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Polebarn/greenhouse heat in S.E. Michigan

ChopinWood Member Posts: 14
Hi All,

I'm in the planning stages for a year-round greenhouse and polebarn to be built this year. At this stage the two structures will be combined, but the exact construction of the greenhouse portion is still up in the air. For the sake of this discussion, lets say the polebarn part is 1,600 sq ft and the greenhouse is 400 sq ft.

Most of the polebarn will be purposed for workshop, and a portion of it will be purposed for a large fish tank 750 - 1000 gal. The fish tank and greenhouse work together in a closed loop aquaponics system, producing vegetables and fish. (with the only input being fish food). To keep everything alive during the winter, the whole thing needs to be kept at around 60F.

So I'm considering different heating systems, and have electricity and NG available.

One type is what a number of farmers use up North: In-floor, forced air. My brother built a good sized workshop/shed (50 x 56) and he's happy with how it works. Just a standard furnace blowing warm air into standard drainage tile beneath an insulated slab.

The heat exits through multiple registers on the opposite side of the shop. It seems simple - but I haven't been able to find much research on the web about it.

So I'm wondering if any of you have seen many of these installs and how they might compare to hydronic radiant heat.

I know I have a TON of variables here, AND issues relating to heating water, but I need to start somewhere. I appreciate any input you can provide... Thanks!



  • LarryC
    LarryC Member Posts: 331
    edited June 2011
    electricity reliability and freeze concerns


    1)  What is the coldest temperature you are designing for ?

    2)  what is the hottest temperature you are designing for ?

    3)  Worst case, how long will the electric power be out ?

    4)  What heat sources will be in the structures ?  People, animals, appliances, etc.

    5)  What is the maximum allowable temperature change for the fish tank water?

    6)  Do you plan on adding additional heat sources?  Solar, wood burning stoves, etc.

    7)  Do you plan on adding additional heat loads?  Domestic hot water, additional radiant walls / ceilings / free standing radiators, outside swimming pools, pavement deicing, etc.


    I would suspect that a water based heating system will be the most flexible with respect to adding additional heat sources and heating loads.  You can have multiple temperature zones to match the load requirements.  Low temperature loads would play well with solar systems.


    A significant concern would be the possibility of freezing, during an extended power outage.  Adding a suitable antifreeze will reduce the risk of freezing at the expense of reduced heat capacity of the water and the more work required to pump the fluid around.


    A well insulated floor slab with multiple loops would perform as your primary heating emitters.  A suitable heat exchanger could heat and cool the fish tank.  Raised seed beds could also be heated with local radiant loops directly below the beds.  Copper piping and plants would not mix due to copper being toxic to plants.  External radiators could help remove heat if the aquarium got too warm.


    A suitable modulating and condensing boiler will allow you to closely match the heat load required to the output of the boiler.  This will reduce the amount of fuel consumed.  They are initially more expensive and MAY be susceptable to power surges and lightning strikes.


    Building envelope insulation should reduce the heat load and therefore allow you to use a smaller heating plant.  If reliability is a significant concern, perhaps two smaller boilers is a better choice than a single larger unit.  A large water mass to help even out the demand for heat during the sping and fall shoulder seasons.


    Leave provisions for future system expansion, and plan for maintenance activities such as component and pump replacements.  Include appropriate isolation valves, take pictures of the radiant loops before they are covered, only use quality parts, use oxygen barrier PEX to minimize oxygen migration into the system.


    Good luck.  It sounds like a fun project.
  • ChopinWood
    ChopinWood Member Posts: 14
    Fun project indeed

    Hi Larry,

    Thanks for that punchlist of items. 

    You are correct that a water-based heating system would be the right choice - it has many benefits like the zoning, including the possibility of heating fish tanks and grow beds.  I was just hoping that in-slab forced air might be cheaper.

    Either way - I do need a backup generator for both heat system and air pumps for the fish.

    The allowable temperature swings in the tank are tied to the fish I ultimately choose.

    As far as the coldest/hottest temp designs - I can see that the average coldest temp is 14, but I'd probably design for zero, then I could fall back upon electric tank heaters if it drops to the rare -15F.

    Having alternate heat sources is a great idea too.

    I appreciate you taking the time to answer.

    I'm off to research builders of heated polebarns in SE Michigan.

    Take care!
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