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

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



Views: 222

Reply to This

Replies to This Discussion

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.


Latest Activity

Landon added a discussion to the group Energy Auditing Equipment for Sale, Trade or to Purchase

WANTED: Blower door complete set Retrotec or Minneapolis

Hi all,I'm looking for a blower door system complete, ready to go. Preferably used, unless you have…See More
1 hour ago
Landon joined allen p tanner's group

Energy Auditing Equipment for Sale, Trade or to Purchase

Discuss the pros and cons of the equipment you are interested in prior to purchase. Post equipment…See More
1 hour ago
ABAA posted an event

ABAA 2018 Conference and TradeShow at Salt Palace Convention Center

May 8, 2018 to May 9, 2018
7th Annual Air Barrier Association of America’s conference and trade show.May 8-9, 2018 at the Salt…See More
2 hours ago
Michael R. Johnson liked Bruce Sullivan's blog post Solving Energy Poverty: Energy Efficient Homes for Those Who Need Them Most
3 hours ago
Home Energy Magazine posted a blog post
4 hours ago
Diane Chojnowski posted a discussion

ACEEE Fact Sheet: The Impact of Federal Energy Efficiency Programs

Federal energy efficiency programs have played a key role in reducing energy use and saving…See More
5 hours ago
Profile IconLandon, Lloyd Elam, Jeremiah Doty and 3 more joined Home Energy Pros Forum
5 hours ago
Khaliquzzaman Katchi liked Khaliquzzaman Katchi's discussion Retrotec DM-2 Pressure Gauge
5 hours ago
Mick Lane replied to Steve Eberly's discussion Infrared camera recommendations for general contractor use
"The real question is what are you going to do with the camera.  Does it need to take pictures…"
6 hours ago
Lloyd Elam commented on Home Energy Magazine's blog post How We Turned Our House into a Giant Foam Box, Part I — Wall Insulation By Wen Lee
"Can't wait to see what your blower door number will be...."
6 hours ago
Danny Gough replied to Jeremy Begley's discussion Manual S for Water Furnace Equipment in the group HVAC
"Well said David Butler. One more thing I would add is if you have some flexibility with the loop…"
8 hours ago
Leslie Jackson posted a blog post
14 hours ago
Steve Eberly posted a discussion

Infrared camera recommendations for general contractor use

Hi, I believe I have searched and found little recent discussion on recommendations for IR…See More
16 hours ago
Jeremiah Doty commented on Jeremiah Doty's blog post A list of 7 important questions to ask potential Contractor's BEFORE hiring them!
"To read more of our helpful Blog's visit us at https://www.jandmservices.us/blog/"
21 hours ago
Jeremiah Doty posted a blog post

A list of 7 important questions to ask potential Contractor's BEFORE hiring them!

A list of 7 important questions to ask a Contractor Before Hiring Them.1) First and foremost when…See More
Todd Williams liked Diane Deyerler's event Energy OutWest Conference
Khaliquzzaman Katchi replied to Khaliquzzaman Katchi's discussion Retrotec DM-2 Pressure Gauge in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"I am prepared to pay $200 and I will pay the shipping to Canada."
Brad Cook replied to Brad Cook's discussion When is a fuel history confidential and why?
"Personally, I just don't see what is so personal about how much fuel your home uses, in…"
Barbara Smith liked Diane Chojnowski's video
Al Tibbs replied to Khaliquzzaman Katchi's discussion Retrotec DM-2 Pressure Gauge in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"I have a DM2 Mark II in VERY GOOD to Excellent Condition. I will sell for $300."


  • Add Photos
  • View All

© 2018   Created by Home Performance Coalition (HPC)   Powered by

Badges  |  Report an Issue  |  Terms of Service