Greenhouse Heater

Overview

A greenhouse heater exists to win a narrow battle: keeping air a few degrees above freezing inside a structure that loses heat through single glazing almost as fast as it gains it. Glass has a U-value around 5.8 W/m²K, so a modest 8 × 3 m glasshouse on a –5 °C night needs roughly 3–4 kW just to hold +2 °C. Electric fan heaters can do this but at mains tariffs the season costs several times the gas equivalent, which is why bottled-propane heaters in the 2–10 kW class dominate hobby and small commercial houses.

The machine is conceptually a small warm-air furnace. Gas burns in the Burner Assembly, hot products pass through the Heat Exchanger & Flue, and the Circulation Fan sweeps greenhouse air over the hot tubes and out through the Louvre Grille. The Control & Ignition group cycles the burner against a set point, and the galvanised Housing & Mounting survives the permanently humid environment.

How it works

Fuel arrives from the cylinder through the Gas Supply Hose and Gas Pressure Regulator, which holds a constant 37 mbar working pressure regardless of cylinder level (a Pressure Test Nipple allows this to be verified at commissioning). When the Capillary Thermostat closes on falling temperature, the Gas Safety Valve opens and the Ignition Module fires ~15 kV sparks across the Igniter Electrode gap.

Gas leaves the Orifice Jet as a high-velocity stream that entrains primary air in the Venturi Mixer, the same Bunsen principle used in domestic boilers. The mixture burns in a ribbon of blue flames along the Burner Tube. Flame supervision is purely thermoelectric: the Flame Thermocouple sits in the flame and its ~25 mV output holds the valve's magnet unit open. If the flame blows out, the voltage decays and the valve snaps shut within 30 seconds, with no electronics in the safety chain.

Combustion products at 600–800 °C pass through four Exchanger Tube runs, each fitted with a twisted Turbulator Baffle that trips the boundary layer and lifts heat transfer by roughly a fifth. On flued models the cooled exhaust leaves through the Flue Collar and a through-roof pipe, with a Draught Diverter isolating the burner from gusty flue pull. Flueless models discharge combustion products into the house itself, acceptable in a leaky glasshouse because plants tolerate, and indeed use, the extra CO₂, but it makes the oxygen-depletion ODS Pilot mandatory: if room oxygen falls below about 18.5 % the pilot flame lifts away from its thermocouple and the heater locks out.

The Blower Motor driving the Axial Fan Blade moves 300–800 m³/h. Forced circulation matters as much as the heat itself: it prevents the stratification that otherwise leaves roof vents at 15 °C while bench-level seedlings frost, and the steady air movement reduces Botrytis pressure on damp foliage. The Motor Bracket keeps the motor in the cool intake stream, and a Fan Guard covers the inlet.

Controls and protection

The capillary Capillary Thermostat is deliberately simple, a liquid-filled bulb placed at plant height, a capillary tube, and a snap-action switch. Frost-protection settings of 2–5 °C are typical; every degree of set point above that roughly adds 10–15 % to seasonal gas use. Backing up the working thermostat are three independent layers: the High-Limit Switch bimetal opens if outlet air exceeds ~90 °C (a blocked grille or failed fan), a one-shot Thermal Fuse catches a welded limit switch, and the flame-failure valve covers loss of combustion. The Heat Shield keeps cabinet surfaces below 60 °C where polythene glazing or shade cloth might touch.

Installation and use

Output sizing uses A × ΔT × U: glazed area times design temperature lift times the glazing U-value, plus a third for air leakage. The heater stands on its Base Foot on a slab, or hangs from the frame on the Hanging Bracket pair to free bench space; either way the louvres point along the path, not at foliage, since 60 °C discharge air scorches leaves within minutes.

Propane is stored outside in 13–47 kg cylinders, with the hose dated and replaced on a five-year cycle. A flueless heater of this class releases about 1.5 L of water vapour per kilogram of propane burned, so growers ventilate briefly each morning to shed humidity. End-of-season service is a burner brush-out, a thermocouple check (replacement is a €10 part and the most common fix), and a wipe of the fan blade, whose dust load is the usual cause of falling airflow.