Need boiler advice please

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Jon E

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Hi, I'm here as a new forum user and could really use some advice. I have a 12-year-old timber-framed home that is covered with SIPs and has an ICF basement. Very well insulated. Live in southwestern VT about 40 miles northwest of Albany, NY. I have in-floor radiant heat in my basement floor slab, and under-slab tubing in two bathrooms on the second floor, and that's all for heat. I have other tubing installed under the entirety of the first floor (staple-up) but it has never been put in service. We keep the house at about 65° in the winter, by having the basement temperature at about 73° and allowing heat to rise. Not the most efficient but it works for now. Domestic hot water use has varied widely but right now it's just four of us in the house, the usual showers, dishes, laundry etc.

Now the fun part. The house has no internal heating system. That's right, nothing. 100% of the DHW and heat is supplied by an outdoor wood boiler (currently, a Heatmaster SS G200, which has a BTU output of around 111K BTU/hr). The wood boiler feeds a 40-gallon indirect tank and heating zones through a plate exchanger. This means that 365 days a year, if I want heat or hot water, I have to feed it firewood. It has worked well for us so far, but health issues and the sheer effort this requires is forcing me to look for alternatives. I have a 500-gallon buried propane tank and a decent supplier, so that is probably going to be my source.

So this isn't so much about "what's my heat loss?" or "what boiler should I buy", but more about an ideal solution. With so many options and products on the market, I don't know where to start. Here's what I would like to accomplish, however:

- I want to keep burning wood for about eight months of the year. Let's say September 15 through May 15. However, if I am unable to load the boiler, or we go on a winter vacation, I want the propane boiler/heater to provide backup heat and hot water. Essentially the wood boiler will be primary and the gas boiler reserve for those 8 months.

- In the summer I want to shut off the wood boiler, clean it out and use propane as the primary source for domestic hot water. I have also considered an electric hybrid heat-pump water heater for this, but it won't give me the backup heating I need in the winter. I want one new system, not two.

- I need a SMALL system. My mechanical room in the basement was never intended to have a boiler in it, and it is small. I could mount a boiler on the wall outside the room, but a floor-mount won't work.

- Highest priority is a copious amount of hot water. My wife does not like cold showers, she likes to burn her skin off every day. A Superstor or other indirect tank as part of the system should be a consideration.

Please tell me where to start! Thanks.
 

Dana

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Cheapest & smallest and easiest to integrate would be an electric boiler, but the operating cost may be higher than propane (or not.) If you're heating primarily with wood there is effectively zero "payback" for going with a more expensive, higher maintenance propane burning solution as your "backup".

Whatever the boiler, size the indirect for the hot water use. There is no standard definition of "...copious amount...", so you're going to have to figure that out. In most cases sizing it for the largest tub you have to fill is more than adequate.

If operating expense matters, most houses meeting the description "...covered with SIPs and has an ICF basement..." can be heated with a cold climate ductless minisplit or two. The Fujitsu xxRLS3H series all have a specified and decent output capacity at -15F outside, 70F inside. The Mitsubishi FHxxNA series have a specified capacity at -25C/-13F. There are others, but those two have the biggest market shares and best support network. They also have a "minimum heat" mode that keeps it at 50F when you're away. (Mitsubishi calls it something else, but provides the same function.) The operating cost pf a minisplit is less than half the cost of propane even at 20 cents/kwh, and about 1/3 of the operating cost of an electric boiler in winter, 1/3 the cost when it's in the 40sF outside.

These ductless heat pumps also provide high efficiency air conditioning and dehumidification for the 50 hours /year you might need air conditioning in your part of VT.

The installed cost of a 1.25 ton cold climate mini-split is usually quite a bit cheaper than installing a gas or propane boiler too. A pair of 3/4 ton or 1 tonners might be comparable the cost of integrating a propane boiler into your wood boiler system.
 

Jon E

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I see your point about the mini-splits. That's supposed to be "Phase 2" of my project. Due to medical condition, my wife is going to require better climate control in the house in all seasons, and it gets too hot in the summer. I did not consider the "heating" aspect of the ductless mini-splits, but that would certainly be acceptable as either a backup during absences or as supplemental heating during sub-zero heating days. So my other option, the hybrid electric heat-pump tank-style heater, would probably be the best choice. I can pre-heat the water going into it by putting a small plate exchanger on the cold inlet side and essentially feeding it warm water from the wood boiler during the winter months. I also like the idea that the hybrid will provide a small level of air conditioning in the basement during the summer months.
 
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