Published: August 13, 2015 - by Ray Wohlfarth

Categories: Commercial Heating

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One of the consideration when replacing a boiler is whether to re-use the existing flue and chimney. While most installations work without problems, there are a couple precautions that you should consider.

What category flue?

To lower installation costs, the installer and designer chose to re-use the existing Category III stack, which meant that it was a non-condensing, positive static pressure flue. The new boiler called for a Category I which was non-condensing with a negative static pressure. After about two heating seasons, the Category III flue collapsed into itself because it was not designed to operate in a negative condition. It was like a beer can being squeezed after consumption. It was a legal mess and could have been a dangerous situation. The entire flue had to be replaced at the expense of the installer and designer.  If you are re-using the existing flue, verify that the size and category is correct. The following is a list of the flue categories that are used for venting boilers.

Flue Categories

  • Category I Appliance operates with a negative vent static pressure at temperatures above condensing temperatures. Typical efficiency range 78% - 83%.
  • Category II Appliance operates with a negative vent static pressure at or below condensing temperatures. Typical efficiency range is over 85%.
  • Category III Appliance operates with a positive vent static pressure at temperatures above condensing temperatures. Typical efficiency range 78% - 83%.
  • Category IV Appliance operates with a positive vent static pressure at temperatures at or below condensing temperatures. Typical efficiency range is over 90%.

Confirm the size of the flue that the old boilers were using. If the old boiler had a draft hood, the flue may be oversized when using a power burner. I suggest making a diagram of the existing flue and consult a venting specialist when reusing the existing flue.

Please Sir, May I Have Another?

When I was young, my parents took me to see the musical, Oliver which was about the adventures of a young orphan in London based on the book, Oliver Twist. If you are removing the boiler flue from the chimney and venting it through the wall, be wary of “Orphans.” When you remove the boiler venting and leave the existing water heater or pool heater in the old chimney, it is referred to as an orphan. This is where the “Seven Times Rule” comes into the play, which is: The flow area of the largest common vent or stack shall not exceed seven times the area of the smallest draft hood outlet. Since most water heaters use a 3" flue, the largest area to connect the water heater should be 49" in area or a 7" round one. You may have to change the venting of the old water heater or install a flue liner for it. The following is a chart that shows the largest common round flue that each can be connected to.

7 Times Rule Round Flue

Smallest Draft Hood Outlet

Largest

Common

Flue

Smallest Draft Hood Outlet

Largest

Common

Flue

3"

7"

9"

22"

4"

10"

10"

26"

5"

12"

12"

30"

6"

14"

14"

36"

7"

18"

16"

42"

8'

22"

 

 

 

If you are connecting to a square or rectangular chimney, you will need to estimate the area. The following is a chart that shows the sizes that the vent can be connected to:

7 Times Rule Rectangular Flue

Smallest Draft Hood Outlet

Remaining Draft Hood Area

Largest Common Vent Area

3"

7.06"

49"

4"

12.56"

88"

5"

19.64"

137"

6"

28.27"

198"

7"

38.48"

269"

8"

50.27"

352"

10"

78.54"

550"

12"

113.1"

792"

14"

153.94

1,078"

16"

201.06"

1,407"

18"

254.46"

1,781"

20"

314.16"

2,199"

 

Allow me to show you an example: If you remove the boilers from a chimney that is 12" x 12" square and want to see if we can leave an old water heater with a 4" flue outlet. The 12" x 12" chimney is 144" This is greater than the rule of 7 sizing for the 4" flue which is 87.92". On this job, we would have to make provisions for the water heater flue. A chimney liner would most likely have to be installed here. You will need to have the price for that in your estimate or design.  Without the chimney liner, the flue gases could condense, ruining the chimney.

Horizontal vs. Vertical   Another rule to remember is that the horizontal vents must be no more than 75% of the vertical height of the flue. If using B Vent, the horizontal length can be the same length as the height of the chimney, as per International Fuel Gas Code, 2006 503.10.9

How tall is the chimney? If you have ever been to a boiler room where they once had a coal fired boiler, the chimney is really tall. I have a theory that the reason these chimneys were so tall was to spread the coal ashes over a wider area.  These stacks take forever to heat high enough to sustain draft. Once it is warm, the draw is tremendous. I once had the chimney draft pull the ball cap off I was wearing when I poked my head in to look. If you are connecting your new boiler to this dinosaur, there could be some real issues. The boiler flue gases could condense until the stack gets warm, causing rollout. Once the stack was warm, the draft could be very high.  A good rule of thumb is that a draft control should be used if the chimney is over 30 feet high. The draft control may range from a barometric damper to a sequencing draft control. It is not uncommon for Category I boilers to have flue gas spillage on the first one or two minutes when firing into a cold stack or chimney. A spill switch should be installed in the event of flue blockage. Always stay safe.

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