Flue Damper Question

[Deleted User]

If the motor driven flue damper closes at about the same time that the gas valve closes, won't a certain amount of products of combustion (Carbon Monoxide) spill out of the draft hood into the room? Has anyone ever seen a study done on this?
It just makes sense that if the flue opening was blocked by the damper, the gases have no where else to go.
Just curious. The dampers are there to save some fuel dollars, but, are they really worth the risk of a possible CO problem.

Steamhead

Good question

Some dampers, like my old Flair, have a delay period before they close. Others close slowly enough that at least some of the CO will be purged before the damper closes all the way. And if the burner is set up properly, it won't make much CO in the first place.

But a fast-closing damper on a maladjusted burner.........

This is one thing to look for next time I crank up the Testo!

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Ken_8

No

There are three primary reasons.

1) the just heated stack/chimney will draw as much as possible - aiding scavenging of the last few remnaining Cu Ft of combusted air that may linger.

2) The vent damper is not a sealed closure, just 97%. With no incoming combustion gases expanding, the over fire and upper boiler will be under slight vacuum and the damper's annular space will allow complete scavenging of any tiny residual byproducts.

3) The vent damper does not snap shut. Once the gas valve circuit is off, the damper slowly rotates to the closed position, allowing ample time for natural scavenging to occur.

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Unknown

Right on Ken

just the answers I would have given.

Unknown

Thinking about

the gas boilers I installed years ago , after the initial heat up when the damper would close slowly , there was always a nasty stench of boiler break in - spilling out the diverter .

Brings up 2 questions - after the break in period , and the nasty smell is burnt out , doesn't some gas still spill out ? Just by looking at the damper it has a small hole in the middle and the ends don't make a tight seal , but still we have that nasty smell after the initial lightoff .

And 2nd , with the nasty smelling fumes spilling out after the 1st burn , how come the spill switch never turned the boiler off ? How much spillage does it take to put that thingy into reset ?

Unknown

Ron

Those nasty smelling odors you are referring to are nothing other than a cornstarch binder that was applied to the burner compartment insulation board to provide protection prior to installation of the boiler. This issue was actually addressed in the I&O manual and we advised firing the boiler for an hour or so with the basement windows and doors open to disipate the odor before leaving the job. We have since changed to a different type of board that no longer needs the protective binder, thus no more odor. It was not anything that was harmful to anyone....just a bit annoying though in some tighter jobs such as the ones that you get blessed with every day! Hope this helps.

Glenn

Unknown

Burn off after installation is

typically a one time deal. After the initial burn off it should not smell gas or anything else for that matter.

The hole in the damper is to be left open if you have a standing pilot, otherwise if spark ignition it should be plugged.

The spill switch works on temperature not odor or gas detection. If there is a continual spillage it will be enough temperature to cause the switch to open.

Another reason why you do not have to worry about dampers causing CO is that most equipment today uses dual seated gas valves and at least one or more of the internal valves is a solenoid which shuts of more quickly than a diaphragm valve.

Unknown

Glenn

Thanks for the info . The companies we installed the gas boilers for years ago did not use Burnham though . I guess it's a common situation with a few manufacturers .

Unknown

Is there a typical

temperature setting in these spill switches ? I wonder if they could work for oil boilers , maybe place one near the draft damper and possibly avoid a puffback ? Actually , I remember seeing a product a few years ago that screws into the sight hole on a boiler and shuts the burner down when it senses positive pressure . Anyone know the name of this ?

Thank you Timmie .

Unknown

Temperature settings vary with manufacturer

I am not sure what those temps are maybe if Jim Davis is hanging around he knows.

Darin Cook_3

Vent Dampers

Didn't the vent dampers come out so the manufacturers could get (something like) 1% or 2% more on the AFUE rating? So the boilers could meet the minimum AFUE rating required to still sell them.





Darin

jackchips_2

Excellent thread, gentlemen.

Thanks for the lesson.

Unknown

Darin the vent damper

intitally was a retrofit device that service techs installed on existing systems with the supposed purpose of saving some flue loss on shut down. They were installed on all kinds of equipment. We had also in addition to electrically operated dampers, Thermal Dampers using bi-metal blades that opened when heated and closed as they cooled. We also had mechanical dampers not many of those caught on. The testing that AGA did back then showed very little energy savings using Vent Dampers as compared with other type energy conserving actions the best being added insulation. Vent dampers gave about 2 to 3% savings on boilers and almost none on furnaces.

The use of vent dampers today is to allow the contiuance of 24 volt standing pilot systems. A lot of the old timers will not install anything but a sit on the floor cast iron system with a pilot. With that they wish it did not have a vent damper as they are not sure how to properly troubleshoot them. I find many jammed open with a jumper across the harness. The vent damper with the pilot gets the efficiency up so it will pass minimum government standards.

If DOE gets new standards in place the days of the thermocouple 24 volt system are gone.

Steamhead

Did anyone ever test

a boiler with and without a damper, on an old chimney that was powerful enough to operate a coal-fired boiler? My experience with these is that they can cool a gas boiler down rather quickly on the off-cycle, and a damper would be much more effective on one of these installations.



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Unknown

The old Ameri-Therm

thermal damper worked great on the old steam jobs with Powerpile systems. The Ameri-Therm opened the fastest and closed the fastest of all the thermal dampers I ever tested.

They have been banned in many locations and are no longer produced. Biggest problem was improper location of the damper. My location made people nervous but it worked great.

I put them right at the breach of the boiler. It never caused a problem with combustion test or CO of all of them I installed.

I still have one on my forced warm air furnace and my water heater. I check them often and they are still working after over 20 years.

I have two spares just in case.

Mark Hunt

I haven't


but you just gave me a homework assignment. (On the heels of a long weekend to boot! Steamhead, you are a wicked task master!)

I'll start noting draft with a vent damper closed as opposed to open with no main burner. Never bothered before.

FHA furnaces could never have benefitted much from the flue dampers, that's quite understandable. Low mass and delayed "fan off" cycle would scrub it pretty much clean of btu's. A large block of cast iron would benefit, keeping the excess btu's in.

Time for a "heated" discussion.

Wouldn't my last statement be a good arguement for low mass boilers?

Let the discussions begin!!!!



Mark H

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Darin Cook_3

I wonder how much standing pilot equipment

still goes in? I cannot remember the last one I have done. Everything we do is HSI or Electronic ignition. I think you are right though, the days of the standing pilot are very quickly coming to an end.






Darin

Unknown

Darin we here

in New England do not give in quickly to modernization. There are still a lot of standing pilot boilers going in around here.

Dale

bimetal dampers and spin valves

yes, there are still alot of bimetal dampers out there, we sold the ameritherm and it worked. Main reason people liked it was to prevent room air from going up the flue. One place had a water heater in an add on laundry room on the back of the house 3 sides to the weather and always cold until the vent damper was installed. We have a policy that if the valve(recalled on LP) is a honeywell V8139 or other spin valve model(made between 1966 and 1972 I think) either the bymetal damper or valve must go. We find alot of the spin valves staying open after the thermocouple is disconnected. I always tell a tech on any standing pilot service call to loosen the thermocouple to see if the main valve closes.

Unknown

Dale those valves

should have had a TOTAL recall LP & Nat Gas. I tell everyone to just automatically change them when they see them. I have had all kinds of problems with them.

Sometime when I have some time I will relate a story about the engineering on those valves and how it also related to the loss of the Submarine Thresher.

Matt Undy

Small CO quantity

Even if the vent damper sealed all the way and closed completely as ssoon as the gas valve closed and the burner was adjusted such that most of the carbon in the gas became CO the quantity produced in the final second or so of burning and that contained in the HX wouldn't be enough to cause a dangerous concentraion of CO unless it was some sort of large industrial boiler in a small room. Its just so little that it will dilute with the room air to a safe concentration. More of a concern would be potential condensation of the water and subsequent corrosion (such as is commmon on draft hoods where bi-metal flue dampers are used).

The somewhat quick closing is desirable to keep air form drafting through the HX and pulling heat in the HX up the chimney. If the final hot gas of the combustion could be held in, that would be better but there is a condensation issue.

Matt

Dale

You are correct

Yes they should all be changed, however Honeywell in it's wisdom did not recall the nat gas models and sadly some customers believe the old if it ain't broke don't fix it idea. When the tech unscrews the thermocouple and the main burner keeps running we tell them replace or we shut off the gas. There was a terrible accident with these valves on LP near here and we are serious about checking them. Actually it was my utility that first noticed the problem with these valves and notified the comsumer products safety commission.

Matt Undy

My mom had one causeing drafting problems on start up. The comustion products on the cold furnace would stipill out and condense on the draft hood for the first few seconds while it opened. This rusted the cabinet under the draft hood. It is now replaced with a straight piece of pipe.

There is nother function of a damper besides just keepign the heat in at the end of a cycle. It also keeps the chimney from drafting as much room air between cycles.

Matt