Non-motoring > Calculating the heat output of a domestic radiator Miscellaneous
Thread Author: ChrisM Replies: 28
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 Mon 22 Aug 16 17:01 
 Calculating the heat output of a domestic radiator - ChrisM
Does anyone know how to calculate, or where I can find a table giving the output of an old radiator?

We have a 40 year old double radiator (no fins) in our hall. It's 900mm wide by 700mm high and it does the job however, I'm about to decorate and it will need painting, so I'm considering replacing with something smarter. The largest rad that I'll be able to fit is 800 x 700 and probably a P+/21 (double with single fin) as we don't want it to stick out more than the 100mm the current one does. Now I know a standard P+ of the aforementioned size will have an output greater than what we have, but there are some nicer flat rads about where the output will be less than a standard P+.

Taking 800 X 600 as an example size of a standard new rad, a K1/11 (single plus fin) has an output of 2,500btu, a P+/21 (double plus one fin) 3,700btu and a K2/22 (double plus double fin) 4,700btu. So it would appear a fin is worth 1,000btu and therefore taking the fins off the K2/22 will give 2,700btu and taking the fin off the P+/21 2,700btu. Using the (perhaps incorrect) logic, an old double has a slightly higher output than a new K1/11. Using the above as an example again, one rad would appear to have an output of 1,200btu without any fins (3,700 - 2,500), so a double with no fins would be approx. 2,400btu. Fairly close to the 2,700btu using the fin method.

Apologies to those who are metricated, but the example works better in old money.

The flat radiators I've been looking at have an output of around 2,500btu for the 800x600 size, so I want to be sure it will cope.
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 Mon 22 Aug 16 18:49 
 Calculating the heat output of a domestic radiator - Cliff Pope
Lifted straight from Wikipedia:

"The basic relationship for heat transfer by convection is:

q = h A ( T a − T b )

where q is the heat transferred per unit time, A is the area of the object, h is the heat transfer coefficient, Ta is the object's surface temperature and Tb is the fluid temperature.]

The convective heat transfer coefficient is dependent upon the physical properties of the fluid and the physical situation. Values of h have been measured and tabulated for commonly encountered fluids and flow of situations."


So the factors you have under your control are basically the temperature of the radiator and its surface area. Easy enough to vary and measure.
Is that all there is to it?
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 Mon 22 Aug 16 19:43 
 Calculating the heat output of a domestic radiator - CGNorwich
I think you are looking at it the wrong way.

Forget about the output of the old radiator. Use an online calculator like this and add

the details and dimensions of your room.

The calculator will then tell you the output of th radiator you require



www.diy.com/help-ideas/btu-radiator-calculator/Dev_npcart_100006.art
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 Mon 22 Aug 16 21:21 
 Calculating the heat output of a domestic radiator - ChrisM
I've used several calculators and have received several answers - the lowest was 60% of the highest. I have yet to find a calculator which has a hall, stairs and landing option in the room type drop down list or one that caters for rooms of an irregular shape, which is what we have.

Given that I have a radiator which is the right size it seems the obvious way, to me at least, to find out its' output.
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 Mon 22 Aug 16 21:51 
 Calculating the heat output of a domestic radiator - CGNorwich
If the room is irregular calculate the volume in cubic metres.


A normal room is room volume (cu m) x 0.04 = KW req

then either increase or decrese by 25% if a high/low heat loss

Hall/ landing/stairs is low heat loss, due to usual lack of outside walls..

Always best to overestimate rather than underestimate.
Last edited by: CGNorwich on Mon 22 Aug 16 at 21:53
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 Mon 22 Aug 16 21:55 
 Calculating the heat output of a domestic radiator - No FM2R
Surely stairs are not only *usually* against an external wall, they also encourage heat flow from downstairs to up, meaning that downstairs can need additional heating.
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 Mon 22 Aug 16 22:08 
 Calculating the heat output of a domestic radiator - CGNorwich
Stair cases normally are located in the middle of a house not against an external wall

Mine certainly are as have been the stairs in all the previous houses I have owned. If they are located against an external wall then don't treat the room as low heat loss.

The fact that the heat rises doesn't really impact on the calculation - it is the room volume that is important although of course the upper part of the room i.e the landing will reach the required temperature first.

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 Mon 22 Aug 16 22:20 
 Calculating the heat output of a domestic radiator - No FM2R
>> Stair cases normally are located in the middle of a house not against an external
>> wall

Are they? I have a house in the UK where one of the stairways is entirely internal, but every other stairway in that house and every other I've ever owned has been against an external wall.

Must just be coincidence I guess.

>> The fact that the heat rises doesn't really impact on the calculation - it is
>> the room volume that is important although of course the upper part of the room
>> i.e the landing will reach the required temperature first.

Well it does rather. It depends where the thermostat for that area is. It depends on whether you are happy for up to be over hot to allow down to reach temperature. Unless you have a door at the stop of the stairs and/or at the bottom you are committed to sharing heat from the downstairs hallway with the upstairs landing etc. etc. etc.
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 Mon 22 Aug 16 22:47 
 Calculating the heat output of a domestic radiator - CGNorwich
Of course some house have staircases against external wall. Most actually don't - I have just checked this out of interest by taking a quick look at an arbitrary 20 houses floor plans on Right Move. 17 were not on external walls.

It's really not an issue though. A I say if the area has external walls treat don't treat it as low heat loss


The capacity of the radiator to warm a space is dependant on the volume of the room. As I said the upper part of the room will warm first. As you say if you are happy for the upper area to be over hot a higher capacity radiator will warm the lower area quicker but if you are just seeking to achieve adequate heating eventually through the entire space it is the volume that you need to consider for your calculations.




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 Tue 23 Aug 16 09:08 
 Calculating the heat output of a domestic radiator - maltrap
In my opinion one of the most important factors when calculating heat loss from rooms is their orientation. North facing rooms need a lot more heat than South fasing ones.
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 Tue 23 Aug 16 09:17 
 Calculating the heat output of a domestic radiator - CGNorwich
Not really. A south facing room will need less heating when the sun is out but on cold winter's night the heating requirements will be the same and hence the same size radiator will be needed regardless of orientation.




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 Tue 23 Aug 16 09:12 
 Calculating the heat output of a domestic radiator - BrianByPass
Searching "output of an old radiator" on Google brings up numerous examples of practical experiences of solving this problem.
Last edited by: BrianByPass on Tue 23 Aug 16 at 09:13
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 Tue 23 Aug 16 10:08 
 Calculating the heat output of a domestic radiator - Mapmaker
>> so I want to be sure it will cope.

Do your bit for the environment. Fit a smaller radiator and wear a jumper.
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 Tue 23 Aug 16 12:06 
 Calculating the heat output of a domestic radiator - devonite
Put in the biggest/most powerful one you can fit in the space, you can always turn it down with a Thermostatic Rad valve till you find the temp you need. You can't turn up a smaller one if you made a slight mistake in your calc and find you fitted one that was supposed to be the right one but wasn't.
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 Tue 23 Aug 16 12:14 
 Calculating the heat output of a domestic radiator - CGNorwich
Pretty sound advice. Always go a bit over the calculated requirement rather than under.
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 Tue 23 Aug 16 13:13 
 Calculating the heat output of a domestic radiator - Ambo
Maybe you borrow or hire one of these or similar and take readings from your other radiators to get a fix?

www.contral.co.uk/contral-cis-380-digital-infrared-thermometer.html?___SID=U

You simply point and shoot to get an instant reading. It also works on your skin, wife, double glazing etc. and the racing laser dot is great for entertaining your cat. I was given a cast off one by a plumber wishing to upgrade.
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 Tue 23 Aug 16 13:25 
 Calculating the heat output of a domestic radiator - Crankcase
I have this one, ambo:

www.maplin.co.uk/p/pocket-infrared-thermometer-n19fr

Considerably cheaper, but still does all you describe. I have indeed used it to measure the temperature of the rads, as well as the flue temperature on the woodburner, for example.
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 Tue 23 Aug 16 13:31 
 Calculating the heat output of a domestic radiator - CGNorwich
That's a thermometer. It measures the temperature not the heat output.
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 Tue 23 Aug 16 13:59 
 Calculating the heat output of a domestic radiator - spamcan61
>> That's a thermometer. It measures the temperature not the heat output.
>>

Presumably the plumber used it to measure temperature difference between radiator inlet and outlet..or to make the punters think (s)he knew what they were doing...

As it 'appens I've got one sitting on my desk at the moment (Fluke 62) so I can point out to the facilities dept. here where the aircon isn't working properly. ..or at all...
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 Tue 23 Aug 16 14:02 
 Calculating the heat output of a domestic radiator - spamcan61
>> Pretty sound advice. Always go a bit over the calculated requirement rather than under.
>>

The one time I did try and do this properly I gave up, on the basis there seemed to be at least two different 'standards' for measuring radiator output and not all manufacturers stated which one they used, or they used a different made up one of their own. So I just went for the biggest that fitted easily in the space available.
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 Tue 23 Aug 16 14:47 
 Calculating the heat output of a domestic radiator - ChrisM
Thanks for the replies. As stated in my original post, I know that replacing what we have with a P+/21 of conventional design and of similar size will be more than sufficient, but I've been looking at some other designs where the output is lower for a given size. I can't go any larger than 800 x 700, hence the need to make sure I don't go under (output wise) what we have already.

So yes, I know I can go over and adjust the output down. A Thermostatic valve is out as the system thermostat is in the hall, hence why I'd like to get close to the current setup which works just fine. Thermostatic valves in other rooms are out too as we have micro bore where the in and out is combined within the one radiator valve. Obviously the new radiator will have two valves.

Last edited by: ChrisM on Tue 23 Aug 16 at 14:48
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 Tue 23 Aug 16 17:48 
 Calculating the heat output of a domestic radiator - maltrap
I think TRV's a better option. A single thermostat turns the whole system on or off depending on the temperature in the hall. Temperature requirements differ from room to room. Combi boilers require 1 radiator without a TRV. You could fit TRV's and turn the thermostat to max or disconnect it.
Last edited by: maltrap on Tue 23 Aug 16 at 17:50
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 Tue 23 Aug 16 18:10 
 Calculating the heat output of a domestic radiator - Dutchie
The best out put we have is from a small oil filled German radiator in the kitchen dining room.

Heat output is way stronger than the radiators from the Central Heating hot water system.
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 Tue 23 Aug 16 18:40 
 Calculating the heat output of a domestic radiator - Cliff Pope
The question of course was how to calculate heat output, not how to assess what size of radiator he needed.
One is to determine how much heat is produced, the other to determine what that heat is required to do.
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 Tue 23 Aug 16 20:35 
 Calculating the heat output of a domestic radiator - ChrisM
maltrap. I've never seen TRVs that fit micro bore where each radiator only has one valve. Would be pleased to be proved wrong.
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 Wed 24 Aug 16 00:23 
 Calculating the heat output of a domestic radiator - maltrap
I'm not familair with microbore systems. But surely all radiators have a flow & return pipe. The TRV would replace the existing valve. If you Google microbore trv it shows how to fit them.
Last edited by: maltrap on Wed 24 Aug 16 at 00:23
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 Wed 24 Aug 16 00:30 
 Calculating the heat output of a domestic radiator - maltrap
The other thing to bear in mind when choosing the output of the radiator, you are governed by the size of the microbore. The bigger the pipe supplying the rad the bigger the maximum output can be.
Last edited by: maltrap on Wed 24 Aug 16 at 00:33
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 Wed 24 Aug 16 10:23 
 Calculating the heat output of a domestic radiator - devonite
Have you considered a vertical slimline Rad? - you could use the height of the hallway, instead of the usual rectangular space, and maybe fit it in a less prominent space such as a corner.

Such as:

soak.com/radiators/vertical-radiators_c23819281.htm?&gclid=CPf-8rrd2c4CFWgo0wodDCkM5g

for example.
Last edited by: devonite on Wed 24 Aug 16 at 10:25
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 Wed 24 Aug 16 10:43 
 Calculating the heat output of a domestic radiator - ChrisM
The problem isn't the microbore itself, it's the fact that my system enters and leaves the radiator at the same point via a combined valve. The inlet side has a tail about a foot or so long (on the ones I've seen) which runs inside the bottom of the radiator directing the flow to the opposite end. Doesn't sound like it would work, but it does. Our largest radiator is a double around 2m long and that works fine. The benefits of microbore are that it's easier to install and there's less water flowing through pipes which isn't where it should be heating the room. Downside is it can have a tendency to clog up and get airlocks. Not issues we've had in the last 17 years fortunately.

I'm not against the idea of a vertical radiator but in general I look at radiators as a necessary evil. They rarely add anything to a room's decor, although some of those I've seen do make a statement albeit at a price I'm not prepared to pay. The current radiator is situated in the centre of the area it needs to heat and I can't see moving it to a corner would improve effectiveness.
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