Advice on indoor furnaces used outdoors

[jakhammer]

I have been thinking about this for years. I want to do wood heat. I currently use a blend of LP, wood pellets, and electric heaters to heat my 1800sqft manufactured home. I tend to keep it on the cool side (66-67 degrees) to save money. I keep my and my sons room warmer with electric heaters at night. Im still spending more than I would like too and would like more warmth in the house. again the reason for wood.

I have been tossing the idea of a stove inside, a furnace outside, or a boiler. Well the boiler is out for now, i cant justify the cost upfront. I dont want anything inside because I dont have a place to put it, dont want the mess of wood, and simply dont want to remodel a room for it. So im left with an outdoor furnace setup. My plan was to buy a used indoor furnace off craigs list and put it into an insulated shed. Run insulated flex duct thru a plastic culvert to the house and then to my heat runs with house return air going back to the shed. That was the plan... but now... I think it might be a foolish idea. So I wanted to ask the advice of those who know more about this stuff than I do.

These are some of the issues I figured I would run into:
Blower cfm of any indoor furnace is probably to low for the distance i will need my heat run to be. Will probably have a 100 foot by the time I have the stove away from the house and then to my farthest duct. Also using flex is not best case and will cause even more resistance.
Upgrading the blower may not work as planned. If the furnace comes with a 1000cfm blower and I put a 1600cfm blower on it, will i get enough heat into the air before it goes into the house?
How big of a unit do I need? I dont want to burn 25 face cord to keep my house 75. at that point i have wasted my money.

Im sure there are way more issues I dont even know about. So is this a bad idea? Im trying to do this on the cheap but not so cheap I cant use it do to wood usage.

Just an example of a furnace I was looking at. A johnson energy systems furnace. it has 2 numbers on it, j-7900 and j-9900. seems like a good deal, but then as i asked questions on how it heated i started to think this whole plan is a bad idea. the furnace says its good for 1500sqft, and granted he was heating 3000sqft and using a pellet stove, but he used 25 face cord running it at night and on the weekends. I dont want that.

So advice is needed. should i throw this plan out and instead go for a true outdoor wood furnace? or is there hope a properly designed system can use an indoor furnace in an outdoor setting?

Chris

 

[stee6043]

This concept has been tossed around several times here over the years. A few have tried it. I think ultimately you're going to find this idea will end up costing your more in the long run. By the time you insulate your shed and run your duct work I bet you could find a used OWB and run some underground pex to a heat exchanger inside for the same money. An OWB will make for a much happier operator in this scenario I think.

I think you'll also find that the setup you're contemplating will be rather inefficient. What you don't want to be is a slave to your furnace once installed. Short of spending thousands on insulating the duct work I assume those will be sources of huge heat loss. I'd also assume you'd need to significantly upgrade the blower to do the job properly.

Is there no chance you could get the furnace inside your house? Have you looked for used OWB's? Have you considered a bigger pellet unit that can do the whole house?

 

[flyingcow]

i used to install hvac duct work, etc. Did commercial installs( mega installs) and house installs. Flex duct over 1ft is a bad idea.It really should only be used for doing ceiling vents hook ups between hard duct and diffuser.

Also trying to push air that far will not work well. I suppose you could hard duct the run and put in a booster fan, but it'll eat up electricity. And be very noisy. Even insulated ductwork in that type of scenario will have a pretty good heat loss.

IMO, take some time on this site and ask questions. I'd look at an indoor (water) boiler, in a shed (future man cave). Pipe underground to house. Look at radiators or low temp emitters. A good design installed with pex is a DIY'er. Even staple up radiant.

Also, pellet boiler with bulk storage?

Yes, these ideas are expensive for upfront costs. But I went with a high efficient gasser to keep my wood usage down. I have to buy my firewood. look at it over a 10/20 yr period and it was a good investment.

Now I am actually looking at putting in a Mitsubishi split heatpump in too. In part for AC in my sleeping quarters. I go to work shortly after midnight and these humid afternoons I have a hard time getting a comfortable sleep. But also plan on using the heat pump for a little bit of heat in the living area. The variable speed compressors have made those very efficient. And some are rated to throw heat whens it's -5f outside.

Sorry, rambled a little.


Welcome to Hearth.com. Real good group here. Nice pellet head site too. Ask questions.

 

[arbutus]

I'm new to wood boilers, but heated a 1700 sq foot DRAFTY house with a woodstove for nine years prior to moving.

I have seen two wood furnaces hooked up outside manufactured homes. Both were low budget installs, but worked and kept the house warm. One was almost adjacent to the home, an indoor model under a metal shed. The other was an outdoor unit about 25 feet from the side of the home. You will have to do some math to see if your ductwork will carry any heat to the far end of the run.


An option that I don't think I saw in your post is a wood boiler with a water to air heat exchanger placed in your existing ductwork. Just the fan in the furnace runs when needed, not the LP burners.


Good advice that I have read again and again is to perform a heat load calculation, so you know how much heat you need, then go from there. The Build it Solar website has a calculator that will give an estimate. Once you understand how the numbers are developed you can tailor it to your house. There are several threads on this site about calculating heat loss. I started one and asked several questions not too long ago, and received helpful answers. It should be available if you search for it here.


The other piece of advice that I have read several times is to air seal and insulate. When you get the calculation done you can easily see how adding insulation benefits your situation, and can make a solid decision on how much insulation to buy.

 

[jakhammer]

OWB are out for now. Even the used ones im finding in the area are more than I can do this setup for. I figure I can have an operational system using a used indoor furnace in a shed for $1600 tops (i have a source for wholesale hvac supplies). I do realize an OWB is the best setup, but it all comes down to cost. I simply dont have the money to do it at this time. However, if doing outdoor forced air isnt a good option I will just save my money.

My pellet stove can heat the whole house, but pellets are stupid expensive. I would be spending roughly $200 a month to keep house as warm as I want on pellets. its not economical.

I have no way of getting a furnace into the/under the house at this point. While I do have a cement floor in my crawl space and I can stand up in it, I have no bilco door to get it into. maybe I should add a door to the project and put a furnace down there? maybe I could even save some cash doing it that way. I never thought about doing that. Then the question becomes how do I run the chimney?

 

[arbutus]

Chimneys can be run through a hole in the floor, hole in the ceiling, hole in the roof, and build a chase around it to make it look nice. Attention to clearance distances is especially needed.

Near a ton of pellets a month. How much will you save going to wood?

 

[jakhammer]

i figure 3 face cord a month on this type setup. if i buy it thats $150 a month. if i cut it, well, way cheaper. im not trying to go wood 100% primary heat source. If I can get 75% of my heat from wood I will be happy, but 50% is ok. My work schedule makes it difficult to heat with wood as a primary without going OWB. or doing it in the house.

 

[jakhammer]

So after thinking about the advise that has been given I have a new plan of attack. Let me know what u think...

Same indoor furnace used in the insulated shed but with a new heat run idea. Instead if insulted duct for 100ft I do a single short run to the end of the house closest to the stove. Maybe 25ft of duct. I think flex will be ok. My buddy's house has 30ft flex run from his LP furnace Plenum to the other side of his house and it works fine. I think I can pull off a short running like that. Then I pull return air from the other end of my house to the shed to complete the air flow and pull heat to the other end of thei house. This I can use large diameter (10-12 inch) flex duct for since velocity is not important. That should help with restrictions. I would still upgrade the blower.

How does that plan sound? I think it will work far better than my original plan. Its still not a owb, its still not as efficient as a stove in the house, but its my best bet to pull off wood heat at this point with the funds I have to work with.

 

[stee6043]

So after thinking about the advise that has been given I have a new plan of attack. Let me know what u think...

Same indoor furnace used in the insulated shed but with a new heat run idea. Instead if insulted duct for 100ft I do a single short run to the end of the house closest to the stove. Maybe 25ft of duct. I think flex will be ok. My buddy's house has 30ft flex run from his LP furnace Plenum to the other side of his house and it works fine. I think I can pull off a short running like that. Then I pull return air from the other end of my house to the shed to complete the air flow and pull heat to the other end of thei house. This I can use large diameter (10-12 inch) flex duct for since velocity is not important. That should help with restrictions. I would still upgrade the blower.

How does that plan sound? I think it will work far better than my original plan. Its still not a owb, its still not as efficient as a stove in the house, but its my best bet to pull off wood heat at this point with the funds I have to work with.

Still sounds like a lot of duct work. At the very least if you end up with most of your ductwork being below your house some of the heat loss should get back into the house assuming your crawl space is insulated. That being said, I think you're going to need a monster blower to get this done.

You mentioned above you'd only be saving $50 a month if you have to pay for wood vs pellets. Based on that the system you're suggesting has a 32 heating-month "payback". That's probably going to be well over five years? Given the amount of work involved in processing a cord of wood vs a ton of pellets I'm not sure you're really saving money in this case. If you're going to cut/split/stack the wood without having to pay for the wood, it might make more sense.

Just my two cents! I think your situation would be different in my mind if you didn't already have pellet heat available. Cordwood heating is going to be a lot more work than pellets...