Howto calculate radiator size for room

[Gasmk1]

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]

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]

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]

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)

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]

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]

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]

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]

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]

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]

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.