Sizing a wood boiler for two hoop houses

[SRNursery]

I'm looking to purchase a wood boiler to heat two hoop houses that are currently under the construction. The hoop houses are located in USDA zone 7a of Virginia and are each 20x50 feet, roughly 12ft at the peak, wrapped in 6mil poly film with 8mm twin wall polycarbonate front and back faces. We hope to keep the hoop houses around roughly 60-65 F during the coldest months around here for hemp plants. The average temp during cold months around here is about 35-40 F. We expect poor insulation due to the 6mil film covering both hoop houses.

Wood supply is no issue as we work with a tree company who hauls all different types of logs back daily and we also have capabilities to cut and split the wood.

The budget is around $7,000. If anyone could point me in the right direction that would be great.

Thanks

 

[stoveliker]

How are you going to be sure you are burning dry wood when burning that much? Are you going to be able to store 2 years worth of split, stacked and preferably covered wood? Wet wood wastes a lot of energy in evaporating the water in it, creates a lot of pollution, and creosote (fire hazard).

 

[E Yoder]

Is this going to be inflated double poly? That would help some, but greenhouses are a big BTU load.
$7,000 is pretty low to get a wood boiler installed.

 

[SRNursery]

Is this going to be inflated double poly? That would help some, but greenhouses are a big BTU load.
$7,000 is pretty low to get a wood boiler installed.

The plan is to have a single poly film with another film of panda plastic covering for light dep. We may explore double poly with a blower if it reduces the BTU requirement drastically.

The $7,000 is for the unit only, we have a guy who will install the pipes and heat exchangers that we are budgeting for separately.

 

[Bad LP]

Is this going to be inflated double poly? That would help some, but greenhouses are a big BTU load.
$7,000 is pretty low to get a wood boiler installed.

Will 7 K even buy a boiler??

 

[E Yoder]

7k might buy a pretty basic model but barely, the EPA restrictions wouldn't apply to a greenhouse hookup so a cheaper conventional updraft model could be used.
An EPA approved gasification model will be over $10k to heat that large of greenhouse space. Maybe more like $12k.

 

[maple1]

I am thinking your wood supply would soon become an issue.

 

[salecker]

Read everything on this site you can for the next few weeks.
Budget is small,good shopping could get you a good used gasifyer if you are patient
Get cutting and splitting now so you can use the free wood next year or maybe even the year after,depending on what wood you get for free.
Line up this years wood befor the demand of cold weather shows up and everyone is looking for dry wood.

 

[SRNursery]

7k might buy a pretty basic model but barely, the EPA restrictions wouldn't apply to a greenhouse hookup so a cheaper conventional updraft model could be used.
An EPA approved gasification model will be over $10k to heat that large of greenhouse space. Maybe more like $12k.

It looks like I ran the budget wrong in my initial post, we are willing to spend over $10k on a unit. Just looking for sizing advice

 

[SRNursery]

Addressing the comments about wood- our parent company hauls several dump trucks of oak, ash, hickory, etc daily from tree service. Wood will not be a limiting factor with the unit

 

[stoveliker]

Addressing the comments about wood- our parent company hauls several dump trucks of oak, ash, hickory, etc daily from tree service. Wood will not be a limiting factor with the unit

The question is not wood. The question is DRY wood. Oak needs two years or more to dry. Otherwise you'll have a dripping creosote fire hazard on your hands.

 

[maple1]

Addressing the comments about wood- our parent company hauls several dump trucks of oak, ash, hickory, etc daily from tree service. Wood will not be a limiting factor with the unit

It also has to be processed.

How much are you thinking you might use? I'm not familiar with hoop houses but that sounds like a huge load. I would suspect 20 cords at bare minimum. Of dry stuff. Straight off the truck would be a lot more. An inefficient boiler more again.

 

[salecker]

Addressing the comments about wood- our parent company hauls several dump trucks of oak, ash, hickory, etc daily from tree service. Wood will not be a limiting factor with the unit

Yes you mentioned that ...
DRY WOOD is what you need,that is going to be your biggest problem for a couple of years till the green stuff is dry after you split and stack it

 

[E Yoder]

I've installed wood boilers to a few hoop style greenhouses here in VA. Have one that's heating two 30x120' hoop houses, double poly, to 70F+ with hydroponic tomatoes.
Has an 800,000 BTU / hr boiler (max BTU), and it just barely keeps up. But has saved him many many thousand $'s in propane. It's an updraft shaker grate model and it does chew through a lot of wood. I'm not going to get into whether that's a bad thing. There's no creosote though with dry or wetter wood I can assure you, it runs wide open.

I'd see what the BTU loss per sq ft is on the average coldest day (design day), then add up the total BTU load. Them compare to the max btu rate the wood boiler can put out. You'll probably want a boiler that can put out twice what the greenhouses load is so you can go 10+ hours between fillups.

 

[peakbagger]

When you get into a boiler that size fuel handling starts to get to be big issue. Wood chip boilers reduce the handling but obviously you need to get the wood chipped. Once chipped, the wood gets moved with a loader into a pit equipped with underpile reclaimers that pull the fuel from the base of the pit and then into a chip boiler.

 

[E Yoder]

Yes, chip boilers are pretty awesome machines, along with a pretty hefty upfront cost to install everything.

 

[HiroariTillWhen]

Yes, I've had a couple installed, it's a big investment, but you don't spend and worry as much as you'd do with other options once it's paid for and installed.

 

[GKFarmer]

I know this is an older thread now but I'm just seeing it and since I have some experience here I thought I'd chime in.
I'm in Vermont and have for the last 20 years heated greenhouses and high tunnels of all sizes in many ways. Currently I've got 2 that need to be heated during the springtime propagating season. One is near identical to your cubic footage, 20x48x10 ft tall. Here are my thoughts....Insulation is critical. The R-value of the 6 mil is not sufficient to prevent convective heat loss. You want double poly with a big space in between. Heat loss is guaranteed to be double without that air barrier and you will be chasing your tail trying to keep a consistent temp in all areas of the gh (remember insulation works both ways). Though they claim heat loss through the sidewalls is nominal I have found that to be untrue. I left a few small spaces in my t track to allow the blower to also inflate the sides. I cannot say enough on this, you can create heat but it won't stay there.
In the 20x48 I use an old Fisher style woodstove with a simple box fan system to distribute the heat. After all these years that has emerged as the most efficient wood fueled system. We grow mainly hot peppers and the minimum temp must be 65. I can achieve this temp when the outside temp is 15f and windy. Yes I do need to go out to load in the middle of the night at times but after a tripped breaker almost put me out of business one night I went all wood and I'm never going to rely on a grid-tied heater in there again. Too much risk.
For the hemp we used a hot water radiant floor system, which again is great until the power goes out.
To sum that all up, it doesn't matter how you heat it, the challenge is keeping the heat there. It's my opinion (from experience) that the wood boiler comes with a lot of concerns from fuel handling to efficiency and if you have a power interruption the heat will easily and quickly escape the single layer poly and you may not be able to recover it quickly enough for your plants. I've gone so far as to cover the whole building with floating row cover on super cold nights. Good luck and if you feel like it, let us know how you make out.