9 Step Greenhouse Heating Setup with a Bio Green Phoenix Tube on a 6-Metre Bench
A 6-metre run of staging will not hold even warmth from a single tube heater. The Bio Green Phoenix range spans 280, 560 and 840 watts, and where you put the tubes changes the electricity bill as much as the wattage on the box. What follows is a working layout, the thermostat trick that controls the cost, and the parts most likely to give trouble.
Why one tube cannot cover 6 metres
A Phoenix tube spills its heat into a plume that fades fast. The 1-metre, 240-watt version warms about 2 to 3 metres of bench before the air slides back toward the ambient reading. Stretch that over a 6-metre run and the trays beside the heater sit 8 to 10 degrees warmer than the ones at the far end. The listed heated area on most tube heaters suggests a tidy blanket of warmth, but the bench does not behave that way.
Two layouts work. You can run two shorter Phoenix units fed from either end of the bench, or you can drop a longer model at the 3-metre mark. The 1.5-metre Phoenix pulls 560 watts and the 2-metre version pulls 840 watts. Put a single 840-watt tube on a 6-metre bench and the middle gets the best of it while both ends stay cool, which is exactly where frost-sensitive seedlings tend to meet the draught off a door.
For a bench this long, two 280-watt or two 560-watt tubes are usually the practical answer. Space them so the warm air overlaps near the centre. That adds cable runs and socket demand, which becomes part of the job the moment the bench grows beyond a short propagation shelf.
Setting the tubes on the bench
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Measure the space below the bench. The Phoenix needs 5 cm beneath the tube and open air above it. Mount the supplied wall brackets under the front lip of the bench where air can move around the aluminium body.
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Divide the bench into two 3-metre zones. A strip of tape at the 3-metre point gives you a reference for aiming both heat plumes toward the centre.
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Fix the brackets at roughly 30 cm and 120 cm along each tube. The body is light, around 1 kg per metre, so the supplied brackets carry the load with ease.
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Run armoured or rubber-sheathed cable to a weatherproof IP44 socket within reach of each heater. Indoor flex trailed across damp staging is the classic route to a tripped RCD in a greenhouse.
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Fit a Bio Green Thermo 2 plug-in thermostat between each socket and its tube. The Phoenix has no built-in temperature control, so direct power leaves it running continuously.
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Sit the thermostat probe at plant height in the middle zone. Air near the floor can read 4 degrees colder than the canopy, and a probe down there keeps the tubes running longer than the plants actually need.
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Set the dial for the crop. Frost protection wants only 3 to 5 degrees. Germinating tomatoes want 15 to 18 degrees, which roughly triples the running hours.
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Power one zone, leave it twenty minutes, then check the far end with a cheap digital max-min thermometer such as a ThermoPro TP50.
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Shift a tube by 20 to 30 cm if one end still lags. The overlap between the warm plumes moves the cold spot far more than a small turn of the dial.
What it costs to run
Take two 560-watt Phoenix tubes. Together they draw 1,120 watts, or 1.12 kW. If the thermostat holds them on for an average of 8 hours through a cold night, they use 8.96 kWh. At a UK unit rate of about 24 to 28 pence per kWh, that night costs roughly 2.15 to 2.50 pounds. Stretch it across a six-week frost window and the total lands near 90 to 105 pounds before the season warms.
The thermostat carries the biggest influence on that figure, because it decides how long the tubes stay live. A unit cycling at 30 percent duty on a mild night uses about a third of the worst-case number. Probe position therefore matters as much as the tube model. A sensor sitting in a cold draught reads low, the heaters stay powered for longer, and that extra running time turns a 50-pound month into something closer to 100 pounds.
Bubble-wrap insulation behind the bench brings the duty cycle down. Horticultural bubble film has larger cells than packing wrap, so it holds more air and lets more light through. Clipped inside the frame on fixings that grip aluminium glazing bars, it cuts heat loss along the back wall, where much of the warmth escapes.
Ventilation is the counterweight to all this heat. A sealed greenhouse held at 16 degrees overnight grows mould as readily as seedlings. An automatic vent opener such as a Bayliss MK7, set to crack the roof vent at 18 to 20 degrees, lets daytime heat escape without a climb up to the house. The heater and the vent work against each other by design, and that tension keeps air moving.
Safety cutout
Use an RCD-protected supply for any mains-powered heater in a damp glasshouse. A 30 mA plug-in RCD between the outdoor socket and the thermostat costs under 15 pounds and trips before a fault turns dangerous.
Matching the warmth to what you grow
Heat should earn its keep. Cordon tomatoes started under a Phoenix at 16 degrees germinate in 7 to 10 days, then need that warmth held until the first true leaves show. After that stage, 12 degrees is enough to harden them without stalling growth.
The cordon pruning method becomes part of the heating plan. Grow one main stem, pinch out every side shoot that appears in the leaf axils, and tie the stem to a cane or string as it climbs. A warm bench quickens the rhythm. A plant that might throw a side shoot weekly in an unheated house can produce one every four to five days at 16 degrees, so the checks come round more often. Miss several shoots and the plant spreads its energy across more stems, fruiting later.
For the small repeated cuts on cordon tomatoes and overwintered box, Niwaki topiary clippers hold an edge well, particularly the GR Pro or Mainichi models. Their narrow blades reach into leaf axils cleanly, which counts when side shoots come off every few days. Topiary under glass moves slower than outdoor hedge work. Box and bay in pots near the heat can push soft growth in late winter, and that growth marks easily, so sharp clippers and a light hand suit it better than powered trimmers. The same bench that helps the tomatoes can wake topiary subjects early, which means February trimming on plants that would normally wait until April. Blade care matters more in the warm, damp air around the bench. Sap and resin from box and tomato foliage gum up clipper pivots fast, so a wipe with a resin solvent every few sessions keeps the action smooth. Niwaki blades are laminated steel and will rust if stored damp, so they go back into a dry box after use.
The same warmth reaches tender plants that need acidic soil. Blueberries and camellias overwintered under glass can slip into early root activity on a heated bench. Warm roots take up nutrients faster, so pots run dry of feed sooner. A liquid ericaceous feed every two weeks through the heated period, mixed at half the summer strength, holds off yellowing leaves without forcing soft growth that the cold ends of the bench cannot support.
Glasshouse arithmetic and tunnel behaviour
The cost figures above suit a modest glass-to-ground greenhouse. A polytunnel loses and gains heat on a different curve, because the skin is thinner and the air volume is often larger. The same two tubes that hold a glass house at 5 degrees may struggle in that extra volume.
Past a certain cubic capacity, a fan heater moves warmed air through the whole space instead of leaning on the bench-level plume from the tubes. That leaves the long bench with a permanent compromise. The warmest air gathers where the tubes overlap, while the draughts keep hunting the edges, and no amount of dial-twiddling closes that gap entirely. The cold corner by the door is the one spot a tube heater will never reach, and that is where the seed trays you most want to protect tend to end up.
