Howto calculate radiator size for room

jaz · Feb 27, 2018

How do I calculate the correct radiator size to heat a room?
There must be a formula somewhere even if its only rule of thumb ?

Our current radiator is taking 4-5 hours to heat the room up to about 20C
This seems excessive to me.
The radiator does get hot and has been bled - no cold patches on it.
(although we do have issues with the boiler this is a long standing problem from installation)

doitmyself · Feb 27, 2018

Try the Myson Heatloss Manager

SimonG · Feb 27, 2018

Length of room x width of room x height of room in feet x (multiplier of 3 for bedrooms or 5 for living area) = rough heat requirements in btu's.

Riley · Feb 27, 2018

There are plenty of apps for if you have a smart phone

Harvest Fields · Feb 27, 2018

Riley said:
There are plenty of apps for if you have a smart phone

I don’t know about you mate. But I find these all give different outputs?

leelister6 · Feb 27, 2018

Rough guide: L x W x H in feet, multiply by 4 then stick 20% on (Btu's).I always err on the larger size for rads as you can always turn a bigger radiator down. You just have to be careful not to exceed your boilers ch output

jaz · Feb 27, 2018

thanks for the quick response guys

I tried the myson site but I'm not certain about some of the figures
(The radiator height doesn't seem right and some of the tech info isn't explained)

So I tried the room volume method and got roughly 1500
This is a kitchen and a bit ventilated so what would be the multiplier
would you say? 4 times that value maybe for a kitchen?
(It needs to be comfortable as its used as a second living space)

ps boiler ch value?

townfanjon · Feb 27, 2018

To do it properly can be a drawn out process, as the lads have said above just download an app .

spanner · Feb 27, 2018

Then is it a single or a double rad??
Boiler central heating: What temperature should a house be? Should you turn yours up?

Last Plumber · Feb 27, 2018

What's the:-
Length, Breadth, height?
Is it a solid or wooden floor?
How many outside walls?
What is the wall, e.g. 11 cavity, 18" solid, fully insulated etc
What room is it? e.g. bedroom, lounge, dining room etc.

Gasmk1 · Feb 28, 2018

all of them are guess work unless you know every detail of your house build. you could use a mears calculator also. i tend to go with the volume method x 6 downstairs and 3 or 4 upstairs thats what i was taught as you can see we all have our own methods but all based on the same theory.

doitmyself · Feb 28, 2018

SimonG said:
Length of room x width of room x height of room in feet x (multiplier of 3 for bedrooms or 5 for living area) = rough heat requirements in btu's.

Out of curiosity I applied your formula to the total volume of my house using 5 as the multiplier. It worked out at just under 50,000 BTU (≈15kW), which was the size of the original boiler. But since the house was built, about 30 years ago, the house insulation has been improved considerably - walls, windows and loft - so the heat loss is now ≈ 7.5kW - half what it was originally.

Applying your formula to a modern well insulated house will result in radiators which could be twice as large as is necessary, which is good for radiator manufacturers and also allows the system to be run in condensing mode even when the outside temperature is well below freezing. But you don't need to double the size of a rad to achieve that.

Gasmk1 · Feb 28, 2018

doitmyself said:
Out of curiosity I applied your formula to the total volume of my house using 5 as the multiplier. It worked out at just under 50,000 BTU (≈15kW), which was the size of the original boiler. But since the house was built, about 30 years ago, the house insulation has been improved considerably - walls, windows and loft - so the heat loss is now ≈ 7.5kW - half what it was originally.

Applying your formula to a modern well insulated house will result in radiators which could be twice as large as is necessary, which is good for radiator manufacturers and also allows the system to be run in condensing mode even when the outside temperature is well below freezing. But you don't need to double the size of a rad to achieve that.

thats all well and good you are assuming insulation is all done properly i recently did some work for a young architect (boiler breakdown) his house is new build 2 or 3 years old, he has had to take the builders to court as there was no insulation in the walls or ceilings even though they had the certificates. also we have all worked in houses that are modern where the rads were sized by the house builders and they are all too small. complaints that rooms not warm enough.

fixitflav · Feb 28, 2018

jaz said:
How do I calculate the correct radiator size to heat a room?
There must be a formula somewhere even if its only rule of thumb ?

Our current radiator is taking 4-5 hours to heat the room up to about 20C
This seems excessive to me.
The radiator does get hot and has been bled - no cold patches on it.
(although we do have issues with the boiler this is a long standing problem from installation)

What was the starting temp for the 4-5 hours warm-up?
The way I see it there are 2 parts to your question - how to calculate the heat output required, then to calculate the radiator size needed.
Nobody has answered the 2nd part.
Output of a rad is about 1kW/sq.m, water temperature ~ 75°C. That's total area, both sides, and taking account of double rads.
I'd suggest o.7-0.8

kW/sq.m for design.

doitmyself · Feb 28, 2018

fixitflav said:
What was the starting temp for the 4-5 hours warm-up?
The way I see it there are 2 parts to your question - how to calculate the heat output required, then to calculate the radiator size needed.
Nobody has answered the 2nd part.

The heat output depends on the difference between starting and desired temperatures; traditionally these have been -1C and 20C. But if the starting temperature is lower than -1C the house may not reach the desired temperature. It all depends on how much headroom has been allowed (typically 10-20%).

The rad output should be the same as the required heat output; but the physical size will depend on the design flow and return temperatures.

fixitflav · Feb 28, 2018

doitmyself said:
The heat output depends on the difference between starting and desired temperatures; traditionally these have been -1C and 20C. But if the starting temperature is lower than -1C the house may not reach the desired temperature. It all depends on how much headroom has been allowed (typically 10-20%).

The rad output should be the same as the required heat output; but the physical size will depend on the design flow and return temperatures.

The question was meant for jaz, as only he knows the start temperature in his specific case. I doubt whether he has a temp as low as -1°C in mind, but if so, and all the room contents, and walls and floor are at that temp, 4-5 hours might not be unreasonable.

Of course t

he rad output should be the same as the required room heat input, but if you're specifying rads you still need a design figure of kW/sq.m. That clearly depends on flow and return temps, my figure is based on typical 75° flow, 65° return.

doitmyself · Mar 1, 2018

fixitflav said:
Of course the rad output should be the same as the required room heat input, but if you're specifying rads you still need a design figure of kW/sq.m. That clearly depends on flow and return temps, my figure is based on typical 75° flow, 65° return.

I assume you mean kW/sq.m of radiator; but why do you need this?

If you need 3kW to heat a room there are plenty of ways of doing this; it just depends on the shape of the room and how many rads are needed. With modern condensing boilers systems should be designed for a return of 55C; and with high resistance heat exchangers the differential is normally about 20C so the flow rate is halved.

fixitflav · Mar 1, 2018

doitmyself said:
I assume you mean kW/sq.m of radiator; but why do you need this?

If you need 3kW to heat a room there are plenty of ways of doing this; it just depends on the shape of the room and how many rads are needed. With modern condensing boilers systems should be designed for a return of 55C; and with high resistance heat exchangers the differential is normally about 20C so the flow rate is halved.

Yes I meant kW/sq.m of radiator, I thought it was clear from the context. How else can you determine radiator area needed, hence select the radiator?

I agree you might want to work on different flow and return temps than the ones I suggested, and if lower figures are used, perhaps to increase boiler condensation and raise efficiency, maybe a lower kW/sq.m is appropriate, but you still need a figure to size the rad.

If there is weather compensation, to reduce flow temp when it's not too cold (again to improve efficiency), it's OK because then you don't need as much heat.

doitmyself · Mar 1, 2018

fixitflav said:
How else can you determine radiator area needed, hence select the radiator?

But why do you need to calculate the radiator area in the first place?

If you know that you require 5kW to heat a room using two radiators then you just select two rads from a manufacturer's catalogue which add up to 5kW and fit the available space. The fact that one produces 0.5 kW/m² and the other 2 kW/m² is irrelevant.

I can see your scenario relevant in a commercial situation, e.g a large office where there are many rads, all the same size and you need to know the output required from each rad; but in a domestic situation the output per m² is irrelevant.

fixitflav · Mar 1, 2018

doitmyself said:
But why do you need to calculate the radiator area in the first place?

If you know that you require 5kW to heat a room using two radiators then you just select two rads from a manufacturer's catalogue which add up to 5kW and fit the available space. The fact that one produces 0.5 kW/m² and the other 2 kW/m² is irrelevant.

I can see your scenario relevant in a commercial situation, e.g a large office where there are many rads, all the same size and you need to know the output required from each rad; but in a domestic situation the output per m² is irrelevant.

OK I was giving first principles, maybe an installer wouldn't go into it, he'd select one from a catalogue, but how do you think the manufacturer prepares his catalogue, if not from known kW/sq.m? And the catalogue would be based on specified flow and return (and possibly room) temperatures.

doitmyself · Mar 2, 2018

fixitflav said:
how do you think the manufacturer prepares his catalogue, if not from known kW/sq.m? And the catalogue would be based on specified flow and return (and possibly room) temperatures.

I have yet to see a manufacturer's catalogue which was based on known kW/sq.m. They manufacture rad in four different types (P1, K1, P+ and K2) to standard heights and standard lengths. The catalogue tells you the output and you are left to choose the rad which is nearest (higher output) to your requirement.

Of course the data is based on specific flow, return and room temperatures. If it wasn't, the rads would not meet the standards laid down by BS EN 442.

jonnyswamp · Mar 2, 2018

Kinell, just chuck a 5ft double on the wall
Or see post 3
Forget the p1ssing contest above

fixitflav · Mar 2, 2018

doitmyself said:
I have yet to see a manufacturer's catalogue which was based on known kW/sq.m. They manufacture rad in four different types (P1, K1, P+ and K2) to standard heights and standard lengths. The catalogue tells you the output and you are left to choose the rad which is nearest (higher output) to your requirement.

Of course the data is based on specific flow, return and room temperatures. If it wasn't, the rads would not meet the standards laid down by BS EN 442.

Of course it's based on kW/sq.m. The manufacturer may not state it explicitly, but there's no other way of doing it.
You can easily work out the area from the published dimensions. Divide the published kW by that to get kW/sq.m.
I looked at Screwfix, and one example is single rad 1.2m x 0.6m, rated 1.231kW. That's 1.44sq.m, giving 0.85kW/sq.m, in good agreement with what I said earlier.
I still say nobody else answered the 2nd part of the original question - having estimated the required heat, how to get some idea whether his existing radiator is adequately sized. He could have looked in a catalogue, equally and probably easier, he could have used the data I posted.

Harvest Fields · Mar 2, 2018

fixitflav said:
Of course it's based on kW/sq.m. The manufacturer may not state it explicitly, but there's no other way of doing it.
You can easily work out the area from the published dimensions. Divide the published kW by that to get kW/sq.m.
I looked at Screwfix, and one example is single rad 1.2m x 0.6m, rated 1.231kW. That's 1.44sq.m, giving 0.85kW/sq.m, in good agreement with what I said earlier.
I still say nobody else answered the 2nd part of the original question - having estimated the required heat, how to get some idea whether his existing radiator is adequately sized. He could have looked in a catalogue, equally and probably easier, he could have used the data I posted.

Yawn...

fixitflav · Mar 2, 2018

jonnyswamp said:
Kinell, just chuck a 5ft double on the wall
Or see post 3
Forget the p1ssing contest above

What makes you think he wants to fit a new rad? Perhaps the existing one is a 5ft double.
He wants to check whether his existing one is up to the job, and presumably take it from there.

Plumbagain. · Mar 2, 2018

Some plumbers can judge the size exactly without any calcs.

jonnyswamp · Mar 2, 2018

fixitflav said:
What makes you think he wants to fit a new rad? Perhaps the existing one is a 5ft double.
He wants to check whether his existing one is up to the job, and presumably take it from there.

If it's taking 5 hrs to heat a room, it's obviously not

jonnyswamp · Mar 2, 2018

Plumbagain. said:
Some plumbers can judge the size exactly without any calcs.

That's exactly how we used to do it
Always worked OK but not scientific enough apparently

doitmyself · Mar 2, 2018

fixitflav said:
Of course it's based on kW/sq.m. The manufacturer may not state it explicitly, but there's no other way of doing it.

If kW/sq.m. is such an important parameter when sizing radiators, why don't the manufacturers provide this information?

If I need 1.75kW to heat a room and the space available is 2 metres long, I can easily select a suitable radiator from a catalogue without needing to know the kW/sq.m.

fixitflav · Mar 2, 2018

doitmyself said:
If kW/sq.m. is such an important parameter when sizing radiators, why don't the manufacturers provide this information?

If I need 1.75kW to heat a room and the space available is 2 metres long, I can easily select a suitable radiator from a catalogue without needing to know the kW/sq.m.

Why should they? Manufacturers don't usually release technical information. But they must have it as there's no other way of sizing a rad, as I've said.
I was just trying to help the OP to deal with his problem. I've explained and answered your comments several times but you don't appear to understand.

Of course there are other approaches.

I unsubscribe from this discussion.

Last Plumber · Mar 2, 2018

Did the OP switch off?

jtsplumbing · Mar 3, 2018

Would help if he told us the size of his room and size of currant rad, baffles me why some people get so involved with complicated formulas and calculations just for a radiator, chaps who have been doing this long enough will know by just looking at the room, if rad is slightly oversized so what, there is a proven way to get over this , its called turning down the valve, and most people can do this without calling a engineer, things are to over complicated just no need for half of this BS.

Last Plumber · Mar 3, 2018

jtsplumbing said:
Would help if he told us the size of his room and size of currant rad, baffles me why some people get so involved with complicated formulas and calculations just for a radiator, chaps who have been doing this long enough will know by just looking at the room, if rad is slightly oversized so what, there is a proven way to get over this , its called turning down the valve, and most people can do this without calling a engineer, things are to over complicated just no need for half of this BS.

If he gave the sizes and info I asked for in an earlier post, I would work it out and tell him what output I would recommend. I thought it might be a help.

Last Plumber · Mar 3, 2018

jtsplumbing said:
things are to over complicated just no need

I would tend to agree with that.

It is one of those long winded calculations which is very methodical and precise. The result however, can still be inaccurate due to all the other factors involved, which are seldom taken into account or cannot be measured.

SimonG · Mar 3, 2018

Better looking at it than for it.

Super Moderator

Plumbers Arms member

Plumbers Arms member

Plumbers Arms member

Plumbers Arms member

Similar plumbing topics