Temperature control systems for polytunnels: everything you need to know

Heating systems are vital in polytunnels for maintaining minimum overnight temperatures and protecting against frost in the early stages of germination – as well as extending the growing season at either end of the year.

The main options in terms of types of polytunnel heater are:

• Gas: the most widely used in commercial polytunnel operations. Thermostatically controlled and relatively straightforward to install and maintain.
• Electric: requires mains supply and is more expensive to run, but can be a practical option where gas isn't available or for smaller, targeted heat zones.
• Biomass: significant upfront capital investment, but strong ROI at scale for operations with consistent heat demand across a large growing area.
• Air source heat pumps: growing in adoption, energy-efficient, and can work well in larger operations – though typically better matched for greenhouse growing, due to the heat pumps working best in well-insulated, airtight buildings. 
• Paraffin: largely declining and now mostly for hobby-scale polytunnel growers rather than commercial operations.

Good ventilation is the most important tool for cooling polytunnels down, as well as managing humidity (and therefore pest and disease risk), and supporting CO2 exchange. In some locations, cooling systems may be needed in addition to ventilation.

The options range from simple manual systems through to purpose-engineered automated setups:

• Roller doors: the most common entry and exit point for air in commercial polytunnels – and can either be managed manually or through motorisation.
• Side vents: panels or strips along the tunnel sidewalls that can be opened to allow cross-ventilation, again can be manual or motorised.
• Roof vents: less common in polytunnels than in glasshouses, but used in some higher-spec structures as an effective way to release heat and humid air that accumulates at the apex. Again, motorised versions are available.
• Automated vent systems: purpose-built systems that integrate roller doors, side vents, and roof vents into a single coordinated setup, designed for automated or remote control. Haygrove's Total Vent is a good example.
• Extraction and circulation fans: extraction fans pull hot or humid air out of the tunnel; circulation fans keep air moving to reduce temperature stratification and humidity pockets.
• Shade cloth and netting: reduces solar heat gain during peak summer periods – typically deployed manually as needed.
• Active cooling systems: pad-and-fan evaporative cooling, fogging and misting systems, and chilled water systems all provide more aggressive temperature reduction when passive ventilation isn't enough. More capital-intensive, typically used only in high-value crop operations or regions with more extreme summer heat.

Manual control means a grower or worker reading temperature conditions and deciding how hardware should respond – setting vents to open, adjusting heaters, deploying shaders. It’s dependent on someone being there to make the call, and to act on it. 

The limitation is straightforward: conditions change faster than manual response allows. And because each adjustment costs time, growers tend to make fewer of them than they should — leaving tunnels in suboptimal conditions for longer than necessary.

A step up from fully manual control, timers trigger hardware responses at set times – opening vents at 7am, switching heating on at night. Simple to set up, and they remove the need for someone to be on-site to make the call.

The limitation is that a timer doesn't know what's actually happening in the tunnel – a timer opens the vents at 7am whether it's a warm sunny morning or a cold wet one, working on a fixed assumption, not what's actually in front of it.

For basic routines they can be useful, but they offer no responsiveness to real conditions.

Thermostatic controllers go one step further than timers: rather than responding to the clock, they respond to a temperature reading. They're available off-the-shelf from most polytunnel suppliers and are a common starting point for growers wanting condition-based automation.

The main types are:

• Single-zone thermostats: manage one zone of the tunnel, triggering hardware (vents, heaters, or both) when temperature crosses a set threshold.
• Multi-zone controllers: manage several zones independently, so each area can have its own threshold and response.

Basic thermostats are limited in what they can do. They're single-variable, reactive, and on-site only – no humidity, no VPD, no weather data, no remote access. The same threshold fires the same response every time, regardless of what else is happening in the tunnel.

Control software manages hardware responses automatically, keeping temperature to pre-defined targets based on integrated monitoring readings  – removing the need for manual intervention for every adjustment. 

The software element enables you to manage the system remotely via a dashboard rather than physical controllers in the polytunnel – but, sophistication varies considerably depending on the platform.

The main types are:

• Single-variable: responds to temperature alone.
• Multi-variable: coordinates across heating, ventilation, fans, and other equipment simultaneously, responding to a broader set of conditions including humidity, VPD, irrigation, and so on.
• Target-based: the grower sets the desired conditions, the software works out how to reach and maintain them.
• Fully automated: the system determines what optimal looks like and manages hardware accordingly, without the grower having to define targets manually.

The key limitation here is that a system managing temperature alone can't account for the knock-on effects: opening vents to cool the tunnel also affects soil moisture evaporation rates, which in turn affects when and how much you need to irrigate. 

When you build up separate platforms and processes for different variables over time you end up with a fragmented picture – multiple dashboards to make sense of, and no clear view of how conditions interact. 

That's the problem an integrated climate control system is designed to solve.

Before evaluating any solutions, get clear on your own situation:

• What's the primary problem you're trying to solve – overheating in summer, frost risk in winter, rapid temperature swings, or all of the above?
• Is temperature affecting yield, crop quality, disease pressure, or labour costs – and can you put a number on it?
• How many hours per week are currently being spent on manual temperature management tasks?
• How many tunnels and hectares are you managing? Do you need to coordinate across multiple sites?
• What equipment do you already have – manual vents, motorised vents, basic controllers, heaters? What can be built on, and what would need to change?
• Is this a hardware gap or a control gap, or both? Do you have the right equipment but it's not being managed well enough – or are you missing the hardware altogether?
• What's your budget, and what does payback need to look like?
• What's your comfort level with technology – do you want to start with visibility and build toward automation, or go straight to a full control system?

• Will this solution still work if you add tunnels, crops, or sites – or will you outgrow it quickly?
• Does it fix the root cause, or reduce the symptoms? A basic thermostat on overheating vents might cut the worst peaks, but won't manage humidity, VPD, or disease risk – the underlying problem remains.
• Does the system manage temperature alone, or does it integrate humidity and VPD for a full picture?
• Does it provide remote access and mobile alerts, or is it on-site only?
• Does it automate to target conditions, or just trigger at a single threshold?
• Does it integrate with weather forecasts to act proactively, not just reactively?
• Is it compatible with your existing equipment, or does it require specific hardware?
• What does setup involve, and does the support team understand polytunnel operations – not just the technology?
• What evidence does the provider have from comparable operations at similar scale?
• How much manual involvement will this still require day-to-day? Is that sustainable at your scale?

The best approach depends on your scale and what you're trying to solve, but the most effective setups combine three things: the right hardware (heating, ventilation, fans), reliable monitoring across your tunnels, and control software that manages conditions automatically to a target rather than relying on manual adjustments or basic threshold triggers. A system like Ostara that integrates all three (rather than managing each in isolation) gives you the most consistent results and the least manual involvement.

It depends on what you're growing. A strawberry crop, for instance, is sensitive to temperatures above 25°C, whereas leafy crops and brassicas are generally more tolerant of temperature variation. As a rough rule, most polytunnel crops need daytime temperatures in the range of 18–25°C and night temperatures above 10°C, but your specific crop requirements should always be the reference point.

Ventilation is the first line of defence: roller doors, side vents, and roof vents, though be careful of manually managed vents that are left in one position for long periods of time, as temperatures can drop significantly overnight even in the summer. Shade cloth can also be used to reduce solar heat gain before it builds up. In high-value operations or particularly hot conditions, active cooling systems like pad-and-fan evaporative cooling provide more aggressive temperature reduction when ventilation alone isn't enough. 

Whichever approach you use, pairing it with temperature monitoring and control software means vents can respond automatically to conditions rather than relying on someone being on-site to make the call.

Glasshouse systems are typically airtight, well-insulated, and often have high-tech monitoring and control systems (e.g. Priva, Ridder, Hoogendoorn) to enable continuous automated climate management – sophisticated, but very expensive to run. 

Polytunnels offer a more flexible, cost-effective growing infrastructure – but that comes with different climate management challenges. The best polytunnel climate control systems are purpose-built for that reality, delivering meaningful monitoring and automation without the complexity or cost of glasshouse systems.

Yes. Hardware-agnostic platforms like Ostara are designed to work with your existing equipment (vents, heaters, fans) rather than requiring you to start from scratch. 

It varies by operation – crop value, current labour costs, and starting point all affect the numbers. Trials with Haygrove showed that manual vent management alone required up to 60 hours of labour per hectare per month. Across a 10-hectare site at £15 per hour, that's £9,000 a month on vent management alone – before accounting for other temperature-related labour like heater adjustments. The same trials showed Ostara maintained VPD within the optimal range for 25% more of the time, with direct implications for yield and crop quality. Most operations will see a return within the first growing season, and compounding ROI from there on.

Yes. Ostara is a fully integrated climate control platform – you choose which hardware and variables you want to manage, whether that's temperature and ventilation alone or the full picture including humidity, VPD, irrigation, and lighting. Ostara is also hardware-agnostic, compatible with your existing ventilation and heating systems. Trials with Haygrove demonstrated 25% more time in optimal VPD conditions and over 60 labour hours saved per hectare per month. Book a demo to see what it could look like for your operation.