Heating Is a Production Decision, Not a Winter Expense
Growers in warm climates build cooling capacity first and treat heating as an accessory. Then a cold snap arrives, root-zone temperature drops out of range, nutrient uptake stalls, and the harvest window moves by two weeks — which costs more than the heating system would have.
This guide covers how to size heat load for a hydroponic greenhouse, how to compare heating options on total cost rather than equipment price, why root-zone temperature deserves its own line in the budget, and what to specify before you request quotes.
Step 1: Calculate the Heat Load Before Choosing Equipment
The basic steady-state load is:
Q = U × A × ΔT + infiltration losses + ground losses
Where U is the overall heat transfer coefficient of the covering, A is the exposed surface area, and ΔT is the difference between your target night temperature and the design outside temperature. Your design temperature is a business decision: heating to −10 °C costs far more than heating to −2 °C, and the difference belongs in the feasibility model, not in an engineer’s assumptions.
Typical U-values by covering
| Covering | U-value (W/m²·K) | Comment |
|---|---|---|
| Single polyethylene film | 6.0–6.8 | Cheapest structure, highest heat loss |
| Double inflated poly film | 3.8–4.2 | The default economics in most commercial projects |
| Twin-wall polycarbonate | 3.0–3.5 | Better insulation, longer life, higher CAPEX |
| Single glass | 5.5–6.0 | Light transmission and durability, poor insulation |
| Double glass | 2.8–3.2 | Premium projects, high CAPEX |
| Single film + thermal screen | ≈2.5–3.0 | The cheapest way to halve the night load on an existing house |
Covering choice is covered in more depth in our covering materials comparison. Note that the covering decision and the heating decision should be made together — a thermal screen often removes more load per dollar than a bigger boiler.
Worked example. A 1,000 m² double-poly house (U ≈ 4.0, surface area ≈ 1,350 m²) holding 15 °C against a −5 °C design night (ΔT = 20 K): 4.0 × 1,350 × 20 ≈ 108 kW, before infiltration. With a well-sealed house and a closed thermal screen, the effective requirement typically falls to 55–70 kW. Without the screen, you buy and fuel a 108 kW system for the coldest night of the year.
Step 2: Cut the Load Before You Size the Heater
In order of return on investment:
- Thermal / energy screens — 30–50% reduction in night heat demand. Usually the fastest payback item in a cold-climate project.
- Sealing infiltration — gaps at gables, doors and sidewalls can add 15–30% to the load. Cheap to fix, expensive to ignore.
- Perimeter and foundation insulation — stops the cold edge rows that always underperform.
- Zoning — heat propagation and young-plant areas to a higher setpoint than storage or packing.
- Night setback strategy — many crops tolerate a lower night temperature if the daily mean is held. This is a crop decision, and it interacts directly with VPD control.
Get the dehumidification side right as well: heating a leaky, humid house wastes energy condensing water you then have to remove. Ventilation strategy is covered in the ventilation design guide.
Step 3: Compare Heating Options on Delivered Heat Cost
| Option | CAPEX | Running cost profile | Best fit | Constraints |
|---|---|---|---|---|
| Hot-water boiler (gas / LPG / diesel / biomass) | Medium–high | Follows fuel price; stable and well understood | Most medium and large commercial projects; the reference option | Boiler room, flue, fuel storage, emissions permitting |
| Forced-air unit heaters | Low | Moderate | Small houses, mild climates, backup duty | Uneven temperature distribution, dry spots near outlets |
| Air-source heat pump | Medium–high | Low where electricity is cheap; COP 2.5–4 in mild cold | Mild winters, sites with good power supply or solar | Capacity and COP fall as outside temperature drops |
| Water-source / ground-source heat pump | High | Lowest and most stable | Sites with water or land for the loop; long-horizon projects | Civil works, hydrogeology, higher up-front cost |
| Electric resistance / infrared | Low | Highest per kWh | Zonal topping, propagation rooms, backup | Only viable where power is very cheap or duty is short |
| Waste heat / CHP | Project-specific | Very low where a source exists | Industrial co-location, greenhouse clusters | Depends entirely on a suitable neighbouring source |
Compare on cost per delivered kWh of useful heat, including boiler or COP efficiency, maintenance, and expected fuel price over ten years — not on equipment price. Fuel price assumptions belong in the same model as the rest of your farm energy budget.
Root-Zone Temperature: The Number Growers Forget to Budget For
Air temperature is what most heating specs address. Root-zone temperature is what controls nutrient uptake and disease pressure — and cold solution is where winter crops quietly fail.
| Crop | Target root-zone temperature | Consequence of running cold |
|---|---|---|
| Lettuce & leafy greens | 18–22 °C | Slow growth, tipburn risk from uptake imbalance |
| Tomato, cucumber, pepper | 20–24 °C | Reduced uptake, delayed fruiting, higher Pythium pressure |
| Strawberry | 15–18 °C | Poor root function, flower and fruit development issues |
| Herbs | 18–22 °C | Stalled growth, extended crop cycle |
Practical measures:
- Insulate the reservoir and the floor slab — the cheapest first step, and often the whole solution in mild climates
- Plate heat exchanger or a dedicated water-source heat pump on the nutrient loop for precise control
- Buffer tanks — thermal mass smooths night dips and lets you charge heat during cheaper tariff windows
- Substrate slabs and beds hold temperature better than exposed films; NFT is the most exposed and the most temperature-sensitive because the solution volume is small
Cold solution and low dissolved oxygen are also the classic entry point for root rot: the plant’s defence slows while the pathogen does not.
Distribution and Control
Even heat beats more heat. A system that holds ±2 °C across the house outperforms one with a 6 °C gradient running a higher setpoint.
- Perimeter plus overhead loop for pipe-rail and bench systems; pipe rails double as heat distribution and transport
- HAF fans to break stratification — warm air at the ridge is wasted heat
- Zone control with independent setpoints for propagation, production and packing areas
- Screen and vent interlocks so the screen closes before the heater fights an open vent
- Graduated setpoints — a hard on/off band wastes fuel and stresses the crop; use proportional control where possible
Planning for the Coldest Night: Power, Frost and Failure
Heating without resilience is a single-point failure on the whole crop.
- Backup generation sized for circulation pumps, controls and a minimum heat output — not for full design load
- Frost protection procedure with a written sequence: who starts the generator, what gets drained, what minimum temperature is survivable
- Pipe and valve insulation, particularly on exposed fill lines, filters and outdoor tank sections
- Drain-down points at low spots so an unheatable section can be emptied rather than frozen
- Alarms on temperature and power with off-site notification; a boiler fault discovered at 7 a.m. is a crop event, not a maintenance ticket
Cold-season disease pressure rises with reduced venting — pair the heating plan with the IPM programme, particularly for botrytis and mildew in a sealed, humid house.
Energy Budget: What to Expect
Annual heating energy varies enormously by climate and structure, but the ranges below are useful for feasibility work:
| Climate | Annual heating demand | Comment |
|---|---|---|
| Warm / subtropical | 0–40 kWh/m²·year | Often only propagation and frost protection |
| Temperate | 80–150 kWh/m²·year | Seasonal heating, screen does most of the work |
| Cold continental | 200–350 kWh/m²·year | Heating becomes a first-order operating cost |
| Cold with high-value crop | 350–500 kWh/m²·year | Requires waste heat, CHP or a very strong crop margin |
Model heating together with cooling: the same envelope decisions drive both, and a house optimised only for summer often has an expensive winter.
What to Send With Your Heating RFQ
- Greenhouse dimensions, covering material, and whether thermal screens are included
- Design inside and outside temperatures, and the return period you are designing to
- Crop, target air temperature and target root-zone temperature
- System type: NFT, DWC, Dutch bucket, ebb & flow — and total solution volume
- Available fuels and their local prices, plus electricity tariff structure
- Power supply capacity and whether three-phase is available
- Backup requirement: generator capacity, minimum survivable temperature
- Emission or permitting constraints on site
- Whether the project is new build or retrofitting an existing structure
These inputs sit inside the wider project list in 12 inputs a supplier needs — missing climate data is the most common reason heating quotes are not comparable.
Five Heating Mistakes
- Sizing to the boiler catalogue rather than the calculated load. Oversized short-cycling boilers waste fuel and wear out early.
- Buying the heater before the screen. The screen is usually the better first dollar.
- Ignoring root-zone temperature. Warm air over cold solution still produces a stalled crop.
- No distribution planning. Cold edge rows and a hot ridge mean you pay for heat the crop never receives.
- No failure plan. One power cut in February without backup is a replant.
FAQ
How much heating capacity does a 1,000 m² hydroponic greenhouse need?
Calculate it: U × surface area × ΔT, plus infiltration. A double-poly house holding 15 °C against −5 °C lands near 108 kW unscreened, and roughly 55–70 kW with a closed thermal screen and good sealing.
Is a heat pump viable for a commercial hydroponic greenhouse?
In mild and temperate climates, yes — COPs of 2.5–4 make it competitive with fossil fuels on running cost. Capacity falls as outside temperature drops, so most cold-climate projects keep a boiler or electric backup for design nights.
Do I need to heat the nutrient solution separately?
If the reservoir drops below the crop’s root-zone target in winter, yes. Insulation and a buffer tank are the first steps; a plate heat exchanger or dedicated heat pump gives you real control.
What is the cheapest way to cut winter heating cost?
A thermal screen. It typically removes 30–50% of night heat demand for a fraction of the cost of additional generating capacity, and it pays back faster than almost any other energy measure.
Should the heating design temperature be the coldest recorded temperature?
Not necessarily. Designing to an extreme means large capital spent for a handful of hours. Most projects design to a practical minimum and accept a managed temperature drop, with frost protection rather than full production temperature, on the coldest nights.
Sizing Heating for a New or Existing House?
Send your location, greenhouse dimensions, covering, crop and target temperatures through the quote form. Our engineers will return a heat load calculation, a screen-versus-capacity comparison and a heating specification matched to your system type and local fuel prices.