Tony Dickerson answers your questions
What's the right size heater for me?
Q
How do I work out the size of heater for my greenhouse? Hugh Davenport, Bidford-on-Avon, Warwickshire
A
A greenhouse with frost protection allows a wide range of tender plants to be overwintered. It also lets you make an early start with sowings from late winter onwards.
For small greenhouses electricity is always the most appropriate heating option. While appearing costly, it’s easy to control and much safer than gas or paraffin heaters, which also produce large amounts of water vapour and carbon monoxide. Both also need adequate ventilation, reducing any apparent savings. Fan heaters circulate the air quickly and can be used to cool the greenhouse in summer. More expensive equipment features an adjustable thermostat, which is desirable. Choose one with a fan that continues to run when the thermostat switches off the heating element. Importantly, only use equipment specifically designed for greenhouse use. The size of heater depends on a number of factors, such as the size of the greenhouse, the minimum temperature you want to maintain and how cold it gets. Most gardeners should aim to maintain a cool greenhouse over winter at a minimum temperature of 7C (45F), which gives a safety margin in very severe weather, which allows frost-sensitive plants to be overwintered.
The heater has to be large enough to heat the greenhouse to 7C (45F) when the outside temperature is -7C (19F). The simplest way to work this out is to calculate the total surface area of glass and multiply by 10 to give the wattage required. A 6x6ft greenhouse is about 162sq ft, which, x 10, gives 1620w. Therefore, a 2kW heater will give you enough heat.
Ensure the greenhouse is well maintained with tight-fitting doors and vents. Bubble insulation also helps to reduce heat loss and draping two or three layers of fleece over plants on benches in very cold spells will give additional protection from frost.
A good range of greenhouse heaters is available from Two Wests & Elliott (tel: 01246 451077; www.twowests.co.uk).