backup hydronic heat source?
 

Looking for info on inexpensive backup heat source for hydronic radiant floor in 800 sg'(two floor) super insulated passive/active solar addition. Worst case heat demand 4,000-5,000 Btu/hr. Elec Coop won't allow off-peak storageelec water heater as not UL boiler rated. Elec Storage furnace, heat pumps boilers all costly for a small heat demand. If the government wants to spur sustainable homes they should be supporting development of affordable backup systems. Ideas?

For back up heat do you really want a off peak tank? unless you know that you are going to have cloud cover the night before you are not going to know that you need to switch on that off peak tank heater so you are either going to have a 12-24 hour lag waiting for off peak night time rates to kick in or you are going to be heating that tank every night and you have a small space so you don't need as much heat!
From what I can find 5,000 BTU is about 1,500 watts in electric heat so a small 5 gallon tank style water heater hooked in as back up is going to be able to keep up with that for the most part.
Otherwise the last straw bale house that I worked on we installed a small electric forced air furnace in the heat recovery ventilation system because the building inspector wanted there to be a furnace of some sort in the building even tho it had active solar hot water in floor heat and that was the cheapest and would provide quick heat as a seldom used back up.

Good thoughts. Maybe a small/med propane water heater might be a good backup and cover me in elec outage. Shouldn't require large(yard size) propane tank to run.

A power vented gas water heater with electronic ignition would allow you to put the water heater on a thermostat so that it kicks on when the house temp drops, with any back up like this you would need to install a pump on the water heater tank and a few check valves so it could be automated.
Heating air in a back up situation is going to provide more instant heat then heating up a mass like your floor for quick thermal recovery and quick shut down of the back up.
If you want to go with gas heat, a small direct vent in wall gas heater might be the way to go, 800 sf house is small enough that if it's well designed you could almost go with a heating system designed for an RV.
Other thing to think about is that on a sunny day the south side of your house is going to be the warmest part but on an over cast day the south side is going to be the coolest as the windows are going to give the greatest heat loss, so even adding a base board heater, hot water or whatever source you choose, might be the best way to give your house an even temp on over cast days.

I think I saw some of that kind on Radiant floor heating by Michael Hackleman Issue #64 . Or can I know your location? So that I could search a backup heat source for hydronic radiant floor for you.