Greenhouse heating

[Cidermanrolls]

For a 6x8, if you have insulation in place, then I would recommend running at the 1kw setting. That way the hysteresis (max-min temperature swings) will be smaller and so your bills will be lower. Marginal, but worth it. The fan will also be on for more of the night, meaning the heat will be better distributed.
Don’t worry about 1kw being too little power. I run my 12x8s with just 1kw, so your smaller greenhouses will be fine. On really cold nights you can always flip it to 2kW.

[el48tel]

For a 6x8, if you have insulation in place, then I would recommend running at the 1kw setting. That way the hysteresis (max-min temperature swings) will be smaller and so your bills will be lower. Marginal, but worth it. The fan will also be on for more of the night, meaning the heat will be better distributed.
Don’t worry about 1kw being too little power. I run my 12x8s with just 1kw, so your smaller greenhouses will be fine. On really cold nights you can always flip it to 2kW.

I agree. You could also use a continuous but unheated fan to move the air around to prevent cold spots, and cause turbulent flow in the GH which would give thorough and efficient mixing of that warmed air.

[Acid John]

Plus that constantly moving air will discourage moulds from forming, another benifit.

[Cactuscat2]

Thank you, I really appreciate all your thoughts. I have tested to see if the thermostat is ok and used the frozen pea method The thermostat is about 2feet from the heater; however the instructions say to place the probe halfway up the middle of the greenhouse, presumably suspended in mid air! This I have managed to do with a length of bamboo but should it be with the plants? Also I will reduce the setting to 1kw, as suggested, thank you.
I have another similar heater in the garage and think it's a good idea to use it just for air flow, as suggested by el48tel. At least the pelargoniums will be happy!

[el48tel]

Thank you, I really appreciate all your thoughts. I have tested to see if the thermostat is ok and used the frozen pea method The thermostat is about 2feet from the heater; however the instructions say to place the probe halfway up the middle of the greenhouse, presumably suspended in mid air! This I have managed to do with a length of bamboo but should it be with the plants? Also I will reduce the setting to 1kw, as suggested, thank you.
I have another similar heater in the garage and think it's a good idea to use it just for air flow, as suggested by el48tel. At least the pelargoniums will be happy!

I think the best place is to place the thermostat probe in the coolest place in the GH. Then you know everywhere else will be warmer. There will be a natural thermal gradient in the GH unless your non heater fan gives fantastic turbulent flow ..... great if you get turbulent mixing ... it gives good mixing and lowers mould growth.
Remember that a small GH cools and heats up more quickly than a larger one .... surface area to volume ratio is larger. Metal frames cool and heat more quickly than wooden ones.

[Chris L]

How I set mine up was to have the thermostat plugged in near the door and the heater plugged in near to the back wall. It means rather a lot of extension cabling under the bench but it means the probe is as far as is practicably possible away from the heater.

The IP65 box is under the staging at the back of the greenhouse and an ext-cable runs under the staging to the front where the double socket has the thermostat plugged into it. There is then another ext-cable plugged into the front of the thermostat that runs to the back of the greenhouse which the heater is plugged into.

[Cactuscat2]

I really appreciate every comment and suggestion, thank you! 2 weeks on and everything seems fine, although no below freezing temps have occured overnight for a while. The lowest has been 4 degrees, despite setting the thermostat to 7. However, I note Chris L's suggestion of a good distance between the heater and thermostat so will implement that tomorrow. At least the greenhouse is still standing after the recent gales, all good!