Heating the Greenhouse | Finishings | Pit Garden Forum

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Heating the Greenhouse
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January 15, 2015
9:35 am
Andrew Blessing
Montana
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The pit garden is designed to use the earth and the sun to maintain heat and it does an admirable job.  As this was our first year with it and we didn’t finish until mid December, we needed a bit more heat to get the starts going and play with our new toy.  I wanted to detail what we’ve tried and discovered for heating the greenhouse.

To set the stage let me explain that we’ve had seeds in the ground now for about a month.  We planted right as the sun was its lowest on the horizon, days were shortest, and nights were dipping below zero many nights.

Doing Nothing

We’ve been afraid to let the new seedlings get too cold at night.  I have though pushed the envelope a bit as I’m curious just how well the pit garden will do with no supplemental heat. Our experience over this past month is that even on cold, windy, and snowy nights, the pit will not drop below 40 degrees without heat.  As mentioned I didn’t feel that was good enough for our starts so I’ve been experimenting.

Just the Grow Lamp

As detailed in our finishing up post, we installed a 1000W grow lamp to give a little extra light during these shorter days.   It’s not just that our usable light drops down to 6-7 hours per day, but many of the days are cloudy and snowy this time of year.  This lamp not only gives the plants more UV to grow, but it also gives off a good deal of heat.  In fact just running the light whenever the sun is not warming the pit maintains a temp of around 52 degrees on our growing surfaces.   Quite honestly I think this is a high enough temp for what we’re growing right now, but I don’t want to burn 1000W’s of energy up to 24 hours per stormy day.

Electric Heater

The first thing I tried was an electric heater.  We had a 1500W unit like this with thermostat control laying around, and I was hopeful that this could keep things warm, and maybe a little drier in the pit without having to run all the time.   I was surprised to find out that it needed to stay on the whole night to keep the same temperature as the grow lamp.  500 more watts, no gain in heat, and the loss of the light for the plants. This solution didn’t stick around long.

Propane Construction Heater

I had a 60,000 BTU construction heater that I also gave a run at.  This proved the concept of propane heat, in that even on low (40,000 BTU), it could take the temp from 45 to 95 in about 30 minutes.  This would then slowly cool off over a few hours before you’d need to fire it again. The only issue was that without thermostat control I had to run in and out of the pit to keep it warm.  Definitely not an all night option.  We lived like this for a couple weeks heating it up at dinner, and before bed with the propane, and then running the grow lamp until breakfast. The advantage was that these two short propane bursts definitely was less expensive than running electricity the whole time.

Ventless Propane Heater — The Winner!

Giving off 30,000 BTU, with a thermostat control (1 on the dial keeps it about 53 degrees), the temp stays warm, we need not lift a finger, and so far we’re looking at about 1/2 the cost in Propane for the coldest nights then electric heat was.  The unit we’re using is a dual fuel unit from Procom.   When the grow light is on the unit almost never has to come alive, and when the light is off it switches on and off as necessary to keep our desired temp.   Now with this heater, and our pit garden automation, the plants are happy without any help from us!

Procom Ventless Propane Heater

The Sun

Using energy to create heat is the necessary evil and backup plan.   The heat sink though with the sun does an amazing all natural job on sunny days.  During the day, I have seen an 80 degree spread between the outside and inside temps when the sun is up and shining bright. Storing this heat energy up in our water barrel heat sinks keeps the temps up at least 5-6 hours after the sun sets.    I’m near certain that by mid February we will no longer need to heat the pit other than on extended cloud covered days.

September 21, 2016
2:44 am
ThomasWinfrey
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September 21, 2016
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Very useful post. Thanks for sharing.

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