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Boiler for outbuilding

Cba Member Posts: 9
I would like to heat a 12x12 fully insulated outbuilding with a boiler and hot water cast iron radiators. The interior ceiling height is 8 feet high. There is power to the building but running water to it is not practical. Can I use an open vent gravity fed system for this where I don’t have water pressure connected to system? My plan was to use anitifreeze 50/50 with water. I don’t need the building every day but on a need to basis. I will be using it for lacquer painted parts that are curing so want radiators. The building’s roof extends past on one side and there is a room there for boiler so it will not be running in the room where the parts are off gassing.
Another thought would be to have pressure to the boiler using a garden hose. Would have a back flow preventer and ball valve and just remove garden hose when not in use. Is not running a boiler every day bad for it?
Any thoughts appreciated,


  • mattmia2
    mattmia2 Member Posts: 8,395
    You can use a pump to purge and pressurize the system from a bucket or trash can, as long as you make it leak free you won't have to add antifreeze/water mix to it. Antifreeze isn't great for systems, but if this is in a freezing climate and you only plan to use it occasionally then hydronic antifreeze is probably the way to go.
  • Cba
    Cba Member Posts: 9
    Yes freezing climate.
  • Zman
    Zman Member Posts: 7,467
    edited April 2022
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 21,535
    Not going to need much of a boiler, if it really is insulated.

    I would seriously consider, though, an "old fashioned" system with a tank up at the ceiling with a vent on it to give constant pressure on the system, rather than using a "modern" expansion tank or even a compression tank. Simpler.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Cba
    Cba Member Posts: 9
    Jamie, your idea is what I had in mind but will consider others also. I still need a circulator pump correct? Above finished ceiling is area like small attic I thought tank would go up there. I have cast iron radiators in my shop and find them hard to beat. I think the antifreeze is a must for my situation
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,244
    Also, with lacquer you’ll need sealed combustion. No room air for combustion air.
  • Cba
    Cba Member Posts: 9
    Steve, boiler goes in separate room. When you say sealed combustion is this separate room or a specific type of burner?
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,430
    Cast iron electric then  :)
  • rick in Alaska
    rick in Alaska Member Posts: 1,429
    Just to clarify; do you already have a boiler setup for your shop heat, and this is just an add on space? If so, is the boiler in an adjoining room?
  • Cba
    Cba Member Posts: 9
    Separate building. It will need it’s own boiler. Building has a concrete floor but I did not insulate under it.
    I built building 25 years ago, fully insulated, drywall, and painted. Just need the right heat. I’m not a huge propane fan but the way I see it that’s the most practical route to get a boiler.
    Zman I checked out that part from Supply house looks pretty good. I like the idea of gravity feed but if that’s not the way to go then I could get that unit. I like the kiss method (keep it simple stupid) but if I have to spend a few bucks that’s ok. I asked about the circulator pump because I read an article that made it sound like years ago they put gravity feed tank up in attic, then cold water came down from there to boiler and pushed steam into radiators. I think I misinterpreted what they were getting at and I need to reread it but can’t find the article. I would assume a pump was needed to get water back to attic.
    I wasn’t sure I would get any responses on this topic, thank you all. Any suggestions on a boiler? My boiler in shop is oil burner so not familiar with a propane one. Direct vent out the wall would be nice but I need to educated on gas boilers.
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,430
    @Cba a boiler for an insulated 12x12 room is extreme overkill - you’ll have a minimum of about 30,000 Btus with a basic boiler for a space that might need 2-3,000 btu on the coldest day of the year. An electric radiator will probably cost ~$50 (they make cast iron ones as well) and would be the right size. 
  • mattmia2
    mattmia2 Member Posts: 8,395
    Before circulators systems had large pipes and large boiler passages and the water rose as it was heated in the boiler and fell back to the boiler as it cooled in the radiators. This has nothing to do with the expansion tank in the attic. Frequently you will see very old systems like this, occasionally you will see systems from the 40's and 50's with modern looking boilers set up for gravity circulation. Gravity systems could have open expansion tanks, compression tanks, or even modern diaphragm type expansion tanks.

    The open expansion tank in the attic was simply to accept the additional water volume as the water expanded when it heated(and to some extent a safety device to keep the boiler from exploding if the water got hot enough to boil). The pressure of the boiler was only the weight of the water to the top of the system, it was open to the atmosphere at this tank.
  • KC_Jones
    KC_Jones Member Posts: 5,430
    I would steer clear of an actual boiler. It's so much overkill I don't think we can effectively put it into words. If the building is properly insulated and relatively tight (12x12 this would be easy), you could heat it with some candles. I'm not suggesting you use candles, just giving you an idea of what dramatic overkill a boiler would be.

    I'm with the suggestions above, use an electric cast iron radiator. If you are handy could probably build it yourself, but here is a link to a company. Also, this option will be cheaper and simpler than what you are proposing.

    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • Cba
    Cba Member Posts: 9
    I have an exhaust fan in there. I can control the fan speed and it has baffles so it can pull fresh air according to how much air I want. So I am bringing in cold air but not a huge amount. I have tried different heaters, the only thing that produces the heat I’m after is a kerosene heater which I think is 22,000 btu’s. I have never used it for drying the lacquer as the open flame is totally out but if I just run it it gets the temperature where I want it. I know this seems excessive to most but off gassing of any finish solvent or water based is not something I want to be around anymore.
  • mattmia2
    mattmia2 Member Posts: 8,395
    You could also put a reheat coil in the intake. You cold possibly use an electric water heater as the heat source or a modulating electric boiler. It depends on how you use it. The electric water heater or boiler that can make the heat you need will not be inexpensive but neither will a propane boiler and all the things you need to run it. It depends on how much you plan to use it if the lower cost of propane vs electricity is worth the added upfront cost.
  • delta T
    delta T Member Posts: 883
    Okay, 12 x 12 x how high?
    How thick are the walls?
    Stud spacing?
    Fiberglass insulation?
    How much insulation in the ceiling?
    You say the slab is not insulated, assuming 4" thick?
    Windows? If so what size? What construction (double pane? single pane?)
    Door construction and size? (solid core, hollow core, window, solid wood, insulated fire door, etc....).
    You mention an exhaust fan. Will this always be running when the heat is needed? How many CFM?
    What temperature is desired inside?
    What is the coldest temperature you expect outside?

    With all this information you can directly calculate how many BTU's are necessary to heat the space. I am in agreement with everyone else that you are unlikely to find a boiler that is even close to small enough for that load. Having an oversized boiler (in this case I'm guessing you would be looking at something like 3 or 4 times oversized on the coldest day, likely 8 or 10 x on a mild day) will cause you lots of headaches and cost you lots of money in the long run. I would say its likely that you could get away with a small 6 gallon electric water heater (instead of a 40, just to save on your glycol mix) and run a series loop with maybe 2 rads total. A standard water heater element is 4500 watts which translates to about 15,000 btu/hr. This would give you over 100 btu/hr for every square foot of your building. The design day temperature for my area is 4 degrees F. I Rarely see any structure need more than 25 Btu/hr/sq. ft. at design temperature. Heat loss is proportional to delta T, so if you want it 70 inside while it is 0 outside, you have a 70 degree delta T (inside temp - outside temp). If you need, lets say for argument, 8000 Btu/hr at design temp (This is a reasonable guess at your structure's actual heat loss), then you would need half that much when it is 35 degrees out (for a 35 degree delta T). Since most of the time in my area it will be much closer to 35 than to 0, it would be crazy to me to put a boiler in for this case. An electric water heater will not mind the short cycling nearly as much, and is easy and cheap to repair and maintain. I would definitely avoid using a boiler as the lowest you are likely to see for a boiler is going to be around 8000 btu / hr. and that would only be for a higher end modulating condensing boiler.

    All this being said, you could just buy a couple oil filled electric heaters and achieve the same thing with out the antifreeze, water heater, pumps and hassle.....
  • I consulted with a local couple who wanted to heat a small cottage with two radiators. The total load was 12,000 BTU's and they used a 40 amp Stiebel Eltron electric water heater. You have to size the radiators at 140° entering water temperature.

    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • neilc
    neilc Member Posts: 2,226
    an open expansion tank ?
    you're gonna smell that glycol there where ever that tank ends up,
    that and the laquer smell , , ,
    leave your shoe laces untied so others can pull you back down from the ceiling,
    known to beat dead horses
  • Cba
    Cba Member Posts: 9
    2x4 walls r-11, ceiling also r-11, no windows, (2) 36” doors with 1-1/2” foam board, t-111 on both sides of doors. I believe that insulation is r-7. 4” concrete floor but I’m thinking putting 2x4’s down and putting 1-1/2” foam board and then cover with 3/4” plywood. Let’s say 100 cfm on fan. 30 degree outside temperature, any colder just not worth it.
    Those oil filled radiators have switch on them and I believe overload protection so concerned with a spark. Originally had cathedral ceiling that is also insulated and drywall. Exhaust fan is at one gable end.
    Inside building on the ceiling there is a 2’x4’ hinged hatch also insulated r-7. This is just below fan at gable end. Hatch has rope that I can open as much as I need. The opening to let a nice mild airflow would probably be 3/4” wide by 48” long. 100 cfm is a guess but could be less or more. You really don’t need a lot just a little airflow.
  • Jon_blaney
    Jon_blaney Member Posts: 313
    Do you have boiler capacity elsewhere on the property? Run insulated pipe to your radiators.
  • Cba
    Cba Member Posts: 9
    So I looked at electric boilers. I would like to spray in this building also. Once the exhaust fan comes on you can’t get enough heat. However when painting a part it only takes about a minute to spray on the paint. Instead of heating the building I’m thinking of building a closet in the building with inside dimensions of 96” long x 30” deep and about 84” tall. I can build a floor and insulate with r19 so the floor of closet is 6” off of the concrete floor. The ceiling of closet will also get beefed up insulation making it also r19. The doors on closet will be plywood with 1” rigid foam in between the ply’s. The doors will actually run horizontal and I may have 5 of them. The idea here is if I am spraying an 8’ molding I would simply fold down a 16” horizontal door, slide in part and close. Parts to be sprayed are also in closet so part is warm. Paint pressure pot will also be fitted into closet so paint stays warm in pot. I looked at electric boilers. One brand has a small boiler which has a rating of 3800 btu’s and uses 1100 watts. That system is only made for radiant heat floor and not a radiator. That would be ok since I am now insulating the closet floor and not the entire concrete floor. As for exhausting fumes probably just a 3” hole somewhere near top going out side wall just to let them escape. I need to maintain 70 degrees for about 72 hours. As I said before I can’t have anything that could spark and boiler would be located in small room enclosure under roof on outside of building. The roof extends on one side like a carport but the area is only about 40” wide. This small room will also get insulated. I also need an explosion proof thermostat. I can’t find a low voltage explosion proof thermostat just line voltage and not sure how that would work with these type boilers. If this sounds doable could someone give me advice on how to run the pex lines and how do I construct floor for best results. Do I run the pex up wall also? Any thoughts appreciated?
  • mattmia2
    mattmia2 Member Posts: 8,395
    you could use a thermostat with a remote thermistor probe. It probably technically wouldn't have an explosion proof rating but a thermistor is a sealed component that changes resistance with temp, it has no contacts to spark. It would connect to a thermostat out of the area of the vapors.
  • Cba
    Cba Member Posts: 9
    I have seen those but just not sure about voltage. The ones I see are 110v and not sure how that hooks to those electric boiler’s control panel.
    I thought for floor 2x6 construction, insulate, 1/2” plywood, pex tubing and then pegboard on top.
  • mattmia2
    mattmia2 Member Posts: 8,395
    Most are 24vac and would need to control an electric boiler through a contactor, usually built in to the boiler.
  • New England SteamWorks
    New England SteamWorks Member Posts: 1,483
    How about a Hot Dawg? No water, super easy.
    New England SteamWorks
    Service, Installation, & Restoration of Steam Heating Systems
  • Paul Formisano
    Paul Formisano Member Posts: 23
    edited April 2022
    I’d just go with a direct vent propane fired sealed combustion wall mounted heater rated for around 30,000 BTU/hr.  Some have a expansion bulb thermostat that will not produce a spark, others are millivolt thermostat and are also safe.  Some have a fan to assist but you probably won’t need one in that small space.
  • rick in Alaska
    rick in Alaska Member Posts: 1,429
    Good idea on the direct vent heater.
    This is the most common one around here, and does really well. And, not too far off on your heat requirements, but you can step up a size if you figure you will have a lot of open vent time for the fumes.
  • thegreatcornholio
    thegreatcornholio Member Posts: 25
    I wouldn’t recommend a boiler for such a small space. 8’ electric baseboard heater would do it. If you think you really got to have hot water heat you could use an electric water heater as your boiler or a small electric tankless..but I’d say keep it simple. 
  • hot_rod
    hot_rod Member Posts: 19,877
    I think you want a spark free system if you are spray painting in there? What about exhaust and ventilation?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream