How To Heat a Timber Garden Building To Optimise Food Production

Gary Parker
Monday, 1st July 2019

Gary Parker from Loosehanger Oak explores how adding different heating systems to timber garden structures can increase food production.

There are already plenty of articles out there which describe how to create the perfect, sustainable timber building for your permaculture garden. But there are fewer articles that deal with renovating or overhauling an existing structure.  In this article, therefore, we will take a look at the different ways in which we can make alternations or additions to an existing timber garden building to heat the structure and increase food yield.

Creating a 'Hot House' or 'Orangery'

One of the great things about a garden building is that it could offer opportunities to grow exotic foods that you would not otherwise be able to easily grow here in the UK.

Passive Solar Design

Before we begin to look at the ways of optimising a garden building to create a hot house or orangery, one of the key things to consider is passive solar design. We are working under the assumption that the garden building already has sufficient glazing to allow plants to be grown inside.

If the garden building does not already have sufficient glazing to the south, it could be worth considering how to get extra light (and heat) into the structure. A new roof light could be the answer where south-facing glazing is not possible. (If heat is not a primary concern, but light levels are low, you might even be able to consider creating an open area in the roof, so rainwater can feed a tank or reservoir beneath – in much the same way as a compluvium in the atrium of a Roman villa.)
In a structure with south facing glazing, you can of course further complement this plant-friendly design by incorporating more thermal mass into the structure. This is most easily done through the addition of a water tank or reservoir, or through the addition of stone, brick or ceramic flooring. These will store the sun's heat during the day and release it at night, helping to temper temperature fluctuations.

Temperature fluctuations will also,of course, be tempered by improving the insulation of your timber garden building. A sturdy timber building can be far warmer than a more flimsy greenhouse or polytunnel, and so the temperatures within can be made much higher during the winter months.

Organic Matter Hot Beds for Winter Growing

Even in a garden building with good passive solar design, extra heat may still be necessary during the winter months if you wish to increase capacity for growing exotic/ tropical plants. In a timber garden building, much as in a greenhouse, hot beds can be created to provide bottom heat for raised beds created within the structure.

To make raised hot beds for a timber garden building:
-      Source or build containers or edging for your beds. Metal troughs for animals could be ideal to contain the organic material that will break down to generate the heat for your raised beds. You could also consider reusing old bath tubs, for example.
-      Fill your raised beds to a minimum of around 30cm depth with straw/ manure mix or straw fed with a nitrogen-rich liquid feed to compost in place.
-      Cover with a 30cm layer of top soil and garden compost (in equal quantities).
-      Water well and wait at least a week before planting up your new hot beds.

A Pumped Water Heating System

Another common way to heat orangeries in the Victorian era was to pass hot water through pipes buried in the grow beds. In those days, the water was often heated using coal-fired boilers. These days, however, we have other options. One option is to install a water heating system that runs from an efficient outside boiler that runs from renewable electricity. This could  be run from solar panels on the roof of your garden building.

Some enterprising gardeners have even succeeded in making their own boilers for a hot water heating system for a greenhouse or orangery.  The simplest way to do so is by utilising an old hot water cylinder or other metal drum/barrel as the heating chamber.

A coil in this cylinder attached to solar panels on the roof will heat the water. This water can then be passed through piping using either a gravity fed or pumped system. Pumps, if required, can also be run with solar power.

A cheaper, though more labour intensive option could involve the use of a wood-fired boiler. In its simplest form, this could simply involve a metal fire box placed below a water heating chamber.

Small bore pipes (2.5-5cm) are best for growing areas, as these can easily be spread through grow beds or under growing benches. Aiming for soil heating rather than space heating is more cost-effective and best for growing plants like tomatoes that require warm soil for planting.

Installing a holding tank to store the water that has passed, heated, through the pipes inside your structure before it is passed back to the boiler could help integrate your water heating system with passive solar design, and pre-heat water before it is passed back to the boiler to be heated to the optimum temperature.

Interestingly, a renewable electricity or wood-fired boiler could also be used to run an outdoors hot tub – the perfect way to relax and unwind after your gardening efforts.
It is worth noting that a sustainable outdoors boiler would only generate hot water when given an energy supply, so would not generate hot water when solar energy was not forthcoming, or when the fire was not lit. It is important to insulate the water heating chamber well so that the heat is retained as much as possible.

Wood fired water heater - could be connected to a water heating system.©Fairbanks Mike Flickr

Heating Inside Beds With a Rocket Mass Stove

Another excellent option for providing bottom heat for the growing areas or benches in a hot house or orangery is to place a heat source actually within the garden building. One interesting and sustainable choice to consider is a rocket mass stove.
A rocket mass stove is a type of rocket stove (an efficient wood burner) which has incorporated ideas the of a masonry heater. These burners have an insulated combustion chamber where wood is burned with high efficiency at high temperatures, and an exhaust pipe in contact with a large thermal mass which absorbs most of the heat generated before it is released into the atmosphere.

Rocket mass stove ©Velacreations/Flickr
The J-shaped design of the fire box and chimney mean that wood is burned hot and clean – the fire burns sideways and with a much higher efficiency than a regular wood burning stove, so little smoke is created. Often, these stoves can be run using just sticks, twigs and pruning materials from elsewhere in your garden.
A rocket mass stove will not only serve as a space heater. The heat from the exhaust can also be used to warm a growing area or a bench seat for sitting on or for germinating seeds from beneath. A soil bed, or cob surround can be created around the metal exhaust to create a growing area or heating potting bench.
Installing such a system in a timber garden building could provide more heat than hot bed or water  heating systems, and could be an ideal solution for those who want to grow tropical/ warm climate edible plants in a UK garden in a sustainable way.
The intriguing solutions outlined above are just some of the options to heat a timber garden building to optimise food production.

Gary Parker keeps the UK’s green construction heritage growing by creating high-quality wooden structures using ancient, traditional methods. 

Useful links

From tree to timber - the traditional approach

Turning fallen timber into a treehouse

Book: Roundwood Timber Framing by Ben Law