Heating Needs in a Greenhouse - Heat Load and Heater Sizing

Heating needs are usually argued about as if a greenhouse were sold by the square metre of floor. It is not. Heat leaves a greenhouse through two paths: it conducts straight out through the covering - roof, end walls and side walls - and it is carried out in the air that leaks through laps, vents and door openings. Both paths are measured in square metres of surface, which means a tall house and a multi-span house with a big roof area lose more heat than a low tunnel of the same footprint. Getting this straight before you buy anything is the difference between a heater that holds your night temperature and one that runs flat out and still falls short.

There is a second reason to start from the cover: the cover is the one part of the heating bill you choose at the design stage and then live with for years. The rest of this page is written for the grower who is sizing heating as part of a build or a retrofit, in the same greenhouse planting technology library as our crop and ventilation articles.

The standard way to size greenhouse heating is a conduction sum. The heat load equals the heat transfer value of the covering multiplied by the surface area of the house multiplied by the temperature difference you have to hold, divided by 1000 to give kilowatts. Written out, that is Q = (U x SA x dT) / 1000, where Q is the heat load in kW, U is the covering heat transfer value in W/m2 per degree C, SA is the surface area of roof and walls in m2, and dT is your night setpoint minus the lowest outside temperature your house has to ride out. This is the method published by the New South Wales Department of Primary Industries in its greenhouse heating guidance, and it is the same table of heat transfer values that most extension services quote.

Two of those three numbers are yours to decide. The setpoint comes from your crop. The outside temperature comes from your site - use the coldest night you expect to design for, not an average winter night. The third number, U, comes from your covering:

Covering material	U value (W/m2 per degree C)
Single polyethylene film	7.5 - 8.5
Double polyethylene film	4.0 - 5.0
Polycarbonate, single wall	6.8
Polycarbonate, double wall	3.5
Glass, 3 mm	7.0 - 8.0
Glass, 6 mm	6.5
Thermal screen, closed	2.8

Run the numbers once and the covering choice stops being an aesthetic argument. Take a house of 300 m2 of surface area being held at 17 degrees C on a night that drops to 1 degree C, so dT is 16. Under double polyethylene film at U 4.5 the load is 4.5 x 300 x 16 / 1000, or 21.6 kW. Put single film at U 8.0 on the same frame and the same night and the load is 8.0 x 300 x 16 / 1000, or 38.4 kW - a heater that is 78 per cent larger, burning for the same plants. That gap is why double-layer film, twin-wall polycarbonate or double glazing pays for itself in a heated house, and it is the reason we would rather see the covering settled on first when a heated house is being planned. Houses on exposed, windy sites lose more again through air leakage, so treat the figure as a floor for the design, not a ceiling. Our film greenhouse covering range covers the single and double layer options behind these numbers.

There are four ways to put heat into a greenhouse: portable heaters, hot water pipes, wired-in electrical heat and solar heating. Each has its own benefit and each has its own failure mode, and the choice is usually decided by house size, fuel availability and how much of the winter you are heating for rather than by price alone.

Portable heaters. The quickest and easiest way to add heat. They can run on natural gas, electricity, propane, wood, oil or kerosene. The catch with the gas, oil, wood and kerosene types is that burning them releases gases that are harmful to plants, so a ventilation path that carries those emissions out of the house is not optional - it is part of the installation. Route the venting so it never sits in direct contact with a plastic film skin, because the heat from the duct can melt the film. When sizing a portable unit, go by the manufacturer's stated maximum and minimum heating space rather than by the nominal output alone; an oversized unit that short-cycles and an undersized unit that never reaches setpoint are both common mistakes.

Hot water pipes. Best installed by professional welders and plumbers, and usually made to measure for the house. The system is a run of pipes around the inside walls, fed from a water intake and circulated by a pump. Heating the water up in the first place costs more, but once the pipe loop is hot the water holds its heat well, and that retained heat is where the running-cost saving of a hot water system comes from. Because the pipe ring sits low in the house it also warms the root zone, which matters for anything being grown on the floor.

Wired-in electrical heat. The simplest of the four to operate - you turn the dial. The problem is cost: electric heat in a greenhouse gets very expensive through the winter, and if the power fails in the middle of a winter freeze you can lose the whole crop overnight. The single most useful accessory for an electrically heated house is a greenhouse temperature alarm, a small device that sounds as soon as the house drops below a level you preset into it. When it goes off you know either that the power has gone out or that the heat needs turning up. For outages, a generator sized to run a portable heater or two is the usual backup.

Solar heating. Cost efficient when the house is sited and designed for it, and enough to carry most plants through a short outage during a freeze. It is not usually enough to keep a house warm through a long freeze on its own, so it is best treated as the base layer of a mixed plan rather than a replacement for the other three.

In houses we supply, the heating loop is quoted as part of the same package as the frame and covering, and retention measures such as screens and extra covering layers belong to the same design conversation as the greenhouse heat preservation methods used to cut the load in the first place.

The commonly quoted optimum growing temperature for most plants is a wide band, from 50 degrees F (10 degrees C) to 85 degrees F (29.4 degrees C), and where you sit inside that band depends on what is in the house. Seedlings do better at the warm end; mature plants are happier cooler. Above 85 degrees F (29.4 degrees C) growth slows and flowers drop, which stops flowers setting and fruit forming, and cold nights cause much the same set of problems from the other direction. Temperature reaches into every part of the plant's cycle - photosynthesis, cell division, water intake and nutrient uptake all move with it - which is why the setpoint is a crop decision before it is an equipment decision.

For heating design specifically, published guidance puts the night air temperature at around 16 degrees C (60 degrees F) for most plants, with daytime settings 6 to 11 degrees C higher. If a house holds more than one crop, or holds seedlings and finished plants at the same time, the honest answer is to zone it: one setpoint and one valve group for the warm benches, a second for the cooler ones. Zoning costs a little more pipe and control at build stage and saves heat every night afterwards.

Ventilation and heating also have to be designed against each other rather than separately, because every cubic metre of air you dump to clear combustion gases or to cool the house at midday is heat you paid for leaving through the same opening. If your house runs both, the control logic and the greenhouse ventilation system should be planned together with the heating side.

Half of a heating plan is not heating. Make the house airtight so it does not lose heat through leaks, and use double or triple glazing where the budget allows, because every extra layer on the cover cuts the U value and therefore the load. Insulate the walls, with the north wall as the priority since that is where most of the wall heat loss happens. The cheapest way to do the north wall is bubble insulation in rolls - the same bubble material used to protect parcels in the post, and some greenhouse suppliers sell a clear grade made for this. The method is simple: spray adhesive on the inside of the glazing and press the pre-cut bubble sheet against it until it sticks.

The other commonly missed surface is the ground. Insulating the foundation of the greenhouse with 1 to 2 inch (25 to 50 mm) thick Styrofoam board stops heat being drawn out sideways into cold soil, and it matters most in exactly the houses that rely on solar heating, where there is no oversized boiler to cover the loss.

Solar gain is decided long before the first panel is bolted on. Orient the house so its long side faces within 20 degrees either side of true south, and if you intend to use larger solar panels, make the house 2 or 3 times longer than it is wide. Decide the position in the garden at the very start of the design, because siting is the one decision that cannot be corrected later by buying better equipment.

Heating is about more than air temperature, because the nutrients you paid for only reach the plant if the root zone is in condition to take them up. Soil pH - the measure of how acid or alkaline the soil is - is the other half of that. If pH is off balance the nutrients the plant needs cannot stay available in the soil, and the root system cannot use the minerals that are there no matter how well the house is heated.

Test the pH rather than assuming it; kits are cheap and widely available online. If the soil is too acidic, the usual correction is limestone or wood. If it is too alkaline, use compost material (avoid manure), sulfur, or a commercially available acidifying plant food. A balanced pH also keeps the beneficial soil microbes alive, and those microbes are what suppress disease and pests in the root zone - when the good microbes die off, the harmful ones take over. It is not unlike the good and bad bacteria in the human gut, which have to be kept in balance to keep disease out.

Bozong Greenhouse has above 35 years of professional greenhouse production behind it. From a small workshop, the company has grown into a professional conservatory manufacturer with a real presence in the market, and we supply complete houses rather than frames alone: cooling and heating, irrigation, fertilization, hydroponics, ventilation, lighting and automatic control are all quoted as part of the same package.

To size a heating system we need three things from you: the house dimensions and covering, the crops and the night temperature you want to hold, and your local winter minimum together with average wind speed. With those, we can run the heat load, recommend a heat source and a covering combination, and quote it as one package. The whole range sits inside our greenhouse equipment catalogue, so the heating side can be ordered together with the frame, cover and vents for the same house.

Standards and market guidance on protected cropping come from bodies such as the NGMA, the US greenhouse manufacturers association, and trade coverage of heated protected cropping is published by Horticulture Week.

Q: How do I work out the heat load for my greenhouse?
A: Multiply the covering heat transfer value by the surface area of roof and walls, then by the temperature difference between your night setpoint and the lowest outside temperature you have to ride out, and divide by 1000 for kilowatts. The heat transfer values we use run from 2.8 W/m2 per degree C for a closed thermal screen to 7.5 - 8.5 for single polyethylene film. Use the real surface area of roof and walls, not the floor area.

Q: Which covering loses the least heat?
A: On published figures, double-wall polycarbonate at about 3.5 W/m2 per degree C loses roughly half what single polyethylene film does at 7.5 - 8.5, and a closed thermal screen at 2.8 beats both. Double polyethylene film at 4.0 - 5.0 is usually the best value on a film house. Whatever you choose, the number that matters is the U value, and the load it produces is Q = (U x SA x dT) / 1000.

Q: If I heat with gas or kerosene, do I still need ventilation?
A: Yes, and it is not optional. Gas, oil, wood and kerosene heaters release gases that are harmful to plants, so the installation needs a venting path that carries the emissions out of the house. Keep the duct clear of any plastic film skin, because the heat from the vent can melt it.

Q: Can I hold different temperatures in different parts of one house?
A: Yes, and on a mixed house it is usually worth it. Seedlings want the warm end of the range and mature plants want it cooler, so zone the benches - one setpoint and valve group for the warm ones, a second for the cooler ones. The extra pipe and control cost at build stage is normally paid back in saved heat over the first few winters.

Q: What happens if the power fails on a freezing night?
A: On an electrically heated house, that is the worst case - you can lose the crop overnight. Fit a greenhouse temperature alarm, a device that sounds as soon as the house falls below a level you preset, so you find out while you can still act. For the outage itself, a generator sized to run a portable heater or two is the usual backup, and solar heating panels will carry a house through a short outage even though they will not hold it through a long freeze.

Q: What do you need from me to quote a heating system?
A: The house dimensions and covering material, the crops you plan to grow and the night temperature you want to hold, plus your local winter minimum temperature and average wind speed. Send those and we will run the heat load, recommend a heat source and covering combination, and quote the heating side together with the frame, cover and vents for the same house.

If you need to ask a question and get some help to find the best greenhouse for you, then just call us on +86 191 5068 3942 or send E-mail to sales@cngreenhouses.com