Chimney damage

exqheat

I was observing the operation of a gas boiler in a 6 family building. The boiler must be 10 years old. I am not sure whether there is a lining in the flue, or if there is one it is a proper material.. I noticed the vent damper closing when the fire was turned off by thermostat. It seemed to me that the flue did not have enough time to vent flue gases after firing stopped as the damper closed right away. Would it be more practical to have a delay in the flue damper closure to allow the escape of harmful exhaust that can condense on the chimney walls and cause damage to chimney?

captainco

That's a fact, which is why I tell my students to disable them.

SWEI

Wouldn't want to lose those AFUE points, would we

delta T

@SWEI do you know of any good data that shows one way or the other if dampers are really that benificial? I am sure there is some increase in AFUE, but is it enough to even pay for the damper itself over say a 10 year life? I haven't ever really looked into it, would be curious to know.

captainco

Its all about the 10 points. I have heard studies that stated 1-2% savings at the most if at all.

Gordy

I think you gain 1 percent, maybe 2. Back in the day it was an incremental step to try to get more efficiency out of a ci boiler. Along with electronic ignition instead of a pilot.

Steamhead

A lot depends on what the boiler is hooked up to. No two chimneys are alike, and until we know under what circumstances the stack dampers were tested, that 1-2% figure means nothing.

If the chimney was built for coal firing, it was designed to pull air through a bed of coal, which required a lot of draft. Pretty much every boiler/radiator catalog I have addresses the point that a coal-fired boiler cannot work well without a good chimney. I've seen chimneys that could pull pets and small children up to the roof.

This level of draft is way too much for oil or gas firing, which is why we have barometric draft regulators, draft hoods and stack dampers. A coal-designed chimney can cool down a boiler considerably between firing cycles, which then takes extra fuel to make up on the next cycle.

If the chimney was built for oil or gas firing, say after World War 2, it probably doesn't draft as strongly. So a stack damper might be less effective in this case.

Another point is when a stack damper closes, the warmed flue gas in the chimney cannot escape as quickly since there is no cold air coming in to take its place. So the chimney stays warmer longer.

We stock replacement motors for the Field/Effikal dampers.

SWEI

The points we're alluding here are part of the AFUE scoring system, which effectively prohibits post-purge cycles longer than 30 seconds. It's absurd, but that's how the game works.

captainco

The basic function of drafthood is to prevent equipment from venting therefore flue dampers had little effect on them.

If flue gases can't escape they will cool and condense in the chimney, not a good idea.

Steamhead

captainco said:

The basic function of drafthood is to prevent equipment from venting therefore flue dampers had little effect on them.

If flue gases can't escape they will cool and condense in the chimney, not a good idea.

Doesn't always work that way though. I regularly observe air drafting through atmospheric boilers on their off cycles, if there is no damper.

exqheat

Not only will not having a damper allow the building to draft heated air up the chimney when the boiler is off, in the milder seasons the whole building can have a steady heat loss when not on. That was one of the reason to provide for immediate fresh air supply at the boiler. In most cases this is not available in older homes, short of leaving a window open in the basement. So the addition of a timer on the draft damper closure, would allow for exhaust escape from the chimney, but retard the energy loss from the ambient heated air in the building. Going forward, how difficult would adding a timer to the damper closure cycle be? surely in this digital world, it could be rather simple to add to dampers going forward. This of course does not help the rotting chimneys in the field.

captainco

If one would measure draft they would find there is usually no draft in the flue during the off cycle, if anything downdrafts.

The whole efficiency rating on a flue damper is based on it closing immediately. This is the same reason that induced draft equipment cannot have post-purge, 5 point deduction on the AFUE.

exqheat

Did they consider the CO risk or the replacement cost for unstable chimneys