I have a small cottage in FL (800 sq ft) and want to use radiant hydronic.  The house is on a crawl space so it will be pretty easy to attach the tubing under the subfloor.  It never gets super cold here but nights can get pretty chilly
(40's F and occasionally in the 30's F).  I'm tired of ice cold floors and tired of forced air heat (always feels drafty)

However,  I don't have access to gas or propane.  Will an electric water heater suffice for my heating needs - short  mild winters?  I'd rather not make the investment right now for an expensive electric furnace/boiler.

The water heater I have now is rated at 4800 watts and almost brand new.

BTW, my heat pump, which is what I use to heat the house now, says that it uses 6500 watts to heat.  And it has been running constantly in this current cold snap (it's 23 years old and on it's last legs) .  Does that mean I'm using 6500 watts constantly (plus the watts for the air handler) every time it's on?  If so it seems it would be cheaper to use two 1500 watt space heaters to heat the living space at half the cost for the time being.

Great website and great info here.  Thanks in advance for any help.

I'm assuming the system (& backup elements?) is using a combined total of 6.5W. When its in the 20s F the heat pump portion's COP is still greater than 1, probably ~2 at 30F so if it's running constantly your actual peak heat load is probably around 6-8KW, ~5kw of heat pump output, the rest from the resistance elements. (Which would be credible at 25-30F with typical FL insulation levels for an 800' house.) If it's running constantly and barely keeping up (or needing supplemental heat),it means yes, it's using the whole 6.5KW.

Electric boilers aren't outrageous in cost, but your average heating bill will be significantly higher using any type of resistance heater (space heater, HW tank or space heater compared to the heat pump, which probably has a COP of 3+ when it's in the 40s & 50s out. To run at the same or better efficiency as the air heat pump you'd be looking at a hyrdonic heat pump.

With a radiant floor insulated to ~R20 from the crawlspace below your heat loss will be lower. It's usually best to seal & insulate the walls of the crawlspace converting it to semi-conditioned space, which gives you the earth-coupling benefit of the thermal mass below, in which case you can insulate under the radiant to ~R12-ish. Insulating the floor/crawlspace will likely cut the total heat load by at least 10%, possibly as much as 20% (depending on the ventilation rate of the crawlspace as-is.), so a 4.8KW water heater and a pair 1.5KW space heaters probably WOULD keep up (but you'd have to skip the AM shower), but at 2-3x the operating cost of supporting the heat load with heat pumps.

Simply sealing and insulating the crawl space walls may be sufficient to quell the ice-cold floor situation, and would reduce the AC load in summer. Subsoil temps in FL are pretty warm (mid to high 60s F in most places), and the floor temp just coasting would be somewhere between the subsoil temp & room air temp. If it's currently a ventilated crawl, the floor temp is somewhere between room temp and outdoor temp on chilly nights, likely ~50F (or even lower on a really cold night). If insulating the crawl cuts the heat load by 20% the duty-cycle of the heat pump will drop significantly while raising the cold night air temp 10F or more. I'd start there first (2" XPS rigid-board glued to the foundation walls caulked/taped at the seams, held in place with furring strips through-screwed into the foundation, and foam sealed the top. Block any ventilation/windows. Insulate the sill & band joist with the same materials, and lay down at least a 6mil poly vapor retarder, sealed at XPS, mastic sealed at any poly seams with 12" of overlap on the joined sheets. Alternatively, use 2" of closed cell foam in place of the XPS. )

If the floors still feel cold, then would be the time to consider a radiant floor. From an installation cost POV it may be cheaper to use a low-voltage electric radiant system than hydronic pump, heat exchanger, tubing, heat transfer plates, etc. but priced it both ways. Operating costs would be the same- it's electric resistance heating, more expensive to run than heat pumps.

Dana,
Thanks for the detailed reply. Kind of shocking to find out that the heat pump is that much more efficient than other types of electric heat. I didn't realize the difference was that great. Doesn't seem like it's worth it if the efficiency is that much lower.

My local gas company has been soliciting new customers. I do have a gas line to my house but the previous owners removed the meter and all piping. But with the incentives it could be attractive. How does NG compare to electric in cost/efficiency?

Closing in the crawl space seems like more of a project than I want to undertake. Would spraying foam on the underside of the subfloor work?

Thanks for your help.

Yes, insulating & air-sealing the floor with spray foam would make a significant difference in floor temperature in winter. But the more insulation you add there, the less earth-coupling you get, which means during the cooling season your AC bills are likely to be higher, which is why sealing and insulating the crawlspace exterior walls is the preferred method. Being earth coupled at the moderate to warm FL subsoil temps give you have a substantial thermal mass working for you year-round.