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furnace vs boiler

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by brian89gp, May 2, 2013.

  1. brian89gp

    brian89gp Feeling the Heat

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    After warping my stove insert and melting the SS liner trying to heat a large house with an undersided stove I am thinking that a properly sized wood furnace or boiler might be a better option. I am looking for the users here to weigh in on the two options for my particular case.

    -Install would be in a basement, flue would be right around 40'
    -Total winter heat usage is right about 130 MBTU per year averaged over a 4 year window
    -Storage for a boiler is not likely unless it can fit down a steep set of cellar stairs and through a 32" wide door
    -The house has a forced air HVAC system
    -I am slowly working on putting in a hydronic system, staple up under floor radiant heat and I am putting in a water chiller system hooked up to water-air coils on the air handlers for AC. There are 3 air handlers currently with a 4th one possible in the future
    -House is 3500 sq/ft, solid uninsulated masrony walls with an insulated attic, and about 130 years old
    -I have natural gas and its cheap, I don't mind having the gas furnaces kick on for make-up heat.
    -Current output of the gas furnaces (there are two) is 160k BTU and they have kept up on the coldest days
    -If a boiler, it would be used for space heating only. DHW is cheap enough to come by with natural gas.
    -I have hopes some day of a solar hot water system to supplement space heating, but it may never happen.
    -If I go with the boiler I would likely replace the gas furnaces with with simple blower cabinets and get a mod-con boiler

    My concerns:
    -Due to the draft from a long flue my insert was very hard to control to the point that I warped the top and melted the liner from constant over-firing. I am worried I will have the same problem with a furnace or boiler
    -The lack of any signifigant storage for a boiler will cause a lot of start/stop
    -Same start/stop concerns for a wood furnace
    -Cost of a gassifier boiler
    -I am in an urban area, whatever goes in will need to have minimal smoke. My EPA secondary burn stove did just fine in this regard for a point of reference

    The house is old and the design and the way it is built does not lend itself to a single central air system without a huge number of bulkheads and other visually intrusive construction to hide ductwork. It has a 15x15' two story open entry room so air stratification is also a huge problem. Having 3-4 smaller air handlers with a central water chiller and central boiler running water to air coils is signifigantly cheaper then 3-4 seperate gas furnaces and AC units.


    So, furnace or boiler or say screw it and go back to natural gas?

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  2. arbutus

    arbutus Burning Hunk

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    In my area wood is expensive and gas is cheap.

    Unfortunately I have do not have gas available, so the choice is between expensive wood, expensive propane, or expensive electricity.

    Wood comes out cheapest, with the total cost of a boiler and hydronic system installed giving about a four year payback over the current electric baseboard.




    What if you use the existing hvac with two separate water to air heat exchangers?
  3. NCFord

    NCFord Member

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    I would think any draft issues could be addressed with a barometric damper, perhaps even 2 if need be. Also, you can add storage with some of the
    tanks from american solartechnics, they can fit almost anywhere.
    I would second the Water to Air heat exchanger for the HVAC, very easy install.
  4. maple1

    maple1 Minister of Fire

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    The sizes of propane tanks available where I got mine were 110 gal (30"x4ft), 330 gal (30"x9ft), 500 gal (36"x9.5ft). Use those sizes to do a full evaluation of the space you might have to put storage before you completely rule it out. The 110 gallon ones were quite cheap ($80) - you might be able to fit a few of those in & plumb them together, depending on the actual space you have. Modularity, sort of.

    A boiler can be made to heat hydronically (naturally), or by forced air (with exchangers) - a furnace can only do forced air. So a furnace will limit your potentials considerably if you are putting hydronic in.

    And a 40ft. chimney would be ideal for a natural draft gassifying boiler. :)
  5. If you read enough threads here you will see most everyone with a gasser and storage is happy with their investment. And have the fewest issues with excessive smoke.
  6. brian89gp

    brian89gp Feeling the Heat

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    The 110gal would fit, the 330gal is too long to fit down and through the cellar door. The basement is large enough for me to do whatever as long as it fits under a 7' ceiling. I would have a preference for pressurized storage to eliminate the need for heat exchangers.

    If going with a boiler, what would be the minimum storage size I would need to go with to prevent short cycling the burn cycle? And I would imagine that if going with minimal storage I would size the boiler smaller to more closely match the constant state heat load vs going with a larger boiler that some do to speed up the water bank recharge rate.

    For the water-air HX it would be easy enough to feed the same HX with both the chiller and the boiler and just have a 3-way manual valve on the feed and return, turn it one way for the chiller system and turn it the other for the boiler system. The house has enough thermal mass that I rarely if ever need to both heat and cool during the same 24 hour period. The HX I have already is sized for 4 tons of cooling which when it comes to heating means it is massivly oversized, according to the heat load of the house and the output of the coil I could probably get away with running the EWT at 120* for most the season and going up to 140* during those one or two cold weeks.
  7. maple1

    maple1 Minister of Fire

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    My boiler specs 8-10l per sq.m. of area, for storage.

    I have 660 gallons, and 2700 sq.ft. - I think that works out close, but I didn't do any calcs, just put storage in that fit where I wanted to put it. I wouldn't want any less. There are differences between boilers, some more significant than others. Mine cannot idle or cycle, it burns all out until the fuel is gone - so it is important to have enough storage and good overheat/dump protection. Others do cycle, although it is generally good to avoid it as much as possible, regardless.

    Personally I would try to take advantage of the tall chimney - I might be a bit biased though.

    Good luck with the researching & decisions - there is a ton of info on here to sort through.
  8. SIERRADMAX

    SIERRADMAX Feeling the Heat

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    What is the cost of heating a year w/ natural gas?
  9. Coal Reaper

    Coal Reaper Minister of Fire

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    x2 on everything maple said. storage gives you a great deal of flexibility on when you are available to fire. you can do a partial load if needed or reload of storage temps require that much BTUs to come back up. there is nothing wrong with a bank of smaller storage tanks. i think there may be 6' tanks available (250gals?) so you might want to look into those options further. look at clarkburg's sig picture. he has several tanks tied together. i would also cross off a furnace from your list. go with a boiler.

    FWIW, if there was natgas offered to my house i would have had to seriously consider whether or not i would have put in such a serious wood heating system as gas is fairly cheap in NJ. and i think just having the fireplace would suffice when i got the urge to play with fire.
  10. If I had cheap natural gas available an outdoor fire pit would satisfy my wood burning urges.
  11. brian89gp

    brian89gp Feeling the Heat

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    Stupid cheap, I would have to buy wood at less then $100 a cord for it to be cheaper then gas. Gas prices keep dropping and its down to around $1000 a year. I planned and put in the stove insert when it was approaching $2000 a year and total install cost including the stove insert and liner was less then $2000, so it made sense.

    A $5000 plus gassifier boiler would take a long time to break even.
  12. maple1

    maple1 Minister of Fire

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    I think you'd be looking at closer to 10k for a wood boiler/storage setup - that's with doing all or most of the installing yourself.

    If you've got the natural gas system all in place, I'd be hard pressed to justify a new wood boiler at those gas prices. Even with 'free' wood - which isn't really free if you factor in time & equipment costs. Maybe even factoring in a new gas boiler also.
    mikefrommaine likes this.
  13. mellow

    mellow Resident Stove Connoisseur

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    Like I mentioned in the other thread I would check out the Drolet Tundra as an option with a baro damper or key damper. It is not out yet but will be this August/September.
  14. BoilerMan

    BoilerMan Minister of Fire

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    I am still a bit of an independant on this, but how long will gas stay cheap? However, I'd have a mod/con with a low temp system in a heartbeat if NG were here, but I'd keep the gasser or at least the stove as a hedge.

    TS
  15. brian89gp

    brian89gp Feeling the Heat

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    I read through the Drolet Tundra thread and will look into it once it comes out. I do like everything I read about it and the size is about right and for the price it seems like it will be a front runner.

    Not sure if I said so or not, but I do have almost all the parts already for a hydronic system, pumps, coils, etc, for the AC chiller system. The pump and water-air coil could pretty easily be switched back and forth between the cold and hot systems with a pair of 3-way T valves. The cost of a boiler would be in the boiler and its storage which granted is still approaching $5-10k. But $10k will buy a handful of evac tube solar panels... Gassifier boilers are what brought me to this forum in the first place and I have a soft spot for them so I am always trying to warrant a purchase, to bad they cost so dang much.

    What are some brands of gassifiers that can ramp up and down some to accommodate for a small water storage volume with price being a main consideration?

  16. The eko 25 might be worth looking into.
  17. __dan

    __dan Burning Hunk

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    With gas available there would be only one choice for me, a condensing gas boiler and all hydronic type zone loads. I'd yank all the duct work and use several window units or ductless mini splits for AC. A wood burning system would be a want, not a must have, like it is with oil.

    Instead of spending 20K on the gasser install, I'd consider first wrapping the house with rigid foam and a new siding layer, flashed into new "new construction type" windows, not replacement windows, Andersen 400's, and probably a new roof and overhangs to go with the new exterior.

    Every house I look at needs new siding, a new roof, new boiler.

    One other thing I would look at is the basement slab. If it's thin and can go pretty easily, or needs to go, I would look at digging the basement floor lower and putting in drainage, rigid foam, and a radiant heated new slab.
  18. laynes69

    laynes69 Minister of Fire

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    We heat with a Caddy, which has basically the same firebox as the Tundra. It's an excellent heater, but I'm afraid it wouldn't be enough for a home that size. Since you already have ductwork in place, I would go for a wood furnace. The Max Caddy would be a better option, and offers a large multi speed computer controlled blower, and a large firebox. With a 40' flue, you will need a barometric damper.
  19. begreen

    begreen Mooderator Staff Member

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    With an existing forced air system I second the vote for a Max Caddy installed properly with backdraft dampers. This is going to mean a lot of wood going into the basement. You might want to think of methods to streamline that process if there isn't outside access.
  20. heaterman

    heaterman Minister of Fire

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    For your climate and the cost of heating with natural gas I have to say expecting payback on a wood burning appliance of any type is pretty dicey at best. You have to look at the whole cost of operation, not just the cost of the boiler/furnace + installation. There are always ongoing costs of operation that go along with it, especially with cordwood.

    If you do choose to go to alternative fuel however, I would quickly eliminate the wood burning furnace option. A wood burner with a load that light is going to spend a lot of time smoldering if there is no heat demand. The on/off nature of a forced air system is the worst for that scenario. With a boiler you have the capability to easily "dump" some of that heat into storage for use at a later time.
    If forced air is your best option I would think about something that uses pellets rather than cordwood because a pellet burner is far more capable of dealing with that on/off cycling.

    If I were in your shoes my choices would be boiler for cord wood or pellets if you are staying with forced air based heating.
  21. brian89gp

    brian89gp Feeling the Heat

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    There is a cellar door, so wood into the basement is a lot easier then wood into the now defunct stove room upstairs.

    The winters here are generally pretty mild and stay in the 20's with the one week it stays in the single digits. My worry with something like the Max Caddy is that it will be running at 25-50% load for 90% of the winter. The regular Caddy or the Tundra is about the right size for 90% of the winter. If I avoid pushing it, the gas furnaces could kick in for the cold snap. Calculated heat load that fits in line with witnessed furnace run time is around 90k BTU @ 20* design temp and 165k BTU @ -10* design temp (plus or minus some as its from memory). Thoughts?
  22. maple1

    maple1 Minister of Fire

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    Furnaces won't work with your hydronic stuff - so you'd have to drop the hydronics if you went with a furnace.
  23. brian89gp

    brian89gp Feeling the Heat

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    Putting in a water chiller AC system so a lot of the hydronics are going in regardless. The wood furnace has the downside that it would only be able to heat the first floor as the other 3 air handlers are elsewhere in the house.

    Pulling out the calculator:
    Using 500 gallons of storage, 180* storage target temp, and 120* for the HVAC coil (coil is huge due to chilling needs), 85k BTU output for EKO 25 and 137k BTU output for EKO 40. Boiler would have a 60* heating delta, from 120* up to 180*
    -Warm shoulder season of 20k BTU load EKO25 3 hour cycle time EKO40 1.6 hour cycle time
    -Cold shoulder season of 40k BTU load EKO25 4.5 hour cycle time EKO40 2 hour cycle time
    -Average winter of 90k BTU load EKO25 24x7 operation EKO40 4.2 hour cycle time
    -Coldest winter period of 140k BTU load EKO25 (undersized) EKO40 24x7 operation

    There would be a mod-con gas boiler to act as backup and pick up the slack.

    On a full load what is the burn time on the EKO 25 and 40? The 25 looks to be just about the right size and the 40 a bit too big, assuming my numbers and math are correct.
  24. Coal Reaper

    Coal Reaper Minister of Fire

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    I think with storage the more important thing to know is how much recoverable btus the boiler puts out on a full load. I my opinion you want to be able to charge you tanks on one full load for most days out of the year. With storage you can also just weigh you wood and burn partial loads depending on how much btus required to raise volume of water X degrees. Btus per hour isnt often usefull other than a reference as to what boiler is bigger than another
  25. SIERRADMAX

    SIERRADMAX Feeling the Heat

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    Chilled water system in your house?! Or do you mean refrigerant?

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