boiler burping particulates into the room

[myusername]

Our windhager pellet boiler was professionally installed late summer. Since then, we've had several incidents where the boiler throws a bunch of particulates into the room. I think this happens during ignition and that the particulates are blasting into the room through the draught damper. It's throwing enough material to make the space smoky/cloudy and set off the smoke alarm across the room (our boiler is in finished space). This burp of particulates is brief and, if I'm at home and notice right away, I can open window and, with a fan, clear it out in a few minutes.
Aside from this, the boiler seems to operate fine. We have good draft up our chimney and I can't figure out the pattern for when/why this happens. Before setting up the boiler, this flue liner was clean and appeared to be brand new. The first time it happened, the boiler had only been in service a few days so it shouldn't be related to something needing a cleaning. I happened to do a thorough cleaning just a week or so ago and we had another burp today.

I believe the particulates are fly ash since there is a fresh little pile right in front of the draught damper.

Has anyone experienced something like this and have ideas for how to prevent this from happening?

 

[chew72]

This is only a guess but upon first read I'm wondering if your house is under a slight negative pressure. Do you have combustion air brought into the same space ? I'm thinking when your boiler starts up the chimney is not heated it won't be enough to overcome the negative pressure of the building envelope. So any smoke or combustible gases on Startup get drawn into the intake, when they ignite that's your little fump dust. Before it starts you can try blowing out a match near the intake and see if the smoke is drawn into the boiler I'm thinking there will be a draft coming out of the intake.

If that is the case and you have an air exchanger just try to balance it to be more neutral. If you don't have an air exchanger and only exhaust fans just try cracking a window before lights next time and see if that solves a problem.

 

[myusername]

I don't think we're under negative pressure and when we've tested similar to your suggestion, the draft is good. However, this only happens intermittently (a few times over 6 months) so it's hard to be sure there isn't something odd going at that precise moment of ignition to create a negative pressure situation. We do not have combustion air as the installer told us that's not something done on these units. I am looking at adding an ERV (an expensive proposition) but that it supposed to be balanced so it won't change the pressure situation.
I appreciate the help.

 

[salecker]

My vote is the lack of combustion air and possible lack of a barometric damper

 

[chew72]

If it's intermittent and you don't have an EVR it could be a bathroom fan running that puts the house in negative pressure or a range hood that's vented to the outdoors. Typically with combustion air supply that helps keep the pressure balanced in the space the unit is in. And you're using unconditioned air for combustion or at least delivering it near the unit.

I'm not very familiar with that unit. Is it auto lighting when it happens? If it usually happens during mild weather it might just be shutting down for an extended period and then smokes excessively so when it lights it's more of a detonation from the combustible gases.

 

[myusername]

It's certainly possible that the wrong combination of things is resulting in negative pressure - probably a combination of internal (various vent fans) and external (e.g., just the right wind conditions across the chimney). The combustion air is conditioned as the boiler is in finished space. We're in NH and it most recently occurred Sunday - the boiler was not off for an extended time. Yes, everything on this model is auto aside from emptying the ash bin and cleaning the unit every few months. And, this happened the first time when the boiler was just a few days old, when it had a few months of wear and fly ash as well as a week after a cleaning.
The unit has a barometric damper and that is where the particulates are blowing into our house from when this happens.

 

[brenndatomu]

The unit has a barometric damper and that is where the particulates are blowing into our house from when this happens.

Is it installed where its supposed to be? There are specific places where they are, and are not allowed to be placed in the pipe.
What brand is it?

 

[myusername]

It was installed by a competent and qualified professional (should he choose to identify himself on the board, I'll let him do so but not out him myself). The damper is windhager branded.

 

[brenndatomu]

It was installed by a competent and qualified professional (should he choose to identify himself on the board, I'll let him do so but not out him myself). The damper is windhager branded.

Well, not trying to stir any trouble, just asked a question.
Not familiar with that brand...couldn't find anything online about it either.
Here is the install guide for the Field brand baro...

 

[myusername]

I appreciate the help, didn't mean to come off any differently. Even professionals might miss some unusual circumstances. The damper is installed similarly to what is shown on the diagram you posted. This seems like a strange one so I'm hoping the collective wisdom of this forum will give me and the installer ideas that could help with this. For a variety of reasons, I'm hoping we can add a combustion air kit but that's not common on this model so it's being investigated.

 

[brenndatomu]

However, this only happens intermittently (a few times over 6 months) so it's hard to be sure there isn't something odd going at that precise moment of ignition to create a negative pressure situation

This would be my guess...there are a ton of things that can affect the pressure in the home...poor air sealing of the upper levels, heat recovery units, bath vent fan, kitchen vent, clothes dryer, radon fan, central vac system, water heater, another furnace, wind speed and direction, etc etc etc

 

[chew72]

My point with the combustion air was that it is unconditioned air. If your going to burning it and send it up a chimney might as well not heat and humidify it etc.

How tight is the house? With a strong gust of wind you will get a large negative pressure on the leeward side of the house. This will suck air out of a leaky house or from bathroom fans if the outlet is on that side. All that air has to be replaced and I've seen a chimmy reverse direction for 2-3 seconds leaving bit of a mess and a lot of smoke. It's quick and strong so I could see it picking up fly ash.

If it happens with no strong winds, I'm back to thinking a build up of combustible gases. Then.. Kaboom it lights.

 

[myusername]

Our house is reasonably tight but not overly so. We have closed cell spray foam in the attic and our blower door test came in around 2300cfm/50.

 

[Mr._Graybeard]

I've had backdrafts in the past from slow pellet ignition, always because the vent and combustion fan were overdue for a cleaning. Since your boiler produced a backdraft when it was new (and clean, I assume), I would make sure the ignition system is properly adjusted. I know nothing about the Windhager, but pellet ignition is usually produced by a heating element; if the element is out of its proper place or defective, it may not be able to ignite the pellets fully before the burn chamber gets loaded with explosive gases.

 

[myusername]

interesting. Thanks. I'm not sure how to do that but will research.

 

[Mr._Graybeard]

interesting. Thanks. I'm not sure how to do that but will research.

I'd get your dealer/installer to look at it. The igniter may be defective, and you should still be under warranty for that.

 

[myusername]

Yes, thanks. My dealer/installer is involved in the diagnosis. I will add this to the things for them to consider.

 

[myusername]

Following up on this. Our installer added a combustion air intake over the summer and the problem has not recurred thus far this season. That's long enough that I believe that solved the problem.
On a related note, we would regularly get some air sucked in via our other flu for our (rarely used) gas fireplace with the associated odor. Out of curiosity, I turned on the fireplace for a few hours to warm up that flu and create a small draft. I left the pilot light on after that and the resultant heat has been enough to keep any draw/odor from coming down that flu. I'm surprised such a small amount of heat is adequate to create the draft. Most years, we don't use the gas fireplace at all so I typically leave the gas valve off.

Hoping this helps someone else searching the forums down the road.