Combustion analysis question

David Nadle

What you say makes sense--what a bummer if the gas company is watering down the booze. But it's a red herring. If we hold BTU/hr flow constant, what you seem to be saying is that the matched rate is primary, then a combustion analysis is done to ensure there isn't a problem like insufficent air. Is that correct?

David Nadle

A contradiction?

Help me wrap my mind around this: I understand the basics of ideal combustion; regulate the fuel so that there is just enough excess air and CO2 is maximized. Let's call this the ideal fuel rate.

Now, the boiler manufacturer may recommend determining the BTUs per CFM of your fuel and regulating flow to match the input rating of the boiler. Let's call this the matched fuel rate.

In the case where the ideal rate is greater than the matched rate (plenty of excess air?), isn't it still better to follow the matched rate, because while you may not achieve the best combustion efficiency you will be using less fuel to obtain sufficient heat?

J.C.A._3

Wow...

Deep question.

My take is to clock the meter, find if it is the appropriate amount for the boiler...at the correct manifold pressure...of course, and work from there.

Altitude, and the ever changing quality of the gas(WOG)will be the determining factors of the INPUT.

Next would be the analysis of the fire and the numbers it was showing me, as long as the previous numbers were right.
(keep in mind that a gas meter just counts what goes through it, if the local gas entity has to pump up the pressure due to high demand, it could be anything from compressed air to L.P.)

Bottom line is... a btu is a btu. Depending on the mix, the meter could spin up to show a proper input...while the product being delivered may contain less btu's than regularly prescribed...Does that make any sense?

If I suspect that the gas is being "subsidized" and there is a question...I'll make it a point to go back and test for proper combustion during a "low load" time of the day to cover my butt. Chris

Jim Davis_3

Nothing is ever equal

Btu input is a theoretical value not a real value. 1 cu ft of gas can produce 1050 btu or 500 btu. We can never produce 100% of the rated btus of fuel into useable heat except at perfect combustion - 0% excess air. If you supply 100 cu ft of gas to an appliance the most useable btus that are produced in our world is between 95% of the btus at 3% O2 or 90% of the btus at 6% O2. If we transfer 80% of 90% we are 72% efficient. HOwever the analyzer only registers the 80%. The only way to truly measure actual performance of any mechanical device is by output only. Therefore the measured output btus should match the manufacturers specs. Input is and always has been an assumed value. No mechanical device can be properly tuned by measuring and setting input. Measuring gas pressure and clocking meters is only to record where the final setting might end up. Oil, LP, Wood, Coal etc cannot be clocked. All fuels burn at their ultimate CO2. Excess air just dilutes the readings in the flue and in reality give us an idea of how many btus we might be producing. The more air the less flame temperature. The less flame temperature the less btus.