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Greenhouse Heating

Overview:

Greenhouse heating is a subject that is often governed by economics and the gardener’s needs, so to this end, I thought I would show you my set up, as opposed to generalising.

Initially I considered whether to heat with Electric, gas, or oil (paraffin)

In the end I chose electric as it was easier to control, plus I could add different pieces of equipment, e.g. soil cables, lighting and purpose made propagators.

Piped gas would have been difficult and expensive to install.

Liquid gas and paraffin, I thought were less user friendly, in so far as, there would be a need to light them, and refill the containers on a regular basis.

Other factors against gas and oil was their combustible nature and the gases and condensation they produced.

Then there was the potential loss of valuable /expensive heat when ventilators were opened to let these gases out.

Initially electricity did seem to be the more costly option compared with oil and gas, however, because of market forces, and that oil and gas were diminishing fossil products, the suggestion was that these fuels would become more expensive as time went on, so I opted for electricity.

My set up

Greenhouse details:

I have one 3m x 2.4m (10’ x 8’) heated greenhouse, and three various sized unheated greenhouses, plus a 2.4m x 0.9m (8’x3’) permanent cold frame, and three 1.2m x 0.9m (4'x 3') collapsible alloy cold frames.

Heated Greenhouse

Unheated Greenhouses

Alloy Coldframe

DIY Coldframe



The 3m x 2.4m greenhouse is used primarily as a propagating area and a place to store plants requiring frost protection during the winter.

The other three greenhouses, and the cold frame, are used as hardening off areas in the spring, and for growing subjects that require protection.

These greenhouses are generally kept empty over the winter months, and the frames used when necessary.

If I were asked what item is of most benefit to the gardener, then I would have to say the cold frame.

The cold frame is often in use all the year round.

Every garden should have at least one, even if it is of the flat pack variety and is erected / dismantled as required to save space.

Insulation:

In winter I insulate my heated greenhouse on the outside rather than the inside.

I use either bubble plastic (bubbles to the glass) or cellophane sheeting as the insulating covering.

UPVC sheeting as used in Polythene Tunnels is another material suited to this task.

The decision for this was made easier because this greenhouse is made of timber, rather than aluminium, so it is much easier to to affix laths to it to hold the sheeting in place.

I find this method of insulating keeps my greenhouse much dryer i.e. less condensation forms and gives me more space.

It also easier to fix in place otherwise I would have to remove and refit the shelving.

Heating:

For economic reasons I have three forms of heat set at various temperature levels rather than having a general temperature throughout the greenhouse, they are as follows;

General heating:

When frost free conditions are required, I operate with a thermostatically controlled fan 3Kw.heater set to give an ambient temperature of 35°-40°F (3°-5°C).

When starting my seed and cutting programme in January/February, I raise the fan heater output to maintain a general air temperature circa 40°-50°F (5°-10°C).

When purchasing the fan heater I opted for a model that doubles as a 'cooling fan' to aid air circulation in the greenhouse during the summer months

In terms of protecting plants throughout the winter months one must consider what temperatures are needed to protect these plants.

As I see it: there is no point in spending lots of money on heating costs for plants that would cost much less to replace, so I would suggest for economic reasons, that you consider storing such plants in your house, or discarding them, rather than trying to maintain high temperatures in the greenhouse.


Bottom heat:

I have a thermostatically controlled heated bench for rooting cuttings or germinating seed that require a temperature of between 50°/60°F (10°/15°C).

The 2.1m x 0.75m (7’x 2.6’) hot bed is a box formed with treated timber and lined with 1” (25mm) thick polystyrene sheets, and filled with river sand.

In the sand, is placed a 25m (80’) 300watt soil warming cable controlled by a Rodstat.

To prevent the wet sand drying out too quickly I cover it with a sheet of polythene.

By doing this I find the condensates that form under it, do not evaporate, this keeps the sand moist at all times, thus giving me good heat distribution.

Plus as a bonus, it keeps the sand clean and any debris that percolates through the drainage holes in the plant containers is easily swept up.

High temperature heating:

When temperatures of 65°F (15°C) and above are required, I use a purpose made 575m x 290mm. thermostatically controlled propagator, which I manually adjust to the desired temperature.

Lighting:

Sometimes, depending upon what I am propagating, I use a time controlled fluorescent light above the hotbed to supplement the poor natural light of the winter months, thus speeding up growing / rooting times.

The timer is usually set to come on roughly two hours before sunrise and switch off two hours after sunset.

Another reason for having these supplementary lights is the insulation mentioned above, although necessary for heat insulation, reduces the amount of natural light getting into the greenhouse.

Shelving:

In the event that if there is ever a need to grow tall plants from floor level the shelving is designed to be easily removed to cater for this.

The floor is laid with paving flags rather than using soil, this consideration has been adopted to eliminate the need for periodic replacement of the soil.

As space in a greenhouse is always at a premium, the bottom shelves have been constructed at a height suited to storing full bags of compost under them.

A few Do's and Don'ts:

Keep the greenhouse as draughtproof as possible by replacing broken panes of glass when necessary.

Keep watering to a minimum.

Although the ambient temperature in the greenhouse might be quite low the root system in relatively dry plant pots is a few degrees warmer, the reverse is the case in over-wet plants.

When outside weather conditions allow, ventilate the greenhouse to reduce the chances of moulds and fungi developing.

Leaving the fan heater running on a cold or cool setting for a few hours will do the same.





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