Greenhouse Heaters & Frost Protection for Winter Growing

Every autumn I have the same quiet panic. The forecast drops to minus three, the greenhouse glass starts to bloom with frost flowers, and I remember that I have a bench full of tender fuchsias, pelargoniums and half-hardy salvias that will turn to mush if I don't act. Over the years I've worked through electric tube heaters, 2kW fan heaters, paraffin burners, rolls of bubble wrap and metres of horticultural fleece — and I've made plenty of expensive mistakes along the way.

This guide pulls all of that together. The honest truth is that there's no single "best" way to heat a greenhouse. The right choice depends on whether you've run a power cable down the garden, how big your structure is, what you're trying to overwinter, and how much you mind topping up a fuel tank in the dark. So rather than crown one winner, I'll walk you through the three main heating strategies — electric, paraffin and passive insulation — with real specifications, sensible comparisons and the kind of advice I wish someone had given me a decade ago.

The Three Pillars of Frost Protection

Before we get into individual products, it helps to understand that frost protection isn't really about one gadget. It's a system, and it rests on three pillars working together. Get the balance right and a modest heater will do a huge amount of work; get it wrong and you'll burn through electricity (or paraffin) trying to heat a leaky glass box.

Most people obsess over the first pillar and ignore the other two. In my experience, lining the greenhouse with bubble wrap before you so much as switch a heater on is the most cost-effective thing you can do all winter. It's the equivalent of putting a jumper on before turning up the central heating.

Electric Tube Heaters: The Efficient Workhorse

If you've got mains power to the greenhouse, low-wattage tubular heaters are my default recommendation for small to medium structures. They run along the floor or low on a wall, producing a gentle, continuous convection current of warm air that keeps frost at bay without the dramatic temperature swings of a fan heater. The EcoHeat range (sold under the LightHouse brand) is one of the most common families you'll see in UK garden centres.

These come in a sensible spread of sizes so you can match output to your space rather than over-buying. The smallest is genuinely tiny and the larger ones step up neatly for bigger benches and small greenhouses.

A few things stand out here. The IP55 rating means these are splash and sprinkler resistant — important in a greenhouse where you'll inevitably be watering near them, and where condensation drips off the glass. That's a genuine safety consideration, not marketing fluff. The larger sizes in the range include a built-in thermostat, which is the feature I'd push hardest for; without one, a tube heater simply runs flat out whenever it's plugged in, which is both wasteful and potentially too warm on milder nights.

The manufacturer claims these use 25% less energy than comparable tubular heaters, and every model is wall or floor mountable with the supplied brackets. For propagation areas and the smallest greenhouses, the 45W and 80W units are wonderfully frugal. The 240W model is rated for a greenhouse of roughly 3.0 × 1.8m, which covers a great many domestic 6×8 and 8×10 structures when paired with decent bubble wrap insulation.

Electric Fan Heaters: Power and Air Circulation

When the greenhouse gets bigger, or when you genuinely need to push temperatures up rather than just hold off frost, a 2kW electric fan heater is the tool of choice. The big advantage over tube heaters is air movement: that circulating fan distributes warmth evenly and, just as importantly, keeps the air moving to reduce the stagnant, humid conditions that breed botrytis and other fungal nasties over winter.

Most greenhouse fan heaters share a familiar specification. They typically offer two heat settings of 1000W and 2000W plus a "fan only" mode that circulates air without heating — useful on mild days for keeping air fresh. Some adjustable models step through 1kW, 1.8kW and 2.8kW. They're built around a robust 'cooker' style heating element and are designed to cope with the humidity of a greenhouse, which is why you'll see IPX4 splash-proof ratings rather than the lower protection on a domestic fan heater.

The thermostat is the heart of a good fan heater. A typical adjustable dial spans 0–85°C, though for greenhouse use you'll set it far lower — just enough to kick in around 2–4°C. The built-in safety cut-off that shuts the unit down if it overheats is non-negotiable in my book; never run a heater without one in a confined glass space surrounded by foliage.

The BioGreen Palma is a well-regarded example in this category, and one figure I find genuinely useful is its air circulation rate: it moves up to 163m³ of air per hour. That kind of throughput is what separates a purpose-built greenhouse heater from a cheap household unit pressed into service. The constant air movement does as much for plant health as the warmth itself.

That bar chart tells an important story. A 2kW fan heater has roughly eight times the peak output of even the largest tube heater. That power is brilliant when you need it — but it also means your electricity meter spins much faster whenever the element is on. This is precisely why insulation and thermostatic control matter so much: a well-sealed, bubble-wrapped greenhouse keeps the fan heater idling rather than roaring.

Paraffin Heaters: When There's No Power

Not everyone has run an armoured cable down to the bottom of the garden, and for those greenhouses paraffin heaters remain genuinely useful. They need no electricity, they keep working in a power cut, and they add a small amount of beneficial moisture and carbon dioxide to the air as they burn. The trade-off is that they require manual lighting, regular refuelling and — crucially — ventilation.

The Bio Green Warmax Power 5 is the benchmark in this category. It's a twin-burner design with adjustable wicks and a generous 4.5 litre fuel tank, delivering 600W of heat. The headline figure is the burn time: a single fill lasts around three and a half days, thanks partly to a long 33cm XL wick that the manufacturer says gives 40% longer burning than standard models. It's rated for greenhouses with a base area up to 5m², which comfortably covers a typical small domestic greenhouse.

If your needs are smaller — a tiny lean-to, a cold frame or a propagation corner — the Bio Green Warmax Mini is a neat little thing. It puts out around 50–100W from a 0.5 litre tank with a roughly three-day burn time, measures just 11cm in diameter and 17cm tall, and weighs a mere 0.5kg. It won't heat a whole greenhouse, but tucked under a bench of cuttings it can keep the immediate area frost-free.

The paraffin compromise

I have a soft spot for paraffin heaters because they're so reassuringly independent — no plug, no power cut to worry about. But they demand discipline. You have to trim and clean the wick, keep the fuel topped up, and resist the temptation to seal the greenhouse completely. There's no thermostat, so a paraffin heater simply burns at whatever rate you've set the wick to, regardless of whether the night turns out mild or bitter. That lack of automatic control is the single biggest reason most UK growers with electricity available choose electric instead.

Head-to-Head: Which Heating Method Wins?

Let's put the three approaches side by side. There's no universally correct answer — it genuinely depends on your situation — but seeing the trade-offs laid out makes the decision much clearer.

My quick rule of thumb: if you have power and a small greenhouse, fit tube heaters with a thermostat. If you have power and a larger greenhouse — or you want active winter growth — go for a 2kW fan heater. If you have no power at all, a Bio Green Warmax Power 5 is the dependable choice, with the Mini for tiny spaces.

Passive Insulation: Bubble Wrap and Fleece

I cannot overstate how much insulation changes the maths. Every watt you don't lose through the glass is a watt your heater doesn't have to replace. Horticultural bubble wrap and fleece are the two cheapest, most effective tools in the whole frost-protection arsenal, and they deserve far more attention than they usually get.

Thermostats: The Component That Pays for Itself

If you take one thing from this entire guide, make it this: a thermostat is the most valuable accessory you can add to any electric heating setup. Without one, a heater either runs constantly (wasteful and expensive) or relies on you remembering to switch it on and off (unreliable and risky). With one, the heat only fires when the temperature actually approaches your set point.

The larger EcoHeat tube heaters and all the proper greenhouse fan heaters include a thermostat as standard. The fan heaters typically use an adjustable dial spanning 0–85°C; for frost protection you'll set it right down so it only triggers a degree or two above freezing. If you're using a smaller tube heater without built-in control, an inline plug-in greenhouse thermostat solves the problem neatly — the heater plugs into the thermostat, the thermostat plugs into the wall, and it cuts power whenever the air is warm enough.

Running Costs and Real-World Strategy

Let's be practical about money, because that's where most winter-growing plans live or die. The wattage figures tell the story. A 45W or 80W tube heater sipping power under a propagation bench costs a trivial amount to run, even left on for weeks. A 2kW fan heater going full tilt on a sub-zero night is an entirely different proposition — which is exactly why you only want it firing when genuinely needed.

This is where the whole system comes together. A bubble-wrapped greenhouse with a thermostatically controlled heater might only call for heat for a few hours on the coldest nights, and barely at all in mild spells. The same heater in an un-insulated greenhouse with no thermostat could run almost constantly. The insulation and the thermostat are what turn a frightening electricity bill into a manageable one.

That rating reflects a properly combined system — insulation plus a thermostatically controlled heater — rather than any single product. Used together, these approaches reliably keep a domestic greenhouse frost-free through a typical British winter, and they do so without an alarming bill.

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