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Calculating flow rates

Jason_13
Jason_13 Member Posts: 305
in THE MAIN WALL
If I know the amount of baseboard in a loop and the temperature drop across the loop is there an easy way to calculate the flow?
Example
15º delta "T" and 42 ft baseboard. Entering water temp 170º exiting 155º
My thought is average water temp is 160º.
BTU per ft (590) x 42Ft. = 24,700
24,700 / (490*15)= 3.3 GPM
Is this correct or am I way off base here?
Any help would be appreciated.
Thanks in advance.

Comments

  • jim sokolovic
    jim sokolovic Member Posts: 439
    That method is...

    correct, Jason. Most people just use a value of 500 in the bottom part of the equation (I forget the fancy math name for that) - it allows them to do the math in their head.
  • John Starcher_4
    John Starcher_4 Member Posts: 794
    The 590......

    ....figure is usually for 180* water, is it not? I'm thinking you need a lower BTU number for the 160* temperature, but you'll need to check the baseboard specifications.

    Starch
  • bob_44
    bob_44 Member Posts: 112
    accuracy?

    Jason, your math is correct but how accurate is your data? Professional balancers do not use this method because of inaccuracy. One, it is very difficult to get dead nuts temps. Two, the output of the baseboard is, for pratical purposes, unknowable to the degree of accuracy necessary. There are too many variables; entering air temp.,air flow, water flow rate, water temp at each point along fin, heating effect factor. I probably forgot a few. A circit setter, a venturi or a pitot tube[a pitot is way too big for baseboard] are the prefered methods. bob
  • Paul Rohrs_2
    Paul Rohrs_2 Member Posts: 171
    I would....

    get the pressure gauge reading of the pump outlet, pressure reading of the pump inlet. Let's say it is 10psi out, 5 psi inlet.

    We know that it is a 5psi differential. 1psi = 2.31ft head
    So the 5 psi differential would equal 11.55 ft of head.

    look at the specific pump curve of your pump and see what gpm rate you actually have for the head loss you have measured. It is an accurate way to get the actual flow rate.

    This drawing is just an example of what I like to see on larger commercial jobs. Residentially, the pump flanges available with test ports on them are ideal for retrieving this info.

    Regards,

    PR
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