Best Heater For Small Greenhouse

Finding the best heater for small greenhouse can feel like a big task. You need something that keeps your plants cozy without taking up too much space or running up a huge bill. As a gardener who’s tried many options, I know the right heater makes all the difference between thriving seedlings and a frosty disaster.

This guide will walk you through the most efficient and compact designs. We’ll look at different types, key features, and how to choose the perfect one for your setup. Let’s get your greenhouse perfectly warmed.

This category is all about balance. The best heater for a small greenhouse provides reliable heat, uses energy wisely, and fits neatly into your growing area. It’s not just about raw power; it’s about smart, targeted warmth.

Small greenhouses lose heat quickly. So, your heater needs to be effective. The ideal choice often combines a small physical footprint with a efficient output. You want every watt of energy to go directly to protecting your plants.

What Makes a Greenhouse Heater “Efficient and Compact”?

Before we look at models, let’s define our terms. Knowing what to look for will help you make a great choice.

Efficient: This means it converts most of its energy (electricity, propane, etc.) into usable heat with minimal waste. It also means it has good thermostatic control to avoid running constantly.
Compact: A design that doesn’t intrude on valuable bench or floor space. It might be wall-mountable, have a slim profile, or be easily stored when not in use.
Safe: Critical for a humid environment! Look for features like automatic tip-over shut-off and cool-touch exteriors.
Appropriate Output: Measured in BTUs or watts, it should match the size and insulation of your greenhouse. Too small is useless; too large is wasteful.

Top Types of Heaters for Small Greenhouses

Each type of heater has its own pros and cons. Your climate, budget, and greenhouse setup will determine which is best for you.

1. Electric Fan Heaters

These are a very popular choice for small spaces. They force warm air out into the room, creating good circulation and preventing cold spots. Many have built-in thermostats.

Pros: Inexpensive to buy, very compact, easy to install (just plug in), and often include safety features. Great for milder climates or as a backup heater.

Cons: Heating costs can add up if you rely on them as your primary heat source in very cold areas. They also reduce humidity, which you might need to compensate for.

2. Oil-Filled Radiant Heaters

These heaters work by heating oil sealed inside metal columns. The heat then radiates outward, providing a steady, gentle warmth. They are silent and don’t dry out the air as much.

Pros: Silent operation, retains heat even after turning off, and is very stable. Excellent for maintaining a consistent background temperature.

Cons: They can be heavier and slower to warm up a space initially. They also take up more floor space than a fan heater.

3. Propane Heaters

Propane heaters are powerful and work independently of the electrical grid. This makes them ideal for off-grid greenhouses or emergency use during power outages.

Pros: High heat output, portable, and works without electricity. Great for sudden cold snaps.

Cons: You must manage propane tank refills. They produce moisture and carbon dioxide (which plants use), but also require ventilation to prevent dangerous gas buildup. Not ideal for tightly sealed greenhouses.

4. Infrared Heaters

Infrared heaters work like the sun. They warm objects, plants, and soil directly rather than heating the air. This direct transfer can feel very effective.

Pros: Heats plants and surfaces directly, instant warmth, and often very energy-efficient for zone heating. No fan, so they’re quiet.

Cons: Heat is directional, so placement is key. Areas outside the direct “line of sight” of the heater may stay cooler.

Key Features to Look For

Don’t just grab the first heater you see. Check for these important features that make a heater truly suited for greenhouse duty.

Adjustable Thermostat: This is non-negotiable. It allows the heater to cycle on and off to maintain your set temperature, saving energy and preventing overheating.
Water & Splash Resistance: Greenhouses are humid. A heater with some level of moisture protection (look for IP ratings) will be safer and last longer.
Overheat Protection: A safety sensor that automatically shuts the heater off if it gets too hot inside.
Tip-Over Switch: Cuts power immediately if the heater is knocked over, preventing a fire hazard.
Long Power Cord: Allows for flexible placement without needing an extension cord, which can be a safety risk.

How to Calculate the Right Heater Size

Buying the wrong size is a common mistake. A heater that’s too small won’t keep up. One that’s too large will cycle on and off too quickly and waste energy. Here’s a simple way to estimate.

Calculate Volume: Multiply your greenhouse’s Length x Width x Average Height.
Determine BTU Needs: For a basic, uninsulated greenhouse, you’ll need about 10-15 BTUs per cubic foot to raise the temperature 20-30°F above outside temps. A well-insulated one might only need 5-7 BTUs per cubic foot.
Do the Math: Volume (cu ft) x BTU factor = Estimated BTU requirement.
Convert to Watts: If looking at electric heaters, 1 Watt ≈ 3.41 BTU. So, take your BTU need and divide by 3.41 to find the approximate wattage.

Example: A 6ft x 8ft greenhouse with a 7ft peak height. Let’s use an average height of 6ft for simplicity.
Volume = 6 x 8 x 6 = 288 cubic feet.
For an uninsulated structure: 288 x 10 = 2,880 BTUs needed.
In watts: 2,880 / 3.41 ≈ 845 watts.
So, a 750W to 1000W electric heater could be a good starting point for moderate climates.

Step-by-Step: Installing Your New Heater Safely

Once you’ve chosen your heater, proper setup is crucial for safety and performance.

Choose the Location: Place it on a stable, level surface away from water spills and plant foliage. For fan heaters, position it where air can circulate freely. Often a central location near the floor (warm air rises) is best.
Secure It: If possible, mount it on a wall or post to free up floor space and prevent accidental bumps. Use the brackets provided and follow the manual.
Plug Directly into an Outlet: Avoid extension cords. If you must use one, ensure it’s a heavy-duty outdoor-rated cord and keep connections dry.
Set the Thermostat: Start with a conservative temperature (e.g., 45-50°F for overwintering plants) and adjust based on your plants’ needs. Use a separate, reliable thermometer to verify the temperature in the greenhouse.
Perform a Test Run: Monitor the heater for a full day-night cycle. Check that it cycles on and off correctly and that the temperature remains stable.

Tips for Maximizing Efficiency

The heater is just one part of the system. These habits will help it work less and save you money.

Insulate: Use bubble wrap on the north wall or on all glazing. It’s a cheap and highly effective way to reduce heat loss.
Seal Gaps: Check for drafts around doors, vents, and frame joints. A little weatherstripping makes a huge difference.
Use Thermal Mass: Place black-painted water barrels or stacks of bricks inside. They absorb heat during the day and release it slowly at night, stabilizing the temperature.
Group Plants Together: Plants create their own slightly humid microclimate. Grouping them helps them share warmth.
Invest in a Thermostat Controller: For ultimate control, plug your heater into a separate thermostat controller. It’s often more accurate than the heater’s built-in dial.

Frequently Asked Questions (FAQ)

What is the cheapest way to heat a small greenhouse?

The cheapest method often involves improving insulation first to reduce heat loss. Then, using a small, thermostatically-controlled electric heater for supplemental warmth can be cost-effective. Solar heat collection via thermal mass is free after the initial setup.

Can I use a small space heater in my greenhouse?

Yes, but choose carefully. Standard household space heaters may not be moisture-resistant. Always opt for a model with safety features like tip-over and overheat protection, and ensure it’s rated for the square footage of your greenhouse.

How do I keep my greenhouse warm at night?

This is when heat is most critical. A properly sized heater on a thermostat is the primary solution. Supporting it with thermal mass (water barrels) and good insulation will help retain the heat generated during the day and reduce the heater’s runtime.

Are paraffin greenhouse heaters any good?

Paraffin (kerosene) heaters are similar to propane—they provide good heat without electricity. However, they also produce substantial moisture and require even more ventilation due to combustion fumes. They are generally less convenient and safe than modern electric or propane options for regular use.

Do greenhouse heaters use a lot of electricity?

It depends on the size of the heater, your outside temperature, and your greenhouse insulation. A small 500W heater running intermittently might not be too costly. But a 1500W heater running constantly in a cold winter will impact your bill. Using a thermostat and good insulation are key to managing consumption.

Choosing the right heater is an investment in your garden’s success. By focusing on a efficient and compact design that matches your specific needs, you’ll create a reliable haven for your plants. Remember to prioritize safety features, calculate your size needs, and pair your heater with good greenhouse practices like insulation. With a little planning, you can enjoy a productive greenhouse all year round, no matter the weather outside.