Carbon monoxide greenhouse heater ventilation
Updated: 22 October 2026
You are about to close every vent because the propane heater only works if the heat stays in. That instinct fights the physics of an unflued flame. This page covers what combustion leaves in a greenhouse, what to open, which alarm to buy, and when to stop and leave, for the blowers in the [gas greenhouse heater comparison](/hub/heating/gas-greenhouse-heater) and for gas heat in general. UK sources include the London Fire Brigade, NHS, HSE and greenhouse retailers. None of them replaces your heater manual. A qualifying purchase can earn a commission at no extra cost to you.
You are about to close every vent because the propane heater only works if the heat stays in. That instinct fights the physics of an unflued flame.
You are about to close every vent because the propane heater only works if the heat stays in. That instinct fights the physics of an unflued flame. This page covers what combustion leaves in a greenhouse, what to open, which alarm to buy, and when to stop and leave, for the blowers in the gas greenhouse heater comparison and for gas heat in general. UK sources include the London Fire Brigade, NHS, HSE and greenhouse retailers. None of them replaces your heater manual. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Use limit | Named cut-out | Flue | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Bimson Power Bimson Power 15 kW | Not a sealed room | Overheat | Unflued | 9.4 | about £119.99 | View offer |
 DAHTEC DAHTEC 10 kW | Outdoor, non-domestic | Flame and overheat | Unflued | 9.4 | about £219.00 | View offer | |
 Bimson Power Bimson Power 30 kW | Well ventilated | Overheat | Unflued | 0.0 | about £169.99 | View offer |
On this page
What stays in the house
An unflued propane heater burns in the same air as the plants and as you. Complete combustion still produces water vapour and carbon dioxide, so glass will steam. Incomplete combustion, or a starved space, adds carbon monoxide, which you cannot smell, and can add ethylene at levels that need not trip a human alarm first. The Royal Horticultural Society lists moisture, ethylene and carbon monoxide as reasons to ventilate, and it limits gas to hardier crops, not tender seedlings in a shut house.
All three amazon.co.uk blowers here are unflued. The DAHTEC text says outdoor or well-ventilated non-domestic use. Bimson reviews say not a sealed room. None names an oxygen depletion sensor. Where another class has one, Hotbox and Eden discussions treat a trip as a ventilation warning, not permission to close vents overnight. That device is not a carbon monoxide alarm. Soot, a yellow flame, or a flame that lifts and roars often means the burner is short of air. Tighter vents make that worse.
What to open
Air in low, products out high. Hotbox points to a low-level airway and a high vent. Greenhouse Stores mentions cracking a roof vent by roughly 10–15 mm on a small gas heater. That is their frost-heater habit, not a clearance for a 10–30 kW blower. Calor’s Super Heat 4.2 kW sheet prints at least 50 cm² at the top and the bottom, and says that model is not for polytunnels. The 50 cm² figure belongs to that sheet. Bimson and DAHTEC do not give a universal area. Which machine you bought is in the propane greenhouse heater buying guide.
For these blowers the page says well ventilated, outdoors, or not a sealed room. Keep a deliberate opening while the flame runs. A fan circulates air. It does not send flue gas up a chimney you do not have. Overnight in a closed hobby house is the line these blowers do not cross. Condensation is water from the flame, not proof the air is safe.
The alarm and the emergency
You cannot smell carbon monoxide, so fit an audible alarm to BS EN 50291, as the London Fire Brigade and GOV.UK recommend for homes using portable LPG among other fuels. Caravan and boat alarms follow BS EN 50291-2. A hobby greenhouse is not a boat. The point is a kitemarked carbon monoxide alarm, not a smoke detector. HSE treats alarms as backup, not a replacement for servicing and ventilation.
Mount it where the instructions say, often head height, and test it on schedule. If it sounds, switch the heater off, open the structure, get people into fresh air, and do not relight until the cause is found. The NHS lists headache, dizziness, nausea, breathlessness and collapse. Collapse means fresh air and 999. A leak you think is continuing is 0800 111 999. The alarm does not replace a vent.
What the rules do not excuse
Approved Document J and the rules on carbon monoxide alarms for fixed combustion appliances apply in domestic homes, not in a private allotment greenhouse. That distinction does not make a plot safe without an alarm or air exchange. Landlord gas checks under the 1998 regulations do not cover a hobby glasshouse you own.
The word greenhouse on a listing is not an exemption. Garage reviews still assume air movement you have to recreate. Stop and service the unit if soot builds, the burner rattles, or the flame pattern is wrong. Calor’s mobile-heater guidance says halt when the burner is sooty or noisy. Insects block stored ports, so cover the heater and inspect it before the season, as Hotbox advises. After an alarm, start again from the regulator match on how to use a gas greenhouse heater.
Common questions
Can I shut the vents overnight?
No, not with an unflued propane heater running. Combustion products stay in the same air as the plants. Hold frost with electric heat, or with a greenhouse-rated gas heater used as its own manual states, or accept a cold night. These workshop blowers are not an overnight closed house.
Which carbon monoxide alarm do I buy?
An audible alarm certified to BS EN 50291, with a UK kitemark or the equivalent marking GOV.UK carbon monoxide guidance calls out. A smoke detector is not that alarm. Caravan and boat alarms follow BS EN 50291-2, and a hobby greenhouse is not a boat. Site it as the maker says, often at head height, and test it on their schedule.
Does an oxygen depletion device replace the alarm?
No. Some low-output greenhouse heaters have one. It responds to falling oxygen, not to every carbon monoxide case, and these blowers do not list one. A trip on that other class is a ventilation warning. You still want an open airway and a BS EN 50291 alarm.
Is carbon dioxide from the heater good for plants?
Controlled enrichment is a different job from a sealed hobby house. That house also gets ethylene, heat spikes and moisture. The Royal Horticultural Society still puts ventilation ahead of free carbon dioxide from a propane flame, and it limits gas to hardier crops.
What are the symptoms, and whom do I call?
The NHS lists headache, dizziness, nausea, breathlessness and collapse. Collapse or serious illness means fresh air and 999. A leak you think is continuing is 0800 111 999, from outside. Switch the heater off, open the house, and do not relight until the cause is found.
Related pages
Sources
Public UK sources named below. amazon.co.uk prices, stars and review counts on these three listings were taken from the listing files read on 20 October 2026. Page updated October 2026.
- RHS, heating greenhouses efficiently (retrieved 20 October 2026)
- Gas Safety (Installation and Use) Regulations 1998 (retrieved 20 October 2026)
- HSE domestic gas FAQ (retrieved 20 October 2026)
- HSE gas safety for employers (retrieved 20 October 2026)
- GOV.UK carbon monoxide (retrieved 20 October 2026)
- NHS carbon monoxide poisoning (retrieved 20 October 2026)
- London Fire Brigade carbon monoxide safety (retrieved 20 October 2026)
- Liquid Gas UK Consumer Guidance Sheet 39 (retrieved 20 October 2026)