Radiator Calcs

[DIYheadache]

Hi

I have tried to calculate the rad sizes but I am just not confident they are correct. Does the figures below seem ok?

It's an 1980's first floor flat, roughly 55m2, with uninsulated cavity walls. Inner block wall is made from bubbly looking concrete type blocks assumed a total external wall U value of 1.4. Floor and ceiling are concrete with 22mm chipboard sub floor, have assumed u values of 1.4 for floor and 1.7 for ceiling. Internal walls are only 2" stud walls with 9.5mm plasterboard, have assumed u value of 2. I have assumed the rooms below and above are heated. Outside design temp is -3C and I am designing for a flow of 70C and return of 50C to condensing combi.

Lounge/dinner (21C) - 5.5m x 3.2m x 2.44m - North facing - 2296W
Kitchen (18C)- 3.2m x 2.4m x 2.44m - North facing - 536W
Bedroom 1 (19C) - 4.5m x 3.2m x 2.44m - 1225W
Bedroom 2/Study (21C) - 3.2m x 2.4m x 2.44m - 850W
Bathroom (22C) - 2.4m x 2m x 2.44m - 463W (No windows)
Hall (18C) - 1.3m x 4m - 430W

Do these fiqures seem right?

Thanks

Pete

 

[fitz1]

go to city plumbing site. bottom of page look for line, heat loss calculator.click on that and away u go.

 

[DIYheadache]

Thanks Fitz1

Just did a quick calc using the site and the figures being given are a lot lower than what I have. Does this calculator take in to account the ventalation losses? or does it just given fabric losses?

Cheers

Pete

 

[fitz1]

not sure only use as a guide against other calcs. did one job from there calcs and have no complaints. plumb center do one aswell.

 

[DIYheadache]

Thanks Fitz1

I have just tried the Plumb centre one and the figures seem to be around the ones I already have.

Thanks

Pete

 

[DIYheadache]

Hi

Is it really worth oversizing the rads on a system with a Condensing boiler?

Just worked out that the rads need to be about 25% bigger to take full advantage of it!!! Only seems to suit new builds or places where the insulation has been sorted out.

View on this one please.

Pete

 

[JohnD]

I like to have the rads at least 50% bigger than needed. With TRVs on them they are a bit more expensive to buy, but I think no more expensive to run, and the boiler can run flat out to get them all hot, which I understand is more efficient. In unusually cold weather, or if I've been away, it has the power to warm the house quickly.

 

[DIYheadache]

Thanks John D

I will be fitting TRVs to all rads apart from the one in the main room with the room stat.

As the one in the main room is on manual valve can you oversize with the rest of the system? My fear being that the main room heats up before the others do, leaving them with no flow. As long as the calulations are correct I don't think this will happen but ....

Thanks

Peter

 

[JohnD]

I suppose that means your room stat is in that room? You could throttle it back a bit with the lockshield valve so that it is the slowest room to heat up. Or, if it is a long room, you could have two smaller rads, with a TRV on just one of them.

 

[DIYheadache]

John D

How long does a room have to be before you need two smaller rads? My main room is 18' x 11' , would it be better to have two?

Thanks

Pete

 

[JohnD]

18' makes it reasonable, especially if it has windows/cold walls at each end, or a long cold wall. But as you have a flat perhaps it only has one short outside wall?

 

[DIYheadache]

John D

Yep, only one external wall at one end facing north with double glazed window. The other walls are all internal bar one which is a dividing wall shared with block next door.

Should one under the window be ok?

Thanks

Pete

 

[JohnD]

If it's the wall 3.2m long, one would do. However, if it does not have a full-length window, there is an advantage in having a small rad each side of the window - it means you can have long curtains without all the heat trying to get out of the window.