How much does cycling losses of a gas furnace reduce AFUE vs. Thermal Efficiency? Loss of efficiency for oversized AC units is well documented, but I haven't been able to find anything conclusive on gas furnaces.

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I have the same question as well.

I was looking at displacing my oil heat with a mini-split/ductless heat pump. The payback seamed good until I calculated in my cycling losses or idle loss and domestic hot water generation. My 53 year old boiler with 20+ year old burner running at 80% efficiency consumes almost 300 gallons of oil annually for cycling & DHW. At 486 gallons of oil used a year, the lowest I can reduce my oil consumption is by an additional 186 gallons without replacing or abandoning my boiler.

What can I use to compare cycling losses between boilers? I need something with more information than an AFUE rating.

Energy Kinetics has some marketing information on cycling losses, which is great to help sell their System 2000 products. I would like hard information, not sponsored by one manufacturer.

If I can save 30% on the cycling losses by updating my boiler I can reduce my consumption by 100 Gallons annually. That’s without savings from an 80% efficiency burn to an 90+ AFUE burn. Talking with a neighbor who changed his boiler to a System 2000 boiler four years ago, he noted he has seen an average of 58% oil reduction. If I had the same results I would use only 204 gallons of oil a year!

Perhaps some of the companies who provide both oil and HAVC service have some solid usage numbers that can be referenced to show boiler upgrade saving.

ENERGY STAR's web site has a savings calculator for Furnaces:

http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showP...

and a similar one for boilers.  Presumably these calculators would estimate *actual* savings in the field available from installing a new furnace "box" (with new AFUE) while the rest of the system, ducts, etc., remained the same.  Which is the usual case.  Perhaps this would shed light on your question. 

Also, DOE is setting a minimum electrical efficiency standard for furnace fans (where there isn't one now).  The furnace fan definitely impacts the total energy use of a home's furnace/heating system, especially since the same fan is often used for non-heating air circulation, and cooling circulation as well.  Comments will be taken until Dec. 24.

http://www.regulations.gov/#!documentDetail;D=EERE-2010-BT-STD-0011...

How efficient of a furnace are you starting with...

My guess is others will say the comfort issues are (temp swings) are more of an issue with an oversized furnace.

Hi Joshua.

AFUE's are a great thing to compare things when all conditions are the same and known.

Your boiler with integrated hot water is not covered in AFUE numbers. Here is a link to a report

by an Alaska outfit that tried to make sense out of AFUE's:

http://cchrc.org/docs/reports/afue_final.pdf 

In the report is a reference to work done by a researcher by the name of Butler who is 

doing work on boilers with integrated hot water at Brookhaven Lab.

You can analyze things a great deal, or you can look at your neighbor's example and see how much similar your homes and systems are, and take his data into consideration.

A 50 year old boiler might have really large air passages with less heat transfer, the passages might not be clean, (soot is a very good insulator). Considering the ages of the equipment you quote I personally doubt that your AFUE is 80%.

I see 50 year old boilers on occasion and their builders plates sometimes rate their input/output capacity at well under 60%.

Kind Regards

Jan Gravemaker

Seattle

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