Boiler size for underfloor heating

We need to heat a new room with partially vaulted ceiling with floor area of 35 square meters and are considering underfloor heating with it's own boiler so as not to labour the existing system. We are considering a small combi boiler that could also supply the kitchen sink. Is there any way of choosing the correct size boiler for the job ?
Your comments are most welcome.

Do you know the power output of your existing boiler?

Do you know the power output of your new UFH ?

I cannot believe that your existing boiler will not be totally adequate.

Who is installing the UFH ? Why don't you want to take his advice?

Tony

SteveP5B, I did exactly this. A small combi is more than big enough for a room of that size of 34 squ metres, unless the ceiling is of cathedral height. Even if they are, the warm floor and heat rising up over you compensates. I did something similar to you in a place I had, with one room. The smallest combis are around are 24kW, there may be smaller but I never came across one, and it will do a kitchen tap OK and more, like a shower. So that part is not a problem. 24kW can heat a largish house. So the room heating side is OK.

Get a combi with a radiator temperature control knob. And get one with a low minimum output, some these days go down to around 3kW. A low minimum output reduces boiler cycling improving efficiency. Piping it up is child's play. The flow from the combi goes to the underfloor piping and the return from the floor goes to the return of the combi. Have a pipe thermostat on the flow set to 50-55C to act as overheat protection for the UFH pipes, if the boiler is set too high. Set the boiler radiator temperature from 35 to 50C to your floor comfort tastes. Once set take off the radiator temperature knob if you can or put tape over it. A had a tamperproof wall thermostat, set to 21 or 22C. People play with controls. The UFH pipes were in a screed with insulation below and to the side walls, like a tray.

I put a gate valve on the return heating pipe to the combi to act as a lockshield type of valve. I left it initially fully open, just the flow straight in and out on full. It worked brilliantly. I thought the combi would cycle and was prepared to adjust the gate valve down slightly to prevent this, by lowering the return temperature. It rarely did. By the time the boiler started to cycle the wall thermostat cut in. Overall it was very simple to do.

Depending on what system you already have, say the boiler is big enough to do the 35 squ metres of room, it may be more expensive to buy a manifold, pump, blending valve etc, than buy a new combi. Plus the inconvenience.

Check if the gas meter can cope with 24kW of combi and the existing boiler. If you do not want a combi, then go for a system boiler of around 12 to 15kW.

Thank you for your reply Ray, seems like my thoughts are worth doing. You make no mention of a pump, but I guess the combi has on inside?

SteveP5B said:

Thank you for your reply Ray, seems like my thoughts are worth doing. You make no mention of a pump, but I guess the combi has on inside?

Click to expand...

The combi has a pump inside. All it needs is to connect the UFH heating pipes to the radiator circuit of the combi. The only external controls are a pipe high-limit thermostat and a room thermostat. That is it. And of course a mains switch. The combi I had had its own time clock. I would advise a thermostat timer and ignore the combi clock keeping it on 24 hrs. Thermostat timers can drop the room temperature at night and bring it up to full room temperature during the day, by setting the temperatures for each time slot. There are many thermostat-timers available and many do quite a lot with many timer zones during the day, weekly times and daily times. etc.

It is very cheap to do.

How big is your existing boiler? How many gas appliances do you have?

Glowworm ultracon HXI 30 modulating. Seems adequate for our needs at the moment, however we live in a 70's bungalow where the heating has been chopped and changed about over the years and would prefer to leave well alone rather than modifying thing even further. At present we have a massive range gas cooker, this will be disposed of in favour of electric so I guess the gas supply will be sufficient for both gas boilers only.
Your point on keeping the water temperature low, is this to prevent the underfloor heating pipe from melting ?

SteveP5B said:

Glowworm ultracon HXI 30 modulating. Seems adequate for our needs at the moment, however we live in a 70's bungalow where the heating has been chopped and changed about over the years and would prefer to leave well alone rather than modifying thing even further. At present we have a massive range gas cooker, this will be disposed of in favour of electric so I guess the gas supply will be sufficient for both gas boilers only.
Your point on keeping the water temperature low, is this to prevent the underfloor heating pipe from melting ?

Click to expand...

As a guide, 62kW is the maximum a domestic gas meter can cope with. 30+24=54. Even plus 10% gets you inside the limit.

UFH runs between 30-50C, any hotter and you cannot walk on the floor. The high limit thermostat set to 55C is to protect the UFH plastic pipes.