If you're about to buy a woodburner, here's what to consider.

Type

Freestanding models are generally the most efficient (for a given firewood load, they return the most heat to a room). They're also the cheapest to install. But if you have an existing open fireplace, an insert model is most often the way to go. Although insert models are not as efficient as freestanders, they're way better than an open fire.

Heat output

Choose a woodburner with a heat output suitable for your home. If you have a non-draughty well-insulated home in the north of the North Island then 10kW should be plenty. A larger house - or the same-sized but less-well-insulated and draughty house - further south will require more heat output. Think in terms of 12-14kW.

In non-open-plan houses there's no point in overheating the lounge while the rest of the house stays cold. Install a heat-distribution system to help spread the heat throughout the house.

Be careful about manufacturers' heat-output claims - some grossly exaggerate what you'll get. Always check the compliance plate (pictured). This has to be on every woodburner that's sold - and it states the tested heat output, along with the efficiency and emissions rate.

Radiant/convector

Radiant burners beam heat directly to objects in the room, and are best for less well-insulated houses with high ceilings. Convectors heat the air then circulate it into the room, and are best for well-insulated, non-draughty houses with low ceilings. All woodburners work in both modes to an extent, but most are designed to be predominantly one or the other.

Controls/cleaning

The controls should operate smoothly, and it should be relatively easy to clean the outer surfaces and empty the firebox.

Flue system
The flue must have a larger diameter outer shield around it where it passes through the ceiling and attic space. Some of the newer designs have this air-gap vented outside rather than into the room (outside venting stops hot air escaping from the room). Some other woodburners get the air needed to burn the wood from outside the house rather than from within the room: this improves efficiency and reduces drafts.

Emissions
The lower the particulate emissions from your fire, the less of a health hazard you'll be causing. You can find this out from the woodburner's compliance plate. See Emission rules for more information about emissions.

Wetbacks

A wetback uses copper pipes to circulate water from the woodburner to the hot-water cylinder and back. They're expensive to install and the hot water cylinder needs to be placed reasonably close to the burner. The payback period for a wetback depends on how you use your woodburner: if the woodburner's not used every day, a wetback is unlikely to be cost effective. See our section on wetbacks for more information about this type of woodburner.

Safety guards

Woodburner surfaces can get very hot and can be a danger to small children. Protective guards are available, and highly recommended.

Building consent

Before you buy a woodburner, make sure your local authority will allow you to install the model you want: some councils will only allow installation of models from their recommended list. You will also need to get a building consent before you install your woodburner (you're unlikely to get one retrospectively). If an illegally installed wood-burner causes a fire, it may invalidate your insurance cover.

Installation

A poor installation job can ruin the heating and emissions performance of the best of woodburners. Check that your installer has New Zealand Home Heating Association certification or is otherwise suitably experienced.

How woodburners work

It's a two-stage process. First, if the wood is hot enough, the combustible creosotes and resins will evaporate out of the timber and will be burnt as gases (if these gases are not burnt completely the result is smoke).

The evaporation of the gases turns the wood to charcoal, which then burns easily and cleanly - and produces most of the heat.

To burn as cleanly as possible, the fire needs to be as hot as possible. It also needs the right amount of air to support the combustion. Too much air cools the fire and smoke is produced. Not enough air has the same effect.

Modern woodburners burn efficiently because the firebox is lined with firebrick material, making a hotter fire. The combustion air is carefully admitted to give the most complete combustion possible. The resulting efficiency (conversion of the fuel energy into heat in the room) is around 65 percent.

Contrast this to the traditional open fire, which admits far too much air, cooling the fire and giving an efficiency of 15 to 20 percent. Some even have negative efficiency - they draw cool air into the house, warm it and send it up the chimney!