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# BTU output with lower supply temps

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.

• 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.

• Member Posts: 232

There probably is a formula, but I have seen different supply/BTU Ft. charts on most of the manufacturers websites or literature.
• Member Posts: 249
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.
• Member Posts: 240
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.
• Member Posts: 240
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.
• Member Posts: 249

I agree with a 37% increase in area. 27% was a whoops.
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