What is hydronic greenhouse heating?

I am a commercial grower of hydroponic tomatoes in basic unheated plastic tunnel houses. My yields are reasonable, but far lower than I am told growers achieve in good heated greenhouses.

I am considering upgrading to a better heated greenhouse. I often see references to hydronic heating as a common method for heating greenhouses. Would you please explain what hydronic heating is?

Answer by RICK DONNAN

Hydronic heating

In its most basic form hydroponic heating is the use of hot water to heat a space. In domestic use water is pumped from some type of heater and recirculated through metal heating panels in rooms. In greenhouses the water is typically heated in a boiler and recirculated through steel pipes usually just above the floor of the greenhouse.

Pipe systems

Hydronic heating water is recirculated through steel pipes usually about 5 cm (2 inches) outside diameter. Particularly for vine vegetable crops they are laid to form trolley rails about 50 cm (20 inches) apart and on supports making them about 10 cm (4 inches) above the floor. (Different countries and manufacturers have different specifications.) The pair of rails is looped, usually at the aisle end of each row.

Hot water is pumped in through a manifold at the wall end of the row, feeding into one side of every pair of rails. The cooler water returns through the other rail into a cool water manifold. The simple way to set up these manifolds is to have them both start and finish at the same end, near the boiler. If done in this way the pipes nearest the boiler have a much shorter flow path than the pipes at the far end of the manifold. Consequently the temperatures in the pairs of pipes will be significantly different, which gives irregular heating.

The way around this is to use the ‘Tickelmann’ layout. Here an extra pipe is installed to start the cool manifold at the opposite end to the boiler. Now the total path through every pair of rails is identical and hence the temperature drop between the inlet and outlet of every pair of pipes should be the same. This also results in the average temperature between the incoming and outgoing pipes being the same at any point along the row. All of this results in giving a uniform spread of heat input across the greenhouse floor.

Trolleys

The pipes can also be used as trolley rails, especially useful for vine crops. They have flanged steel wheels for running on the rails together with rubber tyred wheels for running on concrete. There are several types of trolley used, but the main ones are trolley bins for harvesting and scissor lift trolleys for working the crop. Crop working includes functions such as removing laterals, deleafing, and layering, which is mainly used with tomatoes. Here a reel of string hangs from a wire and is clipped to or wound around a single stem then unwound and moved along the wire as the stem grows. A single tomato plant grown for a year can reach as much as 15 metres (50 feet) long when layered.

Boiler

Boilers can produce either hot water or steam. For greenhouse use it is sensible to use hot water rather than steam because steam is too hot and hence a danger to plants and people. In simple terms a boiler is a large drum with a combustion chamber and tubes running through the drum. Most greenhouses use a fire tube boiler, that is, the water is in the drum and the hot combustion gases flow through the tubes to heat the water. There is also another type called a water tube boiler where the format is reversed.

A range of fuels can be used. The cleanest is gas, but it is possible to use waste oil, coal or biofuels, although these require boiler modifications and often require frequent cleaning. In all cases the combustion products are discharged from the boiler are hot, but it is possible to cool the discharge gases by installing a condenser (also known as an economiser), which also increases the boiler efficiency.

Carbon dioxide

Whatever fuel is used in the boiler it burns to form carbon dioxide (CO2) and water vapour (H2O). Carbon dioxide is the major plant food because it is vital for photosynthesis, hence the CO2 produced could be used as food for the crop rather than just discharged to the atmosphere. For the CO2 to be used this way it must be cooled (by a condenser) and clean. The gases from burning coal and waste oil are not clean enough to use in the greenhouse. Where combustion gases are used as a source of CO2 there should be a NOx (noxious gas) detector installed. The possible dangerous gases are ethylene (to plants) and carbon monoxide (to humans).

Heat storage buffer tank

A major problem with potential use of boiler gas for CO2 enrichment is that most is produced through the night for heating when there is no demand for photosynthesis and little produced through the day when it could be used.

This can be overcome by the installation of a heat storage buffer tank. This is a large heavily insulated tank always full of water and linked to the boiler. The boiler is then run through the day to produce CO2 for enrichment (and hence increased yield). Apart from any small amount needed for heating the greenhouse, the heat is used to heat the water in the buffer tank. Then during the following night the hot water is recirculated through the pipes to maintain the temperature of the greenhouse.

The use of the buffer tank has the twin benefits of providing CO2 enrichment to increase yield, and also reducing the CO2 emissions from the boiler discharged into the atmosphere. b RD

Got a question for Rick? Email him: info@hydroponics.com.au  Ω

PH&G June 2017 / Issue 180


Translator