Indoor or Outdoor Boiler to Heat Garage?

[StihlHead]

An indoor boiler is inherrantly the most efficient due to all losses are inside the building envelope, but so is the wood and associated mess.

An indoor gasser is the most efficient, and with storage the most convientent also.

Well, I would argue that the boiler location is not the main factor in burning efficiency, and starved air systems in typical conventional OWBs are a far greater cause of lost energy (compared to a gassifier) than radiant heat from the firebox/boiler itself and line losses (if the lines are installed correctly). I measured OWB line losses extensively with temp gauges at both ends, and the heat loss was minor, usually 1-3 degrees. I would also argue that any non-gasser stove can become a lot more inefficient if they are stuffed full of wood and then damped down. In these cases, wood stoves just become charcoal makers and the unburned wood gasses escape up the flue. Cat and 2ndary EPA stoves try to not allow this to happen, but if temps go low on either one, the wood gasses escape unburned and the efficiency drops. That is the real advantage of a gasser or Russian firebox. Burn hot and fast, and store the heat someplace else.

Which brings up another point about OWBs vs. indoor gasser/boiler units inside a building, or inside super-insulated building outside: unless you have an OAK you are drawing a lot of heated air into the boiler and up the flue. A super-tightly insulated out building with an indoor gasser in it (a design the OP is considering here) will either starve for air to feed the boiler, and/or it will draw air into the firebox from the heated area of the building unless there is an OAK of some type installed. So the insulating effects are diminished. Similar to an inside boiler unit or stove w/o an OAK. Heated air is going to feed the fire and head up the flue. Lost energy there. The typical OWB air source is another aspect for the OAK debate that people against OAKs bring up all the time: that they do not want super cold air feeding their fireboxes (in any type of wood burning appliance). The thought is that cold air prevents effective burning or reduces boiler/stove efficiency. All OWBs draw air directly from the great outdoors, and they burn just fine.

My background is several fold here. I have designed and installed and used many wood burning appliance, solar systems (passive, active, and electrical) and alternative energy systems. I also have a couple of college and university degrees in engineering and have been educated in thermodynamics and heat transfer. I also have an interest in energy and staying warm in winter.

 

[dook]

Thanks for the info, Stihlhead. Forgive my ignorance, but what does OAK stand for? Outside air something? My thinking is that a boiler in a super insulated shack benefits from being able to use that pre-heated room air for combustion, raising burn temps and getting more BTU per cord than you would if cold outside air were entering the firebox.

Since the application is a garage/shop, wide temp swings are tolerable and I think I would opt for storage and faster,hotter burns.

An older used inefficient boiler used that way might be a better value, the saved money being spent on insulation of the building.
Also, I was thinking that a used inefficient boiler would be less likely to be severely damaged by it's own heat than an efficient used one.

 

[StihlHead]

OAK = Outside Air Kit

Typically a 4 inch air supply from the outside or underside of the house to feed a wood stove or fireplace insert. Required on all HUD homes in the US (manf. or mobile) and for all wood burning appliances in many WA state counties. Much much much debate about them on this site and others, their effectiveness, and new codes requiring them.

As for burning pre-heated air... the energy has to come from someplace to heat that air, and that is from the boiler or the air in your house or wherever the boiler is. In other words, the heated air is not free for the taking. It has to be heated... That may or may not be offset by the radiant heat of the device heating the ambient air that is then robbed by the firebox. I think it would likely not be offset, and it would depend on a lot of factors in the overall system. Any small boiler shack would rapidly use its warm air supply through the air intake damper, and that heat would be lost up the flue. I was reading this afternoon that the EPA has determined that any stove, with or w/o an oak that has new wood put into it that is not run with the damper wide open for about 15 minutes will become far less effective in efficiency and throw out far more particles of smoke during that time. With damper open for 15 minutes that sucks up a lot of ambient air and heat if you are sourcing it from inside a structure. I happen to be a big believer in the benefits of using an OAK, and I have them on my stoves here (I am in a HUD home, and they are required, but I would have added them anyway).

Temp swings are not going to happen in any heated area if you have a t-stat controlled system in any of these options, as long as the boilers/stoves are fed a supply of wood. Typically an OWB hydronic loop is kept at 165 to 185 degrees, and the damper opens when the boiler gets to 165, and closes when it is at 185. They can be set to open and close at lower or higher temps, this is just a typical example of an open system that has a max of 212, the boiling point of water at sea level. Pressurized systems run different and they can run on steam loops, which are at higher temps. That is just the boiler loop temp though. The inside house or shop temp is set by a thermostat, and whatever secondary system that you have attached to the Hx will come on or go off by that setting and not vary because of boiler temp. In a passive DHW Hx, the placement of the Hx will drive a convection water heating loop as long as the Hx is hotter than the water in the DHW. When the temps are equal, the convention loops shuts itself off. As an aside, I do not know of any wood burning system that claims that it is more effective or efficient because it draws on a warm air supply. Great debates, but no facts that I am aware of.

Used boilers are likely going to fail or last a long time based on the way that they are welded, the type of thickness of steel used, and the age of the system. It will also depend on if the boiler water was treated right with an anti-corrosive in an open loop system, and if they were banged up in shipping and the like. I happen to be a fan of thicker plain steel over stainless steel (SS), as they can be repaired far more easily, plain steel is more ductile that SS, and it transfers heat better than SS. SS tends to get brittle, and is very hard to weld in the field. You can get an older style classic Central Boiler new for half what an EPA one costs. $5k last I looked. Some people do not like these boilers, and there is always someone that either installed them or is using them wrong, or does not like the higher OWB wood consumption, etc. and is ranting about them and wanting to sell them. There are many fly-by-night OWB companies out there though, and I would avoid them. Stick with the better brands, like Woodmaster, CB, Wood Dr, Heatmore, etc. if you go with an OWB. With the gassers, Garn, Tarn, Greenwood and some others have been out there a while. Avoid homemade boilers like the plague. I would also avoid pressurized systems, but that's me. You open another can of worms with those systems as compared to open ones.

 

[Floydian]

Well, I would argue that the boiler location is not the main factor in burning efficiency, and starved air systems in typical conventional OWBs are a far greater cause of lost energy (compared to a gassifier) than radiant heat from the firebox/boiler itself and line losses (if the lines are installed correctly). I measured OWB line losses extensively with temp gauges at both ends, and the heat loss was minor, usually 1-3 degrees

IMO, standby heat loss from ANY outdoor boiler, gasser or not, should not be underestimated. All forms of heat transfer get in on the action, 24/7 for however many days a year. I'd be surprised if the avg OWB was losing anything less than 2000 btu/hr. Heck, 4 or 5,000 btu/hr seems well within the realm of possibility. That could equal half a cord or more per year just in standby losses of the boiler itself.

Now let's look at underground losses. A 3 degree loss in underground transit would be completely unacceptable to me. Let's say someone is pumping 8 gpm x 500 x 3T=12,000 btu/hr!
Truly epic losses when figured over an entire heating season.

Imagine burning 2 or 3 cords/yr just in transmission and standby losses

Crunch the numbers people!

Noah

 

[dook]

Thanks, Stihlhead.
In a system like that I think I'd set the temps very wide and fill the firebox once a day. I would consider the hydronic slab to be a "storage system". A 1600 sq.ft. slab 6 or 7 inches thick with 2" foam underneath will stabilize the temp swings somewhat. 24 hours later when I fill the firebox again, it will have to run wide open for quite a while to heat the slab and the storage tank....or so I assume. Would a 100,000 to 150,000 BTU boiler be about right for a well insulated 1600' garage with 3 insulated overhead doors and 3 man doors, and a few triple insulated fixed acrylic windows on the southern exposure? Outside temps will average low 20's f and inside temps 40 to 60. When it goes sub zero I could fill several times a day.

Many modern woodstoves are designed to use firebox temps to preheat the combustion air by routing ducts inside before they feed airwash and secondary nozzles. I assumed the same theory would apply to a boiler in an insulated shack.

 

[Fred61]

Temperature and comfort temperature are two different things. First, I feel 60 degrees is about as warm as I would want a shop unless you're sitting at a bench populating circuit boards or the like. If it's a mechanical or woodworking shop 60 would be tops. But even if it's colder and you have heat rising and coming into your body you're going to feel comfortable at a lower temperature. When the temperature is at 60 and the space starts cooling and heat is leaving your body you will immediately start to feel cold.

 

[EffectaBoilerUser (USA)]

I live in Michigan and although I am not a professional plumber, it is my understanding that if a wood boiler is placed in a garage that the boiler must be in a "boiler room" so as to be accessible only from an outside entrance. Thus, you must not be able to enter the boiler room when in the garage.

I think this is code in more than just Michigan.

However, what I have seen done is to place the boiler in this type of room (even a lean too style room off the garage) but have the storage in the garage itself. By doing so, any heat loss from the tank would go into heating the garage.

Because of the popularity of he outdoor wood stove, it seems like we have just become used to having central heating systems outside.

Having owed and operated my Effecta Lambda indoor wood gasification boiler for 3 years, I would never entertain the idea of having a boiler outside. It is so nice to be able to fill the boiler once per day with 100 lbs of wood without having to go outside.

Another reason I think so many like to have the wood boiler outdoors is that traditional wood boilers created a lot of creosote. However, I have been operating my Effecta Lambda boiler for 3 years now and have never cleaned my chimney. These is absolutely no creosote in the chimney-the only thing in the chimney is very dry ash!

 

[dook]

I live in Michigan and although I am not a professional plumber, it is my understanding that if a wood boiler is placed in a garage that the boiler must be in a "boiler room" so as to be accessible only from an outside entrance. Thus, you must not be able to enter the boiler room when in the garage.

I think this is code in more than just Michigan.

However, what I have seen done is to place the boiler in this type of room (even a lean too style room off the garage) but have the storage in the garage itself. By doing so, any heat loss from the tank would go into heating the garage.

That seems like very good advice.

I've looked at some of the boiler manufacturers websites and it seems that boilers are both very large and very expensive. Paying $7000 plus storage plus hydronics just to do what I could do with a used $200 woodstove (and less wood) just so I can have liability sure is a bitter pill to swallow.

 

[maple1]

You can get into a gassifying boiler for less than $5000 (just the boiler that is).

 

[EffectaBoilerUser (USA)]

If your goal is to heat only one room/garage and you have no plans for future heat loads its tough to beat a decent wood stove.

 

[dook]

If your goal is to heat only one room/garage and you have no plans for future heat loads its tough to beat a decent wood stove.

I agree, but try getting insurance.

 

[Como]

How big is the house and what will the load be?

How often are you going to be in the garage?

 

[StihlHead]

Hmmm... well, interesting issues. We can debate the line loss and radiant heat loss issue all we want, but that takes the thread off on a tangent. The OP here has a dilemma. He cannot have a wood heating device located in the shop/garage, as that wood mean he cannot get insurance on the shop/garage. So he cannot buy a cheap stove and put it in there. He cannot build a fireplace or a fancy Russian fireplace in there either. He cannot put a gassifier in there, or any other type of wood boiler. The storage tank and a hydronic floor heating system can go in the shop/garage, yes, but the boiler, no. So that leaves him with a wood burning appliance that is detached from the shop/garage. So he has a choice of a classic or newer EPA gassifier OWB in its own unit, or an indoor wood gassifier or other type of indoor wood boiler placed in a shed not attached to the shop/garage, which would basically boil down to being an OWB. If it is like Oregon in Montana, that means a minimum of 6 feet away from the building for fire code (and making the insurance co. happy).

He is not heating the house with the device... just the shop/garage. He is stuck with line losses and radiant losses of an OWB, regardless.

 

[StihlHead]

You can get into a gassifying boiler for less than $5000 (just the boiler that is).

Name/brand? Cost?

Smallest new 4030 Classic non-EPA Central Boiler (should be more than enough to heat his 1600 sq ft shop) unit is $5500. All you need on top of that are the brass fittings and valves, the Pex lines, insulation and corrugated drain pipe or thermoPex to run underground, a mixing valve and/or a flat plate Hx, A Taco pump or two, the flooring Pex, and 120 V. wiring to the OWB and inside pump if you have one, as well as a foundation of rock and/or concrete.

Here are several used CBs for sale in the Midwest for as low as $3,000:

(broken link removed)

 

[StihlHead]

IMO, standby heat loss from ANY outdoor boiler, gasser or not, should not be underestimated. All forms of heat transfer get in on the action, 24/7 for however many days a year. I'd be surprised if the avg OWB was losing anything less than 2000 btu/hr. Heck, 4 or 5,000 btu/hr seems well within the realm of possibility. That could equal half a cord or more per year just in standby losses of the boiler itself.

Now let's look at underground losses. A 3 degree loss in underground transit would be completely unacceptable to me. Let's say someone is pumping 8 gpm x 500 x 3T=12,000 btu/hr!
Truly epic losses when figured over an entire heating season.

Imagine burning 2 or 3 cords/yr just in transmission and standby losses

Crunch the numbers people!

Noah

Amusing... but my mistake, my notes (I have not lived there for over 4 years now) are actually unmeasurable to 1/3 of a degree (not 3 degrees). ThermoPex claims less than 1 degree loss per 100 ft of line. My ex's boiler lines are about 50 ft, from the Hx in the house, but only 10 feet are in the ground, the rest of the run is in a well insulated attic of the garage. Line losses were far less than a degree on average.

I would guess that a half cord a season went into thermo losses of my ex's OWB (radiant heat and line losses). We burned 10 cords on average of mixed species wood, mostly fir, oak, alder and maple. So a 5% loss is not unreasonable, and hardly epic. Cords are highly variable in heat value though. A tight stacked cord of dry madrone has far more heat value than a loose stack of damp green cottonwood. However, I bet that one or two cords went up the flue in unburned wood gasses when the OWB is in off mode making charcoal. Its the nature of the OWB, and that is where the epic losses are in them (of you want to use that term). She has an unlimited supply of firewood though, so it did not really matter. The offset cost in electricity saved paid for the system in 4 years of service, inefficient as it is. Burning 1-2 cords less in a gasser would be about right, and another one cords less in an inside gasser would be about right as well. Give or take, rough order estimates. My ex's place had no room for an indoor gassifier or for water storage tanks. It also had no room for a Russian fireplace. It was already plumbed with a PEX hydronic floor heating loop. The layout of the house (sprawling single level ranch home) did not lend itself to using an inside wood burning appliance. So we opted for the OWB, and were happy with the results, inefficient as they are. Sometimes efficiency is not the bottom line, as it seems to be with everyone here. Cost is usually a big issue, as are the requirements and limitations for the heating device being designed.

 

[dook]

How big is the house and what will the load be?

How often are you going to be in the garage?

The house will not be heated by the boiler. Just the garage previously described. I would probably spend less than 20 hours a week in it but want to keep the contents from freezing and keep the dogs comfortable. They don't live in the house.

 

[maple1]

Name/brand? Cost?

Smallest new 4030 Classic non-EPA Central Boiler (should be more than enough to heat his 1600 sq ft shop) unit is $5500. All you need on top of that are the brass fittings and valves, the Pex lines, insulation and corrugated drain pipe or thermoPex to run underground, a mixing valve and/or a flat plate Hx, A Taco pump or two, the flooring Pex, and 120 V. wiring to the OWB and inside pump if you have one, as well as a foundation of rock and/or concrete.

Here are several used CBs for sale in the Midwest for as low as $3,000:

(broken link removed)

Make that less than $4000

(broken link removed to http://www.newhorizonstore.com/Products/88-eko-gasification-boiler.aspx)

 

[maple1]

I'm in the process of designing my next home and trying to figure out how to heat the garage in a manner that will not cause insurance companies to reject me.

I understand that a woodstove in a garage is not kosher, although most rural people around here do just that without problems. There are no codes in this rural Montana county, the only objective is qualifying for insurance.

I've read many threads here about heating garages and NFPA standards and narrowed things down to this:

It seems that Outdoor wood boilers have a reputation of using MUCH more wood to heat a given area than indoor ones because heat is not radiated inside the structure to be heated. Not knowing much about boilers, it seems to me that if OWB's radiate that much heat, the issue is insulation.

Would it be more efficient to build a super-insulated shack 2 feet from the garage to house an indoor boiler and a couple cords of wood?

So how will you be heating your house?

 

[dook]

Smallest new 4030 Classic non-EPA Central Boiler (should be more than enough to heat his 1600 sq ft shop) unit is $5500.

http://centralboiler.com/models.php Looking at the cutaway view of that boiler...it seems like it would be easy to build one, with a flat top (maybe fins) and deflector plate in the back to re-route exhaust and an extra large door to facilitate construction. I like your ideas about using heavy steel instead of stainless. I can't weld stainless well enough not to have leaks.

Looks like $1100 in steel if 5/16 is used.

 

[dook]

So how will you be heating your house?

Woodstove and a mass of rockwork around it. A living room without a fireplace or woodstove just ain't right, especially in a log home. I like to be able to see the fire. I have a woodstove in my current home and love it. I easily maintain 73-74 degrees with about 3 cords a year. In sub-zero spells it might drop to 68 in the morning.
There is only one reason I can't have a woodstove in the garage....lawyers. I don't even want fire insurance, just need the liability.

 

[maple1]

I was just wondering as, if you were thinking about using some sort of outdoor or 'self-enclosed' wood burning device to heat the shop - why not also consider running a couple of lines to the house & heat or help heat that also? You could also use it to heat your DHW. You could still have the stove in the livingroom - but using the outdoor unit as much as you could in the house too would reduce stove wood consumption a lot, and supplement your DHW heating (also not sure how you're heating your DHW though?). Think maximization of investment, I guess.

 

[dook]

You could still have the stove in the livingroom - but using the outdoor unit as much as you could in the house too would reduce stove wood consumption a lot,

As I understand it, the opposite would be true. I would use less wood heating with the indoor woodstove than the OWB. If the house was going to be on a slab, it might be worth it to keep the slab warm, but it will be a crawl space and stem wall design.

I had considered DHW, but I live alone and don't use enough to justify piping it that far just for winter use.

 

[StihlHead]

http://centralboiler.com/models.php Looking at the cutaway view of that boiler...it seems like it would be easy to build one, with a flat top (maybe fins) and deflector plate in the back to re-route exhaust and an extra large door to facilitate construction. I like your ideas about using heavy steel instead of stainless. I can't weld stainless well enough not to have leaks.

Looks like $1100 in steel if 5/16 is used.

If you have your own fab shop you could make one, great. Many home made OWBs and many OWB companies have failed for subtle reason though. The boilers and/or fireboxes crack or the welds fail and they leak over time. You also need a feedback controller and solenoid unit for the damper and a tight fitting door. I have the supplier for the control modules that CB uses someplace. They are cheaper there (about 1/3 the price) than CB. You also need a good 12' plus stainless flue and cap (most OWBs have really short flues, and that is not good), and a stand and stuff like that. Boilers are not that easy to make tight to last, but... if you are good at welding and steel fab work, go for it. I would use as thick a steel plate as you can manage. Thicker is better. You also need to have an anti-corrosive in the boiler water to keep the steel from rusting out. CB has about the best stuff on the market for that (as tested by some of my chemist buddies that have CBs). Inside the firebox rust is not an issue, as creosote does not corrode steel that much, but it can trap water that can. You could build the unit on a tube frame on a slab inside an well insulated out building and forget all the trim. Basically a firebox with a boiler around it, a door, a damper someplace for air intake that flaps closed to control the burn, secondary injection air ports off the damper feeding steel pipes at the top with holes drilled into them making EPA wood stove secondary burners to improve efficiency. You could plumb in one or two in/out threaded ports for water flow. The smaller CBs have a pair of each for two separate hydronic systems.

The baffles at the top of the CB classic fireboxes are designed to trap and burn more wood gasses at the top of the boiler, but I would imagine that they are a PITA to fabricate. You could use a flat top boiler with the drilled pipe air injectors like the typical EPA wood stove instead. Use 3/4 inch threaded pipe for that and it would be easy. You could make the boiler jacket/tank as large as you need, but for this application I would imagine that you do not need much more than 150 gallons.

Yes, I can hear the people in New England and New York screaming from my yard about building a home made OWB, 3,000 miles away...

 

[dook]

Thanks, Stihlhead,
If I can't find a good used boiler when I get ready to do this, I just might go that route. When you say I shouldn't need more than 150 gallons, do you mean not including a storage tank?

 

[Floydian]

Hi dook,

Will the garage be off grid? Will it have a good potential for passive solar gain? You mentioned triple pane acrylic windows on the south side of the building-Do these have a high solar heat gain coefficient?

If grid tied, what is your Kwh rate?

Noah