RMH for Year Round Growing in Solar Greenhouse in Nashville, TN

I am planning out a solar 24 x 72 ft greenhouse for year round growing in zone 6. I will need supplemental heating at least Dec, Jan, and Feb of each year. The greenhouse will be a Chinese designed greenhouse with a large thermal mass back wall and double poly glazing. It will have gravel aisles and grow boxes built up from the soil. I am thinking about installing a RMH to help keep night time temps above about 55 F during the winter months. If I install the exhaust stack parallel to the thermal mass back wall buried straight down the aisle, how long should I make the exhaust stack to remove as much heat as possible before exiting the greenhouse? Will 1 RMH be effective at keeping this greenhouse above 55F during the winter? Should I install 2? Thanks, Brad

Hi Brad,
Are you doing double-wall or any kind of insulation for the greenhouse at night? The difference in heat loss between single and double (glass or plastic) is pretty steep. I don't know your climate that well either, so I don't know what your nastiest cold snap would be and so on. But a large greenhouse is pretty good at losing heat compared to even the most poorly-insulated house of the same size.

You could easily go 30-40 feet with one 8" system, maybe up to 50 feet if the exhaust is inside the greenhouse and stays warmer than outdoors (and exits vertically). It's possible a single system would work out fine, but the heat will be much more concentrated by the barrel and cooler at the exhaust end. Depending on the heat loss, it might not be enough.

I'd think about two systems, one on each end, with the barrels having about 36" clearance around them at each end. You could build them assymetric (one longer system, one shorter and wider with the pipes doubling back) and then you could control the heat in 'zones' where more sensitive plants might have their heater run longer / more often. Or you could build them nose-to-tail so that if it turns out you only need to run one, it's well-placed to heat evenly.

If the whole thing is a single zone, consider running the two heaters perpendicular to the thermal mass wall with their barrels up by the thermal mass wall (assuming it's non-combustible). This will charge the thermal mass with a lot more heat on those low-sun days. Could cramp your aisle plans, and there may be other ways to get similar heat distribution.

Just some thoughts.
We do have a draft set of greenhouse heater plans, but until my beta-test sites report back I can't sell them as "proven" plans. (I'm selling the draft for $20 by request, though, so let me know if you want a copy).

Yours,
Erica W

Hi Erica,
Thank you for your reply. I will be using double plastic poly that has a small blower to inflate the two layers of plastic. Here in Nashville it can get down to 0 F in Jan-Feb with an average temp of about 28-30 F. Here is a link to our average temps = http://www.weather.com/weather/wxclimatology/monthly/graph/USTN0357 The back and side walls will be made of on-site slip-formed limestone walls about 12" thick with 4" (R-20)of Dow blue board insulation on the outside of the stone using the stone as thermal mass.

I like your idea of running two systems head to tail pushed up against the long rock thermal wall with the second system barrel just about in the middle of the greenhouse. I am concerned about the smoke. Most plants, especially in a closed environment do not do well around smoke from a fire. Are others using this heating technology successfully in greenhouses?

Concerning your draft set of plans, what is included? Are details on your web site? When do you expect a report from your beta-test site?

Thanks,

Brad

Erica, over the last couple of days I have continued thinking about this RMH in the greenhouse I will be building on my farm. I was wondering if you and Ernie know of a way to capture the heat off the RMH by heating water and then piping the hot water via buried PEX tubing into the large back rock wall of the greenhouse. When I build that slip-formed rock wall I could bury PEX tubing into it and if we can heat water and pump it through PEX buried in the wall then that wall could radiate heat for days even if the sun did not shine. Do you know of anyone doing this?

I just listened to the latest podcast with you, Ernie and Paul. I would be very interested in a ship-able core. Thanks for your help! Brad

Brad Howard wrote:Hi Erica,
Thank you for your reply. I will be using double plastic poly that has a small blower to inflate the two layers of plastic. Here in Nashville it can get down to 0 F in Jan-Feb with an average temp of about 28-30 F. Here is a link to our average temps = http://www.weather.com/weather/wxclimatology/monthly/graph/USTN0357 The back and side walls will be made of on-site slip-formed limestone walls about 12" thick with 4" (R-20)of Dow blue board insulation on the outside of the stone using the stone as thermal mass.

I like your idea of running two systems head to tail pushed up against the long rock thermal wall with the second system barrel just about in the middle of the greenhouse. I am concerned about the smoke. Most plants, especially in a closed environment do not do well around smoke from a fire. Are others using this heating technology successfully in greenhouses?

Concerning your draft set of plans, what is included? Are details on your web site? When do you expect a report from your beta-test site?

Thanks,

Brad