Does anyone know about chimneys?

How cool would it be if this was an in-flight shot of the tail of the jet bike I built in the garage over winter?

Although it is notionally summer here now…notionally…and we have just had a week of beautiful sun (probably more than we saw all last “summer”), we still have bouts of quite cold weather when our drill is to put the fire on when I get up at 6 to warm the house up and again in the evening if necessary…

Our problem is that the large wood-burner in the lounge that heats the house has taken to smoking continuously and, just like with teenagers, you just can’t tell it not to. We had it refurbished at the beginning of the year, including replacing the flue, removing the damper on the flue and replacing all the seals around the doors. We did this at the same time we re-roofed the house but a few months before deciding to re-roof we asked if ‘hats’ could be placed over the tops of the upper arms of the ‘H’ cap to stop rain water washing soot and much from the inside of the cap onto the roof where it had, over time, created an ugly stain running from the base of the flue down to the edge of the roof. At the time I was surprised that the hats were so small and so close to the opening of the H-cap but figured that our installers knew what they were doing.

You can’t really see if in the picture but there is also a layer of wire netting stretched across each external opening to prevent birds either dying in the cap or building their own little home sweet homes in it – for the first four or five years that we were here, we never had a problem with birds then all of a sudden the chimneys were like avian condos…the gauge of the netting is half-three-quarter inch so is unlikely to be THE problem but may be a contributing factor to the possible lack of draw across the top of the H…

We didn’t start using the wood-burner over winter til around May because, although we had a non-summer, it wasn’t that cold, just wet. But we were surprised that after only a couple of months the wood-burner started smoking worse and worse and so we got the flue swept – twice because the first time they did go hard enough and left a thick layer of creosote scaling on the inside of the flue. The folk who swept the chimney took the hit for not doing the job properly the first time around but were also critical of our wood supply as too wet. Looking back, I think that this is a stock answer as the same wood supply also feeds out wood-burners in the laundry and the guest house, neither of which has any problems with smoking. We always has a wet and dry side of the wood shed and it is possible that they only looked at the wet side i.e. the stuff drying for next winter.

I have been doing a ton of research into this issue on the net and have read up about positive and negative drafts and pressures inside the house. I have tried the fire with windows and doors open to see if it is a draft issue and doors/windows open or closed makes no difference other than to help clear the smoke that pours out the door every time we open it to add more wood.

I keep coming back to the two things that have changed: replacing all the seals on the wood-burner and adding the hats onto the H-cap. From my research it seems that the better sealed a wood-burner is, the more efficiently it operates which brings me back each time to the hats. What I’m wondering is if they need to be lifted higher above the mouth of the H for better wind flow across the top and to offer the least amount of resistance to smoke coming through the H-cap – the caps are, after all only there to stop rain washing soot etc onto the roof…

So I’m hoping that perhaps someone in our global community might actually be a subject matter expert on this topic and be able to offer up some definitive advice that doesn’t include:

  • pre-warming the flue by stuffing newspaper up it and lighting it,
  • getting better wood (there is nothing wrong with our wood supply)
  • replacing the wood-burner or
  • just hardening the heck up…

7 thoughts on “Does anyone know about chimneys?

  1. The fix to your problem is actually quite simple. The H cap that you have put on the chimney flue is the culprit. Flues and exhaust vents need a slope in order to draft properly. I don’t know what that slope will be without seeing your chimney and familiarizing myself with the New Zealand building, fire, and chimney codes. However, there are standard rain cap/spark arrestor combinations that easily fit on your flue. My recommendation: Get rid of the H cap and go with a standard rain cap/spark arrestor. Here are some example:


  2. P.S. Another problem with that H cap is that the diameter is reduced from the flue to the H cap. Couple that with the 90° bend, and a bottleneck is created that won’t let all the smoke exhaust through the flue as rapidly as it should.


  3. Ah-ha! I think I see what’s happening. I included a link in my first comment so it went to your spam or needs approval folder. My rewrite did, too, since I included the link again. Use the first, more detailed comment and deleted the second one.


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