# calculating the correct size rad for a room

Discussion in 'Plumbing and Central Heating' started by jointy, 16 Oct 2012.

1. ### jointy

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i have a living room about 7m by 3m,how do you do a simple calculation to get the right size rad for each room anybody,thanks,jointy

3. ### Dan Robinson

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Plenty of onlone calculators, although they tend to over egg the pudding:

Like these.

4. ### Armo74

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You need to calculate the fabric heatloss: Heat conducted through walls,ceilings,floors.

Then the ventilation heatloss: Warm air leaving the house which is replaced by cold air which then has to be re-heated, which can be worked out by calculating the room volume and the specific air change rates per hour also taking into account the density of the air under constant conditions.

You also need to know the U-Value of the walls which is the number of watts that flows through an area of 1 square metre when subjected to a temperature difference of 1 degree celcius.

So to calculate the fabric heatloss you have to know the wall area and times it by it's U-Value also taking into account the inside and outside design temperatures this needs to be done for each heat losing surface.
Allowing for window and door surface areas.

Or you could do it on a heatloss calculation website

5. ### D_Hailsham

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Agreed.

I recently analysed the results of using 11 online calculators to calculate the rad size for the same room. The differences were interesting - to say the least. They ranged from 1008 Watts to 3070 Watts: a 1 to 3 ratio between smallest and largest.

I also delved into the methods used, which varied from the crude (room volume x a number) to the fairly sophisticated (U values and standard room temperatures).

In the end I concluded that the HomeSupply Calculator gave the most reliable results, without resorting to a detailed calculator such as Stelrad Stars

6. ### jointy

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i did a level 3 plumbing course last year and have a jtl book and have gone by the example given but came up with a figure that i thought was unrealistic as they show plans of a bungalow with one bedroom,i have a three bedroom house do i calculate all 3 bedrooms?ie heatloss?thanks,jointy

7. ### Agile

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I have a 2000mm x 600 mm radiator!

What size room do I need to put it in?

8. ### Agile

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Do you remember the four calculations one day at Priority 2-3 years ago?

9. ### Dan Robinson

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I barely remember last week...

However in preparations for designing my new heating system I have just heatlossed my house.

4 bed detached with assumed insulation up to scratch... total load - 9255W at mean flow of 55 and outside temp of -3.

Annoyingly the rooms with the highest losses are the ones I don't want mahoosive rads in. Single biggest is the kitchen at 3354W.

Gonna pour a G&T and think about it.

11. ### Agile

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Your total heat loss adds support to my many advice postings here saying that many houses only need a 12 kW boiler.

12. ### Terry lambert

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I heat losses a four bed house last week and it came out at eight kw.

I had to do it twice just to make sure.

It makes you wonder, just how many oversized boilers there are out there

13. ### Dan Robinson

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Indeed - and I totally agree.

Just refined the calcs and added some other areas I wasn't going to bother to heat. Totalling 57 cubic metres over another 21 square metres...

Brings the loss up to 12178 watts.

Adjusted down to -5 as well.

I still haver a 24kW boiler, but through the TS it doesn't matter about turn down ratios.

Playing with settings at the moment and have the boiler set to 68 and a store temp of 58. Hot water is still perfectly OK. Wonder how low it will go?

Now, If I could be a7sed I would do a whole house calc and see how far off it is.

Interestingly my "student digs" method wasn't that far off either. Although that tends to round up to the nearest quarter kW.

14. ### D_Hailsham

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The heating load is not dependent on the water temperature; the size of rad is.

15. ### Dan Robinson

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You think?

I was doing my calcs as part of a complete design. My rads are being sized as I described.

16. ### D_Hailsham

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Yes!

The physical size of radiator will vary with water temperature because the output varies with temperature, but the heat requirement will not change.

Say a room needs 1kW according to the heat loss calculator. You can provide this by putting in a 1kW rad running at 75/65, or you can put in a 1.6kW rad and run it at 60/50. Both rads will be giving off 1kW.

The boiler is sized according to the actual heat required, i.e 1kW, not the nominal size of the rad (1.65kW).

If you have worked out that you need 12178kW of rads at a mean temp of 55C, the actual heat loss will be 7545W. So you would install an 8kW boiler not a 13kW.

What does the Whole House Boiler Size Calculator say you need?

17. ### Dan Robinson

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Will have a look when I'm out of the bath.

But I also need to size my boiler for hot water. It has to top a TS up that is running (as part of an experiment) very cool relatively. Those in the CC will be bored of it all by now, but I am playing with ways of fine controlling it even possibly having two temperatures in it. Effectively getting weather compensated heating. With all the benefits of super hot water and a boiler that pours condensate like a race horse pees.

18. ### DIYnot Local

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