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dkiernan
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Posts:34
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| 04/01/2009 6:22 AM |
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I have been looking at alternative energy ideas for my home since last summer. Currently, with the tax incentives, both state and federal I have been thinking about putting in solar panels for domestic hot water. I now heat my hot water from an oil fired boiler. The hot water is stored in an Amtrol hot water tank that is separate from the boiler. My question is this. Since the boiler runs all winter(I live in RI) am I getting this hot water as a free by product of heating my house?
The one installer I talked to so far, said I should save between $500-$600/yr. in hot water heating costs. This would give a payback period of 7-8 yrs. I would appreciate any help on this topic. Thanks.
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Dana1
Registered Users
Advanced Member
Posts:702
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| 04/01/2009 1:53 PM |
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Posted By dkiernan on 04/01/2009 6:22 AM
I have been looking at alternative energy ideas for my home since last summer. Currently, with the tax incentives, both state and federal I have been thinking about putting in solar panels for domestic hot water. I now heat my hot water from an oil fired boiler. The hot water is stored in an Amtrol hot water tank that is separate from the boiler. My question is this. Since the boiler runs all winter(I live in RI) am I getting this hot water as a free by product of heating my house?
The one installer I talked to so far, said I should save between $500-$600/yr. in hot water heating costs. This would give a payback period of 7-8 yrs. I would appreciate any help on this topic. Thanks.
There ain't any free lunch- energy that gets drawn out of the indirect-fired tank in the form of hot water gets replaced only by burning more oil to heat up that which is drawn in. Whether you're also burning oil to heat the house doesn't change this fact. (It's the first law of thermodynamics, for those who slept through HS science. :-) )
Unless you use a HELLUVA lot of hot water, or your boiler is some miserable antique with a sub-60% combustion efficiency it's unlikely that you're you're even paying $500-600 year for hot water. (Got 6 kids who all take daily showers?)
For example, at my house (family of 3, in Worcester MA) we burn maybe 10 natural gas therms (about 7.5 gallons worth of oil) per month in the summer season. The incoming water is much colder in winter, but mid-winter water heating energy requirements is not likely more than 3x the summer water heating energy. Order of magnitude, we probably spend at most an average of 20 therms/month for hot water, x 12= 240 therms in a non-condensing ~80% combustion-efficiency old-skool tankless HW heater (with standing pilot that burns 7 therms/month to boot, whether we use HW or not). This is the energy equivalent of ~180 gallons of heating oil. Even at $2.50/gallon that doesn't quite add up to $500. YMMV.
Then consider that the "solar fraction", the percentage of water heated by your panels isn't going to be 100%, optimistically you can figure on 60% annually, best-case, unless it's so oversized it needs active heat-dumping to keep it under the boiling point in the summer. For me, that'd be under $300/year. Again, YMMV. (DO you have 6 kids who shower daily? :-) )
That said, you can still get good value out of a really stripped down 1-2 panel low temp solar HW system pre-heating the water in a tank preceding the Amtrol to 60F+. By running the solar colder (60-120F instead of 120-180F) you get better collection efficiency out of the panels themselves, since running cooler they give up less of the captured heat to the colder outside air. But whether that's a better value (even with subsidy) than other potential efficiency improvements at your house (insulating the basement, air sealing the rim joists & attics, etc) is highly particular to the house. But unless it's been thoroughly analysed and retrofitted from a heating efficiency point of view, it's unlikely that solar HW is going to be your first-best efficiency or carbon-footprint reduction investment. Unless you're in a certified Energy Star house, it's likely that you can achieve greater annual fuel savings for that level of cash outlay by something other than solar technology.
Do you burn under 1000 gallons/year total for a family of 4? Then a full-on solar HW system might be the next-most cost effective measure to take, but not likely before then. Get a professional energy audit (complete with infra-red imaging and blower-door air leakage tests), explain that you have $XXXX to spend on efficiency and need to figure out the best place to spend it. Alternatively, the Lawrence Berkelyl Nat'l Labs house model is pretty good if you take the time to measure accurately and fill in the profile correctly:
http://hes.lbl.gov/
It's based on weather data for your zip code- tweak the 101 answers in the profile until the projected energy use is within 30% of historical reality, then look at the "upgrades" cost-benefit analysis it generates. (Fill in real numbers for what various items cost when you have quotes or better estimates) Save your session number- you can come back an fiddle with it as things change. But with quasi-real numbers filled in, anything with simple-payback in 5 years or less is probably a financial no-brainer.
Also, once you have it estimating within 30% of your actual oil use, then see how big a fraction they're guesstimating your hot water portion is, then compare 60% of that dollar number to what your solar installer was telling you. (The estimate it gives me is ~35% high overall, but estimates $322 total for the hot water portion, which may be low, but can't be 50% of reality.)
FWIW: Indirect fired hot water heaters like yours tend to be far more efficient than standalone tanks even when running off an old 80% AFUE boiler (most oil-fired boilers less than 20 years old are more efficient than that), and by increasing the average duty cycle of the boiler, boosts it's effective AFUE efficiency a smidge.
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