiator system is running all the time at 30-40C anyway, can I just run the underfloor heating off that with a pump and blend valve?

Second question is, for the underfloor heating it seems you can either bury it in the screed or have an overlay system. It seems like it's standard to do under the screed for new build and overlay for renovations. I'm going to dig up the kitchen floor, so I can do either. The screed systems take a long time to respond though, compared to the overlay systems, which seems like a significant advantage to overlay to me? What's the downside? Just price?

 

[SJB060685]

If you want to continue using opentherm or any other modulating controls, or low temperature systems then some form of hydronic separation will be needed as the lower your flow temperature gets the UFH will pull more flow rates, which can starve the rads when both are in use.
Alternatively if its reverted to simple on off controls then this wouldn't be needed and could be zoned, obviously that's a step back though.
System needs a bit of thought. With regards to screeds or underlay I cant really comment but others with experience will be along shortly.

 

[exedon2]

I will leave it to others to comment re controls. The underfloor itself get it properly designed we are polypipe registered installer's but all the manufacturers will do you a proper design/layout.
As for type we only install overlay as a first reserve last resort basis, really fast response is not required on underfloor its not how you operate it!.
The much larger heat sink of a screed system for my money is always preferable.

 

[patstew]

Thanks for the replies.

Shouldn't the scenario you suggest just result in the thermostat modulating the boiler higher? Isn't it basically the same as a TRV opening/closing? I suppose since the blend valve keeps the floor at constant temperature, roughly the same amount of heat would come out of it whatever the thermostat did, so the nest would be just controlling the radiators, so I would need some kind of thermostatic control of the ufh.

What's the actual benefit of a 'larger heat sink' as you put it? More even floor temperature?

 

[SJB060685]

Yes with multiple zoned opentherm and compatible boiler you can zone it.

 

[patstew]

By multiple zoned opentherm, do you mean getting a new heating control system entirely, like EvoHome or something?

 

[SJB060685]

Evohome does do multiple opentherm zones yes.

 

[exedon2]

Underfloor heating should be controlled with back set temperatures typically no more than 4 C below desired room temperature. I have both screed and overlay systems in my own property when up to temperature the screed system runs far less typically only been on less than 5 hrs a day maintaining 22C with a backset of 19C for 7 hrs overnight.
You should also consider type of screed pumped systems work well also polymer modified screeds.

 

[Vee]

Honeywell Evohome best solution. Far superior to any Nest.