Sizing the Heating System: The Core Calculation
A greenhouse heating system must be sized for the design low temperature of the site — not the average winter temperature, and not the temperature recorded most frequently during winter. The design low is typically the 1% or 2.5% annual exceedance temperature published in climate data tables for a given location. In southern British Columbia, a design low of –10°C to –15°C is common. In the Prairie provinces, design lows of –35°C to –40°C are used for sites outside major urban areas. In Yukon and the NWT, design lows can reach –50°C.
The basic heat loss calculation combines transmission loss (heat moving through the glazing and floor) with infiltration loss (warm air leaking out, cold air leaking in). For a well-sealed polycarbonate greenhouse, infiltration typically adds 25–35% to the transmission loss figure. For older or less carefully constructed structures, the infiltration component can equal or exceed the transmission loss.
Forced-Air Heating Systems
Gas and propane unit heaters
Gas-fired forced-air unit heaters are the most common heating solution in Canadian hobby greenhouses and small commercial structures up to approximately 500 m². They are straightforward to install, available from multiple Canadian suppliers, and can be sized incrementally — multiple smaller units staged to the controller are more reliable than a single large unit at maximum output. Natural gas is the most economical fuel in regions with distribution infrastructure (most of British Columbia, Alberta, Ontario, and Quebec). Propane serves locations without natural gas access and is the standard fuel across much of rural Canada.
Standard-efficiency gas unit heaters operate at 80–82% thermal efficiency. High-efficiency condensing units reach 92–97% efficiency, recovering heat from the flue gas that standard units vent to atmosphere. At current natural gas prices in Canada ($0.25–0.40 CAD/m³ depending on province), a high-efficiency unit pays back its premium installation cost in fuel savings within 3–7 years in a commercial-scale greenhouse operating through a full Canadian heating season.
Air distribution
Unit heaters located at one end of a greenhouse create a temperature gradient that leaves the far end 3–5°C cooler during cold weather. For short structures (under 15 metres), this is often acceptable. For longer structures, either a second unit heater at the far end or a perforated poly tube distribution system solves the gradient problem. A poly inflation tube — a clear or white polyethylene tube 300–450 mm in diameter running the full length of the greenhouse at ridge height — distributes warm air evenly along the length of the growing space and simultaneously inflates to serve as the tube itself. At costs of $0.80–1.20 CAD per linear metre, it's an economical solution.
Radiant Heating Systems
Hot-water radiant floor heating
Radiant floor heating — PEX tubing embedded in or below a concrete growing slab, fed by a hot-water boiler — is the preferred system in high-value commercial greenhouses growing tomatoes, peppers, or cucumbers. The system heats the root zone directly, which improves nutrient uptake and growth rates. Soil-zone temperature of 18–21°C maintains root activity even when air temperatures are kept lower overnight, reducing overall energy consumption by 10–20% compared to air-heating-only systems at equivalent growth rates.
The capital cost of radiant floor installation ($25–45 CAD/m² including tubing, manifolds, and boiler share) is higher than unit heater installation, and the system requires professional installation and annual boiler servicing. The payback period for the efficiency premium depends on natural gas prices and the crop value, but in commercial tomato or pepper production in Ontario and British Columbia, it consistently falls within 4–8 years.
Overhead radiant tube heaters
Gas-fired infrared tube heaters mounted overhead at ridge height provide radiant warmth to plants and bench surfaces without heating large volumes of air. They are particularly effective in high-ceilinged structures (over 3 m clear) where the air volume is disproportionately large relative to the growing area. Because they heat surfaces rather than air, infiltration losses affect them less than forced-air systems — a useful characteristic in older structures with significant air leakage. They are not suitable as the sole heating system in frost-sensitive structures, because they do not protect unheated air zones from freezing.
Biomass and Alternative Heating
Wood-fired biomass boilers connected to in-floor or perimeter radiant circuits have become practical for Canadian greenhouse operations with access to waste wood, wood chips, or agricultural biomass (corn stover, switch grass). The capital cost is higher than propane or natural gas systems ($40,000–120,000 CAD for a commercial-scale biomass boiler installation depending on size and fuel handling equipment), but fuel costs can be 60–75% lower than propane at equivalent energy output, and the system qualifies for various federal and provincial clean energy rebates.
For hobby and small commercial greenhouses without access to bulk fuel delivery, electric resistance heating via baseboard or in-slab elements is the simplest installation but is rarely economical in Canada given electricity rates in most provinces. Heat pumps — air-source or ground-source — are an increasingly viable alternative for small greenhouses in milder-climate regions (coastal BC, southern Ontario), with COPs of 2.5–4.0 reducing effective heating cost relative to resistance heating.
Ventilation System Design
Why ventilation matters year-round
The intuitive greenhouse problem in Canadian conditions is heat loss. But overheating — and the humidity management challenges that come with it — is equally significant for crop quality. On a clear February day in British Columbia, solar gain through polycarbonate glazing can raise interior temperatures above 30°C by mid-morning in a greenhouse with inadequate ventilation, even with outdoor temperatures of –5°C. Botrytis (grey mould) and powdery mildew both thrive in stagnant, humid air; ventilation that keeps air moving is the primary cultural control tool for both.
Natural ventilation: ridge and side vents
A continuous ridge vent running the full length of a gable-roof greenhouse — typically occupying 15–20% of the roof area on each side of the ridge — allows warm, humid air to escape by buoyancy (warm air rises). Side vents or roll-up sides at 0.5–1.2 m above grade allow cool replacement air to enter. The temperature differential between floor level and ridge drives a natural airflow that can achieve one to two complete air changes per minute during warm periods without any fan energy. In commercial Venlo structures, alternating ridge vent sections provide similar natural ventilation at scale.
Ridge vent sizing for natural ventilation follows the rule of thumb that the total vent area (combined ridge and side) should equal approximately 15–25% of the floor area for effective summer cooling in most Canadian locations. Structures in high-solar-gain climates (interior BC, southern Prairie provinces) should target the upper end of this range.
Horizontal Airflow (HAF) fans
HAF fans are small, continuously running propeller fans (typically 200–350 mm diameter, 40–150 W) positioned to create a horizontal, circular airflow pattern throughout the growing area. They do not ventilate — they do not exchange interior air with exterior air — but they maintain air movement around leaf surfaces, which reduces the humid boundary layer that promotes fungal disease. In tomato production, consistent HAF operation has been shown to reduce Botrytis incidence by 30–60% in humid fall and early winter conditions compared to unventilated control sections.
HAF fan spacing follows the guideline: one fan per 30–50 m² of floor area, positioned 1.8–2.4 m above bench height, oriented to create a circular airflow pattern (fans at each end pointing the same direction along the ridge, or in a figure-eight pattern in wider structures). At 40–80 W per fan, HAF systems are cheap to operate and are considered baseline equipment in any commercial greenhouse producing crops susceptible to foliar diseases.
Exhaust fan ventilation (mechanical)
Mechanically exhausted greenhouses use motor-driven propeller fans in one gable end wall, with motorised inlet louvers in the opposite end. This provides positive airflow control and can achieve precise air exchange rates regardless of wind conditions. Fan capacity is sized to achieve a minimum of one complete air change per minute during maximum summer heat load. For a 500 m² greenhouse at 3 m average ceiling height (1,500 m³ volume), that requires 1,500 m³/min of fan capacity — typically achieved with two to three large exhaust fans.
Year-Round Growing Schedules
Light as the limiting factor in winter
For most of Canada north of 49°N latitude, the limiting factor for year-round crop production in winter months is not temperature — it is light. Daily Light Integral (DLI), expressed as mol/m²/day, determines how much photosynthesis is possible in a given day. In December and January, natural DLI in Vancouver is 5–8 mol/m²/day; in Edmonton, 4–6 mol/m²/day; in Calgary, occasionally 7–9 mol/m²/day on clear days due to high solar angle reflection from snow.
Most fruiting crops (tomatoes, cucumbers, peppers) require 12–15 mol/m²/day minimum. Leafy greens and herbs can produce usefully at 8–12 mol/m²/day. Without supplemental lighting, fruiting crop production in Canada is typically viable from April through October in unlit greenhouses, with seedling propagation and low-light crops filling the winter months.
Supplemental lighting for continuous production
High-pressure sodium (HPS) or LED grow lights extend the viable growing season. LED fixtures have largely displaced HPS in new installations since 2022 due to their higher photon efficacy (2.5–3.0 µmol/J for commercial LED versus 1.7–2.1 µmol/J for HPS), lower radiant heat output, and longer rated lifespan. Fixture mounting height, target PPFD (typically 150–400 µmol/m²/s for leafy greens; 400–700 µmol/m²/s for fruiting crops), and daily photoperiod are set through the climate controller.
Operating LED supplemental lights during the winter electricity rate period (typically off-peak hours from 7 pm to 7 am in Ontario's time-of-use system) both reduces electricity cost and provides night heat input that partially offsets nighttime heating demand — a useful secondary benefit in cold-climate greenhouses.
Sample year-round production calendar (Zone 6, southern Ontario)
| Period | Crop Focus | Heating Need | Lighting |
|---|---|---|---|
| Jan–Feb | Microgreens, herbs, seedlings | High (continuous) | Supplemental LED required |
| Mar–Apr | Transplant production, early leafy greens | Moderate–high | Supplemental useful |
| May–Jun | Tomatoes, cucumbers, peppers (fruiting) | Low (frost protection) | Natural sufficient |
| Jul–Aug | Fruiting crops (peak), summer herbs | None (cooling focus) | Natural sufficient |
| Sep–Oct | Fall leafy greens, late fruiting crops | Low–moderate | Natural sufficient |
| Nov–Dec | Leafy greens, herbs, propagation | High (continuous) | Supplemental LED required |
For structural considerations that affect heating system placement and ventilation duct routing, see Greenhouse Frame Types: Steel, Aluminum, and Wood Compared. For how glazing material affects heating load calculations, see Choosing Greenhouse Glazing Materials.
Last updated: May 12, 2026.