Can I add a wood burning stove to radiant heat boiler system?

Question

We just moved into a house that was built in 1905 and got our first utility bill... it was almost $800. ( $600+ of it was gas, from boiler). We are aware of our energy usage habits (bring temp down at night, etc), our previous bills in other homes have never been this high. I am assuming it is because it is an older house and has old windows, thin insulation, and poor sheathing.

We have now have access to a lot of wood (now on a farm). I am wondering can I add a supplemental wood burning stove to act as primary boiler for the radiant system? I'd also just buy a cord of wood to help get us running. Ideally I could use both boilers, for when I am out of town and I don't want to worry about my wife managing the kids and bringing in wood, it could just run the natural boiler. (I'd just not route the water through that system.)

I'd like to replace windows and add insulation over time, but we just spent a good chunk of money on the down payment and moving. I am now trying to find ways to bring down our energy bill.

Answer 1

To answer your question in a sentence: yes, wood-burning boilers are a thing, and everyone I know who owns one has it installed as a supplementary system to a gas or oil-burning boiler. So possible, yes, but a good idea? Maybe.

1. They consume a huge amount of wood. If a traditional wood stove will consume 1-2 cords per season, an outdoor boiler will consume 10-15 cords. Do you really have that much wood available? If you are planning on buying the wood, it's flat-out not worth it to get a wood-fired boiler. Oh, and the wood has to be seasoned and dry.
2. They are really polluting. Burning that much wood means that you are putting that much smoke and particles in the air you breathe. I don't know about you, but part of the reason I live in a rural area is to get fresh air. From the state of MA: "Research by the State of New York has found that even when used properly, one of these units can emit as much fine particle pollution as: 12 EPA-certified indoor wood stoves, 1,000 homes with oil heat, 1,800 homes with natural gas heat"
3. Outdoor boilers are expensive and the jackets tend to leak. During the summer the thing is sitting attracting humidity and corroding. This is only OK if you know how to weld and can do repairs yourself.
4. Installation requires digging a trench to bury the insulated pipes going to/from the house. You want the smoke as far as possible away from your house, but the farther away, the longer the trench, the longer you have to walk to feed the beast, and the more heat will be lost through the pipes. Your neighbors will also not appreciate if you put the boiler close to the property line.
5. Zoning and permitting might not allow for wood-burning boiler in your area. Where I live they are legal but only allowed in farms and commercial buildings.

Outdoor wood-burning boilers only make sense if you are a farmer with access to a wood lot, have a tractor to pull the logs, have a splitter, lots of space to stack the wood, and you have several large buildings that require heating. Everybody else should just put their money on efficiency and insulation, and eventually get minisplits.

Answer 2

Adding a wood burner is, itself, expensive and complex. You can (in most places) do that, but it may not be the best economic use of the money.

Improving air-sealing and insulation is generally much less complex, and won't increase your insurance bill (which adding a wood burner can do) or void your insurance (which adding a wood burner without informing your insurance company can do) and has a faster payback.

i.e. Long before you replace windows, (quite expensive to do) you use window film insulation kits (not very expensive) to add a layer and reduce drafts, and caulk any remaining drafts, having an immediate impact on your energy use.

Depending on the house, it's also pretty common management in a large old farmhouse to reduce the number of rooms you fully heat in the winter, or to install movable insulation panels or insulated drapes in some windows for the winter to reduce overall energy use.

Adding attic insulation is generally quick and easy with a fast payback. Walls are more effort.

It's fairly common for people to ignore the "boring" solutions of insulation and air-sealing in favor of more expensive system upgrades, but unless they have already been done, they have consistently been shown to be the most impact for the least investment of additional money.

Answer 3

Yes, there are methods to combine two boilers together or even 2 boilers and a solar heating system.

We used an "intermediate" tank to receive the hot water output from any of the heat sources and had relays to allow control of the central hearting pump without either of the gas or wood boiler confusing the other.

Answer 4

I am wondering can I add a supplemental wood burning stove to act as ...

the less expensive, and better way, is putting in a traditional wood burning cast iron stove where it simply heats the surrounding air. I've have never been cold in such a home that has one. Trying to be slick and specifically having a supplemental wood burning boiler incorporated with your existing hydronic system is... well you can of course do it.

however be aware current politics and environmental concerns. Some states have banned wood burning... you did not mention which state you live in... I thought the people here were mostly law abiding, and while this is not electrical NEC related I am surprised no one has mentioned this! I don't know what to think either responses here are reckless regarding legalities and environmental ethics, or the wood burning ban slash environmental crisis global warming thing is a bit of a charade.

Answer 5

How new is your current boiler? Because if it needs replacing soon anyway, getting a wood pellet burner as it's replacement (whether the math still checks out for supplemental installation is another thing).

You'd also need a pelletizer, wood chipper (far finer than most regular gardening chippers) [or a combination machine], and a place to store the dry pellets throughout the year.

You also need a LOT of wood over the years, definitely more than will grow back on your property unless you have a significant area of actual forest. Look up wood pellet prices in your area because they often are somewhat comparable to oil or natural gas.

Why wood pellets? Because their near-uniform size enables your hvac to run fully autonomous like a gas or oil boiler. The small size and (because of indoor storage) increased dryness also mean they burn far cleaner and more efficiently than other approaches.

The downturn is significantly higher setup costs, especially if you want to make the pellets yourself, and it "costs" a fair bit of basement space (that also needs to be fairly dry) for the pellet bunker.

Answer 6

Do the low-tech things first. Get a bit of tech to help with that: invest in a thermal camera to find the cold spots in your home. Then you can focus your efforts on fixing the worst things first.

Likely suspects:

• windows - caulk the frames, cover with film
• doors - add sealing strips and a sweep, or use a 'snake' if that isn't an option
• attic - insulate it.
• water heater - add a blanket?

You can also close off some rooms that you're not using. If you have some walls that are particularly bad drafty/leaky, cover them with heavy curtains.

Boring, low-tech basic and cheap stuff. But the payoff is nearly immediate.

If you're determined to use wood, a wood stove makes more sense. It's much easier to install than a wood boiler and way more efficient and sustainable.

Then there's this: how are you going to keep that boiler stoked? At least with a stove you can cache the wood inside.

Finally, consider a mini-split. These are surprisingly affordable (maybe $2.5-3k for a 1.5 ton unit) and can be less to operate than gas if your winters are milder (they will be more expensive than gas if you have freezing temps.) If you’re in the US, some states offer rebates, and there's Federal tax credits for them.

Answer 7

The UK and Australia there's such a thing as 'back boiler', which is a wood stove that also includes pipes for heating water, and is located in your living space. Showroom example:

(source)

I had to look hard for a picture showing the pipes, because on most of them they look just like a normal living room wood stove but with an extra compartment at the rear, example:

Examples might be in the 15-30kW range. Aside from having to tend to the fire in order to get hot water, there are two downsides. One is that the relatively cool water causes more condensation of tars at the heat exchangers. The other comes with older back boilers which were often built into fireplaces and abandoned. If there is a small amount of water in the heat exchanger it can boil to steam and then explode - you would need some way to avoid this with some kind of over temperature alarm. Also you need to make sure you never heat the water above a safe maximum temperature (~60C) to avoid scalding.

I'd not want to rely on one for full time hot water heating (firing and cleaning any stove is manual work), but they can be a useful contribution if you have free wood, a large hot water demand or habitually run the fire anyway.