Heating a Greenhouse

Answered by Harrod Horticultural Posted in Category Greenhouse

Dear Martin

I have recently had a greenhouse/orangery built, made from painted cedar, stone floor and measures 12 x 18ft. it is single glazed has automatic roof opening windows, double doors and 3 side windows.

Ideally I would like to keep bird of paradise plants, geranium’s and canners in there over the window months.
I am looking to put a heater of some sort on for the winter and really don’t know what size heater, whether electric or gas, or whether they are going to prove very expensive to run.

Any comments or advise you could give me would be most welcome.

Kind regards
Christina Tozer

Dear Christina

Many thanks for your recent enquiry concerning how best to keep your new – and very attractive sounding – greenhouse/orangery heated this winter.

The plants you list in your message – pelargoniums, cannas and bird of paradise – all require cool but frost free conditions to survive the winter in their various states – and this normally means the greenhouse needs to be kept at a minimum of 4-5°C, which I’ll get onto later. I’ve calculated that your 12ft x 18ft will require 4 kilowatts of power to maintain a frost-free climate but with the introduction of bubble insulation (again, more on that later) this required output will drop.

Let’s start by taking a look at the two forms of power for greenhouse heaters and I’ve based the information on a few of the models we supply; the Black Forest Catalytic Heater, Tropic Electric Fan Heater and Stainless Steel Fan Heater.

If you’re considering using propane gas as a power source then look no further than the Black Forest Catalytic heaters. These models are superior to their gas and propane fired counterparts in theory, but we can’t comment on the results of using one of these heaters as a true comparison would be very difficult to set-up. You’d need to almost replicate the conditions – same plants, same weather, same feeding regime and same greenhouse – to obtain worthwhile data so we are really guided by the technical specifications, which are impressive.

The main advantage of using the catalytic heaters over gas or propane is the efficiency and cost saving. The catalytic burner has a thermal efficiency of over 99% and the warm air it produces is also enriched with CO2, which of course is ideal for growing plants. Normal gas, propane and paraffin heaters often do not attain complete combustion which can lead to a build-up of carbon monoxide, do not pump CO2 into your greenhouse and are generally less efficient to run. You’ll find that the gas consumption of the 3.5kw catalytic heater when operating on full burner (0.297kg/hr) compares favourably to similar sized models and this size of heater will just about keep your 12ft x 18ft greenhouse frost free. We would advise lining your greenhouse with bubble insulation as this form of ‘double glazing’ can further reduce your heating costs by up to 50%.

Electric greenhouse heaters are obviously cleaner, safer, quieter and practically maintenance free to run and calculating the running costs of these heaters is also relatively straightforward; just check the power rating (the versions we’re considering are the Tropic and Stainless Steel Fan Heater, 2kw and 2.8kw respectively) and then compare this to hourly rate per kw (kilowatts) you are charged by your electricity provider on one of your electricity bills and you should get a fairly good idea what running them will cost you!

To make things even more economical, both these heaters are thermostatically controlled so they can be set to operate when the temperature in the greenhouse reaches a chosen point; some manufacturers term this the ‘frostwatch’ setting and the heater will keep the greenhouse temperature from falling below 4-5°C. In a greenhouse your size, any 4kw rated heater (or 2x 2kw models) will keep the structure frost free whilst the more powerful 2.8kw model has the capabilities, when operated in tandem with a 2kw heater, to provide increased levels of heat if desired. You may also want to consider sectioning off an area of the greenhouse with a bubble insulation curtain to create a ‘hot area’, which you could use for propagating and housing more delicate plants and seedlings.

Apart from the difference in power output as mentioned above, the 2.8kw Stainless Steel heater comes with the option of positioning in the beams of the greenhouse via the hanging chains supplied. This is particularly beneficial in keeping greenhouse humidity levels - much loved by common plant diseases such as botrytis and damping off- down by circulating air at this is the main advantage of mounting in the roof, along with using the heater on the cooling fan-only setting in summer – plus your heater doesn’t take up valuable floor space. On the other hand, standing or mounting the heater on the floor does place it nearer (and most importantly the thermostat) the seedlings so the temperature in the plant zone is more accurately monitored so you might want to position the heater here in the winter and pop it up in the roof for the summer! 
There’s plenty of heater and greenhouse information here for you to digest and if there’s anything else you’d like me to cover, please don’t hesitate to ask.

Many thanks once again for your enquiry and the best of luck with overwintering plants and growing in your new greenhouse both this winter and into next year.

Kind Regards