Boiler combustion venting


By Roy Collver

The venting of gas and propane flue products is a serious business. Safety is paramount.

Plastic venting materials all fall under Type BH special venting systems and, as early as January 2006, directives were issued by the Interprovincial Gas Advisory Council and from various provincial safety authorities to enforce and clarify the requirements for ULC S636 certification of these venting systems. Ten years on there can be a bit of confusion, but the vast majority of these systems comply with rules based on:

  • National codes and standards
  • Manufacturer’s certified installation instructions
  • Local jurisdiction requirements and revisions

There can be a bit of tail chasing inherent in this pecking order. National codes generally defer to the appliance manufacturer and the venting manufacturer’s installation instructions, but conflicts arise. A common clarification reads: “where a conflict exists between the manufacturer’s certified installation instructions and the Code, the requirements of the Code shall prevail unless otherwise approved. Therefore, instructions that conflict with Code requirements can be modified by the authority having jurisdiction for reasons of safety and performance.” Got that?

This article is intended to be educational and I have no authority to direct you as to details. Readers must make decisions on their own, based on local requirements. I will however, pass on some simple advice I received from my favourite code official, some 40 years ago – advice that has worked every time. When in doubt, ask for direction.

Ask everyone involved and work through any issues that come up. My personal path to enlightenment starts with the appliance and the venting manufacturer’s certified instructions. Read them both – identify any potential conflicts or areas of confusion, and get them resolved. In my experience, both parties are eager to clarify any issues.
While reviewing the manufacturer’s information, compare with the requirements of the B149 gas code to ensure there are no conflicts (you should pretty much have the venting section memorized). Again, if anything pops out at you, go back to the manufacturer to resolve the variation – get it in writing and then seek approval from the local authority having jurisdiction before you proceed. There may be local conditions or requirements that you are unaware of.

Don’t be shy about expressing your own opinion, but at the end of the day when it comes to safe and proper venting – take no chances.

On long horizontal runs it may be necessary to provide for condensate removal prior to vent risers.

Common problems

Here are the some common issues I have seen recently:

Omissions, misinterpretation or conflict between manufacturer’s instructions and code requirements sometimes occur. The direct vent, air intake piping material issue is a classic example. Many appliance manufacturers just don’t address this detail in their instructions and the code is silent on it.

When there is lack of information and the B149 is silent on the issue and a code official isn’t sure – they will (correctly) choose caution every time. As reported in the last issue of this magazine – some code officials have demanded that inlet air piping be identical to the vent piping.

However, one Canadian boiler manufacturer’s instructions read:

“For the inlet air – four-inch Schedule 40 PVC, CPVC, ABS, or PPs piping of any type is permitted.”

That clause is what the code official is looking for. If you bring it to their attention, they should approve the installation accordingly. Unfortunately, many manufacturers don’t detail this in their instructions and, although they should be able to provide a letter of clarification, there is no guarantee it will gain approval. If you disagree with a code official, there is an appeal procedure in most jurisdictions, but often it is more practical to comply for the job in dispute and then work with the authority and the manufacturer to try and clarify the issue before the next job.

Attention to detail

Failure to follow manufacturer’s installation instructions is another common fault. The devil is in the details, as they say, and S636 certified products have very detailed and specific instructions. Appliances that use these systems also have specific requirements of their own – more important details.

When using atmospheric venting it was common to look at venting possibilities before even choosing an appliance. Pressure venting is much more flexible, but you should still approach the planning the same way. Read, read, read the details – and plan ahead so you don’t find yourself backed into a corner.

Proper support is essential. This manufacturer can supply a special base elbow and support.

Common failures

Here are some specific fails that I’ve seen:

1) Improper support of plastic venting: Proper support methods are highly detailed in the venting manufacturer’s installation instructions. Follow them to the letter because plastic can easily sag, creating areas where condensate can be trapped.

2) Using old chimneys or B-vents as a chase can prove problematic when it comes to correct vertical supports, but most vent manufacturers have approved work-arounds. Vertical venting risers may need special base elbows or hangers to eliminate weight from damaging the appliance.

3) Improper slope-back leading to condensate trapping: Purchase a level – read the instructions – comply. The old minimum quarter inch to the foot may not apply to the venting system you are using; it may require more slope so, again, read the instructions.

4) Excessive horizontal runs leading to water “standing wave”: The problem can arise even in properly sloped horizontal vents that elbow up 90° to a vertical rise. Long burner runs can push condensate up the vent where the high flue gas velocities will hold it at the elbow. Customers may hear a “swishing” sound in the vent, and the appliance might shut down on high fan pressure. Increasing the slope angle might help, in extreme cases you may need a trapped drain elbow at the bottom of the vertical rise.

5) Improper condensate removal: Includes improper condensate traps, drains, neutralization, etc.

6) Termination issues: This is the most common fail I see. The individual appliance manufacturer’s details rule here and they can vary widely from one appliance to the next. There should be specific illustrations showing vertical and horizontal clearance of air intake piping from exhaust termination.

The requirements you see in the B149 will apply here, but the appliance manufacturer may have additional requirements and the local codes may also have additional requirements (snow levels, clearance from lot lines, etc.). The more stringent requirements normally apply.

7) Expansion problems. Plastic pipe expands when heated up and can bend, causing stress and possible sags. Type BH venting systems should list details as to vent lengths versus expansion and detail how to deal with it if it becomes a problem.

8) Vent size versus length issues. These specific details will be in the appliance manufacturer’s instructions and you can’t push the limits at all. If you are a few feet over, you have to go up to the next size, but only if the manufacturer allows that option.

9) Joinery issues are another common fail. The vent manufacturer’s instructions must be followed to the letter – don’t get creative with this one. On glued systems, if they say you must use their primer and glue, use their primer and glue. If they say use their fittings and pipe only – do so. On gasketed systems, if they provide gasket lubricant and instruction? You get the idea.

Again, plan ahead. Make sure you have all of the materials you need. Most wholesalers I talk to are pretty good about returning in-stock venting material if you order extra. Take no chances.


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