Collection: Heater For Greenhouse

What a Greenhouse Heater Is For

A greenhouse heater holds temperature above frost so overwintering plants and early sowings survive cold nights. It is not there to make the house warm, and treating it as though it were is how running costs get out of hand.

For most growers the target is modest. Holding three to five degrees protects tender stock through a British winter and costs a fraction of holding ten. Knowing which you need depends on what you are keeping, and that decision should come before choosing a model.

The Three Fuel Types

• Electric is thermostatic, produces no water vapour and is the only type safe to leave running unattended. It needs a proper outdoor rated supply.
• Paraffin needs no power and suits sites without electricity, but produces water vapour and fumes and requires ventilation while burning.
• Bioethanol burns cleanly with no soot or ash, but is not thermostatic and must never be left unattended in a closed structure.

Where power is available, electric is the straightforward answer. The electric heater range covers thermostatic and fan assisted models with output figures listed.

Matching Output to the House

Size the heater against the cubic volume of the greenhouse rather than its floor area. A tall house with the same footprint as a low one holds considerably more air and needs more output to hold the same temperature.

Glazing matters too. A glass house loses heat faster than one glazed in twin wall polycarbonate, so the same volume may need a larger unit. Gaps at the base and missing glazing clips lose more heat than most owners expect, and are worth fixing before buying a bigger heater.

Insulate Before You Heat

Lining the inside of the glazing with bubble sheet cuts heat loss substantially for a fraction of what the equivalent heating would cost across a winter. It reduces light transmission slightly, which is the trade off, though winter light is usually not the limiting factor for dormant stock.

The protection range covers bubble sheet and horticultural fleece. Many growers also partition the house and heat only one end, which is the single most effective saving available at larger sizes.

Placement and Airflow

Warm air rises and collects at the ridge, leaving the floor cold where most roots are. A fan assisted model distributes it, or a separate circulation fan will do the same job at lower cost.

Keep the unit clear of foliage, staging and anything hanging, and site it where air can move freely around it.

Knowing Whether It Is Working

A maximum and minimum thermometer is the cheapest item in this entire category and the most useful. It records the overnight low, which tells you whether the heater held the temperature you set or whether it was overwhelmed.