Designing a house in Rhode Island. Location is offshore so climate is tempered. Under the code we're zone 5 but eastern tip of Long Island is 18 miles west of us and that's zone 4. Design temp is +5. January average temp is 35. Winters are very windy. House will be off grid, solar PV with propane generator backup. Construction is full basement, first floor is slab on bar joists, second floor is trus joists frame with 1 1/2" gypcrete with radiant in both. Walls are essentially double stud with 2" XPS on exterior - total R about 42. Roof is R 65. Under slab R20 and basement walls R22 (ICF). Total heat loss (conduction only) figures to about 14,000 Btu's per hour. I haven't yet calced the infiltration/ventilation load but figure it will be about 8000 to 10000 Btu's for a total peak load of say 24,000 Btu's ignoring passive solar gain. House is designed with about 7.5% of the floor area in south facing windows. I realize my radiant water temps will be very low so slabs won't be "cozy warm" but other benefits point toward radiant. Question is: with loads this low what is the best way to heat this house? There are no 25,000 Btu oil boilers I can find. No NG available. Propane is expensive here ($5.65/gallon) oil is perhaps the best buy per Btu. Should I (can I) heat with an oil fired domestic water heater? Smallest of those have inputs in the 100,000 range. Can I integrate domestic HW, radiant water and perhaps some solar thermal all into one system? (I have available roof area for solar thermal but thinking of that as "future") All suggestions welcome.

You can use a water tank to buffer the excess capacity of the boiler. But why not a heat pump along with heat recovery from the generator? Perhaps run a diesel generator on oil (longer life, better efficiency).

I would build with both interior and exterior air barriers. For example, house wrap and taped foam respectively.

Heat pump would far exceed my solar generation. I'm going off grid since we have VERY expensive power here ($.61 / kWh). The hope is the generator will be used very seldom. I wouldn't have it at all except I will be renting during the summer and vacationers will not tolerate changing their lifestyle when the batteries are low. Noise is an issue with the genset so I'm hoping propane can be quieter and cleaner. It will be in a separate shed so capturing the heat may not be worth the effort as hopefully most of its hours will be during summer. So there would be a small boiler with a buffer tank and then the radiant and the DHW would draw heat from that via two heat exchangers? Or would the buffer tank contents circulate thru the radiant with temp control via a mixing valve and DHW would be via an indirect heater on a separate zone off the boiler? Having trouble getting my head around this....

capturing the heat may not be worth the effort as hopefully most of its hours will be during summer.

Because of the vacation/renter scheduling? Normally, cogeneration is a good fit as heat production often comes during cold dark hours.

The typical diesel generator is louder than the typical propane one. But, I expect that you want a separate, fairly soundproof building, so that can become a non-issue and then diesel looks best.

I would separate the heating system from DHW. With a radiant floor, the former needs lower temps and, with the right source, lower temps are more efficient. For DHW, consider heat from the generator and/or a heat pump water heater.

Suggestions.
Something used predominantly in the summer in a high cost energy environment makes me think of a few things.
1. You can probably make a strong case for solar DHW heating. Much more efficient per btu than PV if you are using it to heat stuff. Sized right it could be 80-90% of the btu’s needed for hot water in the summer at peak rental time. Pair this with a second tank heated by the boiler.
2. Oil boilers are tough in an efficient house. There is a minimum nozzle size before you have problems with clogging. Most guys I know don’t like going smaller than 0.75 gallons/hour on the nozzle this translates into around 100,000 btu unit. If your MAX heat loss is in the 25000 btu/hour range you have a lot of choices with small gas boilers, none with oil (4 x oversized). Somegas boilers have min fire rates as low as 15000 btu’s hour. I know propane is expensive, but I am assuming that in the winter the place will be mostly unoccupied ie just enough heat to keep it from freezing so you can figure out how many Heating degree days you would have with a 45-50 deg base temp, instead of 65 and figure out how much propane you really need for the winter. It may be small enough to deal with the high cost of propane.
3. What every you do you only want one tank, so go all propane or all oil as far as generator and heating system go.

I recommend that any house that will be unoccupied in a cold winter be designed so that it can easily be left unheated. A small air compressor makes emptying the pipes easy.

Any oil boiler would be insane for a house with an I=B=R calculated heat load of 14BKTU/hr.   At 5-6x oversizing just the standby-loss of the boiler might be enough to heat the house most of the time (except for cold-start tolerant units.)  But either way, an oil boiler is going to be a problem to be designed-around rather than a solution to a heating problem.  If you go that route, only direct-vented sealed combustion units would be appropriate, since the air leakage of the vent & combustion air on other units add a  real fraction to your low heat load.  Peerless makes a 0.5gph unit that might work, as well as the Buderus Logano series, but there are fuel-quality issues that can muck with tiny 0.5gph jets and condensing oil boilers to consider too- it may be too risky to go there unless you KNOW there is the local fuel & support to run & maintain them.  They're rediculously oversized for the load though.

Smallest in class propane mod-cons have a min-mod that low, and if you allow 25-50% oversizing for infiltration it might make some sense. With a radiant slab that cold efficiency could be made to average in the high 90s.  A condensing propane tank HW heater like the Polaris would hit in the mid-90s, and may be the "right" solution.  At heat loads that low, even at $6/gallon heating with propane won't break the bank, but it won't be free either.

BTW: The 99% outside design temps for Block Island/Long-Island are more like +10F rather than +5F, and your heating/cooling balance point is likely to be closer to base 60F than 65F, not that it moves your load numbers enough to matter.  The newer design temp listings are 4-5F higher than older listings, in part due to warmer average winter temps over the past 15 years, but also from a subtle difference in how the 99% number is defined- it's similar to what had been listed as the 97.5% number, which is a perfectly valid number to size for, particularly in tighter higher-R houses.  If the place is unoccupied in winter and you're only maintaining 50-55F indoors, the heat load number will shift much more significantly. 

Fuel use depends on your as-used HDD base, since your average load during the shoulder seasons drops to about zero at base 50, but at base 60 or 65 it's still a real number.

Posted By jonr on 06 Sep 2012 03:17 PM
I recommend that any house that will be unoccupied in a cold winter be designed so that it can easily be left unheated. A small air compressor makes emptying the pipes easy.

The January mean temp on Block Island is about 30F and cold snaps where daily highs are below 25F for several days are all but unheard of.  Worries about a tight well insulated house with passive solar tempering freezing-up are a bit misplaced there compared to even 50 miles inland, since the cold snaps are highly correlated with clear weather and higher solar gain, and the lows are moderated by being surrounded by water.

With a lot of spare time you could simulate how well it would coast through the coldest weeks of the past decade.