Check vent length with elbows

What is going on

Every vent system has a maximum equivalent length, and fittings count as extra length. This is a worked example for the 60 KBTU/H direct vent model using 2 in pipe. Do the math before you cut pipe, not after, and keep the fitting type in mind, because swapping long sweep elbows for standard ones can push a run over the limit. Vent length is a code and safety item. If the total is over the maximum, change the layout or pipe size rather than hoping it works.

Likely causes

• New direct vent install with many elbows
• Replacement furnace on an existing vent run
• Change from long sweep to standard elbows
• Vent that pressure switch trips on because it is too long

Check in this order

1. Add up straight pipe length (example 31 ft)
2. Multiply the number of long sweep 90 degree elbows by 5 ft (example: 5 elbows = 25 ft)
3. Add straight pipe and elbow equivalents (example: 56 ft)
4. Compare to the model maximum (65 ft for 60 KBTU/H). Acceptable if at or below
5. If standard elbows are used instead (10 ft each), the same run becomes 81 ft and is not acceptable
6. If over the limit, re-route, remove elbows or step up the pipe size per the manual
7. Recheck the total after any change to the layout

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