Backdraft

Bob Harper

Mark, before you are taken to be a "liner basher", lets review a few of your points:
-Big straw versus swizzle stick. Here you make an assertion that all liners are downsized. Incorrect. Liners should be sized according to the combined firing rates, types of appliances, stack height, and other conditions. If you apply the same pressure gradient on a large conduit versus a smaller one, you will get higher flow on the larger with a slower velocity while vice versa with the smaller diameter. Therefore, if the total CFM being moved is adequate even with the smaller liner, then the higher velocity would be a good thing. Remember, when an appliance first fires, it has to push out that heavy column of cold, dense air in the stack to get things moving north as it were. A smaller conduit has less volume to expel thus can prime quicker. Think of the stack air as a giant push pop. You match the lienr with the intended function. The appliance collar should not be the only gauge.

- Mfr's appliance collar6", how use 4" liner? In the first place, you should not be downsizing more than one size in general. How, oil appliances are made to accomodate a wide range of firing rates. The same appliance may have to handle anywhere from 0.65-1.85 gph or more. Are you going to put a 7" liner on a 0.65gph boiler when a 5" is plenty? Smaller liners heat up quicker than larger ones and rob less heat thus provide more heat to generate draft.
-corrugated liners have same vent capacity as smoothe walled flues? Who the hell ever said that? The codes clearly state to de-rate corrugated liners 20% from smooth walled plus another 20% if there are offsets in the flue.
-cold backdraft? Agreed-the type of flue liner has nothing to do with a cold backdraft. Fix the house and the chimney will work. If you have a 34MBTU water heater firing into an 8x8 flue all summer, it may never vent up the chimney. How about furnaces and boilers on setback T-stats? Long wet times... Meanwhile, when the water heater fires, it tries to dry that cold, soggy masonry flue but since it is adding moisture at the same time, it takes awhile. Maybe it never does dry the flue. Until all the moisture is evaporated, the flue surface will not rise above 212F. As long as the flue is cold, it will wick heat from the stack gases thereby reducing draft. All that thermal mass of a masonry chimney is killing the draft. Only when the outside of the chimney reaches equilibrium with the flue surface will it quit robbing heat from the stack.

There is no comparison btw metallic liners versus tile flues. All masonry chimneys should be relined-period. The heating equipment vented into those chimneys should be limted by the maximum liner that will fit down the chimney. If you can only get a 6" down, then do the arithmetic and don't overload it with a big boiler and 76K water heater.

All installations should have a combustion analysis, measure for makeup air and provide as needed, and
perform a Worst Case Depressurization test. All duct work should be sealed. The home should be surveyed for upper level bypasses and pressure leaks. The chimney should have a Level II inspection and a mesh guard on the cap.

Mark, you've made a point about the perils of downsizing. I can make the same arguement that 98% of the heater flues I see are oversized. So, if I size my liner correctly, do you still say terra cotta lined chimneys are better than metallic liners? What I don't find is metallic liners collapsed blocking the flue or full of holes leaking into the home.

We're on the same team here but have different game plans.

Joe K_2

Backdraft

The mason just installed a 7" liner for my 40' unlined interior chimney in preparation for the install of a new Peerless steam boiler (4 section EC)in my old victorian.
I noticed that there was a draft coming out of the open bottom of the slinkey where the boiler will connect. I thought it would go the other way...
Should I be worried about 'backdraft' and CO2 for the new boiler? If so, what to do?
JK

jp_2

2 drafts?

I would think you have 2 drafts, warm interior air going up and cold air falling down. cold is what you are noticing.

Bill Nye_2

When

When you add heat , boiler/burner, the draft will go the other way , Up.

Brad White_134

You might also have

had other fans running, dryers, what-have-you.

Mark Hunt

Depressurization


is definitely the issue and it will remain an issue even after the new equipment is installed. Adding heat to the vent system will not cure depressurization.

I was told today of a CO poisoning that happened to a woman after a chimney liner was installed in her home. The manufacturers specs said that the liner had enough capacity to handle the attached load. Well,............it didn't and the people in the home got dosed when their water heater vented into the house.

The lady is a reporter for the Washington Post and she is NOT happy. She and Jim Davis will be having a chat.

For some reason, many seem to think that chimney liners solve everything. Which is easier? Drinking a milk shake through a big straw or drinking it through a swizzle stick?
How does making the "draft hole" smaller make draft better?
Why would a manufacturer put a 6" vent on a unit when a 4" vent would work better?
How can a corrugated pipe that has bends in it possibly have the same venting capacity as a straight, smooth interior pipe?
If a chimney liner "back drafts" before the heating appliance is installed, how will the "heat" from the new unit get into the liner to "heat it up and cause draft"?

The steam boiler I installed a few weeks ago was vented into the SS liner we installed. The existing chimney was/is unusable. From the moment the liner got to the bottom of the chimney, air was going up. No heat added, just draft. Even the old chimney drafted after weeks of having nothing vented into it and we were not in the dog days of summer.

A "worst case depressurization" test should follow EVERY installation of a combustion appliance. Water heater manufacturers have been stating this in their instruction manuals for YEARS. It's true. Go read one. A plumber I spoke to at ISH was amazed at this when I pointed it out. She had never read the manuals. For more years than I care to think about, neither did I.

Without testing, we only guess.

Sorry for the rant Brad, wasn't aimed at you or anyone else here. It was aimed at the misconception that all things are equal and that all parts are interchangeable. If it worked here, it will work there. No questions asked.

No questions asked until the EMT's get involved that is.

Mark H




To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Unknown

Chimneys

Very well put Mark!

Mark Hunt

EH EH EH


Look again Bob.

What exactly did I say?

I said that there are people that believe that a liner solves all problems. If you read my post Bob, I mention that I installed a SS liner in one of my recent jobs. So there is no liner bashing, just "liner mentality" bashing.

A liner will not solve a depressurization issue. Period. A liner will not solve a draft issue. Period. As far as sizing goes, if you read the liner manufacturers specs, their product does with smaller dimensions what B-vent and All-Fuel can't. I didn't make this up Bob, it's in their literature.

I gave you an instance where the manufacturers specs said a certain size liner would work. It didn't. People got poisoned with CO. This was verified by VERY knowledgeable people in the field. I can almost guarantee you that there will be a story about this in a national news paper. Am I bashing the manufacturer of that liner? NO! I am saying that it doesn't matter WHAT a manufacturer says! WE are the final judge on what is right and what is wrong through TESTING. How could a liner manufacturer know all of the variables that may exist in each application? They can't so they default to "the average".

Do a simple "base line" test of the pressure in a basement relative to the atmospheric pressure. You will find that basements are already negative even with NO exhaust units running. Simple physics, stack effect.

You said, " Remember, when an appliance first fires, it has to push out that heavy column of cold, dense air in the stack to get things moving north as it were." When does a water heater go cold? When does a water heater stop drafting due to "cold air" dropping down the chimney? Have you EVER tested a water heater that only drafts while it is burning? What about "stand-by losses" Bob? What are they? Where did the heat go?

Bob, I respect you more than you could know, but there should be no column of cold air to push out if a chimney works. We could type all night about what makes a chimney work, but I can tell you this, the connected appliance ain't it. The connected appliance is attached to a chimney BECAUSE the chimney works not FOR the chimney to work.

I'll do one more straw analogy. One straight straw vs. one of those twisty, loop-di-doo models. Which has more resistance? Both have equal ID's and both are smooth. One has twists, one is straight. Which is easier to move through?

One more question. I have an atmospheric water heater in a mechanical room. Code says that I must provide adequate make up air for the water heater. I vent the water heater with a 4" vent into a 6" b-vent chimney. I provide make up air with a 6" single-wall vent pipe through the roof and it terminates 10' from the b-vent chimney and drops to within 6" of the floor in the mechanical room. How does the air know which is "in" and which is "out"? Are you saying that it is the attached appliance that decides which pipe "sucks" and which pipe "blows"??

Draft is pressure, not heat. Reducing the size of a draft hole increases resistence, this does not increase draft it reduces it. Causing turbulence reduces draft, it can't increase it. If you have a chimney that does not draft, making it smaller does not make it better, it makes it worse.

But how many of these "miracle" fixes get tested? HA! NONE!

You are a top notch guy in my book Bob, and I mean no disrespect. I am merely bringing an alternate view to our discussion.

I wish you the greatest peace.

Mark H



To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Bob Harper

mutual respect here

Mark, I was merely pointing out it sounded like you came down kinda heavy against liners in general.

We agree on a lot of things such as the need for combustion analysis as well as developing the house pressure profile. I think we all get sucked into a bit of a trap when discussing venting because there are just so many variables, you cannot take them all into consideration on every case.

I enjoy our disussions here and it is great to have nationally recognized experts such as yourself here to test our ideas on.

So, while me may not agree on which has the greater influence on flow resistance: smaller diameter corrugated liners versus larger surface area of bigger liners, I think we can agree you cannot rely on any damn book, chart or other printed reference alone. I think we agree the bottom line is, does it work? and the only way to prove that is with testing. As I say in my classes, I never met an appliance that can read a manual. They will do what they want to. We can perform sophisticated tests to ascertain if said appliance is operating "properly" or at optimum efiiciency and so forth but we cannot always take into account every variable. For just as we pack our gear, the weather will change or the damn gas company will change the mix on their fuel or the homeowner will plan to remodel the home, changing the dynamics of the home. So, until we can mount a gas chromatograph and micromanometer to the appliance, we'll never be able to set an appliance perfectly for all conditions. However, we still need to do within reason to account for what can reasonably be expected.

take care my friend,
Bob

jp_2

ALL posts CORRECT!

given the lack of info, all the posts have valid points.

draft is only caused by 2 things:

1.)temperature difference

2.)mechcanical means-fans, blowers

wind doesn't count cause its not always windy.

so everyone was correct.

my woodstove has no draft until I light a fire.

jp_2

question Mark?

with the statement you posted:

""I was told today of a CO poisoning that happened to a woman after a chimney liner was installed in her home. The manufacturers specs said that the liner had enough capacity to handle the attached load. Well,............it didn't and the people in the home got dosed when their water heater vented into the house.""


has it been established that the sole problem was the liner?

interesting to hear the details.

question #2: you said:
Do a simple "base line" test of the pressure in a basement relative to the atmospheric pressure. You will find that basements are already negative even with NO exhaust units running. Simple physics, stack effect.

guess I do not know this simple physics, can you explain?

Mark Hunt

JP


Yes, the issue was that the liner was too small for the total connected load even though the lit said it would work.

RE: Stack effect.

Warm air rises and exits a structure at or near the top. Cooler, denser air enters at or near the bottom to fill the void. The pressure at the bottom is negative while the pressure near the top is positive(relative to atmospheric).
The taller the structure, the more magnified the effect. It was explained to me during my building science courses that in high rise buildings the pressure at the base of the building could be negative enough that doors that opened outward could not be pushed open and doors that opened inward could not be pushed shut. I was also told that rotating doors were used for this reason. Newer high rise buildings have vestibules. We enter a "short" stucture that disconnects the "high" structure from the pressure difference.

In a typical two story home with a basement, I usually found the basements negative in the range of -1 to -3 pascals. During our worst case tests, we would watch to see what effect, if any, mechanical exhausts had on the pressure. In a few instances we found huge problems. Worst one to date was a home where the HO had a commercial gas stove/oven installed. Over it is a 1000cfm exhaust hood, as per manufactuers specs. When ever they cooked they ran the exhaust. 1000cfm had to come from somewhere and it was the chimney. Both his water heater and boiler would have the flue gas sucked from them and into the house. You could literally feel a breeze coming from the draft diverters.

Mark H

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Brad White_9

Stack Effect Big Time

To illustrate your point, Mark, we did a study on ways to reduce infiltration at Boston's Prudential Tower, a height of 750 feet.

There is a smoke relief shaft going nearly the full height of the building to relieve smoke from each floor as needed.

On a cold day, the updraft volume was measured at over 50,000 CFM. The lower shaft stack pressure relative to atmosphere was about -1.75 inches (that's negative).

What is interesting about stack effect is that it exists; here is the explanation/conundrum I gave to my students:

We all accept that fluids (gasses or liquids) always flow from high to low pressure. There is higher pressure at the earth's surface due to the weight of air above it (14.696 PSI Absolute). As air rises it becomes less dense, lower in mass hence pressure. There is therefore a natural tendency for air to rise, or is there? The air aloft is cooler due to adiabatic cooling (losing roughly 4.5 degrees per 1,000 feet elevation). So there really should be a tendency for that air to fall, right? So there is a bit of a stand-off here.

Along comes a chimney- it is a soda straw or a large smoke stack which immediately connects the two disparate pressure zones. The higher pressure is down low, the lower pressure is up high, thus flow is established.

Appropos of nothing, I thought you might appreciate this.

Brad

Mark Hunt

ZOINKS!


"Fizzy lifting drinks" would not be a good idea in that place!

Thanks for that example Brad!

Mark H



To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"