January 07, 2014

Greenhouse Plants Survival in Record Lows

At the end of November I wrote about how I heat the greenhouse. We had a couple of nights back then where temps hovered around freezing. Heating a Greenhouse in a Coastal South Climate.

We had a real test last night. Predicted low was 18 or 20 degrees, depending on your source.

By 10pm we were looking at a 7 degree difference in temperatures outside and in the greenhouse. I went out and fine-tuned the little thermostats on the two heaters until there was a wider difference and rising.

During the night we got up and looked at the remote temps. At 2:40am it was 26 degrees outside and 38 in the greenhouse. The next time I got up I forgot to write down the time but I think it was around 5am. We were looking at 38 degrees in the greenhouse and 23.3 outside. The eventual low was 23 outside and the greenhouse held.

When the sun came up, the greenhouse commenced to rise. By 10am  I unplugged the heaters so the settings would be constant tonight. When the sun starts down in the west I will plug them back in.

Is there a contingency plan if the electricity fails? There's a kerosene heater. We have generators.

Many of the plants in the greenhouse, like hyacinth bulbs I'm forcing can take short bursts of freezing temperatures without permanent damage. Some things, like cuttings that have rooted in water are expendable if worst came to worst -- there are always more. I might have to choose which of the 5 Epiphyllums would get to come into the house so I would have a stock plant and the same with multiple Schlumbergeras. A plus for us is that freezing temperatures are short-lived here when the sun comes up.

A final note about the heaters. Why are there two? Why do we run them on low? There are two so they can run on low rather than running one on high. Our belief is that prolonged running on high can possibly burn out the wiring. In the event that one should fail for some other reason, there's one left that could run on high overnight.


Janie Jurkiewicz said...

I have the same type of heater...can you explain what you mean about burning out the wiring?

Alison said...

Thanks for the update and the link to your post about how you heat the greenhouse. I think I remember reading that when you first wrote it, but what stuck with me was the use of thermal mass.

Jean Campbell said...

Janie, I can't speak with much authority concerning the use of electricity but He Who Knows tells me that running a heater at the 750 watt setting (low) pulls much less current through the wires, prolonging their life.

I asked him about the thermostat and he said that when the setting is such that it never cycles off, it makes no difference. It can only put out as much heat as it can, running all the time. If that isn't enough it won't get any hotter.

At that point, I might have to turn at least one heater up to High, but that didn't happen last night. The Greenhouse stayed above freezing with both heaters on Low setting.

I am heating a 10 x 12' space and only enough to keep the temperature above freezing but not up to 40 degrees in those couple of hours right at daybreak when it dips so low.

If we lived where there were freezing temps every night through the winter, I would look at a different way to heat. By the weekend we'll be back to nights well above freezing -- until the next freeze.

Jean Campbell said...

Thermal mass mediates the temperatures. As soon as the sun goes down, thermal mass starts giving up its heat and it won't last the night.

Thermal mass helps somewhat to keep sunny days from getting too hot in the greenhouse and on cold days takes up heat and holds as long as possible. Come freezing temperatures, it will quickly get the same inside as outside.

Today it was 81 degrees in there about 2 pm. The cat loved it. At 5 pm when the sun disappeared it fell quickly to 65 while the outside is in the mid thirties. By 8 it will be plunging both outside and in. Low expected is 20.

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