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Cold versus hot combustion air-
If you look at the three T's of combustion: Time, Temp. & Turbulence, you would think that a hotter flame would be more efficient. Ask any woodstove mfr. if he wants cold air injected into his combustion chamber. They now have baffles on the intake air to preheat it and wash the glass to reduce soot blocking the view of the flame (yes, I said soot because it is inside the combustion chamber).
Now, look at it from a race car driver's perspective- If he compresses air, he drives off heat but it densifies the air to a point so he gets more horsepower per unit volume of air delivered. Look at the converse- combustion struggles as you go up in altitude, which is why combustion airplane engines went to superchargers for high altitudes.
Now, back to the home. Is there really significant difference in the density of 70 degree room air versus cool or cold outdoor air? Not a whole lot.
So far, we have discussed combustion efficiency. That's not the same as overall heating efficiency. If you use room air for combustion, you must have adequate makeup air infiltration to replace stack losses. That means cool air pulling the home down some thus driving up the heat demand. Now, if I install a hard ducted MUA kit, that cool, dense outdoor air is piped directly into the combustion chamber without cooling the home down or removing conditioned air from the home. Therefore, the AFUE rating may reflect the difference while the combustion efficiency may hardly see a flicker. You won't know unless you test with a combustion analyzer though.
A word on drawing intake air for Cat III and IV furnaces directly from the home---don't, even if allowed by the listed instructions. Drawing your combustion air from the combustion appliance zone (CAZ), will depressurize the CAZ. If other combustion appliances are present, esp. a draft hood equipped water heater, it may backdraft. I see no logical reason to not draw MUA directly from the outdoors when dealing with these units. This is one case where I don't give a dang or not if it is listed this way, I still recommend it be corrected. Codes and listings don't guarantee performance and they lag behind current knowledge.
Brian, GAMA is the Gas Appliance Manufacturer's Association--not a company. Efficiency ratings are based on testing by independent approved testing laboratories to a national standard such as the Gov't Annual Fuel Utilization Efficiency or AFUE. You can have a high combustion efficiency with a much lower AFUE do to a variety of factors. Some of these factors, such as heat exhanger design and materials are part of the appliance while other factors such as the fan speed, duct flows and pressures and thus Delta T will affect overall heat transfer. If the fan is running too fast or the ducts moving too much air, it can strip away enough heat to exacerbate condensation in some units. The opposite case is with low speeds, you overheat the unit causing premature failure of components. An undersized, clogged or otherwise improper vent or chimney can have a similar effect. Each unit has a range of acceptable numbers that affect the residence time of flue gases in the heat exhanger so the unit operates as deigned and within acceptable parameters. Here, there is a growing debate over how acceptable mfrs. specs are as DavidR can attest.
The bottom line is all the more reason to have a pro come in and inspect first then test using a combustion analyzer and other equipment. The days of the old timers eyeballing the flame are over.
HTH,
Bob
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Keep the fire in the fireplace.
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