BTU use

Brad White_191

I would question your assumption of efficiency- yes you may achieve those numbers sometimes on the combustion side. Oil may well get into the 86-87% range with a good 3-pass but your standard 1-pass might hit 84% on a good day. Likewise the propane- if your return water temperatures are near room temperature, you might see 96%. Unless you are running radiant, I would not expect to see those numbers except on a mild day.

Now, moving away from combustion efficiencies, your annual system efficiency is likely to be 10 to 15 percent less. Modulating equipment will be higher in efficiency, on-off control will be lower.

The size of your appliance is not likely to exactly match your heat loss so there are inefficiencies right there.

For our purposes, I would figure the propane in a Mod-Con with low temperature supply to be about 88% annual average, otherwise maybe 85%. The oil I would take at 78 percent if a good 3-pass, 75% of a standard configuration and 60 percent if an older design which is over-sized. I am assuming hot water in all of these.

Whew.

Now, I do not know where in MA you live (I am in Auburndale) but for eastern MA, inside 495, I would figure about 6,000 degree days. Worcester and west, maybe 7,000. If on the coast, maybe 5,600.

If we use 6,000 degree days and your 70K heat loss is based on a 70 degree delta-T inside to outside, this is what I get:

(Oil at 140,000 BTU's per gallon at 78% efficiency, propane at 92,400 BTU's per gallon and 85% efficiency).

Oil: 791 gallons per year. At $4.75 = $3,757 per year.

Propane: 1,100 gallons per year. At %3.19 = $3,281 per year.

At those prices, I expect the cost of Depends to spike anytime soon.

[Deleted User]

BTU use

I'm trying to figure out if I should switch from oil to propane. Currently, propane is $3.19/gal and oil is $4.75. Using an online heating fuel comparison calculator, and adjusting for appliance efficiency (86% for oil and 96% for propane), I find oil will cost me $39.82 per million BTUs and propane will cost $36.38 per million BTUs.

I'm kind of embarrassed to ask, but how do I determine how many BTUs my house consumes in a year (in order to calculate total cost difference b/t the fuels)?

House is located in Mass, is approx 2900 sq ft, w/ a heat loss calc of ~70 kBTUs/hr.

[Deleted User]

thanks for the info, brad. just curious, how did you arrive at the 791 and 1,100 gallon figures? That's exactly what I was trying to find on the net without luck and thought I'd post here for some help.

Brad White_193

If I told you....

I would have to tell you!

Well, OK. I will

Multiply your heat loss in BTUH by the degree-days for your area then by 24 (hours).

(BTUH x DD x 24)

Divide all this by the delta-T used to derive the heat loss (say it is 70), multiplied by the decimal efficiency (say 0.85) and then by the BTU per unit of fuel used.

(Delta-T x Eff. x Fuel Unit)

The outcome of this is multiplied by what is known as a Cd factor (usually 0.60 for our area), which represents solar gains, internal gains and otherwise unquantifiable factors which offset the heat losses.

The product of all this is your annual fuel unit usage.

[Deleted User]

Thanks so much...knowledge is king!

FWIW, for anyone following this thread, the Energy Information Admistration provides historical, residential, energy prices for oil, gas and propane by region in a downloadable format. Since gas is not an option for me, I estimated the cost of oil vs propane since 1990. On average, it would cost approx $560/yr more to run a propane boiler using the inputs noted in this thread.

http://tonto.eia.doe.gov/oog/info/hopu/hopu.asp