Looks like my 400W panelheater setup can only hold the temperature about 5C above the outside temp. Last night's 4C outside caused my greenhouse to drop to the lowest point of 9.6C. The light then came on at 3am and got it up to 12C but still means that between about midnight and 3am I was at just under 10 degrees which isnt cool.
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You can see the drop on the last two days where instead of staying above 14C it first went down to 11C the night before then 9.6 last night.
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I'm gonna rearrange my heater setup this weekend, will place it at the lowest point of the greenhouse, and will see if I can find some large rolls of bubble wrap as I've seen tons of people using it on their greenhouses as insulation.
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But for next year I think I'll be rethinking my heating setup completely.