Woodburners test results
Compare heat output, emissions, specifications and more in our database of over 150 models.
Heating & Energy
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Choosing the right woodburner depends on your house and whether you want a wetback. Getting the best performance depends on the size and type of wood you burn. Another option may be a pellet burner. They’re convenient and provide low-pollution carbon-neutral heating. But the high cost of the pellets in some areas has slowed the take-up of these heaters.
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The large amount of radiant heat produced by woodburners suits many of our houses – especially older, poorly insulated and draughty homes with high ceilings. A properly sized and installed woodburner can heat the whole home.
Many people also like the toasty radiant heat a woodburner produces. Some models can heat hot water and all of them can be used during a power cut. Many have a flat top surface for heating a kettle – or even for emergency cooking.
Burning wood is carbon-neutral because it's a renewable resource, but burning it cleanly is the key to making it enviro-friendly.
Pellet burners have some real advantages – if you can source pellets at a reasonable price.
Most pellet burners look like a conventional woodburner and are available either as a free-standing model or an insert into a fireplace. You can even get a basement-furnace model for central heating.
But pellet burners burn only compressed wood pellets, which you buy in 15kg or 20kg plastic bags or in bulk. The pellets are loaded into a hopper at the back of the unit and are fed into the fire through an automatic feed system.
Adjusting the rate the pellets are consumed gives you control over the amount of heat produced.
Compared to sources like firewood, there are a limited number of wood pellet suppliers in New Zealand. Check there is a manufacturer near you before investing in a wood pellet burner. If you have access to cheap firewood it’s probably best to go with a log burner.
The NES sets standards for emissions and efficiency. The emissions figure is the number of grams of smoke particles for every kilogram of dry wood burnt. The NES requires that this be less than 1.5g/kg. If you live in Canterbury or Nelson the rules are even tougher.
The efficiency figure is the percentage of the theoretical maximum heat contained in the firewood that the burner delivers to the room. The NES requires this to be 65 percent or greater for woodburners in urban areas.
All new woodburners must be tested for compliance with the NES - and this must be done by an approved laboratory. The manufacturer sends a sample model to the lab, which notes the critical dimensions of the model and tests its burner's efficiency and emissions.
If a model passes the tests, it's NES compliant. The manufacturer then labels all production of that model as such.
However, several tests – including our own – have shown the emissions figures obtained in the lab can be exceeded many times over when the woodburner is installed in a house and is burning real firewood.
That means NES-compliant woodburners are still probably significant polluters. It also means regional authorities may have poor information when trying to assess the pollution impact of the woodburners installed in an “airshed” in their area.
Install a pellet fire and the emissions from your burner would be close to the laboratory test results (because the fuel you burn is the same as the test fuel). Pellet fuel is produced from sawmill waste, often sawdust and wood shavings. The waste is compressed, then dried. No glues or other binders are used, so the pellets are 100 percent wood.
Burning wood is carbon-neutral. The carbon dioxide emitted from burning wood is absorbed by growing trees. And so burning wood doesn't contribute to the build-up of atmospheric carbon dioxide, unlike burning fossil-fuels.
Verdict? A pellet fire is probably the cleanest and greenest home-heating system available.
This means, for instance, a burner that automatically adjusts its airflow to minimise emissions during the burn cycle cannot be allowed. Additionally, the test doesn’t include the start-up period – when there’s often the most pollution.
Once the testing is completed, the internal design of the burner isn’t allowed to be changed. So it’s illegal to retro-fit any device to the burner, or its flue, that might improve performance.
The upshot is that the NES test which was meant to improve the cleanliness of woodburners has stifled design innovation. We’re aware of several technological developments that have been “caught” by the rules and so haven’t been implemented.
A new standard called the Canterbury Method (CM1), sponsored by Environment Canterbury (ECAN), could be the solution. This CM1 standard won’t replace NES – but because it requires better emissions and efficiency performance than existing standards local authorities will be able to choose to use it.
The new method has been developed in response to the needs of people rebuilding houses after the Christchurch earthquakes. It allows CM1-approved woodburners to be installed in some areas where woodburners had been banned for new houses.
The new method takes a different approach from the NES: it specifies the desired emissions and efficiency but not how to achieve them. It also tries to mirror how people actually use their woodburner. So it has a full-burn test cycle that measures a woodburner’s performance through light-up, warm-up, burn, refuel and die-down.
We know of two woodburners that can pass this test:
Both of these burners have received resource consent to be installed in Canterbury properties where a wood fire could not previously be used.
We look forward to more innovative and clean burning products being developed under this new standard.
This means it's trickier to state a definitive heat output for woodburners than for other types of heater, because different testing methods will give different heat outputs – even for the same woodburner.
To be sold in urban areas, all woodburners must meet the NES (national environmental standard) for emissions and efficiency using the AS/NZS 4012/4013 test method, or meet the new CM1 standard. The NES test involves running the woodburner until it has a glowing ember bed. Then a precise amount of kiln-dried and knot-free pine is placed into the firebox, and the heat output is measured until the wood burns back down to an ember bed.
We use the NES results for scoring the heat output of woodburners because it gives a consistent and repeatable result for comparing models. But it often understates what you could achieve in your home – sometimes by 30 percent or more. This is especially pronounced with larger woodburners as they aren’t allowed enough time to fully heat up.
For some models, our database also shows the results of the New Zealand Home Heating Association (NZHHA) test method. This refuels the woodburner every 20 minutes, so the heat output will be higher. These results reflect the maximum you could achieve in your home. When buying a larger woodburner (greater than 10kW) that you’d like to keep on all day, we recommend consulting the NZHHA results if they’re available.
Tip: Keep the woodburner refuelled often and use dry firewood of the right size (110mm diameter or less). Control the heat output by adjusting the amount of wood that’s burning rather than by using the air control.
The maximum output of most pellet burners installed in a living room is in the 9 to 11 kW range. But overheating the room is less likely with a pellet burner because the heat output is controllable over quite a wide range – usually 1.9 to 11 kW.
Firewood must be seasoned (less than 25 percent moisture content) and must also be the right size (fits through a 110mm hole) to burn with maximum efficiency and minimum pollution. That means buying firewood several months (hardwood dries more slowly than softwood) ahead of winter and storing it (where air can circulate but it’s sheltered from the weather) … or paying more for dry firewood.
"Space-heating efficiency" is the efficiency of converting the wood's heat energy into space (air) heat. The water heating from the wetback isn't included in the efficiency calculation – so when some of the wood's energy is going into water heating it means that relatively less is going into space heating.
This means that some woodburners are compliant without a wetback but not compliant when fitted with one. This is because the heat removed by the wetback reduces the woodburner’s efficiency to below 65 percent.
Wetbacks are expensive to install and require the hot water cylinder to be placed reasonably close to the burner. The payback period for a wetback depends on how you use your woodburner. If the woodburner is not used every day, a wetback is unlikely to be cost effective.
One problem with woodburners is they can over-heat the lounge while the rest of the house remains cold. This isn’t healthy. The most common commercially available heat transfer systems use ducting to move warm air. We think the rule on wetback “efficiency” is holding back development of woodburner systems that could distribute heat around the house using a wetback and water radiators.
Tip: If your house is not open plan, think seriously about installing a heat-transfer system.
Calculate the right size woodburner for the area you want to heat. Measure the floor and ceiling areas in square meters (length times width in metres). Usually the ceiling area is the same as the floor. Measure the wall and window areas in square meters (height times width in metres). Enter the areas you have measured into the calculator below.
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