Heathloss / Radiator size Calculator / BTU / Watts

[moobear]

Dear All,

I'm aware that there are several postings on the subject, however I feel (stupidly?) that I need further advise and clarification.

To cut a long story short, I'm refurbishing my flat and as part of this I'm changing all pipework, rads and boiler.

My plumber advised me that I'm better off calculating anew the BTU/Watt requirements for each room, instead of relying on the size of the (very) old radiators that are being replaced.

So I first went to http://www.heatandplumb.com/acatalog/radCalcs.html and their calculator told me that my bedroom needs approximately 712 Watts rad.

Armed with that knowledge I spent hours looking at different rads, makes, models, shapes, sizes, BTUs, prices etc. etc. etc. and juuuust when I was about to start placing an order, I 'fell' onto this page http://www.radiatorcentre.com/Heatloss/heatloss.htm and it threw my whole research out of the window.

Basically, the above page told me that according to my room sizes, I need 1374 watts rad - which is about twice as much as the calculator at Heath&Plumb.

I wonder does anyone have an informed opinion on what could I be doing wrong and which calculator's guidance should I follow, please?

The particular bedroom in my case has the following particulars:
height 2.7m (9')
area - length 3.6m (11'), width 3.4m (10')
window area - 2.5 sq.m. (26sq.f.)
its on the ground floor so it has a solid concrete floor
above the room are my neighbors - so a heated room
I have just one outside wall and its brick cavity. In all fairness, its a North-facing wall (not all calculators ask that!)
The windows are Plastic Double Glazing.

To add to my misery, the B&Q calculator ( http://www.diy.com/diy/jsp/bq/templ...=/bq_advice/planning_tools/btu_calculator.jsp ) shows 4158 BTU - about 1215 Watts - same as the one on eBay

The one at http://www.radcalcs.com/ (recommended by your website in the Sticky FAQ) shows me 4627 BTUs (1356 Watts)

The one at http://cultfollower.net/radcalc2/BTUcalc.htm shows me 694 Watts requirements... and I'm slowly loosing my mind...

I also tried the manual (?) method - height by length by width by 4 - this gives me 3960 BTU...

How could a simple things like that range between 694 Watts and 1356 Watts... whatever the approximation, I don't see how the difference could be almost twice!

Any help will really be greatly appreciated and will certainly keeps my sanity and the misus off my back.

Kind regards
MB

 

[gas4you]

The answer I get and would use would be 1180 watts. (industry standard Mears calculator)

I would possibly add 10% for a north wall if it was exposed to the elements.

 

[baxpoti]

To cut a long story short,

I'd hate to see the long version!

 

[gas4you]

 

[moobear]

Hey gas4you, thank you very much for your reply, I will go with about 1300 watts single convection, even though we are talking about global warming

baxpoti - can I send you the long version via email?

Thank you
MB

 

[holty]

try heatweb.com then flash tools. but remember to turn external temp as -1

 

[moobear]

Hi Holty,

Thank you for taking the time to answer my query.

I did find this earlier, but could never work out the adjacent room temperature... I know I can cycle through them, I just don't get which bit is my room and why there are two adjacent squares to the right...

Anyhuu, I'll go with gas4you's suggestion (thanking him again thus)

However, could someone explain why such seemingly reputable site like heatandplumb.com will have a totally wrong BTU calc?

Thank you all for taking the time to deal with my misery

Cheers
MB

 

[holty]

the two adjacent squares to the right, are. think 3d. the last square would be the roof.click on the cross ,on your right. the smaller version.and on clicking on one square repeatedly. it will bring the color code of the room adjacent.

 

[Agile]

I really dont understand what kind of plumber you are employing if he apparently tells you to do all the calculations.

If he is replacing the boiler and radiators then one expects he is CORGI registered and would do the rad sizing hemself.

In any case an experienced person can wander round the flat and tell you what size rad is needed in each room by experience.

Many of the calculators give oversized rad sizes but that does not matter. there is no harm done by oversizing and it can give quicker heat up.

Do ensure you have nice TRVs fitted to all the new rads except in the room where the stat is fittted.

Tony

 

[moobear]

Holty: Thanks for the reply... now I've got it, finally.

Agile: some week ago, he told me what he sees as a scenario, but I wanted to choose my own style of rads and he wasn't to sit with me for hours whilst I browse different sites.

Thing is, he did mention what he thinks sizewise, but said that I can derive better figures using a good calculator.

He did mention 'good'... but I just went for the first I can find... that was the heatandplumb.com one ... and the rest is the previous posts.

I had since contacted him again and he said that I should go for a double rad 600mm by 1200/1300mm long. Or see what is the BTU of a such rad and go for the same BTU whatever size/shape I fancy.

This last opinion is similar to the figures gas4you is quoting, so I reckon my plumber (yes, Corgi) is ok... I just need to learn to listen to the whole sentence

Kind regards and thank you all for your replies
MB

 

[Agile]

Its not necessary to be very exact with sizing as long as you are not too small.

1400 w with a 10' square room and only one outside wall is about right.

But you can fit around 2 kW with a TRV as it will give faster heatup and be controlled by the TRV anyway.

It gives better heat distribution to use a longer single panel rad than a short double ( as many customers seem to prefer ).

Tony

 

[D_Hailsham]

How could a simple things like that range between 694 Watts and 1356 Watts... whatever the approximation, I don't see how the difference could be almost twice!

The reason for the wide difference is in the assumptions that these calculators make about the construction of your house, i.e the U-values, and the temperatures required.

I did a quick calc with the info you gave using the Barlo calculator and it worked out about 1350 kW for a room temp of 19C (bedrooms tend to be colder - some suggest 16C) and outside temp -1C. But I still had to make many assumptions, e.g that the walls are brick outside and Thermalite inside.