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Length of flue


Darren
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What is the maximum length of a flue pipe. I did a house yesterday that had an oil fired steam boiler with a flue about 14 feet long AND a propane fired water heater with a flue about 18 feet.

I'm under the impression 10 feet is max; is this standard for both oil & gas? is propane different?

FYI, the oil flue was recently replaced due to sever rusting, the new one did not have a damper installed. Kinda tells a story by itself.

Darren

Lost in NJ

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Darren,

Arnie Green, who is a retired oil dealer, legendary home inspector, and about the nicest guy you could ever hope to meet taught me that the smokepipe connecting a heating system to a chimney can be as long as 2/3 the height of the chimney.

This has always struck me as an odd rule of thumb because chimneys work on equal parts science and magic. Why 2/3? Why not 1/2? Why not 3/4? It is safe to say shorter is always better.

For me, it boils down to common sense. If it looks longer than about 2/3 the height of the chimney, if there are signs that it hasn't been drafting properly in the past, if it puffs back when I'm inspecting it, then I say so and tell my clients to get it checked out. Especially, as you point out if there is no barometric damper.

Best,

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Darren, there is no maximum vertical rise that I am aware of. That 10 ft dimension you see in Code Check is for comparison of the horizontal run to the vertical rise, not a maximum. Single wall horizontal run (>45 degrees from vertical) cannot exceed 75% of the rise. For B type vent the horizontal can equal 100% of the vertical. The horizontal must have a minimum 1/4" per ft rise. There are many code issues of importance concerning gas and oil appliance venting this being only one of them.

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Darren,

Some folks interchange Flue and Chimney. The higher the more stack effect, the better it will draw.

The exit point should be 10 feet away from anything. In other words 10' above the ridge, side of the roof, neighbor's house etc. Another way to put it is, hook a tape measure on the top of the flue and if you can measure less than 10' to anything then the flue is too short.

Bruce

:)

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Gentlemen;

Thanks for your replies. The length I was talking about was horizontal; they ran 14 & 18 feet to the chimney. It was a one story house with a low attic. I figure the chimney was only about 18 feet high.

I did call out the damper, the fact that the flue pipe had already rusted out and the length was too long.

I was under the impression the appliance should be installed within 15 feet of the chimney. Last month I attended an CE seminar (ASHI sponsored), thats where I got the 10 foot rule.

Guess it comes down to if it looks wrong it probably is wrong.

Darren

Lost in NJ

www.aboutthehouseinspections.com

(check it out, let me know what ya think)

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Hi Bruce,

I think you've right, it isn't drafting correctly. However, in regard to the propane/oil in the same vent I think you've erred there. According to my HVAC CodeCheck, it is okay to vent oil and gas into the same flue (IRC 2426.5.6.2/UMC 809.0). The same text, when discussing Gas-Appliance Venting says that no solid fuel and gas-burning appliances can be connected to the same vent (IRC2426.5.7.1/UMC 802.4) and no common connection with a solid-fuel chimney (IRC 1801.11/UMC815.2.2.9).

Also, the length of the horizontal run, although very long, might not have anything to do with why it isn't venting well. Darren didn't say whether the connector is a single-wall or double-walled vent. If it is a double-walled vent and no longer than 18 ft. with an 18ft. vertical vent, it would not have too long a horizontal run - at least not according to the code. It could be the type of flue - single-wall vs. double-wall - it might be the tightness of the area where the furnace is located, it might even be that a cold air return or a clothes dryer are too close to the furnace and water heater or they are too close to each other. There are lots of possibilities.

One thing that I don't think HVAC guys do very much, but are supposed to since about 1990, is to check the amount of draw on a flue and determine the combined total BTU/HR of all appliances connected to the vent, determine whether there is sufficient make-up air in the space where they are installed, and then, if so, compare the total combined BTU/HR usage to the NFPA vent sizing tables that were published back then to serve as a guide for flue sizing. They're still doing things the old way - using vent ratios - to determine whether a vent is too large or too small.

If you've got a copy lying around, refer to the February 2003 issue of JLC for a great article by Bob Dwyer and Mark Gronley on page 75 about this topic.

ONE TEAM - ONE FIGHT!!!

Mike

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Bruce,

Gas & oil can share the same flue as long as the gas enters higher than the oil.

The pipe was single wall; the boiler was in the basement and the pipe ran thru the crawlspace (new pipe was in contact with ground). The crawl was completely open, air temp was same as basement.

Mike, what your saying (1:1)conflicts with what Jim says (2/3:1). And where did the 10 foot rule come from. My head is starting to spin. Guess if it was easy, everyone would be doing inspections.

Heres another question, does the pipe size come into play? Boiler has 8 inch while water heater is 4.

I tried looking on Rheem & Bradford-White web, no mention of pipe length.

Darren

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Hi Darren,

There is a rule in the codes which states that an uninsulated horizontal connector may be equal to or less than (<) 75% of the total length of the vertical leg of the vent, and an insulated (double-walled) connector can be equal to or less than 100% of the total length of the vertical leg of the vent (IRC 1803.3.2/UMC 815.2.2.7).

I've never heard of a 10ft. rule.

Vent pipe size does come into play, but so does the total BTU/HR usage of the appliance, the total cubic feet of the area where the appliances are located, their sources of combustion air, whether other air-scavenging appliances are located within the same space, tightness of the building, temperature of the space where the furnace and water heater are located and the type, size and location of the flue which they are venting into. On top of that, minerals in the soil of that crawlspace may very well shorten the life of that pipe that's in contact with the soil and cause it to cool more rapidly as well.

That's why you need to understand the science of what is going on with combustion and the flue and what conditions must work together for the furnace to vent properly. The article that I cited above is an excellent place to garner the basics of this issue, so I'll see if I can upload it to the downloads section for you guys. First I have to check with my boss at JLC and see if that is okay.

For you folks who're new to the business, a lot of what we need to know in this business is grounded in science. A background in the trades isn't necessarily going to give you all of the tools that you need to do this job well, and it'll be easy for yourhead to "spin", if you don't have a good understanding of basic physical science as well as building science issues.

I recommend that you spend as much time as you can studying these issues. There are some great sites to do so. You can subscribe to JLC and Fine Homebuilding. You can frequent this and the JLC Building Science Forum. You can visit Joe Lstiburek's website at Building Science Corp., or you can hang out on the Home Energy Site and read as much as you can. You can also take college courses in physical science and building science.

Whatever you do, don't stop trying to learn everything you can each and every day you're in this business or you'll get left behind by technology.

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by Darren

Bruce,

The pipe was single wall; the boiler was in the basement and the pipe ran thru the crawlspace (new pipe was in contact with ground).

Darren

This is extremely problematic and most likely the root of your problem. Single wall pipe in contact with anything will radically cool and prevent proper drafting.

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Crusty:

I said the new pipe is in contact with earth, I have no idea (one can only assume) about the old pipe. Just like I can only assume the old pipe had a damper installed.

Mike,

From Carson-Dunlop 'Home Reference Book' I got 8 years ago when I attended training from a large franchise (that I can no longer refer to):

14.1 Exhaust Flue

Length; Depending upon the location of the heating system relative to the chimney, some exhaust flues are too long. Exhaust flues from oil furnaces should be no more than ten feet in length.

Thats why I asked if there is any difference between gas & oil.

Darren

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Hi Darren,

That's one I've never heard before. However, if all oil flues could not be more than 10ft. in length there are literally millions of heating systems that are violating that rule. What's up with that? Sometimes you can't believe everything that you read.

ONE TEAM - ONE FIGHT!!!

Mike

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