Ufh loading question

[Sharp Point]

Evening everyone, i've been asked to fit some underfloor heating, approx 800 square metres over 4 floors . I work on the basis of 100watt per m2 for working out heat load, so that works out to be 80kw load. Now that seems to be a massive heat input for a dwelling, but just like electricians work on the 'factor of diversity', would a heating system be ON at the same time through out the entire property? Please discuss...


Oh and thats not taking into consideration hot water load or losses!!!

 

[missplumb]

You will know better than us as to what the usage of the building is. If it's offices then I would imagine heating will be on the same time on all floors, if it's a big house then could probably time it so it's not all on at the same time, but, you should size it to run it all at the same time and apply diversity as your customer will be pretty peed of in the winter when it doesn't warm up cos you put an undersized boiler in!!

 

[kris]

Is there UFH going in every floor ??

 

[Nostrum]

You can't just assume the heat loss as 100w/m2. A new build to reasonable standards could be 35w/m2 ?

You'll need to do a full heat loss calculation. At 100w/m2 you'll be pushing underfloor heating by the time you've got floor coverings down!

 

[Sharp Point]

You can't just assume the heat loss as 100w/m2. A new build to reasonable standards could be 35w/m2 ?

You'll need to do a full heat loss calculation. At 100w/m2 you'll be pushing underfloor heating by the time you've got floor coverings down!

Thanks Nostrum for your input. I think you may have misinterpreted what i was describing, I was referring to the heat input per m2 of floor area, i've installed a few houses now with ufh ( only been employed by builders to fit, they provided the kits) the builders have bought through merchants or specialist ufh suppliers and i have raised the question of heat input needed per m2 to calculate pipe sizing and they mostly recommend an input of 100w per m2.
These have been designed by john guest, polypipe etc

I assume it takes quite a bit of energy to heat the slab and the thermostat takes care of the rest, am i on the wrong lines?

 

[Nostrum]

You size the boiler on the leat loss of the building, not the amount of pipe in the ground or floor area. There is no rule of thumb unless you're working for a housing developer who will do all they can to squeeze past current building regs!

Size the building and calculate heat loss then specify the boiler on the heat load requirements. If required you can then divide the heat loss by the floor area too give you a W/m2 value.

Personally I don't add any additional amount for hot water, especially if it's a large boiler.

 

[Chalked]

With a fairy modern insulated building with 10mm insulation, on screeded floor.a quick calculation works out at about 10kw per floor for 200m2
So, 40 kW plus hot water.

that size system would be better served by a low loss header, then a mixing set with weather compensation.

ps, don't let the builder dictate your job.

 

[Sharp Point]

With a fairy modern insulated building with 10mm insulation, on screeded floor.a quick calculation works out at about 10kw per floor for 200m2
So, 40 kW plus hot water.

that size system would be better served by a low loss header, then a mixing set with weather compensation.

ps, don't let the builder dictate your job.

Thanks Chalked, that sounds more like it to me. Yes i will inquire about a low loss header, who are the people to talk to?

The dwelling has just been roofed, the client built it himself over a seven year period. He originally submitted plans before the step up in current insulation levels, so only 50mm kingspan in floor and 75mm filled cavity with cavity batts.

 

[Sharp Point]

You size the boiler on the leat loss of the building, not the amount of pipe in the ground or floor area. There is no rule of thumb unless you're working for a housing developer who will do all they can to squeeze past current building regs!

Size the building and calculate heat loss then specify the boiler on the heat load requirements. If required you can then divide the heat loss by the floor area too give you a W/m2 value.

Personally I don't add any additional amount for hot water, especially if it's a large boiler.

Thanks Nostrum, your advice is duly being soaked up. when will architects start giving out heat loss calcs with their plans????:cool3:

 

[Chalked]

O

Thanks Chalked, that sounds more like it to me. Yes i will inquire about a low loss header, who are the people to talk to?

The dwelling has just been roofed, the client built it himself over a seven year period. He originally submitted plans before the step up in current insulation levels, so only 50mm kingspan in floor and 75mm filled cavity with cavity batts.

50mm increases your load to 12kw per floor