Thread: Advice required on wood heater repair

Hi all,

A warm one in Melbourne today mostly kept me out of the shed so I turned my eye to some domestic duties..... sort of;

I figured it was a good day to give the wood heater a good clean and yearly inspection. I knew some of the heat bricks were looking a little worse for wear and also the heat baffle that sits above. These are reasonably easily replaced.

One thing that will also need replacing it what is called the air tubes. These are two tubes with holes along their length. They introduce tertiary air into the firebox to help complete the combustion process to help reduce particulate pollution and increase the efficiency of the heater (stated to be 85%)

Anyway, as can be seen from the pics, one is particularly badly rusted and the other not far behind. I'm keen to replace these since it may affect the effeciency of the heater. I'm sure I could just purchase a new manifold but since it's a bit of tube tack welded to a plate I would feel ashamed if I didn't put to use some of my equipment in the shed to make a replacement myself.

These things glow cherry red when the fire is up to temp so they cycle through quite a wide range of temps on a daily basis in the cooler months.

Question is this;

Should I make them out of stainless, either 304 or 316? The extra cost would be bugger all. I'm thinking they may last a bit longer than the 5 years that these did.

They are only held and fixed at one end. The other end just rests on a plate so thermal expansion and/or buckling will pose no issue.

Thoughts anyone?

Simon

First pic is sideways. Sorry.

Simon

Happy for anyone with better metallurgy skills than me to correct me, but my first instinct would be stainless of whatever grade should give better life span.

WRT the first picture, I thought all flues ran horizontal and tape measures hung in mid air on vertical surfaces.

Hi Simon,
I am not sure if I would use either.

Many years back I worked in a shop where they were doing some sort of industrial heater. Tubes inside it were made of stainless steel,but I was told it was a special grade to better resist the carburisation inherent in the combustion / heating process.. It was some stainless grade that was specially obtained.

I am thinking it may have been this stuff.

https://www.australwright.com.au/sta...eel-grade-310/

Much better suited for what you want it to do.

Grahame

Hi Simon,

You might find that using the high temperature stainless steel is far more costly than you might want to pay for ! In addition you might also discover that they cannot be welded without specialist filler rods.

My two P worth would be to simply replace the tubes with whatever mild steel tube that you can obtain cheaply and not have any issues welding them. Not to mention that most varieties of stainless steel will work harden if you look at them wrongly ! So that would be something to take into account.

Thanks guys.

Baron, the only machining involved is cutting the tube to length and then drilling the holes.

I'm tempted to give 316 a go. If for anything just out of interest to see how it performs.

I would use just standard 316 electrodes.


Grahame, that material looks way too expensive for a humble wood heater.

I was thinking that even 316 would be fancy!

The standard tubes are just ms. Nothing fancy.

Simon

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Hello from BC.
5 years is not bad life for air tubes in a combustion chamber. I would just replace them with low carbon steel tube. If your hellbent in using SS tubes the correct electrode for SS to MS is 309L. 316L tube running at that temperature is going to loose a lot of chrome to oxygen depletion. Watch the length as SS will expand much more than carbon steel.
BC

Thanks everyone for their input. I couldn't help myself, ended up making the part out of 316. They were 3/4" OD and 485mm long so the local steel place charged me $11 for the tube. If they evaporate after a single winter then I'll re-think the SS idea!

There were 3 bolts holding that plate to the outside of the heater, through bolted and screwed through the 6mm plate. Of course they rusted and became impossible to unscrew without shearing them off. I drilled them out and replaced them with SS bolts as well.

Thermal expansion is not an issue in this application. The design at least has allowed for such as the tubes are merely resting at the other end about 1/2" short of the heater wall. They are free to slide in and out as they heat up and cool.

Simon

Originally Posted by steamloco1954

316L tube running at that temperature is going to loose a lot of chrome to oxygen depletion.

I'm interested in this. What will happen to the stainless in an oxygen deficient environment?

Simon

Originally Posted by simonl

I'm interested in this. What will happen to the stainless in an oxygen deficient environment?

Simon

If these are air distribution tubes I would have thought that air would have oxygen. P & W JT8-D engines had tertiary air holes drilled in the combustion chambers in the late 60's to get rid of the exhaust trail. This improved fuel efficiency by around 3%.

Hi Simon,

It goes rusty !

Could you put a coating on them to extend their life?

Thanks Baron, I should have guessed that!

Harry, a coating would be impractical. Given they can glow cherry red with a hot fire, they can reach temps of around 600 - 650 degrees C, maybe hotter. Would have to be a very special coating to withstand that.

Also, bare in mind that replacements tubes can be bought for maybe around $150. It's not a massive engineering feat to replace them.

I'm actually keen to see how the SS tubes perform.

Hopefully I won't have to drill out these bolts if/when I need to replace them next time. I was going to use an antisieze paste on the bolts but not sure if it would last.

Simon

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G'day Simon, I'm late here but have been doing a similar job on our wood fired central heating furnace myself. I think that I would have used heavy walled pipe such as steam pipe or scaffold tube myself, but as you have said, the material cost for the 316 wasn't really a factor you needed to account for so it is worth trying just to see how it goes. It certainly looks nice ATM, and time will inform you of its success or otherwise. Good luck,
Rob

Hi Rob,

The thought of a heavier type steam pipe crossed my mind but I'm constrained by the OD having to be 3/4" in order to fit in it's designed spot. Heavier would mean a smaller ID (assuming I could get it) and I'm not sure I fc that would have affected the air flow dynamics or characteristics.

Not sure how critical the ID is for the design to keep it's stated efficiency.

Simon

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