9 Step Greenhouse Heating Setup with a Bio Green Phoenix Tube Over Winter

Begin with the volume calculation, since the wattage follows from it. A 6x8 ft glasshouse holds roughly 11 cubic metres of air. Greenhouse suppliers often use a heat demand rule of about 50W to 70W per cubic metre to hold a 10C to 12C lift above outside temperature in single glazing. For 11 cubic metres, that means about 550W to 770W of sustained output to defend a 5C minimum when outside air falls to minus 5C. The Bio Green Phoenix 1,000W tube has enough output for that size with some headroom. In the same house, a 2,000W version mainly creates more on-off cycling from a hot tube, with little gain in steadiness.

Tube heat in a small frost-free house

The Phoenix is a tubular convection heater: an aluminium element enclosed in a tube, set low so it warms air by displacement. A fan heater throws heat quickly and dries leaf surfaces, which suits a propagation bench checked twice a day. The tube works more slowly, with a casing temperature around 60C to 70C and a rising convection current that helps keep cold air away from the glass and the pots.

Foliage touching the casing scorches less readily at that lower surface temperature. Running costs are easier to predict as well. A 1,000W tube drawing continuously for 8 hours uses 8 kWh. At a UK unit rate near 25p per kWh, the cost is about GBP 2.00 for a night under continuous draw, and a thermostat can cut that figure roughly in half on a mild night by switching the element off.

A 2,800W fan heater can use the same energy in about a third of the time while chasing the same setpoint. It also cycles harder and leaves wider temperature swings between cut-in and cut-out.

Seal, insulate, and set the tube low

Close the gaps before fitting the heater. A glasshouse loses much of its heat through air leakage at the door, the vent, and the overlap between glass panes; the glass itself is only part of the loss. Run draught tape along the door frame and clip loose panes back into their glazing bars with W-clips.

Line the inside with horticultural bubble insulation, using the large-cell type sold in 750mm rolls and fixing it to the aluminium frame with proprietary clips. Figures published by the RHS put the heat-loss reduction from insulation at roughly a third to a half. That saving brings an 11 cubic metre house comfortably into the range a single 1,000W tube can hold.

Position the tube low and central if the bench layout allows it. Where staging splits the space, two shorter tubes along opposite sides give a more even spread. Heat rises, so a low mount lets the convection current travel across the floor before it climbs.

Keep the casing at least 50mm from timber, staging, and plastic trays. Phoenix wall brackets set that clearance automatically. If the tube is used on the floor, fit the supplied feet so the underside can vent; resting it flat on a paving slab blocks that airflow.

Thermostat work, including probe placement

The tube has no built-in thermostat worth relying on for frost protection. Bio Green sells the Thermo 1 plug-in unit, a socket-pass-through controller with a remote probe and a dial calibrated from 0C to 40C. Put it between the wall socket and the tube, place the probe in the canopy near the plants, and set the dial to 5C for hardy overwintering or 2C when the aim is simply to keep air from freezing.

Accuracy depends on where the probe sits. Clip it at plant height in the centre of the house, away from the tube and out of direct low-angle sun coming through south-facing glass on a clear winter morning. Too close to the element, the probe reads warmth early, switches the heater off, and leaves the far corner exposed. In a cold draught by the door, it keeps the heater running to satisfy a reading the plants do not experience.

Run the heater through one full cold night with a separate min-max thermometer in the coldest corner. In the morning, compare that corner with the intended setpoint and adjust the dial up or down until the coldest spot reads as planned.

An 8x10 ft house changes the load

An 8x10 ft greenhouse holds around 19 cubic metres. At 60W per cubic metre for a 10C lift, sustained demand comes to roughly 1,140W, already beyond the continuous output of the 1,000W tube. Bubble insulation pulls the real requirement down toward 600W to 750W on most nights, so the 1,000W tube copes with ordinary frosts. On a still night at minus 8C under clear skies, demand climbs and a single tube may run flat out without quite holding 5C. The fix is either the 1,500W Phoenix or a second 1,000W tube on the opposite wall, both governed by one Thermo 1 so they switch together. Two tubes also spread convection more evenly across a long house than one tube working twice as hard at one end.

Power, RCD, and the first week

Feed the heater from a socket protected by a 30mA RCD. A greenhouse is a wet electrical environment, with condensation forming on cold surfaces. The standard arrangement is an armoured cable buried 450mm to the greenhouse and an outdoor-rated socket inside.

If an extension lead is used from the house, choose a fully unwound outdoor reel. A coiled reel under load can overheat.

Avoid chaining the heater through a timer that can override the thermostat. The thermostat needs final control over whether the element is live.

Set the system up a week ahead of the first expected frost. Leave the min-max thermometer in the cold corner and read it each morning against the overnight low from a local forecast. That trial week shows whether the wattage and insulation are holding the setpoint while there is still time to add a second tube or another roll of bubble film.

Ventilation remains a winter job on mild days. A sealed, insulated house can run warm on a sunny December afternoon and trap humid air. Botrytis spreads on overwintering pelargoniums and fuchsias in those conditions. Crack the vent or the door for an hour around midday when outside air is above 8C, then close up before the temperature falls in late afternoon. Night heat will protect against frost, while daytime airflow reduces the grey mould that kills more overwintered plants than frost does.

Check the probe contact and the RCD test button monthly. A probe slipped behind a tray can report the wrong temperature for weeks before a cold snap exposes the problem, and an RCD that has never been tested may fail to trip when it matters.

Catalog area claims

Supplier listings quote a heated area in square metres for each tube size. Those figures assume an insulated house and a modest lift over outside temperature, and catalogues rarely state the assumptions. The same 1,000W tube is sold as suitable for 6x8 ft and 8x6 ft houses in different catalogues with no mention of glazing or target minimum.

What the first hard frost still has to prove

The result comes from sustained watts matched against the volume being held above outside temperature, with insulation included in the calculation. A clear-sky radiative frost at minus 8C can make a harder demand than a cloudy night at minus 10C because cloud cover slows heat loss from the glass.

The coldest corner gives the useful reading because it shows what the plants actually endured, especially when the probe is warmer than the far side of the house. Catalogues cannot answer one awkward detail from the first frosty spell: whether the corner farthest from the tube matched the number at the probe.