How To Heat Your Greenhouse

Heating will be a deciding factor on the variety of plants you are able to grow in a greenhouse and the number of plants that can be kept over winter. Here, Robert Hall goes through the pros and cons of the different types of heating that are available.

Created by Robert Hall on Monday, 16th of January, 2017.
Updated on Friday, 17th of March, 2017.


Greenhouse Heating For The Winter

You have a choice of tree main types of greenhouse heating, electric, gas and paraffin. Of course all have their advantages and drawbacks, and it is important to take these into consideration when choosing a method that will suit your needs and situation perfectly. 

A minimum temperature of 7C is needed to keep most tender plants happy, so you must think about insulating and heating the greenhouse if you want to make the most of it throughout the year.

Electricity

Easy to use and reliable, electricity is clean and doesn't create any odour or fumes. Again high temperatures can be achieved and controlled fairly accurately, and cost effectively, by a thermostat. Fan heaters provide more uniform heat and can be used to cool the greenhouse during the summer.

  • Clean, no odour or fumes
  • High temperatures can be achieved
  • Temperature can be controlled accurately and cost effectively with a thermostat
  • Fan heaters provide uniform heat and good circulation
  • Fans can also be used to cool in warmer months
  • Professional installation is required to fit waterproof cables and sockets

Gas

Propane, butane, even mains gas are easy to use although again harmful fumes may be a problem. Initially more expensive than paraffin, gas can be controlled by a thermostat and high temperatures may be obtained if required. You always need a spare gas cylinder at hand.

  • Propane and butane are easy to use
  • High temperatures can be achieved
  • Temperature can be controlled by a thermostat
  • A spare cylinder must always be available
  • May give off harmful fumes
  • Causes condensation

Paraffin

These heaters can be economical to buy but, without a thermostat, temperature control is difficult and they need daily maintenance with frequent re-fuelling. Other disadvantages are that humidity and condensation might be a problem and toxic fumes, that can be harmful to plant health, may be emitted.

  • Cheap to buy but running costs may be high
  • Temperature control is difficult
  • May give off harmful fumes
  • Requires frequent re-fuelling and daily maintenance
  • Creates humidity and condensation

Solar energy or wind turbines may become a consideration in the future but at the moment they are far too expensive due in part to the poor heat retention qualities of a greenhouse. Spent cooking oil is used occasionally commercially but is messy, smelly and not really suited to small scale operation.

Insulation

Once you've heated your greenhouse, you need to retain the warmth. Clear polythene could be used for double glazing your greenhouse, but it has little insulating effect and clings to the glass when wet, further minimising its potential.

But when manufactured with the addition of clear bubbles, polythene becomes extremely attractive. The bubbles trap pockets of air, creating the same effect as sealed units in double glazing by providing a still air barrier. 

Bubble wrap can be fixed into any metal greenhouse by using special plastic fastenings that slot into the channels of the aluminum frames. It is then held in place with a cap that secures it tightly ensuring a 1" insulation gap. With a wooden greenhouse you can simply use drawing pins. 

Remember to place the bubbles towards the glass, and ensure good ventilation. Never completely seal up a greenhouse. Once in place you will discover that during daylight hours the warmth from the sun's rays passes through the bubble glazed lined glass to heat the greenhouse.

When the outside temperature falls, the effect will be to retain this free heat. This results in a 70% increase in daytime temperature during the cold season, and a 60% increase at night. 

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