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BTU output with lower supply temps
Tom Giedraitis
Member Posts: 44
Is there a method to determining the output of a radiator when supply temp is say 150 degrees versus a manuf listed output for a temp of 180?
I'm trying to oversize radiators so I can run lower temps and still meet design demand.
I'm trying to oversize radiators so I can run lower temps and still meet design demand.
0
Comments

Is there a method to determining the output of a radiator when supply temp is say 150 degrees versus a manuf listed output for a temp of 180?
I'm trying to oversize radiators so I can run lower temps and still meet design demand.
0 
There probably is a formula, but I have seen different supply/BTU Ft. charts on most of the manufacturers websites or literature.0 
Heat output formula
Heat output in Btu / hour is a function of the surface area of the radiator times the difference in temperature between the room and the radiator.
For the 180 degree case the temperature difference is: 180  70 = 110
For the 150 degree case the temperature difference is: 150  70 = 80
80 / 110 = 73% as much heat output at 150 degrees compared to 180 degrees.
If you increase radiator surface area by 27% you are back to the same heat output you started with because the temperature reduction and area increase offset each other.0 
Watch the math...
> Heat output in Btu / hour is a function of the
> surface area of the radiator times the difference
> in temperature between the room and the radiator.
> For the 180 degree case the temperature
> difference is: 180  70 = 110
>
> For the 150
> degree case the temperature difference is: 150 
> 70 = 80
>
> 80 / 110 = 73% as much heat output at
> 150 degrees compared to 180 degrees.
>
> If you
> increase radiator surface area by 27% you are
> back to the same heat output you started with
> because the temperature reduction and area
> increase offset each other.
If you start with cooler water and get 73% capacity, to get back up to 100% means 1/.73 or 1.37, so the surface area needs to be 37% larger, not 27% larger.0 
Watch the math...
I agree with your basic premise (UAdT), but if you start with cooler water and get 73% capacity, to get back up to 100% means 1/.73 or 1.37, so the surface area needs to be 37% larger, not 27% larger.0 
I agree with a 37% increase in area. 27% was a whoops.0
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