View Full Version : Combustion Air Source and Cat IV Efficiency

Michael Thomas
08-17-2007, 08:41 AM
At a meeting last night it was mentioned that when the manufacturer of a CAT IV appliance allows drawing the combustion from the interior of the structure instead of directly from the exterior of via a dedicated intake pipe directly connected to the appliance, that if you elect this option a 90+ efficient appliance can drop into the 80+ category.

I've tried looking through the installation instructions for several manufacturers, but I've found no reference to this efficiency drop.

Can anyone point me to a source confirming or refuting this?


Brian E Kelly
08-17-2007, 09:22 AM
What kind of a meeting was this? The eff. does not drop due to pulling combustion air from with in the building vs outside. Think about it , the combustion air has no thing to due with eff. of the furnace. The eff. of a furnace has to due with how much of the total heat can be taken out of the exhaust gas along with other factor's. The main way that GAMA ( the company that determine each and every furnace's eff. independent of any manufacture) is the temp of the flue gas. On a high eff. furnace the flue temps are in the 85-100 degree area and a 80plus furnace the temps. range in the 300 range. I might be a little off on the temps but that is the easiest way to explain. I hope this helps.

Michael Thomas
08-17-2007, 10:12 AM
The argument was two-fold:

1) Colder exterior air burns somewhat more efficiently.

2) There is greater heat loss up out the vent as conditioned interior combustion air is being removed from the structure.

We need a snopes.com for home inspectors!

neal lewis
08-17-2007, 11:07 AM
2) There is greater heat loss up out the vent as conditioned inner combustion air is being removed from the structure.!

I can't believe that there is greater heat loss as air is going out the vent. The house is being depressurized, so that affects air infiltration. But, how does that affect the actual efficiency of the appliance itself?

Brian E Kelly
08-17-2007, 11:59 AM
It seems like you are trying to start something, because any HVAC person I have ever worked with or known would laugh at the statement that colder air burns better. And the statement that there is a greater heat loss up the vent as conditioned inner combustion air is being removed from the structure.
My question still stands what kind of meeting were you at? As this might explain their thinking.

Bob Harper
08-17-2007, 12:26 PM
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.