Retrofit Underfloor Heating to Suspended Concrete Floor From Below

[_Chris]

Hi all,

It's my first post here so here goes.

I use an old mains gas combination boiler and conventional radiators for heating and hot water. The boiler is a Myson Midas and is still working well after over 20 years service. My house is split level and on 3 floors - a basement, a suspended "beam & pot" ground floor and a conventional first floor with timber joists.

Most of our living is done on the ground floor where the concrete beams and infil blocks have been screeded with concrete and either quarry tiled or carpeted. Because the house is built on a slope parts of the concrete ground floor are accessible from a void at basement level and I was wondering if it would be possible to heat the floor directly above the void by installing underfloor heating to the "ceiling" in the void  to heat the floors in our kitchen and study and still retain (and use) the radiators.

My knowledge of underfloor heating is very limited but one thing that I think I  know is that the temperature of the water flowing through underfloor heating pipes is much lower than the temperature of the water used to heat conventional radiators.

Would it be practical to take the hot water returning from the radiators, which would have cooled somewhat, and pass it through the underfloor heating before returning to the boiler? The underfloor heating would need to be in intimate contact with the blocks above and could be insulated below using loft insulation to avoid heating the void.

Does anyone have any experience of a mixed system such as this or am I just being completely wacky!

Thanks,

Chris

The concrete floor has beams and blocks that are 6" thick with a 2" screed over.

[dhaslam]

It should work well.  You will need a lot of  insulation under the pipes, something like 150mm of high density insulation or nearly double that in fibreglass or rockwool.  It will take a few days to warm up once you start heating but it should make a big difference to comfort.  Water temperature needs to be limited to  something like 35C with a mixer valve and it would  probably be best to arrange that the flow temperature can be adjusted roughly in line with outside temperatures.  Internal thermostats could be used just on the radiator circuits for short term temperature adjustment. 

[_Chris]

It will take a few days to warm up once you start heating

Yes, I was wondering about that. I've just been reading that some suppliers recommend not letting the floor temperature drop below 3°C to 4°C of outside temperature because of the time it takes to warm the floor up again. As I will be heating the whole thickness of the floor, some 7" or 8", it's going to take a long time to heat up from cold, as you say, as I'm not too keen on keeping the floor heated when the house is empty.

On the other hand, it will be a large heat source and once warm may hold its heat long enough during the day if the house is empty, and again overnight after the heating has been on in the evening.

I suppose most underfloor water heating pipes are buried in just a couple of inches of screed. Do you know how long these stay warm for before they go stone cold and do people tend to keep their floors above a minimum temperature all winter or just switch on the heating when needed?

[dhaslam]

Normally underfloor pipes are buried in the  floor and have all round heat transfer, about 6 watts per metre at 35C  is normal.  There will need to be more pipe when the transfer is mostly to air underneath the floor.    Concrete stores  about five times less heat than water by weight  but about 2.5 times less by volume  so  a square metre of 200 cm floor takes  200/1000  X 860 /2.5 watts per degree  to heat, just about 70 watts for one degree rise per hour.      At 100mm centres there would be 10 metres of pipe  per square metre so it might heat the floor by one degree in two hours.   Obviously it isn't possible  to vary the temperature on a short term basis but you could have some variation over the day if sufficient time is allowed for warm up.
         

[_Chris]

Thanks for that.

I'm thinking that it's turning into a big job what with hot & cold feeds, mixer valves, temperature sensors, manifolds, controls etc.

Would there be any benefit in just lagging the underside of the floor with say 150mm of loft insulation. I know that warm air rises so insulating a loft is clearly beneficial but is enough heat lost through the floor to make that worth doing?

Thanks for your patience