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PostPosted: Mon Jul 31, 2017 2:24 pm 
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I have a vailant ecotec plus 838 combi boiler (no tank etc) and I am installing wet underloor heating in an existing part of my house where it meets the new extension. It will be one zone. I have a two port manifold as we will be covering a floor area of around 35 square meters. My understanding is that each port should not exceed a floor area of 20m (100 linear meters of pipe) otherwise the heat output will drop. We are using high output triple layered pipes which are meant to be 20cm distance from each other.

My house central heating runs on a thermostat (wireless) and the underfloor heating runs on a wireless thermostat also. My gas safe engineer said we would use one thermostat in total and if temperature of radiators was too much we can turn down (as we have TRVS)

Surely we can have two thermostats and have different temperatures for the underfloor heating and the rest of the house.

The under floor heating shop said to tell the gas safe engineer he needs two 22mm 2 port motorised valves. The engineer told me he couldnt use them as they are for system boilers and mine is not.

I then called their technical helpline and they told me he needs to plum/wire the underfloor heating as combi s plan

Does this make sense and is there a diagram I can show my engineer to help please?


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PostPosted: Mon Jul 31, 2017 3:27 pm 
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1. You just treat the UFH as a big radiator you can control it with a TRV or a linear motor and a digital or dial stat so its only on when the heating is on but is controllable, Its do able with a standard stat and zone valve - but iirc there are specific products on the market like the Honeywell MT8 (iirc they fit on a standard TRV body http://www.honeywelluk.com/products/Und ... ating/MT8/

2. If you want the UFH to control the boiler that's where it gets complicated (basically 1 boiler, 2 systems or zones)

I'm sure someone more qualified will be a long shortly

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PostPosted: Mon Jul 31, 2017 5:20 pm 
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Can't see what he is talking about.

One 2 port for the UFH manifold/pump and the other 2 port for the radiators.

Nothing hard at all about that, as you have been told, wire as an S plan.

One stat to control each 2 port valve, then the orange wire back to the junction box then to the 240v switched live on the boiler.

Or you can connect the grey and orange wires on the valves to the no volt side of the boiler if needed.

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PostPosted: Mon Jul 31, 2017 7:19 pm 
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I wouldn't usually say this but I'd get a different engineer if he's that behind the times he can't work it out :cb

They need to be controlled separately because the Ufh will have to be on at different (longer) times than the rads. Any Honeywell S plan diagram will do for the wiring you just substitute the Ufh thermostat where the cylinder one goes

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PostPosted: Mon Jul 31, 2017 10:43 pm 
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The temperature of the heated floor surface is obviously much lower than that of a conventional radiator (65°C to 80°C), with the absolute maximum being 29°C for a solid floor, or 27°C for a timber floor.

This means for under floor heating you need two thermostats, one which keeps the floor temperature within limits and one to control the room temperature, these do not need to be electric, in fact likely better if not electric. With electric thermostat control it has two modes, on or off, with hydraulic control it can gradually increase or decrease flow, so hydraulic is the preferred method, or for the best regulation an electronic head on a hydraulic valve.

I have not gone into the valve for under floor heating, but the eTRV head for a radiator has two sensors, one measures water temperature the other air temperature so the reading is compensated for the heat from the radiator, I am sure if wanted the version for under floor heating could restrict water if getting too hot, but not something I have needed to do.

The electric underfloor heating is useless, maximum output keeping within the temperature limits both electric and wet is 100W per square meter, but the floor has to be on quite some time to get to 29°C it is not allowed to over shoot so it has to warm up slowly, in mothers wet room at least 2 hours before tiles feel warm. The wet system is better but units like this Image are not cheap, it would seem these units control the temperature of the water, my son tells me his underfloor heating is not controlled, it is used to cool the hot water produced by the back boiler once the domestic hot water tank is satisfied, there is a problem with solid fuel, you can't simply switch off the boiler, so either he runs off hot water, or he uses it to heat house.

He says the heat takes so long to get from water to floor that as soon as the pump starts he stops feeding the Rayburn and the fire goes out before the floor gets too hot.

He also tells me that the plastic pipe expands a lot when hot, so he needs extra expansion vessel if used as a closed system, he was considering a header tank as it is considered safer should the water boil, but he is using solid fuel. I am sure there is a calculation saying what size vessel is required?


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PostPosted: Tue Aug 01, 2017 1:45 pm 
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The Honeywell mt8 is a linear motor so fully controllable the same as the new-ish eTRV

eg.http://www.honeywelluk.com/products/Und ... -Zone-Kit/

http://www.honeywelluk.com/products/Und ... ing/HCC80/


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PostPosted: Mon Aug 14, 2017 10:37 pm 
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Like_knowlege wrote:
I have a vailant ecotec plus 838 combi boiler (no tank etc) and I am installing wet underloor heating in an existing part of my house where it meets the new extension. It will be one zone. I have a two port manifold as we will be covering a floor area of around 35 square meters. My understanding is that each port should not exceed a floor area of 20m (100 linear meters of pipe) otherwise the heat output will drop. We are using high output triple layered pipes which are meant to be 20cm distance from each other.

My house central heating runs on a thermostat (wireless) and the underfloor heating runs on a wireless thermostat also. My gas safe engineer said we would use one thermostat in total and if temperature of radiators was too much we can turn down (as we have TRVS)

Surely we can have two thermostats and have different temperatures for the underfloor heating and the rest of the house.

The under floor heating shop said to tell the gas safe engineer he needs two 22mm 2 port motorised valves. The engineer told me he couldnt use them as they are for system boilers and mine is not.

I then called their technical helpline and they told me he needs to plum/wire the underfloor heating as combi s plan

Does this make sense and is there a diagram I can show my engineer to help please?


I agree with Razor as always. Others are off on a tangent, why I don't know???!!!!
Simply for the control of two zones(radiators & UFH) you'll need two zone valves/motorised valves. The UFH operates at a different temperature & requires a 'set back' temperature.

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PostPosted: Wed Sep 13, 2017 11:48 pm 
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I assumed that by having a separate thermostat in the room where the underfloor heating would be fine. All I want is for that room to be at a constant 20 degrees. How can I ensure that the actual pipes are not above 29 degrees. There is nothing in the product literature to have a thermostat fitted where the pipes are to ensure that the temperature does not exceed 29 degrees.


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PostPosted: Thu Sep 14, 2017 8:18 am 
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You will need an UFH manifold of some type that blends the flow and return water and has a thermostat itself on it that will keep the water in the UFH to your required temperature.

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