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BTU Reduction When Closing the Damper on Hot Water Baseboard

smithgroup Member Posts: 2
edited June 2022 in Radiant Heating

I've searched the internet for hours and cannot find this data anywhere. How much is the BTU per foot reduced by closing the damper?

My Slant-Fin baseboards are rated at 480 BTUs per foot at 1 GPM with 170 degree water.

My question is how much will the BTUs drop, per foot, if the damper or louver is closed?

Isn't there data on this somewhere? BTU rating open vs. closed damper?

Many thanks in advance!!!


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,175
    edited June 2022
    I can't remember where I heard this. This is something that I just understand to be true because I heard it so long ago that I just assume it it be fact.

    85% of a baseboard radiator's output is convection and 15% is radiant heat.

    I must have heard it from the service manager at my first job so many years ago. I believe that is a generalization and not an exact measurement.

    To be completely accurate, some sort of test must be completed to determine the exact measurement for each different brand. But what would be the point. Of course, this is just the type of nonsense that the Federal Government might spend half a million on in order to use up the money so some research agency won't loose its funding for next year. So call your congressman if you really want to know.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • hot_rod
    hot_rod Member Posts: 19,980
    If anyone, I would suspect the manufacturer has that data. Did you dig deep at their site?

    I suspect it would be close to the output with low temperature supply. At 110F supply that board has a 140 btu/ ft output.

    Used to be a time when the Hydronics Institute tested fin tube equipment that manufacturers submitted. That kinda went south when GAMA took over IBR.

    I think manufacturers do their own rating now. I don't know if a damper closed test was done, however.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,085
    Hi, How about doing some testing on your own? Try measuring supply and return water temps with dampers open and again with dampers closed. Now you can calculate how many BTUs were given off. Divide by the linear feet of baseboard and you'll have a pretty good number.

    Yours, Larry
  • smithgroup
    smithgroup Member Posts: 2
    Good suggestion, Larry Weingarten.

    Unfortunately, I don't have a way to measure return water temp or what my GPM flow rate is.

    I would think Slant-Fin would have BTU data for both opened and closed baseboard housing, but I cannot find it anywhere on their website.

    Anyone from Slant-Fin on here?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,175
    edited June 2022
    I still think the Fed could spend our tax dollars to get the answer. But could we really believe the results?
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics