Active vs. Passive Dampers

What are they?

Dampers are air controls in the kiln chimney. By controlling the air flow through the chimney, this affects the air flow through the kiln. Increasing the air flow through the kiln gives more oxygen to be involved in the burning of the fuel, whereas restricting the air flow may result in insufficient oxygen, and so reduction.

Active Damper

This is a plate (often a kiln shelf) the width of the chimney that is slid into the chimney to control the air flow, in the same way that a valve may be used in a piping network.

Following Bernouilli's principle, air accelerates through the gap as the damper is closed, and then decelerates on the other side. The more the damper is closed, the less air can get through, and so the less air enters the kiln.

Passive Dampers

These are bricks that can be taken out of the chimney, either above or below the active damper (if present). With the accelerated motion of air in the chimney, it's pressure is below atmospheric pressure, so air is sucked in through the damper holes. This may cause the following effects:

  • Introducing air, so the greater total amount of air requires an increase in velocity and/or increase in density
  • Introducing more oxygen, which can burn off carbon and carbon monoxide reducing pollution and releasing heat
  • Introducing cold air, which will cool the exhaust air from the kiln, increasing its density
  • Possibly some energy loss from the mixing of the air flows, which could slow the air and increase back pressure

What's the Difference?

As far as I could see, both types of damper act as a restriction to the air flow through the chimney, so they increase the back pressure and reduce air flow through the kiln. But many wood firers claim that they act differently, and prefer one over the other.

To try and understand this, I asked which type people use and why in the Wood Fired Kilns and Pottery group on Facebook, which generated some lively responses that I have summarised here (excluding those that veer off topic)

Claims for Passive Dampers

  • Results more heat to back of kiln
  • A passive damper is more effective than an active damper in regards to drawing less air into the kiln (i.e. more reduction).
  • A passive damper allows additional oxygen to enter the chimney thus aiding in completing combustion of excess carbon (produced by reduction) causing a reduction of smoke exiting the chimney
  • It interacts with gusts of wind or steady breeze, whatever you have, and of course the active damper can’t.
  • The passives change the speed at which the heat is passing through the kiln so if the air is going slower then the flame tends to rise to the top of the kiln more go around corners and take it's time getting out. If the bottom is cold then you speed up the flame so more passes through bottom of the kiln.
  • A passive damper shortens the flame length. The tip of the flame is the hottest so it can help heat up the back end of the kiln. You can watch the flame get shorter when you pull a brick.
  • A passive damper is like poking a hole in a vacuum hose where an active damper is like clogging a vacuum hose. Poking a hole creates another air inlet that reduces the vacuum effect.
  • A passive keeps the majority of flame in the kiln
  • The passive is like poking a hole in a straw when you are sucking on it. The more holes in the chimney; the less draw ; the kiln effectively draws less oxygen into the firebox and slows combustion as well as flame path thus changing flame pattern on the work and sometimes evening out temperature in a cross-draft kiln.

Claims for Active Dampers

  • Closing the active damper on a long kiln is counter productive to heating the back of the kiln
  • Active damper chokes down the air flow by closing the exiting gases, while passive doesn't choke the air, just slows the air down at the primary air ports ...encouraging the fire to burn a longer flame inside the ware chamber seeking out oxygen.
  • The active dampers restrict the amount of air coming into the kiln and therefore the amount of reduction that you get.
  • The active damper can create turbulence in the flame path and keep the unburned particulate in the kiln longer creating a fast build up of excess carbon that results in reduction. I often use active dampers if I feel like too much heat is leaving the chamber too quickly and I am struggling to gain temp or reduction.

Ways of Using them

  • I like to set the active damper a little stronger (more open), then adjust the passive damper to fine-tune.
  • To maximize control, it’s good to mud every crack on the entire kiln surface several times during the firing, because they’re all mini-passive-dampers of course, and they expand as it heats. Just the thinnest coat of mud possible that still seals the cracks, so that mud only cracks in one place when it heats. And the next crack is smaller.
  • If you have a lot of wind, the kiln will reduce a lot on its own so close the passive dampers
  • I use passives to fine tune the heat rise between to top and the bottom of the kiln.... If the bottom of the kiln is getting a little hotter than the top then adjusting the passives will control the speed at which the heat passes through the kiln directing it more to the top or visa versa as needed.
  • Another advantage of having passive as well as active dampers is that the passives are a great way to watch the flame in the chimney and check it for evenness across both side.
  • I like a combination, but as much passive damper as possible. I found that a little passive damper can eliminate smoke out the chimney by allowing combustion inside the chimney.
  • Active usually but passive to fine tune. Passives reduce the velocity of the air leaving the kiln.
  • In my kiln closing the active damper does indeed reduce the draft. So does opening the passive damper. Both seem to be able to cause the same effect in my kiln. IMO, so far nobody has offered any convincing rationale for why either one would cause different behavior in the kiln chamber(s). Why would it matter HOW you slow the draft?
  • Not passive in my opinion. Your chimney is your kilns’ drive controller. The controller would react much better when its got hotter. Hotter is better for this process. Cooling your chimney with cold air to push back and create reduce atmosphere is dynamic affect however you can do that with different ways. Looks like he creates a hot chimney so the air expands and sucks more air into the kiln

End of Firing

Close and seal active damper and any passive dampers below it, to stop any airflow through the kiln.
Open any passive dampers above the active damper to minimise any draw from the chimney, and introduce cold air to cool it down.

Next Steps

Lots of opinions from Facebook, some conflicting, and no clear explanations.
So I'll devise some tests for the April firing and report back.