Compare boiler and furnace efficiency

I currently have an oil fired furnace that has an efficiency rate of 81%. I am considering replacing my current hot water heater and this furnace with an oil fired boiler (Biasi B-10) that has a net output 69K. It uses a .65 nozzle or 91K BTU's of oil to obtain that net output. This seems to indicate an efficiency of about 76%, but the manufacturer rates it as 86.6% efficient.

How do I compare real world use of these two heat source and the respective efficiency?

 

What you want to look for on just how good is the furnace or a boiler. Look at the HSPF of the unit. Not what size the nozzle is or Btu input.

 

I thought HSPF was a heat pump rating and AFUE was for fossil fuels.

The AFUE on the furnace is 81%, thus the net putout is 81% of the input. The AFUE on the boiler is 86.6% but the stated net putput work out to be 76% of the input.

Will the boiler be more efficient that the furnace and will be be differnt than the 5.6% difference in the ratings?

 

There are a couple of different "net output" figures. You are probably looking at IBR. There is also DOE net output which is always higher.

 

read this tutorial on AFUE ratings:

http://energykinetics.com/pdf/AFUE%20position%20paper.pdf

AFUE only tells part of the story. It is a starting point, but it does not reflect real world operating conditions. Almost all of the new boilers will be efficient if they are properly sized to the heat loss and installed correctly. My backup multifuel system is 78% AFUE, and my Biasi B10-4 was tested at 87%. But, the savings when burning oil is a lot greater than the 9 percent difference based on the AFUE ratings! Another major issue is properly sizing the boiler to your heat loss. From your info, it sounds like your going with the smallest Biasi made, the B10-3? I assume you had a heat loss done?

 

Grady,

Yes, I am looking at the lower IBR number which I have been told is the number to match to the heat loss. It is about 76% of the input amount and that is where my confusion comes from.

 

Also, where did you get those figures for the Biasi B10s?

My book shows the B13 is rated at 67MBH gross, 58 Net with a .55 nozzle. The B14 is rated at 110 MBH gross, 96 MBH net with a .90 nozzle.

 

radioconnection,

Thanks for the link. I will check it out.

I have done my my own heat loss calc and I will require the installing contractor to do one also so that it is "his" number. I did my own just so I would know what is going on. I am doing some tighting up of the home, plus I thought both my cooling and heating units were grossly oversized. They are!

When you state that the savings is greater than the 9% differnce I assume you mean that with oil there is much more BTU's per gallon that it is cheaper to heat with oil that propane? Or, are there other benefits?

 

Looking at too many things at once. You are correct with your numbers. Doesn't change the question though.

 

I'm sorry, Perhaps I didn't understand what the question was? I thought you were referring the to discrepency on the net vs. gross not working out to be 86%. The figures in my manual are in that range; and the service tech measured it at 87% when he tuned it last year. Regarding real world differences, I believe the AFUE is the steady state rating for the boiler if it is continuously running. Perhaps Who or one of the other regulars could add to this. AFUE doesn't account for idle time heat loss.

Pete

 

mchild, the IBR rating would be what you couild get steady state if the boiler was outside of heated space and also had piping utside of heated space.

So what are you wanting to heat the house up with is you get a boiler? Sorry if I missed it? Rads or are you just going to an air handler?

 

Who,

So if the boiler is within the home I should size it off the DOE net number? This is contrary to other advice I have received.

The system I'll be using are two heat pumps (up and down) for the primary heating and the boiler will serve as auxilary/emergency heat to the heat pumps. I will be using hydro coils in the duct work for that and then I'll also have the indirect for DHW.

Thanks.

 

mchild, yes, here for permit reasons the IBR is used for some strange reason (lack of home hydronic heating knowledge is my guess). But if the whole heating system is within the heated envelope the the net is better. The IBR has pickup factors for heatlosses that would all be lost within the envelope. The only heat you should be losing is what goes up the chimney - not what you lose through the boiler jacket and close piping (essentially that's what IBR nets out).

What's your heatloss and and what are your winters like? Average outdoor winter temps and design temps?

 

Who,

Near the foothills of the Blueridge Mountains about 55 miles West of Washington, DC.

Design temps are 90/8 (summer/winter) and indoor is 78/69. Heat loss works out to be about 65K. If the winter design is dropped to 2* the heat loss goes up to 70K. Since I will have two heat pump systems (up and down) I'm not terribly worried about having to provide emergency heat for both at the same time. But, having the boiler capable of providng all the heat would be a good fall back if electric rates spike and it becomes cheaper to heat with the boiler and lock out the compressors at some point when it gets too cold and the heat pump COP drops too low.

Before I actually jump off and do all this I am going to have a blower door test done to confirm my infiltration rate.

 

The reason I ask what you average winter temp is was to get a sense about how mild the winter is there. With milder winter weather and larger shoulder seasons a modcon on propane might be worth checking out for cost comparisons. Efficiencies on these boilers change with teh weather where oil boilers are opposite. With oil, peak efficiency is at design and it falls from there, often quite a bit during shoulder weather. With a modcon their worst efficiency is at design conditions although even then they tend to be pretty efficient.

Even here in teh Toronto area only about 10 days a winter tend to be really cold (under 20°F).

Food for thought while you're considering options...

 

Sorry I did get the HSPF and AFUE mixed up . To much AC and H/P here
You might want to go to http://warmair.net there you can compare the fuel cost for where you are.

 

I looked at mod/con early on in my quest but passed on them as the net cost of a BTU was so much higher than oil. There is only about a .30 cent difference between propane and fuel oil (oil higher) in my area. How else should I be comparing the two to determine true operating cost?