I hear all the time that furnace has 90% - 95% efficiency.

How is the efficiency of furnace counted? When it says 90 % efficiency, what's meant by that? 90 % measured against what? What's 100 %?

Thanks,

Jignesh

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Essentially for every dollar thrown at it you get 90 - 95 cents worth of heat, the rest goes up the flue or... As for the testing you might want to pop over to ENERGY STAR as they have the criteria listed for the testing there as I recall

Adding to Sean's comment... The reason a 90%+ furnace uses a PVC flue (as opposed to metal) is because the flue gas is cooler, compared to a conventional 80% furnace, which loses 20% up the flue. By design, higher efficiency furnaces extract more heat from the combustion air prior to exhaust.

Note that the AFUE rating (annual fuel utilization efficiency) does not account for the electricity that powers the blower motor, inducer fan and electronics.

You also may also want to account for the distribution losses as you move the heat from the exchange to the living space.   It's not uncommon to lose 10-20% through leaks and lack of insulation on the ducts / plenum.

NO combustion based heating system can ever be 100% efficient, according to the Second Law of Thermodynamics.

Energy can neither be created nor destroyed, but only converted from one form to another, The efficiency of a furnace or boiler is a measure of how many BTU/hr in the chemical energy of the fuel can be converted to heat energy that is useful to do work (such as heating the house or heating hot water). The more heat energy that goes out the flue, the less is available to do work, and the lower the efficiency which is measured as "AFUE", or Annual Fuel Utilization Efficiency. The AFUE is based on laboratory testing and adjusted for seasonal variation in efficiency.

Condensing furnaces and boilers cool down the combustion (or flue gases) to condense the water vapor. It takes energy to convert liquid water to a vapor (the "Latent Heat of Vaporization"), and that energy is given up when water vapor is cooled to the point that it condenses. These condensing heaters typically have AFUEs of >95%, BUT if the flue gases are not cooled enough to condense, you won't achieve those higher efficiencies. This is a problem with many boiler installs, where the boiler return water temperature is not cool enough for condensation and the rated efficiency is not attained.

As noted above, AFUE does not include distribution or electrical inefficiencies.

Great question - what the efficiency rating means is one of the most common questions we get asked. In simple terms, if something is 90% AFUE - 90% of the fuel you consume is being used for heat inside the home - the other 10% being wasted. Higher the AFUE the better!

I like looking at system efficiency - Lets say the you have a 95% gas using furnace 100,000 BTU's in put.  1000 CFM into building You use a hood or SP or  some way of testing CFM into building  supply grills total.  Temp diff X 1.08 X CFM = BTU's.   50 X 1.08 X 1000 = 54,000 BTU's  54/100 is 54%

System Efficiency is great.  Not much use, if you don't intend to fix the various problems with the system.  Not much use for comparison shopping for a new unit. If you are energy modeling, it would give you a great check on the duct testing results and the AFUE rating so you can adjust them to observed  conditions.

Installation and proper sizing is important to getting the best performance out of your furnace. If the furnace is rated to be a certain efficiency, it can only achieve that ideal if it sized correctly, etc. Even a condensing furnace only operates in condensing mode only part of the time. A high efficiency furnace that does condensing is "squeezing" more of the heat from the combustion process and putting it into your home, rather than dumping it outside your home in the form of exhaust. That's good. It saves you money on your utility bill, because you get all the heat you need while burning less petroleum-based fuel.

Furnaces and all HVAC Equipment is rated by AHRI.  You can get the certification as to the actual ratings from their website.

AFUE is the essentially the difference between Input Energy and Output Energy.   So a 100k  BTU/hr  furnace,  is the Input amount of Energy.   The output is all heat (Energy) leaving the heat exchanger and entering the duct system of a forced air unit or the distribution system of other type systems.  The rest went up the flue or got lost in the furnace case or warming and cooling the heat exchanger.   AFUE is Annual Fuel Utilization Efficiency. It is a standard way to label the performance from unit to unit. Test conditions may not duplicate actual conditions. This is not an actual tested number, it is a calculated number.

OK lets say you have a gravity air system with a 80% burn on heat exchanger with out the 8" pilot assembly burning 25 therms of nat gas a mo.   The whole system to keep the flue warm and burner from condensating is just 36.6%    To me its total system not just burner   

That is why gravity systems are much less popular then 50 years ago.   It could easily be improved by installing a chimney liner in that huge masonry chimney to reduce the amount of draft needed and the need for keeping it warm.

You could install a draft inducing fan to accomplish the same thing.

Or you could move to some type of heat pump system and drop the fuel fired combustion inside the home. 

Staying with a system that comes in at less the 50% efficiency is not sustainable for many families.

This is a great discussion.  I'd like to ask a question about the distribution contribution to system efficiency.

I remember a matrix (from BPI, I think) that gave duct distribution efficiency based on insulation/no insulation, inside/outside of conditioned space and sealed/not sealed. I can't find my copy. Does anyone have one?

The reason I'm asking, I'm working on a commercial building with 95% furnaces and all ductwork in an attic space above the ceiling insulation.  This is in Montana and the snow melt and ice dam problems are tremendous.  I'm making a proposal to the owner about a fairly major re-insulation project and would like to justify the cost.

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