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

 

[Nostrum]

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

Sometimes they will provide the SAP calcs if it's been done.

 

[Sharp Point]

Sometimes they will provide the SAP calcs if it's been done.

Nostrum, can i ask if you know of an easy to use online heat loss calculator? Last year when i did my oftec, we we're set a whole house heat loss calc task and my brain actually split in two. It was only a tester, now if i was taught how to do the if's/ but's and maybe's i would learn.

 

[Nostrum]

Stelrad stars is pretty detailed. Once you get into it it's easy enough although takes a little time. Off plan it normally takes me a couple of hours to model the house.

 

[Chalked]

Nostrum, can i ask if you know of an easy to use online heat loss calculator? Last year when i did my oftec, we we're set a whole house heat loss calc task and my brain actually split in two. It was only a tester, now if i was taught how to do the if's/ but's and maybe's i would learn.

Whole house calcs won't give you an output for ufh, but will give you a boiler loading.
Why don't you send a copy of the plans to a ufh manufacturer ( not speed fit or polypipe) they just say, put pipes at 200 or 100 centres.
Most will give you ( if you buy from them) a installers plan and calculations for boiler load. I bet your builder just uses speedfit and polypipe because his supplier sells it.

 

[Sharp Point]

I

Whole house calcs won't give you an output for ufh, but will give you a boiler loading.
Why don't you send a copy of the plans to a ufh manufacturer ( not speed fit or polypipe) they just say, put pipes at 200 or 100 centres.
Most will give you ( if you buy from them) a installers plan and calculations for boiler load. I bet your builder just uses speedfit and polypipe because his supplier sells it.

Hello chalked, I've just emailed the ufh company. UK underfloor heating in Sheffield and raised the question with their technical guy and he recommended a heat input of 70watts per m2 for a modern new build levels of insulation, but says smaller rooms may require a form of supplementary heat.

 

[Nostrum]

70w/m is current minimum requirements for building regs ie Taylor whimpey standards!

If this is a new build and the property is 800m2 you would think that the cost of the build would lend itself to a higher level of insulation?

 

[Worcester]

Brand new 2015 builds are around 40W/m2, old leakies 100w/m2, 70w/m2 is a good guess, however that is all it is. (We've got one, new build 400m2 house to do, heat load is 12kW)

If like Nostrum and us you live and breath renewables, we have to do full room by room calcs for every job

We find on almost every retrofit property there is is always some cold spot or other, which can easily be supplemented by running a rad off one of the ufh circuits (just its own pair of pipes off the manifold). A GOOD designer / installer will also use a spiral / snail pattern and modify the spacing to heat cold zones ' high heat loss area appropriately (We often close up the spacing alongside patio/bifold door)

Get a copy of this: CIBSE - Domestic Heating - Design Guide 2015 (Domestic Building Services Panel)
and this: CIBSE - Underfloor Heating: Design & Installation (Domestic Building Services Panel) 2012
and this: CIBSE - KS08 How to Design a Heating System
Invaluable

If the guy has spent 7 years getting to this stage, he's not going to be happy if the ufh doesn't work properly. A proper heat loss design is imperative. The stars system will give you a good start.

 

[Sharp Point]

Thanks for your input guys, so really this outfit from Sheffield are just stabbing in the dark? Come to think of it they didn't even ask me for any insulation levels. I must say though, I do install the snail pattern and add more pipes per surface area near' Glassworks'

 

[Sharp Point]

May I add, like I did in a previous post, the insulations are pretty poor....50mm king span floor, 75mm cavity batts in between block/ brick,faced internally with dot n dab plasterboard.

 

[Worcester]

Found this: http://www.eceuk.com/assets/Documents/KS08_How_to_Design_a_Heating_System.pdf

 

[Sharp Point]

Right chaps, taking your good advice.....which is duly appreciated. I've got a technical guy coming from a another underfloor heating company, coming to size the system AND carry out full heat loss calcs(I'm to scared to really, in case I cock it up) I have stated that full calcs to be submitted with quotation.

Thanks

 

[Worcester]

With a project that size, that's a sensible approach.

 

[ShaunCorbs]

if i was you i wouldn't submit the calcs because all the work you have done they could just give them calcs to another cheaper heating/plumber, and you wouldn't get the job