Where to REALLY Install Barometric Dampers on Multiple Boilers (Avoid These Deadly Mistakes!)

RayWohlfarth · January 23

For years, it was common to install a barometric damper at the far end of the flue. This video explains why its a bad idea and where to properly install them on multiple boiler projects.

https://www.youtube.com/watch?v=zLsN_bo_LoQ

Mad Dog_2 · January 24

The best explanation I've heard. Awesome Ray Ray! Mad Dog

RayWohlfarth · January 24

Thank you @Mad Dog_2

captainco · January 26

Mother Nature controls draft in a flue based on pressure difference indoors versus outdoors and by temperature difference of flue gas and outdoor temperature. This is a constant change and then we can throw in wind and rain.

The amount of air that is pulled into the equipment is affected by this change and affects the combustion. This would affect safety and efficiency. The purpose of a barometric is to maintain a fixed flow or draft through the equipment. In the case of residential gas equipment, the draft should be -.02"w.c. Commercial gas equipment will on occasion require up to =.05" w.c.

Prior to the development of gas equipment, all barometrics were single acting for oil, wood and coal. When gas arrived a problem developed because of standing pilots. With wind a downdraft conditions pilots would blow out. In the beginning there were no safety pilots so when the pilot went out gas would continue to flow. Not a good thing. I have searched for some time to determine which came first, the double acting barometric or the drafthood with no luck. But it didn't take long to realize a fixed device (drafthood) cannot control variable conditions whereas a double acting barometric can.

What about safety? Does allowing equipment to continue to operate when the flue is blocked or there are severe downdrafts a smart thing? Lets add some spill switches for safety. If that is the case why do 90+% of drafthood equipment not have spill switches. No those are not spill switches on new equipment,, they are blocked flue switches, spillage still allowed.

The picture in the Field Controls literature with 3 boilers and barometrics mounted in positions A, B & C is a mechanical nightmare. I have tried for 40 years to get rid of the picture. Understand that no one from Field Controls has ever been in the field testing that configuration. In that picture C is the worst location but not because of the location but because of the design. You cannot have smaller flue connection down stream.

I always wondered why oil equipment has single acting barometrics? Don't we want an oil burner to stop if the flue is blocked? I can see with wood or coal a single acting barometric may help snuff the flames with a clogged flue but not oil.

I did do a test with Field Controls and AGA in 1993 to prove how dangerous drafthood equipment operated. Anyone that has been in the NCI Combustion class and still using drafthoods are careless, negligent and unprofessional.

RayWohlfarth · January 26

@captainco I was wondering why they allowed double acting barometric dampers on gas appliances. It never made sense to me. Than ks for schooling me

JohnNY · January 29

Ray, thank you for this informative video. The value of your continued dedication and contribution to our industry are beyond measure.

JohnNY · January 29

@captainco Jim, thank you for that post. Great info following Ray's excellent video. I appreciate you both.

Intplm. · January 29

@RayWohlfarth Excellent video. This goes far beyond the manufacturers instructions too. Very well explained.

captainco · January 29

article - History of draft diverters.docx

Here is an article I wrote in 2020 which defines drafthoods and their creation versus barometrics.

RayWohlfarth · January 30

Thank you @JohnNY and @Intplm. Much appreciated

Bob Harper · January 31

Air entrained in through a bullhead baro. flows laminar and thus fails to mix well with the stack gases. This negates the purpose of the baro.- we want it to mix so it can cool the stack gases, which in turn reduces the draft pressure. The laminar air is flowing laminarly at higher velocity past the tees from each boiler. This tends to obstruct the entrance of stack gases as the dilution air becomes a bully blocking stack gas entrainment. The higher velocity of the dilution air from the baro. will also entrain more air through each boiler due to the Bernoulii Effect. Ray did a nice job discussing the bad things that can cause.

If there is a flue blockage, each boiler becomes the relief port- not the baro. Remember a single acting baro. i s designed to flow into the stack but has stops to prevent it from becoming a x2. You literally have to remove the stop to make it a double acting.

I'm glad Jim made the distinction btw "spillage" vs. "blocked flue". The ANSI test std. allows for 10 continuous minutes of "spillage" (read- allowable) before it becomes "backdrafting" (a legal liability). So called "spill switches and their mountings are designed around this 10-minute std. If you replace a switch with one of another temp. rating or mounting, you change the response time. Even the angle a switch is mounted to the throat of a baro. or hood can have significant impact on its response time. Most switches I've encountered on residential equipment use 250F switches. That does not tell you the response time. Also, these switches go bad. Where a switch sees frequent spillage, the plastic or Bakelite casing degrades until it fails. Likewise, the bimetallic element can work harden and change its response. I've seen solderless connectors cooked to where they fell off the terminals.

The NCI class will discuss the pros and cons of switch placement and the follow of switches on draft hoods.

My preferred setup for multiple boilers is a baro. about 18" above the flue gas collar or roughly 2-3 duct diameters minimum from the collar. The connector should enter the common vent/ chimney connector from an angle. Most people improperly refer to this as a 'wye'- it is technically a 'Siamese'. Ask any firefighter. A wye forks- a Siamese merges together. This angled entry allows for a balance of converging gas streams with some mixing but not blockage nor excessive venturi effect on the boiler.

If you see soot on the throat of a baro. stop and fix the venting failure. I see most techs ignore this sign just as they ignore melted grommets and scorching on the top of CAT I water heaters. These are red flags that cannot be ignored. Clean these areas once photographed so you can better determine if there is re-occurrence vs. problem corrected. Ideally, combustion analysis the proof.

They ought to put little stickers on the inlet throats of baros. stating "One Way" pointing inwards so the air knows which way to go.

RayWohlfarth · January 31

@Bob Harper Thanks for the lesson Loved the sticker comment LOL

Where to REALLY Install Barometric Dampers on Multiple Boilers (Avoid These Deadly Mistakes!)

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