Best Paraffin and Non-Electric Heaters for Shoulder-Season Nights

What this guide covers, and what it does not

This is about the shoulder season: roughly the six weeks either side of your first frost, when nights drop below freezing but days are still mild and the house does not need heat around the clock. It is a different problem from midwinter heating, and it has different answers.

In that window you typically need heat on maybe ten to twenty nights, for a handful of hours each, to hold a small structure a few degrees above freezing. You do not need a permanently installed system. You need something you can light at dusk on the nights that matter, which is precisely where non-electric heaters make sense — particularly in a greenhouse with no power run to it, which describes most hobby greenhouses.

For the electric side of the argument, and for the running-cost comparison, see paraffin vs electric heat for October nights.

The four types worth considering

Paraffin greenhouse heaters

The traditional answer: a wick burner in a metal body, often with a blue flame burner and a heat-spreading top plate. No power needed, no moving parts, runs unattended overnight on one fill. Output is modest, which for a small house is appropriate rather than a limitation.

The trade-offs are real. Combustion produces water vapour, so a paraffin heater raises humidity noticeably in a sealed house — which is a problem given that damp still air is what causes gray mold. It needs the wick trimmed and the fuel kept clean, and a badly maintained or badly fuelled wick heater soots up, which both wastes fuel and coats your glazing.

Propane and butane radiant heaters

Higher output, faster to reach temperature, and cleaner burning than paraffin in general. Cylinder handling is the main practical friction, and the higher output makes thermostat control more important, not less — an unregulated propane heater in a small greenhouse can swing the temperature wildly.

Many portable propane heaters are explicitly rated for indoor or enclosed use with an oxygen depletion sensor. Buy one that is. A heater rated outdoor-only does not become safe indoors because the space is small.

Candle and tealight heaters

The internet's favourite cheap solution: tealights under a stack of terracotta pots. Be honest about what this is. A tealight produces a very small amount of heat, and a cluster of them produces a slightly less small amount. In a tiny, well-sealed cold frame or a very small greenhouse it can lift the temperature enough to matter on a marginal night. In anything of real volume it will not.

It is also an open flame, unattended, near dry material. If you use this approach, use a purpose-made enclosed candle heater rather than an improvised pot stack, and site it where nothing can fall onto it.

Thermal mass, which is not a heater at all

Black-painted water barrels or drums along the north wall absorb heat during the day and release it slowly at night. Zero running cost, zero fuel, zero risk, and genuinely effective at damping the overnight low in the shoulder season, which is exactly the window this guide covers. It will not hold a house frost-free through a hard freeze, but it will often carry you through the marginal nights of September and October without lighting anything at all.

Sizing without overbuying

Two questions decide the size. What volume are you heating, and what minimum are you holding it to?

The volume answer is usually "less than the whole greenhouse". Curtain off the end you are actually using with a bubble film or fleece divider, and you can halve the space a heater has to work on. This is the single most effective way to make a small heater adequate.

The minimum answer is the one people get wrong. "Frost free", meaning a couple of degrees above zero, is a modest ask that a small heater can meet on the nights it is needed. "Warm enough to keep growing" is a fundamentally different commitment, running most nights for months, and in the shoulder season it is almost never what you actually need — because below roughly ten hours of daylight your plants are not growing anyway, no matter how warm you keep them. That is covered in the 10-hour light rule, and it is the reason most hobby growers should be buying for frost protection rather than for growth.

Combustion safety, in full

This section is not boilerplate. Fuel-burning heaters in enclosed spaces kill people every year.

• Fit a carbon monoxide alarm in the greenhouse, at head height, and test it before the season starts. This is a cheap item and it is the only thing between you and a gas you cannot perceive.
• Provide permanent ventilation. Combustion needs oxygen and produces exhaust. A greenhouse sealed for winter with a burning heater inside must have a deliberate, permanent air path — a vent cracked open, a louvre left clear. Never seal the last vent because a heater is running.
• Use the correct fuel and only the correct fuel. Paraffin heaters take clean paraffin or specified heater fuel. Fuel contaminated with water or the wrong grade produces soot, fumes and a badly burning wick.
• Never refuel a hot or lit heater. Extinguish, let it cool, then fill outside the greenhouse.
• Maintain clearances. Keep the heater clear of staging, benches, fabric, fleece and foliage on every side, including above.
• Test-fire in September, in daylight, with the doors open, before you depend on it. Stage four of the fall changeover sequence exists for exactly this reason.
• Buy indoor-rated equipment with an oxygen depletion sensor and a tip-over cutout where the type offers one.

What to buy

The humidity side effect nobody warns you about

Burning paraffin produces roughly as much water vapour as it does heat, by volume. In a well-sealed greenhouse that water has nowhere to go, and it condenses on every cold surface overnight. You have then created the exact conditions — still, saturated, cool air on plant surfaces — that gray mold needs.

The fix is not to stop using the heater. It is to accept that a fuel heater and a sealed house are incompatible, and to leave a deliberate vent path open whenever it runs. That vent costs you a little heat and saves you the crop. Read autumn condensation and gray mold for the full picture, and consider covering the plants rather than heating the air where you can.

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Common questions