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Ben-gee

Esteemed
Plumber
Messages
1,808
Rads are sized by working out the heat loss from a room, there are various calculators to do this available on the internet or your plumber’s merchant will do it.
The more information you use the more accurate it will be; room dimensions, target temperature, windows, external/internal walls, construction type, what’s above, what’s below, north facing...
This is approximate order of importance.

You end up with an amount of energy required to heat that room, this can be reported in watts or btu, it doesn’t matter - they are both just units of energy and you can switch between the two with simple maths. (Just like miles / kilometres, or kilograms / stones etc )

For simplicity stick to one lot of units, watts is most commonly used now (uk).
 
Messages
1,813
Most commonly used in the UK is watts or kilowatts. Watts is an SI unit for power and energy is measured in joules. A 1 kw radiator will be delivering 1000 joules per second of energy. If you know the power requirement you can easily convert to btu's (British Thermal Units).
As above the more information you have the more accurate your heat loss result will be but there are a lot of variables to consider.
 

Knappers

Plumber
Gas Engineer
Messages
552
As above its to do with heatloss.
There are plenty of online calculators, you want atleast to be using one that asks about the building materials, size of widows and external walls.
Heat loss and rad sizing are not the same tho make sure you use rad sizing tool.

Technically what your doing is working out the heatloss (usually with a 20degC differential between inside and out) and then adding an amount for heating the volume of air in the room by 20deg within 1 hour.

It doesn't hurt to oversize rads but you should aim to oversize all rads by the same percentage (say 20%). If I were sizing replacement rads then I'd check to see if existing rads are oversized and adjust to suit, balance is the key to a happy system.
 
Messages
23
Most commonly used in the UK is watts or kilowatts. Watts is an SI unit for power and energy is measured in joules. A 1 kw radiator will be delivering 1000 joules per second of energy. If you know the power requirement you can easily convert to btu's (British Thermal Units).
As above the more information you have the more accurate your heat loss result will be but there are a lot of variables to consider.
So what would a 1000x500 single rad kw output be??
 

Ric2013

Plumber
Messages
2,839
So what would a 1000x500 single rad kw output be??
You really need to check manufacturer's specifications as it depends on the radiator (and also at what temperature you are running it - the modern trend is to assume the average radiator temperature is 50°C above the desired room temperature (known as Delta T 50) but some retailers still use higher Delta T ratings, and you may even run at a lower Delta T to improve boiler efficiency, especially in less cold weather). The hotter the radiator is run, the greater the output.

But, if you just want an idea for now, to be able to compare likely outputs of radiators or your existing radiators, this link is quite helpful: Radiator Power Output - SimplifyDIY - DIY and Home Improvement Solutions - http://simplifydiy.com/plumbing-and-heating/radiators/power . Do note that there are "single panel radiators" and "single panel single convector radiators" (i.e. finned), so do look at the right chart. I don't know what Delta T that link is written for, but if you take it as a rough guide, it will give you some idea.
 
Messages
23
You really need to check manufacturer's specifications as it depends on the radiator (and also at what temperature you are running it - the modern trend is to assume the average radiator temperature is 50°C above the desired room temperature (known as Delta T 50) but some retailers still use higher Delta T ratings, and you may even run at a lower Delta T to improve boiler efficiency, especially in less cold weather). The hotter the radiator is run, the greater the output.

But, if you just want an idea for now, to be able to compare likely outputs of radiators or your existing radiators, this link is quite helpful: Radiator Power Output - SimplifyDIY - DIY and Home Improvement Solutions - http://simplifydiy.com/plumbing-and-heating/radiators/power . Do note that there are "single panel radiators" and "single panel single convector radiators" (i.e. finned), so do look at the right chart. I don't know what Delta T that link is written for, but if you take it as a rough guide, it will give you some idea.
One more question
Does say an old 90000btu oil boiler have a smaller burner and or boiler than say a 120000??is the popular 90 of old be like a 26 kw now??hope makes since
 
Messages
1,813
You would need to look at individual brand radiator catalogues to get an idea of outputs. There's a number of factors and correction factors to consider when sizing a radiator.
With regards to your oil boiler, the burners can be range rated to usually three outputs, say 18kw, 25kw and 33kw for example. This is done by installing different nozzle sizes and pump pressures. 90000 btu's is roughly 26kw.
 

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