Photo courtesy of Modine Manufacturing

A lot of heat in small boxes can help feed the world, and make it a more beautiful place.

Unit heaters can ensure greenhouse owners their crops of vegetables, flowers or other ornamentals won’t freeze, even in the most northern climes. But these heaters must be handled with care, much as you would handle a freshly sprouted seedling.

Advances and technological evolution in both greenhouses and unit heaters can substantially extend growing seasons. Proper use of modern unit heaters can allow greenhouses to operate in the harsh winters of Michigan or even Northern Canada, allowing for nearly year-round growing seasons once the weather gets too cold for outdoor crops.

From grandma’s to the gargantuan

These heaters, which are most economically fired by propane or natural gas, can be used in the smallest mom-and-pop greenhouses — or the one behind your grandmother’s house — and the largest commercial growing operations. Most manufacturers offer sizes ranging from 150,000 to 400,000 BTUs. Regardless of the output needed to keep greenhouse temperatures at an even 75° or so, these heaters have a relatively small footprint. These small boxes can provide a lot of heat, but they, like plants, need a little TLC. If one conks out, and there is no backup, there’s a good chance of losing an entire crop in the depths of winter. That means livelihoods depend upon properly functioning equipment. So, make sure it’s installed properly, for starters.

Expert installation required

Some growers insist upon installing their own heaters. This is not advisable. For one, it might be an improper size for the greenhouse. A professional contractor can do the math to determine what size best fits a greenhouse. There are a lot of variables in determining the type and power of a unit heater, including the materials used in the greenhouse sheathing. Most greenhouses have glass walls, but some consist of plastic, or have blankets that cover the top half of a greenhouse so it retains more heat.

General pre-season maintenance is crucial to ensuring your greenhouse heaters run efficiently and effectively.
Photo courtesy of Modine Manufacturing

Vortex of heat

The locations of unit heaters within a greenhouse also play a role in their efficiency and effectiveness. Large growers will want multiple units to ensure redundancies in the event a heater goes out overnight. Some tender plants are very sensitive to cold; they can succumb quickly. The heaters should be placed in an even pattern around the interior perimeter for maximum efficiency. The intent is to basically form a pattern in which each heater is blowing toward the next. This will create a vortex of hot air around the perimeter, washing the walls with heat. This will keep the walls warm and will maintain the most even heat throughout the greenhouse. One reason unit heaters are preferable for greenhouse heating is the redundancy factor. Again, if one central heat source fails and there is no backup, you are getting a head start on next spring’s compost.

Despite the fact that they are literally intended to support life, the environment within a greenhouse can be hard on a unit heater. There are high humidity levels and a range of airborne dust and dirt particles. Considering the fact that these boxes provide livelihoods, it is crucial to inspect and maintain them throughout the year, but especially at the onset of a growing season. Picking a proper unit in the first place is key, but maintenance is important to guarantee the unit performs up to par for a long time.

Maintenance and inspection

The burners should be pulled out and inspected every year. Growers should also make sure the heat exchanger is clean and the motor runs properly. Oil should be applied as needed. One of the biggest costs for a greenhouse operator is gas, so make sure gas controls are working and the pressure is set correctly. The most effective unit heater is gas-fired — though there are still some fuel-oil and electric heaters in use — and it’s also the easiest type to install and maintain. To truly ensure efficiency and limit unnecessary gas consumption, correct pressure settings are key, and these should be checked every year before cranking up the heater. Pressure can fluctuate over the years and seasons.

The condensate trade-off

Two of the most popular heaters have efficiency ratings of 82 percent and up to 97 percent.

The less-efficient model is popular on the greenhouse market — it is fairly easy to install and maintain. The 97 percent model will certainly save a grower money and pay for itself over time, but there is a caveat — the higher-efficiency units produce condensate. While it may seem a simple prospect to simply route the condensate back on to the plants, the water in most cases is slightly acidic so condensate should always be routed to a building drain. Another area that growers should be aware of is the fact that water will accumulate in drains during operation, and if they are not run during the coldest months, that condensate can freeze and cause potential issues. Draining the condensate when these units will be shut down during colder months is an area growers need to be mindful of. These high-efficiency units require a little more care, but they will pay for themselves much more quickly than other units.

Venting flue gases can also be a challenge with greenhouse unit heaters. That is another task best left to professional contractors. Improper ventilation is dangerous for both people and plants.

Gone are the days of shoveling coal into a boiler to ensure a greenhouse stays warm during winter. Modern unit heaters can protect greenhouse produce and flowers to guarantee the cream of crop in the darkest of winters.

Jamie is product manager for Modine Manufacturing Co. in Racine, Wisconsin.