'On demands' do have the delay and waste situation described above but for low-use periods you might want to consider one of the less expensive ones offered at Sportsman's Guide.
I got the cheapest propane version, to use in a camper I have as a guest house. The more expensive ones are about $250 and should be enough for a regular household. If you are handy, you can plumb a spigot line from the hot-water circuit to the outside of your house fairly cheaply and even if you hire a plumber it shouldn't take more than 2-3 man hours.
The whole thing will be much less than the couple thousand that will be wanted for an inside installation. When the propane tank gets chilled it won't provide as much energy to the heater but you still have your boiler to switch on. It's just a question of turning a few valves when you decide which unit to use.
If this suggestion doesn't work out for you, you can still get some of your money back by selling it and you'll have a hot water outlet which will come in handy for cleaning your grill and other gear.
I mounted mine on a post a couple feet away from the trailer wall. I would suggest a similar installation but the larger ones do offer more conventional options. Mine is removed in the off-season but you may choose to leave it in place but winterized.
Good luck!
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