Calculating radiator size for awkward room

[Lobster]

I posted a while ago with requests for help about insulating the sloping roof of my daughter's bedroom - thanks for all the help: plasterer finished yesterday and all looks hunky dory (hopefully so will my gas bill next spring).

Anyway - now I need to install a new, appropriately-sized radiator. Trouble is I've tried a few "calculators", however they don't cover the complex shape structure of the room; cross-section looks like this (except the slopey bits are shallower than this):

________ / \ / \ / \ | | |____________|

Also there's a dormer window built into one side of the room.

For example:

- The slopey ceiling is made up of 2mm plaster/9mm plasterboard/50mm polystyrene/50mm Kingspan/0 sarking/clay tiles.

- Flat ceiling is 20mm lath-and-plaster/200mm Rockwool

- Vertical external walls are 2mm plaster/9mm plasterboard/50mm polystyrene/50mm Kingspan/200mm solid brick

I'm sure your average heating engineer would just eyeball the room and fit any old rad he knew would be at least OK, however I'd like to get it reasonably right! How do I go about this?

Thanks David

[Dave Fawthrop]

|I posted a while ago with requests for help about insulating the sloping |roof of my daughter's bedroom - thanks for all the help: plasterer |finished yesterday and all looks hunky dory (hopefully so will my gas |bill next spring). | |Anyway - now I need to install a new, appropriately-sized radiator. |Trouble is I've tried a few "calculators", however they don't cover the |complex shape structure of the room; cross-section looks like this |(except the slopey bits are shallower than this): | | ________ | / \ | / \ | / \ | | | | |____________| | |Also there's a dormer window built into one side of the room. | |For example: | |- The slopey ceiling is made up of 2mm plaster/9mm plasterboard/50mm |polystyrene/50mm Kingspan/0 sarking/clay tiles. | |- Flat ceiling is 20mm lath-and-plaster/200mm Rockwool | |- Vertical external walls are 2mm plaster/9mm plasterboard/50mm |polystyrene/50mm Kingspan/200mm solid brick | |I'm sure your average heating engineer would just eyeball the room and |fit any old rad he knew would be at least OK, however I'd like to get it |reasonably right! How do I go about this?

Put one twice the size you think would be right, and fit a TRV.

[John Stumbles]

You can calculate it by hand (or on a spreadsheet) adding up for each surface its areas * U value of the material * temperature differnce across it; then add the volume of the room * no of air changes per hour * temperature difference (inside - outside) * 0.36.

[Christian McArdle]

It won't need a very big one, unless the room is huge. You didn't state dimensions.

Christian.

[HLAH]

"Dave Fawthrop" wrote

That's pretty much our plumbers strategy. It makes sense to me.

H

[John Stumbles]

Price of steel these days it's better to calculate it and get it right. Often I've been suprised how small (and sometimes how large) a rad needs to be for a given room.

[The Natural Philosopher]

You don't mention size, but what you have is better insulated than most of my upper rooms and a kilowatt sorts those out.

[Lobster]

TBH I suspect that the cost difference between my largest "guesstimated" rad wouldn't be vastly different to the ideal size, so that's not my main motive - would just rather keep it neat and not clog up the room with more hardware than required.

Anyone know of a Noddy guide to calculating U-values? Once tried doing it from first principles before, without a software calculator, and came unstuck!

For the purpose of rad sizing/U-value calcs, does it matter whether the sloping area is considered as "wall" or "ceiling"?

(BTW floor area is (IIRC) something like 3.5 x 3m (I'm not at home now) but the flat area of the ceiling is probably half that?)

Thanks David

[The Natural Philosopher]

I cvan tell you instantly by comparison with here, that something around a kilowatt (3500 BTU/h) will be ample. TRV it of course to make sure you do not overheat.

I have less good insulation and the one room my 1.2KW isn't quite enough is about 5x3 meters, north facing, with not much heat coming up from below as its over a guest area thats not in use often.

I'd say one meter length of double radiator would be ample. Or two smaller if it fits better aesthetically. I hate radiators..I fitted hot air blowers instead. Yes they whirr, but a kilowatt is a 18" square panel in the wall..

Anyway, pick whatever suits from e.g. here

[John Stumbles]

On Thu, 14 Sep 2006 05:17:55 -0700, Lobster wrote:

Google :-)

There are a few standard U values used in the calculators and you can pick up some others online

Here are some I made up earlier :-)

Surface Construction U source Wall Brick 220 + plaster 13 2 Brick 335 + plaster 13 1.6 Block heavy 200 + air 25 + pboard 10 1.8 Block heavy 200 + air 25 + exp poly 25 + pboard 10 0.79 Block light 200 + air 25 + exp poly 25 + pboard 10 0.46 brick + cavity + brick 1.68 Warmfill brick + cavity + concrete block 1.55 Warmfill brick + cavity + clinker block 1.4 Warmfill brick + cavity + insulation block 1.05 Warmfill brick + bead-filled cavity + brick 0.49 Warmfill brick + bead-filled cavity + concrete block 0.47 Warmfill brick + bead-filled cavity + clinker block 0.45 Warmfill brick + bead-filled cavity + insulation block 0.42 Warmfill Windows Wooden SG 5 Wood/uPVC DG 2.9 Low-emissivity Double Glazed 1.9 Pilkington 12mm gap, Pilkington 'K' glass Argon filled/low-e glass DG 1.6 Pilkington Aluminium, no thermal break, single 6.4 Aluminium, no thermal break, double 4.3 Aluminium, thermal break, single 5.8 Aluminium, thermal break, double 3.7 Floor Suspended, 1 air brick, bare 0.61 Suspended, 1 air brick, covered 0.59 Suspended, >1 air brick, bare 0.82 Suspended, >1 air brick, covered 0.68 Solid on earth 0.36 Roof uninsulated 0.7 guess fibre 60mm 0.55 fibre 80mm 0.43 fibre 100mm 0.36 fibre 60mm + sarking underlay 0.51 fibre 80mm + sarking underlay 0.4 fibre 100mm + sarking underlay 0.34 fibre 60mm + felt on sarking board 0.48 fibre 80mm + felt on sarking board 0.39 fibre 100mm + felt on sarking board 0.32

Nope

[Lobster]

[snipped U's]

Thanks a lot John - I'll fire up Excel over the weekend and have a play! David