Finding the best heater for a small greenhouse is the key to protecting your tender plants without wasting energy or precious space. As a gardener, you know that a sudden frost can wipe out months of work, so choosing the right heating solution is critical.
This guide will walk you through the most efficient and space-saving options available. We’ll look at different heater types, how to calculate your needs, and smart tips to keep costs down. Let’s get your greenhouse cozy and productive all year round.
Best Heater For Small Greenhouse
This category is all about balancing heat output with a compact footprint. You need a unit that effectively warms your growing area but doesn’t get in the way of your plants or your workflow. The “best” choice always depends on your specific setup, climate, and what you’re growing.
For most small greenhouses, efficiency means direct warmth with minimal heat loss. Space-saving designs mount on walls or hang from the ceiling, keeping the floor clear. We’ll break down the top contenders so you can make an informed decision.
Understanding Your Greenhouse Heating Needs
Before you buy any heater, you need to do a little math. This prevents you from purchasing a unit that’s too weak (and fails to protect plants) or too powerful (wasting money and potentially overheating).
Start by calculating the square footage of your greenhouse. Simply multiply the length by the width. Next, consider the difference between the coldest outside temperature and your desired inside temperature. This is called the temperature differential.
- Greenhouse Size: A 6′ x 8′ greenhouse has 48 square feet.
- Desired Temperature: If you want 55°F inside and it gets to 20°F outside, your differential is 35°F.
- Insulation: A well-insated greenhouse with double-walled polycarbonate needs less heat than a single-layer polyethylene film house.
- Plant Requirements: Seedlings need more warmth than overwintering hardy perennials.
As a general rule, you need roughly 10-15 watts of heating power per square foot for a moderate climate. For our 48 sq ft example, you’d look for a heater between 480 and 720 watts. In colder zones, aim for the higher end of that range or even 20 watts per square foot.
Types of Efficient Heaters for Small Spaces
Each heater type has its pros and cons. The right one for you depends on your access to power, your budget, and how hands-on you want to be.
Electric Fan Heaters
These are a popular plug-and-play option. They use a heating element and a fan to circulate warm air quickly. This helps prevent cold spots and reduces humidity buildup on leaves, which can lead to disease.
- Pros: Affordable upfront cost, easy to install, provides good air circulation, often includes a thermostat.
- Cons: Can dry out the air, running costs add up in very cold climates, need a reliable outdoor-rated electrical outlet.
- Best For: Smaller, well-insulated greenhouses with easy power access. Great for occasional frost protection.
Radiant or Infrared Heaters
Radiant heaters work like the sun. They emit infrared rays that warm objects, plants, and soil directly, rather than heating the air. This is a very efficient method as you’re heating what matters most.
- Pros: Silent operation, heats surfaces directly (feels warmer at lower air temps), doesn’t reduce humidity, no moving parts to break.
- Cons: Can create uneven heating if not positioned correctly, less effective in very drafty structures.
- Best For: Targeted heating of benches or specific plant zones. Excellent for drafty greenhouses where heating the air is inefficient.
Propane Greenhouse Heaters
These heaters burn propane to generate heat. They are completely independent of the electrical grid, making them ideal for remote greenhouses. Modern vented models are much safer than older unvented ones.
It’s crucial to choose a heater designed for greenhouses, as they produce carbon dioxide and water vapor as byproducts. Plants can use both, but proper ventilation is non-negotiable to prevent dangerous gas buildup.
- Pros: High heat output, works during power outages, adds beneficial CO2 and humidity.
- Cons: Requires monitoring fuel levels, needs ventilation for safety, involves ongoing fuel costs.
- Best For: Larger small greenhouses or areas with frequent power cuts. Essential for off-grid setups.
Oil-Filled Radiator Heaters
These are electric heaters that heat sealed oil inside their fins. The oil retains heat well and provides a gentle, consistent warmth. They don’t have a fan, so they operate silently.
- Pros: Safe to the touch (great if you’re worried about accidental plant contact), silent, provides steady heat even after cycling off.
- Cons: Slower to warm up an area, heavier and less portable, can be less efficient in very drafty spaces.
- Best For: Well-insulated hobby greenhouses where safety and silent operation are priorities. Good for maintaining a steady baseline temperature.
Critical Features for Space-Saving Design
In a small greenhouse, every inch counts. Look for these design features to maximize your growing area.
- Wall-Mountable: This is the biggest space-saver. Mounting the heater on a side wall or end wall keeps it completely off the floor and away from water splashes.
- Low Profile: A slim, flat design that sits close to the wall protrudes less into the space.
- Ceiling Hanging Kits: Some radiant and fan heaters can be suspended from the greenhouse frame, providing excellent, out-of-the-way heat distribution from above.
- Integrated Thermostat: A built-in thermostat avoids the need for extra, dangling wires from a separate unit. It keeps the heater from running constantly.
- Swivel Base: For floor models, a base that rotates allows you to direct heat where it’s needed most without moving the whole unit.
Step-by-Step Guide to Installing Your Heater Safely
Proper installation ensures efficiency and prevents hazards. Follow these steps.
- Choose the Right Location: Place the heater near the center of a long wall or at one end, pointing down the length of the greenhouse. Avoid placing it directly above plants or where water can drip on it.
- Secure It Firmly: If wall-mounting, use the provided hardware and anchor it securely into the greenhouse frame, not just the glazing material. For hanging heaters, ensure the hanging kit is rated for the weight.
- Power Safely: Use a GFCI (Ground Fault Circuit Interrupter) outlet for any electric heater. This is a critical safety feature for damp environments. Use outdoor-rated, heavy-duty extension cords only if absolutely necessary, and never run them under mats or where they can be damaged.
- Clearance is Key: Maintain the manufacturer’s recommended clearance from any combustible materials—this includes wooden benches, fabric plant covers, and dry leaves.
- Test Before You Trust It: Set the thermostat a few degrees above the current temperature and let the heater cycle on and off. Verify it’s working correctly before a cold night arrives.
Maximizing Efficiency: Tips to Reduce Heat Loss
The most efficient heater in the world can’t compansate for a leaky greenhouse. Before you size your heater, take these cost-free steps to improve efficiency.
- Seal All Gaps: Use silicone caulk or greenhouse tape to seal cracks around vents, doors, and frame joints. A draft is your biggest enemy.
- Add Bubble Insulation: Horticultural bubble wrap can be attached to the interior walls. It lets light in while creating an insulating air barrier.
- Use Thermal Mass: Place black-painted water barrels or stacks of pavers inside. They absorb heat during the day and release it slowly at night, stabilizing temperatures.
- Install a Thermal Blanket: For night use, an interior layer of row cover fabric hung from the ceiling can trap a layer of warm air directly around your plants.
- Check Door Seals: Ensure your door closes tightly. A simple draft excluder can make a big difference.
Smart Heating on a Budget
If you’re on a tight budget or have a very minimal setup, consider these low-tech solutions. They can be used alone or to supplement a primary heater.
- Heat Mats: Perfect for seed starting trays. They provide bottom heat directly to the root zone, which is very efficient for germination.
- Christmas Lights: Old-fashioned incandescent Christmas lights (not LEDs) give off a surprising amount of heat. String them around sensitive plants on very cold nights.
- Compost Heat: An active compost pile placed inside the greenhouse generates a steady, gentle heat as it breaks down. It also produces CO2.
- Solar Heat Sink: Create a simple heat sink by stacking clean, gallon milk jugs filled with water painted black. They will collect heat all day and radiate it at night.
Maintenance and Seasonal Care
To keep your heater running safely and efficiently for years, a little maintenance is required.
- Pre-Season Check: Before the first frost, inspect the heater for any damage, clean dust and cobwebs from intakes and heating elements, and test its operation.
- Regular Cleaning: Wipe down the exterior and gently vacuum any vents or fans monthly during heavy use.
- Inspect Cords and Plugs: Look for fraying, cracking, or discoloration, which can indicate a hazard in the humid environment.
- End-of-Season Storage: If you remove the heater for summer, store it in a dry place. For propane heaters, ensure the tank valve is fully closed and disconnected.
FAQ: Your Greenhouse Heating Questions Answered
What is the most cost-effective heater for a small greenhouse?
For most gardeners with electricity, a simple electric fan heater with a thermostat is the most cost-effective upfront. For long-term running costs in mild climates, its efficiency is good. In off-grid situations, a properly vented propane heater is the only cost-effective choice.
Can I use a space heater in my greenhouse?
You can, but you must be cautious. Only use a space heater rated for outdoor or “damp” environments. Never use a standard indoor home space heater, as they are not safe for the moisture and dust present in a greenhouse and pose a serious fire risk.
How can I keep my greenhouse warm without electricity?
Focus on capturing and storing solar heat. Use thermal mass (water barrels), insulate heavily with bubble wrap, and employ row covers inside as a second layer. A compost pile or even a few candles in a safe, enclosed lantern can provide emergency warmth on a critical night.
Do greenhouse heaters need ventilation?
Yes, especially combustion heaters (propane, kerosene). They require fresh air intake to burn safely and to prevent a buildup of harmful gases. Even electric heaters benefit from slight ventilation to manage humidity levels and prevent mold growth on plants.
What temperature should I aim for in a small greenhouse at night?
This depends entirely on your plants. For protecting frost-tender plants like tomatoes or peppers, aim for a minimum of 45-50°F. For overwintering hardy plants like kale or dormant perennials, keeping it just above freezing (35-40°F) is often sufficent. Seedlings need warmer temps, around 60-65°F.
Choosing the best heater for a small greenhouse is a practical decision that pays off in healthier plants and a longer growing season. By focusing on efficiency and a space-saving design, you ensure your heater protects your plants without taking over your garden. Start by calculating your needs, consider your power options, and don’t forget the simple, low-cost tips to reduce heat loss. With the right setup, you’ll enjoy a vibrant, productive greenhouse no matter what the weather brings.