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Clocking My CGi

knotgrumpy Member Posts: 208
Today I took a few minutes and clocked my gas meter while only my W-M CGi boiler was running.  The boiler is rated at 133,000 btuh input, but by monitoring the gas meter I came up with an actual figure of 124,800 btuh.

This past year I completely rebuilt the distribution side of my system and am looking to replace the CGi with a modcon this summer.  I've installed an hour meter and have been monitoring my run time to get some ball park figure on if my boiler is close to the right size.  Using the DOE output rating on the plate of 112,000 btuh, the highest figure I came up with during sustained below zero weather was 89,000 btuh.

If the input rating is not accurate (for whatever reason), then I'd assume that the figure of 112,000 btuh I used the calculate btu's used to offset heat loss in my house is not accurate either.

I guess there are a few questions:

1) Assuming I am clocking the gas meter correctly (and I think I am) Why the discrepancy? Gas pressure?

2)  The whole idea of recording my boiler's 'on' times was to get an empirical idea of how to size the new boiler.  If the input on the plate is wrong, can I just de-rate the output figure printed on there by the same percentage that the input is off?

Thanks in advance,



    EBEBRATT-Ed Member Posts: 9,731
    gas pressure

    I would trust the gas meter. Believe me, the Gas Co doesn't give away free gas. Your gas pressure is probably off a smidge. Your firing the boiler at about 93% capacity so your output is roughly 105,000.

    If you had the instruments you could get the boiler efficiency but what you are doing will give you accurate results. You could double check it by doing a heat loss of your house but I feel you are in the ballpark.

    If you go mod-con the boiler will modulate to match the load just have to be in the range of the boiler you need.

  • Tim McElwain
    Tim McElwain Member Posts: 4,477
    When determining

    boiler firing rate with the meter the formula is:

    Let the meter 1/2 foot dial make two rotations and accurately clock that with a stop watch. Let us say it took 20 seconds, divide that into 3600 (number of seconds in an hour) that comes to 180 Cubic feet per hour. The boiler is rated in BTU's so to get BTU's from Cubic feet you must know the local BTU per cubic foot. That is usually on the gas bill it will say so many ccf at 1025, 1025 is the average BTU sent out for that particular area. You would multiply 180 times 1025 to get your BTU  184,500.

    It is also a good idea to check gas pressure at the outlet of the gas valve in most cases it is 3.5" W.C. unless you have a negative pressure gas valve. Do not use a water filled manometer on negative pressure gas valves as they will suck the water out of the manometer.

    Hope that helps. 
  • knotgrumpy
    knotgrumpy Member Posts: 208

    My heat loss calculations came out to around 100,000 btuh, so I think I was a little bit high on that.

    Now I get to pick a boiler.  Lot's of choices...
  • knotgrumpy
    knotgrumpy Member Posts: 208
    Thanks Mr. McElwain!

    I used 1040 btu/cf and used the 1/2' dial like you suggested.

    Appreciate the response!