Bad Cold Weather "Flue" Season! : (
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Ed,
The blower fan creates the higher pressure required inside the chimney. That pressure is greater than pressure outside the building and, therefore, no air will enter the chimney while the blower is operating, provided it produces sufficient pressure to be above atmospheric.
The problem is that it will likely maintain an undesirable positive pressure IN the chimney at all times if it is to work acceptably well. Furthermore, we don't know what the positive pressure requirement is for this chimney under all operating conditions. It would be possible for the fan to create so much positive pressure that combustion is negatively affected. The boiler is not designed to have any real positive pressure above it other than the very slight amount that the heated exhaust gas can create.
It's a risky proposition on both ends of the spectrum.
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so, just for laughs …….. if I go there and shut off every power vented fan including dryers, in the entire building ……… the very strong downdraft should cease immediately? Right?
…. then I could turn on just the large common hall fans, then likewise just the bathroom fans ….. to find the offending fan group (maybe both!)
ERROR: I didn't yet make a valid test with just the dryers running (I didn't know about the bathroom and hall fans at the time)
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@LRCCBJ, I posted that diagram of the higher outside pressure and the lower inside pressure to illustrate that the "FREE" blower inserted into the clean-out of the chimney is NOT a good idea. the path of least resistance for the air flow is from the blower discharge, up the chimney, out the flue connector (in the wrong direction), then out the draft hood, thru the boiler room then to the inlet of the blower fan. I believe that the only correct fix is the Chimney Top power vent. The lower cost fix using a Draft Inducer will solve the problem by creating a proper flow direction, but it will put the chimney tiles in a positive pressure condition.
This will work but is not the best option.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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That is an exercise in futility. there is no single offending exhaust fan. It is a combination of several of the many fans. Think of it this way. you have 100 exhaust fans. every one can move 10 CFM out of the building. Turn on only one of them and you do not have a problem. now turn on #17 and you still don't have a problem. then randomly select another and you still don't have a problem, but eventually you will get enough of the 100 fans @ 10 CFM per fan to add up to the amount of negative pressure inside the building that will cause the Chimney to become an air inlet to balance the negative pressure inside. So selecting one group will not solve the problem and you will no longer have the exhaust that the group fans were supposed to solve, be it bathroom odor and humidity, or clothes drying or what ever other exhaust fans are installed.
Adding the proper amount of make up air for the total exhaust should have been included in the plans that the original engineer designed. Any additional exhaust devices that were added after the building was finished needed to have an engineer's approval in order to prevent the problem you are having now. That was clearly not done.
Now you need to put your own "bandaid" on to solve Your problem. …and I'm sure that you will not be paying an engineer to approve your fix. You are just going to do it and get paid for the successful fix.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Before you lose someone, have a pro perform a level II inspection complete with internal video scanning. The chimney should have a stainless steel liner sized for both appliances. Install barometric dampers on the vent connectors with spill switches. Then, fix the building- close the holes in the ceiling and provide makeup air if needed.
If you decide you need to power vent, it must be up top putting the entire venting system under negative pressure with a pressure switch and interlock it to a dedicated control for both boilers. Intervex/ Exhausto have the fans and controls. It is against code to pressurize a CAT I chimney.2 -
Hi Bob! …. thanks!
I decided to remove the very old atmospheric boilers and install a group of tankless condensing water heaters enjoining the existing 120 gallon storage tank.
I can drop all the 2" PVC exhaust and air intake pipes straight down the chimney into this utility room.
With over 54 exhaust fans (roof top grouped bathroom fans, commercial dryers, large common area ventilation fans, etc.) putting the entire building under a large varying negative pressure, I now feel this is not a safe environment for an atmospheric vented heating units.
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What happened to all the make up air for all the exhaust fans? IDK how old your hotel is, but all the ones we've built in the last 5-10 years have fresh air going into the rooms & hallways, either from a MUA (with heat recovery) or by opening the outside air intakes in the PTACs/VTACs.
The building sounds seriously out of kilter. Even if you solve the backdrafting issue by sealed combustion, the underlying issue still remains.
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The building is at least 60 years old.
I'm sure the roof top grouped bathroom fans were added to the existenting common vent stacks about 10 years ago.
The hallway fans were installed around 5 years ago.
I'm gonna look for any air intake grills this week to get a better understanding of what's going on there.
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Hello Rick,
You should be using Dwyer Mark II manometers to measure your Hg. accurately. You can use the Dwyer Mark II Manometer to measure the positive pressure as well as it had a dual gauge using it with the correctly sized pitot tube.
Atmosperic boilers should never be in the same room with clothes dryers unless they are walled off with an fire resistant access door which is how my fathers laundromat was designed.
With both residential and commercial dryers the issue is always the pressure gradient being water gauge pressure; Hg. and the ambient air temperature.
Cold air is always more dense because of those pesky molecules.
My father commercial laundromat was designed with the covered dryer exhaust vents with the heated moist air always exiting from the bottom of the wall mounted cubes which were above the equally sized air inlets that were covered with the same type of cube that had an open screened inlet bottom.
The pressure gradient/ water gauge Hg./ negative pressure/combustion air draw is always equivalent to however many dryers are running.
The original 1972 twin WEBEN atmospheric boilers were gas fired with copper coils at the top of their fire boxes and they had twin air inlet grates in the exterior wall and the exhaust flue was through the roof.
All you need to do is install air inlet grates through the walls if possible with the equivalent square area to allow combustion air to enter at zero Hg/one atmosphere of pressure determined with the Dwyer Mark 2 Manometer and the correctly sized pitot tube.
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Hi Leonz!
Not as bad nowadays, but not that long ago, when I would get a service call there, if I didn't have to step over six cops pinning some guest down on the floor in the middle of the hallway …….. It would have been unusual! : (
I'll bet the addition of installing powered vent bathroom fans on the roof and large powered vent fans in the hallways ……. was to clear the building of the problematic "second hand smoke"! : )
In troubleshooting this downdraft issue, I did shut down all the dryers and basement outside wall fan.
The downdraft remained the same! you can stand 10 feet away from the exhaust entry port into the chimney an feel a strong air flow on your face!
I'm sure now that the negative pressure in the entire building is mainly from the many 1st and 2nd floor exhaust fans.
…….. mostly via the 4ft x 4ft laundry elevator shafts on each side of the building.
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……. as I think about this more and more:
In the other boiler room on the other side of this building were two Copper Fin hot water boilers w/ internal exhaust fan inducers.
Thinking back on past visits there, I would notice a short whiff of flue gas at start-up …… but would quickly subside.
These boilers were fan forced into a class 1 (block and clay liner) chimney which I now understand is against code to do so. (no positive pressure for that type of chimney)
Two feet next to this chimney, there is another same size chimney (long abandoned) that was used back in the days where they used to burn their trash.
I get the same strong downdraft from this chimney as well.
I'll bet ya that this abandoned chimney was sucking the rooftop exhaust from the boilers back down into the building! : (
I have since replaced these boilers with a group of tankless condensing units with all there own roof top 2" PVC exhaust and air pipes.
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……. looking back now:
Both these utility room areas are quite large.
When in these large open areas, you get absolutely no sensation if your in a strong negative pressure zone …. you feel no draft at all ….. zero!
Thru my lack of experience, when I exposed the vent port into the chimney,
the very strong downflow suggested to me this can only be the prevailing winds rushing in down the chimney!
……… experience is your friend! : )
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……. thinking:
Might be a good idea for them to replace the large hallway exhaust fans that simply vent out to the roof with commercial "smoke eater" filtration units.
…. probably pay for itself in heating and air-condition savings.
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Did you check to make sure that the kitchen make up air unit is running? We run into this all the time. Kitchen exhaust hoods are sized to exhaust a 300 CFMs per foot of exhaust hood. When the make up air units fail we normally will get a no gas at cook line equipment service due to the E3point CO system.
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Hello Rick, another clue is to this is how difficult is it to open an exterior door.
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Hi, After reading through this thread, two things come to mind. If the water heater is to remain atmospheric, it needs to be isolated from the main building pressure, and live in its own zone. That might be a "fun" job. The other thing is "stack effect". It might be playing into this as well. I think I'd be looking to convert the heater into a direct vent somehow, essentially removing it from the variable building pressure zone. If it's near an outside wall, it shouldn't be too difficult.
Yours, Larry
ps. On a cold day, having a look at everything (particularly from the outside) with an IR camera might give some useful info.
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What Larry has given you is sound advice, using dans analogy about being the ball in a hydronic hot water heating pipe water like air is lazy and the denser cold winter air will always sink and the hot air will always rise and then sink to the ground as it becomes colder.
With coal stokers and tight basements/houses, the solution is to provide a large enough PVC pipe being 3-4 inches in diameter coming from fresh air to the floor near the squirrel cage fan or turbine used to provide an unrestricted combustion air source.
If this building is suffering from a high negative pressure gradient you have to equalize the air pressure to reach 0 Hg.
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