Outdoor wood water heating system wanting to convert to electric

I have an outdoor wood heating system and it works great. This unit heated the water that then ciculated in to the house and ran in to a heat exchanger that is in my propane forced air furnace. this allowed me to heat with wood and if needed i could still use the propane forced air system if i had a pump failure, took a winter trip, or just didn't feel like feeding it wood. The problem is i am getting old and i'm getting tired of cutting wood and feeding this unit all winter long. Since i have a fair amount of money invested in this system i would like to add an electric water heater/boiler to this system. I would most likely drain the wood heating unit and just circulate the electrically generated hot water through the existing heat exchanger in the forced air furnaces plenum. The heat from the wood boiler was hard to beat. it also heated the potable hot water heater with a unit called a "side arm" and It would be nice to somehow retain the side arm as the hot water heater is propane and propane cost big $$ these days. One of the things that bugged me about the outdoor wood heater was the circulating pump that ran 24/7 all winter long. I think if i can find a suitable water heating unit i can save a bunch of money. I have a 1800 sq/ft walk out rambler in Cambridge Mn. I went through a tank of propane a month (380 gal @ $2.00 gal = $765.00 a month or $4590.00 per heating season) or 11 cord of wood ($1000.00 per heating season) so the wood was a great savings and the house could be any temp we wanted all winter and it made great hot water for showering as well. So what is a good electric hot water heating system that will keep up with the winter here in Minnesota. this would need to circulate thru the heat exchanger when the forced air furnace calls for heat. Is the electric unit going to be close to the efficiency of the wood boiler? I'm envisioning the electric water heater to only run when the forced air furnace tells it to. Is there a unit that can heat the water and keep up with the demand? Any help i can get would be great. I am tired of chucking wood. Wood is a young mans method thats for sure.

Clayton S said:

Is the electric unit going to be close to the efficiency of the wood boiler?

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Electric heaters are 100% efficient in comparison since they do not have exhaust stacks releasing heat into the atmosphere, but that is not what you really mean to be asking...

Clayton S said:

I'm envisioning the electric water heater to only run when the forced air furnace tells it to. Is there a unit that can heat the water and keep up with the demand?

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Your wood stove and heat exchanger likely produced a higher temperature at your forced-air furnace than a typical electric water heater will produce, but I believe there are electric water heaters specifically intended to do what you want done...and possibly even with the sidearm. It has been quite a while since I looked into that (and ended up doing nothing), but I will look again if nobody else here comes up with any suggestions. Or if you wish, do a Google search for "electric boiler".

FWIW, a typical propane fired WH is probably in the 55-60% efficiency range. A good, properly sized boiler running on propane would be in the order of 90-95% efficient, or close to a 50% improvement in efficiency. That's still expensive compared to $1000 for wood. Had you considered a pellet stove or (if they make them) a pellet fired boiler? Some of them have a pretty big hopper, and even have auto-ignition. Not sure the longest they'd run without someone filling them up, but it may be a lower cost option. Where I live, electric would likely be more expensive than propane, but the rates on either energy can vary quite a bit by region and sometimes by town.

The cost of running an electric boiler (which is probably what you'll need, given your fuel use numbers) will probably be close to the cost of heating with propane, and probably MORE. What are rates per kwh, delivered?

You can probably heat a large zone at ~1/3 the power use with a cool-weather ductless mini-split heat air source pump like the Mitsubishi MSZ-FE18NA or a Fujitsu AOU-15RLS2H. The Mitsubishi FE18 puts out a sligthtly more heat than the 15RLS2H, but stops running at about -18F (to self protect), whereas the Fujitsu is rated for operation at -15F but keeps on chugging away well in to the -20s. In my neighborhood either would be running about $4.5KUSD installed, give or take. There is a similar Fujitsu RLS2 (not -H) series that will be cheaper, would still run down in the negative teens, but isn't really designed to work well there. If Fujitsu, the RLS2H series or nuthin' in your neighborhood.

Either one will be more than 2x as efficient than an electric boiler or hot water heater averaged over the season, and your only hope of even touching your wood-boiler's operating cost. There is a hydronic output air source heat pump (Daikin Altherma) that works with hydronic coils in air handlers, but I suspect the coil your air handler is at least 2x as big as what would be reasonably served by even the largest Altherma, and the $15-25K price tag might be a bit daunting. You could buy 3-4 good-sized mini-splits for the money.

jadnashua said:

FWIW, a typical propane fired WH is probably in the 55-60% efficiency range. A good, properly sized boiler running on propane would be in the order of 90-95% efficient, or close to a 50% improvement in efficiency. That's still expensive compared to $1000 for wood. Had you considered a pellet stove or (if they make them) a pellet fired boiler? Some of them have a pretty big hopper, and even have auto-ignition. Not sure the longest they'd run without someone filling them up, but it may be a lower cost option. Where I live, electric would likely be more expensive than propane, but the rates on either energy can vary quite a bit by region and sometimes by town.

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At typical atmospheric-drafted propane tank would run about 82-84% in raw combustion efficiency, which is about where it would average if used as a space heating combi-heater. They only run in the 55-60% range in an EF hot water heating test, where the vast majority of the time it's standing-by rather than burning.

Some reasonably efficient pellet boilers are starting to show up (mostly European imports), but they're fairly expensive to install, and the fuel isn't exactly cheap either. Ductless air source heat pumps are usually cheaper to run than pellet boilers, and far cheaper to install than a full-on pellet boiler, according to RMI's financial analysis (see figure 2).

In my neighborhood at 15 cents per kilowatt hour heating with ductless heat pumps is less than half the cost of heating with $3/gallon propane. MN has some of the lowest electricity prices in the midwest, and you can probably come in at about 1/3 the cost of heating with $2 propane, despite a somewhat lower seasonal efficiency compared to ductless heat pumps in New England. Here in centreal MA they would average about 280-300% efficient, but in Cambridge MN you'd be looking at 260-280% seasonal efficiency, due to the lower mid winter outdoor temperatures. At -15F they're only ~180% efficient, but at +45F they're over 400% efficient, and in-between it's, well, in-between.

The key issue in this case is coming up with sufficient capacity more than raw efficiency. While I have relatives in the Pacific Northwest heating the whole house with a single -FE18, from the fuel use numbers here it's clear that a single mini-split won't come close to doing the whole shebang.

The reason i asked is my dad has a radiant floor heating system that was propane fueled. they turned in to "snow birds" and since they wouldn't be home to check the propane level in the tank they added an electric boiler that was just supposed to help assist the propane unit. they had me check their propane tank every month and the level never dropped. they said their electric bill was only $50.00 a month more than usual when away so they assumed the propane was doing all the heating. this was not the case. the electric was keeping up and the propane never ran. they are now using the electric all the time and his heating bill is very low. I would like to have this kind of ease of use and low cost. he said the unit was $1500.00 which sounds like it is very affordable especially if i can add it on to my wood heating system. I have some 55 gallon plastic drums i can use for storage if capacity is needed. of course i would insulated the drums to help retain the stored heat. I appreciate all the feedback you guys are giving me. Thanks

Utility rates vary all over the place from about $0.03/Kwhr to closer to $0.18/Kwhr or more, so even moving from one town to the next, what's economical may change when it comes to energy choices. But, keep in mind, those snowbirds probably don't have the thermostat set very high while they're away and that certainly helps. AN electric boiler is probably the simplest boiler available, and the price is reflected in that.

The delivered per-kwh pricing is critical for making this decision. While MN average electricity pricing is half that of southern New England, it's not the same everywhere, and it matters.

An electric boiler delivers 3412 BTU for every kwh of power use. A 95% condensing propane boiler delivers about 86,450 BTU/ per gallon.

If your electricity costs are 7 cents/kwh, it's the rough equivalent of heating with $0.07 x (86,450/3412)= $1.77/gallon propane.

If your electricity costs 5 cents that's the equivalent of $.05 x (86,450/3412)= $1.27/gallon propane.

How much does propane actually cost in your neighborhood? (EIA sez it's about a buck-sixty.) How much does electricity cost? (EIA sez about 11 cents.) But the prices of either vary dramatically from utility to utility, supplier to supplier within a region or state.

Eleven cent electricity (the MN state residential average) is equivalent to $2.79 propane, which is more than a buck over the state average price for propane. A generic recommendation to heat with an electric boiler would thus be a bit premature. Do the math on your real prices, using your real propane boiler's nameplate efficiency. But it looks like your electricity pricing would have to be under 6 cents to have any real cost advantage with propane, unless your propane burner is the world's least-efficient model (and probably dangerous.)