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Fire or Heat pump?


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its easy to maintain the fire at night. i either put a couple of large logs (half a tree round) on before bed (around 10pm) and then when i wake in the middle of the night 3 or 4am (because of child) i just restoke it. I hardly ever sleep through the night so no issues there!

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  • 2 months later...

Most log fires can barely do 74% efficiency, and the clean air regulations now require combustion to be at least 65% efficient.

I've come across something called a rocket mass heater which uses small amounts of wood, and is very efficient .. probably similar to a pellet stove. The emissions are said to be only CO2 and water. You can choose to use them for heating, or cooking. Not really both as the design is slightly different. The rocket mass heater achieves its efficiency using a tall chimney, and hotter temperatures so that all the gases are combusted as well. All the gas is then passed horizontally thru a large thermal mass which then continues to radiate heat many hours after the fire has extinguished. So, similar to a Kang except the Kang takes the exhausts directly from the furnace rather after a secondary combustion chamber. Not sure if one built one here whether one could get the rocket mass heater permitted.

There's a video from the local permaculture guys showing how to built a rocket heater ( for cooking ) using 5 cinder blocks. Probably not suitable as cinder blocks would likely crumble eventually under the heat. And there's a video on youtube showing how to build one using 16 (I think) chimney bricks.

I removed my chimney in my renovations, and now wonder if I should have left it there, and converted the fireplace to a rocket heater!

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Because pellet fires are auto feeding and the pellets are dry. A whole lot easier to regulate than a fire where you can get the fuel off the side of the road if you like.

That adding a log at certain intervals is a strange one. Whats the reasoning behind this? Or is this just because of the low emission fires that cant be fully dampened so you have to add logs to keep them going?

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The whole approval thing was a bit of a farce from the beginning. A whole lot were approved after testing without a wetback then people installed them with a wetback (which takes a lot of heat from the combustion chamber) and the emissions were a lot higher. There have since been a few invented which cannot pass approval, even though their emissions are far better than the approved ones, because their loading methods are different to those required by the test. They need to change the test but they are too busy supporting the land developers that want to get rich from creating thousands more sections than the City needs. Christchurch is the only city I know of where the expensive land is by the airport and the cheap stuff is by the sea.

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Guess that makes sense .. if you live near the airport, you have the best chance of getting out!

Having a wetback around the combustion chamber seems to be a poor design because as you say, that lowers the energy available for combustion. The rocket mass heater mentioned above insulates both the first combustion chamber, and the insulates the chimney where the combustion of the gases occurs. The heat is extracted after the combustion has been completed by directing the exhausts horizontally through the thermal mass before venting to the outside with an exhast temperature of about 50 deg C.

There are a few videos on youtube showing people using these to heat their glasshouses where they have their aquaponics setups. If it were legal, you could even direct some of the CO2 from the combustion back into the greenhouse as a form of CO2 injection. ( Greenhouses get low in Co2 due to lack of air circulation )

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there are cheap sources of fire wood which i think makes a wood fire a very good option. I buy 1 ton bundles of off cuts from a local saw mill for $20 each and cut it up on a tractor driven saw. It only takes around 45 mins to cut and load 60% of my 8x4x4 trailer so is extremely time and cost effective. I have also lately got a few unwanted forklift pallets which are easy to cut up but burn really fast as they are generally low density thin wood but still a cheap source of fuel. If i ever replace a fireplace i would do what i could to get a wetback in, it works really well in practice.

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My aunt, in Hokitika, has a woodburner with wetback. She says her power bills average $65 per reading :o

that's a bit more then my parents winter bills. Ironically their summer ones are higher, due to have the HWC turned on.

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