Cheapest way to heat a greenhouse?

Hi

What's a cost effective way to heat a green house? As spring was so cold I've only been harvesting cukes and toms for the last three weeks and many tomatoes are still to ripen.

With the nights now getting chilly I feel it may all be wasted effort so I'd like to warm the greenhouse at night.

I can run electric to the greenhouse but would rather not, and I'd rather have something I can put on a timer or is thermostatic.

Advice much appreciated!

 

Electricity is cheapest if you want a modest amount of heat. You do need to buy a highly accurate thermostat (about £50) to achieve the saving though, but you can then use a £20 fan heater if you like. Getting electricity fitted to the greenhouse is not cheap though ... armoured cable to get there, small distribution box in the greenhouse with safety cutouts, and an earth stake ...

But you can add lights, or propagator, or a misting bench, or ... all sorts really!

Soil heating can also help (in allowing lower air temperatures). An old trick was to put the gas or paraffin heater under the metal staging so that it heated up, and from there the soil in the pots.

Insulation is important. You can line the greenhouse with bubblewrap, for example, or get a cheap "blow away" 4-shelf tower and put the plants in that, with a small heater, and zip it up at night.

Bottle Gas or Paraffin require that you light them each night when you think it will be cold. So they will be running even when the temperature is not critical (in the case of gas the heater will have a thermostat, but the hysteresis will be awful, probably 10C from On to Off, which is what costs dearly). Both Gas and Paraffin produce huge amounts of condensation which is a problem in itself. You run the risk that the fuel runs out, but similarly of a power cut with electric.

Caravan paraffin heaters are much more sophisticated and might be suitable for a greenhouse. Not cheap though.

My greenhouse, with electric fan heater, stays within 0.5C of target. The fan blows the heat into all corners, so no hot or cold spots.

 

Thanks for those comprehensive replies...

In other words - it's not simple! I think I'll start with the bricks / water bottles idea as I have loads of both.

Mine has an automatic opener for the top vent which is great as it will ventilate if gets too hot... but as it's an oil expansion one it takes a while to close thus making it hard to trap the warm air in late afternoon. Perhaps I should take that off?

 

Or adjust to open later (and close earlier) this time of year? If the sun is full pelt on the greenhouse until 10am without the windows opening it will most probably only get to "nice toasty warm" rather than "cooked"!!

Or, as you say, take it off. I close my greenhouse at 3pm now unless it is very sunny and then maybe 4pm latest.

 

I'm not sure how to adjust it!

 

I disable mine overnight otherwise the heat rising from the heater opens the vents, letting all the hot air out and causing the thermostat to turn the heater up causing the autovent to open the windows wider and in the morning you find the roof vents open wide and the greenhouse heater on full blast.

 

Well what d'ya know...

http://www.gardman.co.uk/sites/default/files/products/instruction_sheets/08870_Auto_Vent_Inst.pdf

 

Any idea how accurate these are? Bio Green Thermo 1 Thermostat £39.95 delivered http://www.jungleseeds.co.uk/contents/en-uk/d106.html#p1565. Am I correct in assuming you need to set the heater thermostat on max and the desired temperature on the separate thermostat it's plugged into?

P.S. The Thermo 2 (extra £12) looks handy as it can also control an electric heater's fan only facility in summer. Might put this one on my Xmas present list.

 

Can only find the more expensive product on the Bio Green website, so maybe the cheaper one is discontinued. I'd expect a digital control like that to be accurate to within 0.5C (spec is likely to be more vague, but in practice they are very good - I tested an array of 30 of similar solid state temperature sensors (which I wanted for some monitoring equipment) as I needed to calibrate them, they were all within 0.1C of each other

You could email the company to check? I think the only difference is that the more expensive one has an option for a Cooling control in summer - turn ON when temperature reached (Summer), rather than turn OFF (Winter)

http://www.biogreen.de/en/thermostats/thermo-2/index.php

Make sure you put the sensor amongst the plants where you want the temperature to be right, albeit not completely shielded by foliage!