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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,