4 Jun 2016

Wood burners: a buyer's guide

From This Way Up, 1:20 pm on 4 June 2016

As we head into winter, This Way Up looks at the best home heating options with George Block of consumer.org.nz. This week - wood burners.


Photo: (kaboompics.com CCO)

Read an edited extract of the conversation:

Simon Morton: We're in a large retailer and we’re surrounded by [wood burners], George. The first thing that strikes me is that most of them are square, most are of them are a dark blacky-brown colour. The only variable is the fact that some will sit inside a wall. Over here they almost look like cassettes. They’re inserts versus these stand-alones. Which are more efficient?

George Block: Generally these stand-alones which sit on the ground and radiate heat out from all sides are the most efficient. But those inset cassettes are definitely a good option if you’ve got an old open fire to replace or you’re replacing an old night store heater or gas fireplace.

The other distinction they make is radiant and convector. Radiant fires shoot heat out from the front. They’re the ones that you sit around with a glass of cognac. The convector ones are a bit more… not like a heat pump,but they warm the air. So they’re less toasty out front, but they provide more ambient heating.

Generally, in drafty houses radiant heaters are a good option ‘cause you can sit in front of it, warm up quickly. But if you’ve got a well-insulated, modern, airtight home, we think convectors are worth considering, as well.

So the first thing you have to work out as a consumer when you’re coming in to look at a fire is how big a fire to get – when I say big, how much heat it’s going to give out – is that the critical thing?

Absolutely, yeah. Just like heat pumps, get the right amount of kilowats for the room you’re heating. In the north of the north island in a well insulated house, 10 kilowatts is going to be fine. But if you’ve got a drafty home down south or a big, huge house in Queenstown to heat up, for large spaces you’re going to need 14 kilowatts and upwards. If you’re heating a whole home by way of a ducted heat transfer system, look upwards of 20 kilowatts.

Right here we’ve got a 26-kilowatt beast. It is a lovely looking fire – this is made in New Zealand, nearly $3,000 – versus one over here, 14 kilowatts $1,600. Quite a lot of difference in price, nearly twice the price for the bigger unit. We’re talking quite a lot of capital here, aren’t we?

Yeah, that’s right. But that said, a big heat pump is going to set you back about $3,000 plus $500 - $1,000 to install. So they are comparable to heat pumps and natural gas fires. And they also cost about the same amount to run, even if you are buying firewood. But the fact is, there is more hassle, so it’s not an option for everyone. What we’ve found is that kiwis still love their wood burners. We recently asked our Facebook followers what was the best sort of heat and wood burners won hands down. People love the radiant toasty heat. They don’t like the hairdryer effect of heat pumps. So there’s definitely still a place for wood burners. They’re going to be here for years to come.

Another number on these fires, I can see there’s a plate here attached to it and it says 67%. This is the ‘overall average efficiency burning soft wood’. What does that mean?

That’s the number that they’ve come up with in the National Environmental Standards test. This NES test we’ve got a lot of issues with. It doesn’t give an accurate measure of how efficient it is in real life. That’s because they burn this ideal wood, this flat no-knots radiate that’s burnt in ideal test conditions. When we’ve tested a wood burner with anything like real firewood those efficiencies and emissions figures are far lower. But these percentages are valuable as a comparative tool. So this one - 67% - this one is definitely going to be more efficient, you’re going to get more heat from a piece of wood.

So the critical thing is get the highest efficiently you can afford and higher means you’re going to get more efficient wood burning.

That’s right, but the most important thing to consider is the raw heat output.

What about emissions? How do you measure emissions on a wood burner?

If you look on any wood burner label, you’ll see a figure that says ‘so and so grams per kilogram’ and it’s how many grams of your firewood is going into the atmosphere in terms of particulates. That’s a real hazard because people breathe in those pieces of firewood, basically, and it can cause all manner of respitory issues. Timaru – where they’ve got a lot of wood burners – has the worst air quality in Australasia.

Surely a big factor would be the quality of the firewood in determining those emissions. If you’ve got damp, bad firewood you’re going to have higher emissions. Is it as simple as that?

Yeah, basically it is. You’re going to need to burn well-seasoned firewood. That’s firewood with moisture content less than 25%. For soft woods like pine you’re going to need to dry it for 6 to 12 months. For things like gum and macrocarpa and manuka you’re going to need to dry it for anything up to 18 months.