Electric oil heater in greenhouse

[It never rains..it pours]

Anyone ever used one of these in a greenhouse. Set one up in greenhouse last night, all I had in the dont use but dont throw away box. Cable under decking to summer house plug was easier than expected. However it still got down to -2c last night (coldest been all year, rare to see much below 0), but outside was -4.0c. Wondering if I need to turn it on conntinuous, rather than rely on thermostat as some delay factor in warming with them. Or mabye 750w wasnt enough and need to turn, both elements on so 1500w.

[nickdub]

Rather depends on what plants you have in the greenhouse.

[It never rains..it pours]

Yes I should have mentioned that Nick
Geraniums, potatoes in tubs, peas in gutters, strawberrys in tubs, raspberrys in tubs, asparagus in buckets. Bar the geraniums and the asparagus (run out of room) they are hopefully going to produce ahead of the outdoor plants and are looking a lot further ahead at present.

Found below link and calculated need 1350 btu (400w) approx (doesnt seem alot to me) to keep at 0c, so will just put the 750w on constant rather than thermostat tonight and see

https://hartley-botanic.co.uk/guides...er-calculator/

[Thelma Sanders]

In the cold, last night - I think your heater would have been on all night anyway and did all it could to stop the inside temp from being the same as the outside, but that 750W just wasn't enough to keep temperature up.
For greenhouse to remain frost free, it is usual to set thermostat to at least 5C. Anything approaching 0C can kill tender plants completely, and kill foliage on hardier plants (although they will regrow from the roots eventually).

[4Shoes]

What temperature is the stat set at?

Should not matter if you have 1500w or 750W when using the thermostat. It will switch both off on reaching set temperature. The 2nd element will heat up the space faster.

If not at temperature, the plants will suffer.

Do you have a smart meter? you will be able to see when it switches off and on

EDIT.... Mind was on the stat ..rather than have on constant...
Found below link and calculated need 1350 btu (400w) approx (doesnt seem alot to me) to keep at 0c, so will just put the 750w on constant rather than thermostat tonight and see

Yes, you are right if not enough power, will not heat the space.

[Thelma Sanders]

What temperature is the stat set at?

Should not matter if you have 1500w or 750W when using the thermostat. It will switch both off on reaching set temperature. The 2nd element will heat up the space faster.

It would matter if the 750W on its own couldn't keep the temp above freezing IYKWIM.
With both on the heater may keep the temperature up enough and turn itself off when/if the setting is reached.

[nickdub]

Yes I should have mentioned that Nick
Geraniums, potatoes in tubs, peas in gutters, strawberrys in tubs, raspberrys in tubs, asparagus in buckets. Bar the geraniums and the asparagus (run out of room) they are hopefully going to produce ahead of the outdoor plants and are looking a lot further ahead at present.

https://hartley-botanic.co.uk/guides...er-calculator/

I would :-
a) bring the Geraniums in to the house over night
b) cover the potatoes and asparagus with anything insulating eg old blankets
c) not worry much about peas in gutters, strawberries in tubs or raspberries in tubs

keep the heater on just in case it helps, but don't expect it to do miracles.

[4Shoes]

It would matter if the 750W on its own couldn't keep the temp above freezing IYKWIM.
With both on the heater may keep the temperature up enough and turn itself off when/if the setting is reached.

Yes of course..

[It never rains..it pours]

Thank you all for the help, much appreciated. Bit too dark to move the heavy geranium pots tonight so have decided to be less mean and as advised put the heater on 1500w along with whacking the thermostat up. Think one issue is the heater thermostat is designed for house temperatures rather than greenhouse. Don't have a smart meter but one thing did notice is it didn't take long to heat the greenhouse up so hopefully not too expensive to run

[nickdub]

No worries - if this cold spell continues it might be worth knocking the geraniums out of their pots and sticking them in carrier bags, until more normal temperatures return.

[Scarlet]

...... hopefully not too expensive to run

I think you are probably spending more than the cost of replacing with new plants in the Spring. I’d either move them in or just try covering and keep your fingers crossed.

Just bring one or two geraniums in and take some cuttings?

[veggiechicken]

I agree with Scarlet.
Most of the plants in your GH are frost hardy and would normally be outside all winter. I leave my geraniums in an unheated GH over winter.
Cover the geraniums but don't worry about the rest - unless you're rich!

[It never rains..it pours]

Thank you for the advice. Last night was -1c low and -4c outside. Will leave setting as it is as not minted and gereniums will have lump it or will buy new. Have put a socket power usage reader on, so will see what it uses tonight.

[It never rains..it pours]

Last night greenhouse (8 x 12) low was 0c and outside -4c. Heater used 8.8kw last night 6pm to 8am. Conclusion for me is its ok to defend 0c for a few nights, only get a few heat would be needed here, but 5c would be rather expensive if not impossible and buying new plants in spring maybe more economic

[nickdub]

Thanks for posting the up date - always interesting to know how things turned out.