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Wood Boilers for Greenhouse Heating
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Paul Betz, High Mowing Organic Seeds Sales Associate and owner of High Ledge Farm, VT
 Greenhouses
play a big role on our farm. Spring plant sales account for 20% of our
gross sales, and our tomato house provides beautiful tomatoes from early
July till October. Once the plant production push is over, benches come
out and more tomatoes go in, onions and winter squash cure and are
stored there in the fall. We had been heating from start-up till the end
of May, and then turned the heat back on in mid-October. All told, our
propane use was around 800 gallons annually; small for some farms but a
lot for us, even with a pre-buy contract. We had been looking into a
fuel switch in the winter of 2008, but the cost of reinvesting in a new
heating system when we had one that worked fine seemed like a lot for
us. Then, in early April 2009, we lost our house, greenhouse, barns and
much farm infrastructure in a fire.
We have been rebuilding our farm for a few years now. When
we looked at our heating options, we knew we didn’t want propane, and
being tied to a fossil fuel and the whim of the market was becoming
harder for us on a lot of different levels. We had looked at corn or
pellet stoves, but the access to fuel seemed to have potential problems.
We have no room to grow the corn we would need, and I have yet to find a
wood pellet anywhere on the farm, but we do have lots of woods around
us, so it seemed to make sense to use our on-site resources.
Outdoor wood boilers have come a long way. While some older
models are heavy polluters with their particulate emissions, the new
models are gassifiers, and burn clean. When the unit is at operating
temperature, the only indication that it is running is some ripples of
heat at the top of the stack. They are also able to capture more of the
potential energy in each piece of wood. My boiler requires dry hardwood
to operate, so fuel management is definitely a consideration, and should
be a part of any decision to pursue this kind of a system.
 The boiler heats 440 gallons of water to 185 degrees, which is then pumped underground through insulated piping.
Last
season, there was also a 275 gallon insulated tank in the greenhouse,
which was constantly circulating in a loop with the boiler. When I
initially designed the system, I wanted to have extra storage, mainly
for use at night when I anticipated the greatest demand on the boiler.
It turns out that the heat exchangers and boiler are pretty well
matched, and I plan on taking the extra tank out this spring. From the
plant house, the water is pumped into two zones, with each greenhouse
representing a zone with its own thermostat.The time that the
greenhouses were heating, from April 1st to May 31st, I used 8 cords of
wood. This is a lot of fuel, and it was a sight to watch my wood pile
shrink that quickly, but in previous years I was using close to 800
gallons of propane.
While I was just heating the air last season, I will be
putting in some under bench heating for the 2011 season. At some point I
will swap in better quality piping from the boiler to the greenhouses. I
went with a lower cost option, and I lose heat in the time it takes for
the water to travel, which translates into more fuel use. I may also
resize my heat exchangers. Right now they are adequate, but with a
really cold snap I can't always get the temperature in the greenhouses
where I want it. Under bench sidewall insulation (also on my pre-spring
to-do list) should help the heat I have go farther.
I
like the fact that my system is using atmospheric carbon to heat the
greenhouses. It does require a lot more work though. Having a truck roll
up and deliver millions of BTUs that work with the push of a button
sure was convenient. One of the reasons I wanted a boiler is that I can
run the heating end of the system pretty accurately with a thermostat,
and I don’t have to worry about loading a wood stove in the middle of
the night. I do feel like the system adds a few hours a week to my
schedule, including getting it lit, loaded and cleaned. Having a
tractor sure helps though. I have gotten used to the extra time, and it
doesn’t bother me.
I did get help, both with engineering and financial
assistance, through University of Vermont Extension. Here’s a link to
the study online: http://www.uvm.edu/vtvegandberry/CaseStudies.html
There are a few other boiler installation studies there as
well. I am excited about the choice that we made to install this system.
If you have specific questions I would be happy to answer them.
All my best,
Paul
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