How to Avoid 9 Critical Boiler Venting Mistakes

Indoor gas-fired boilers require exhaust vents to remove harmful byproducts of combustion, ensuring the system’s safety and durability. Furthermore, when installing boilers indoors, proper venting or a louvered air supply is required to deliver clean air to the combustion chamber. Boiler venting requirements can differ by brand, application and applicable codes. Therefore, consulting the manufacturer’s installation and operation manual (IOM) and adhering to local code requirements is of utmost importance.

The following overview outlines key pitfalls to avoid when venting gas-fired boilers.

Mistake #1: Improper Vent Selection

Selecting the appropriate vent material is crucial for the specific applications of gas-fired boilers. Venting categories play a significant role in determining suitable vent pipes. Here’s a breakdown of the different categories:

• Category I appliances operate at a non-positive vent pressure and have an exhaust gas temperature of at least 140°F (dew point). This is a non-condensing mode suitable for use with type “B”, “L” and positive pressure stack venting.
• Category II appliances operate with a non-positive vent pressure and exhaust gas temperatures lower than the dew point. Because of the low exhaust gas temperatures, the system runs the risk of being in condensing mode, so a corrosion-resistant material is required for the condensate. Category II appliances require a UL 1738 listed product.
• Category III appliances operate with a vent pressure that is positive and above the dew point. They require a UL 103 listed product produced for positive pressure capable of handling increased exhaust gas temperatures under positive pressure.
• Category IV appliances operate with a positive vent pressure and with exhaust gas temperatures below the dew point. A sealed UL 1738 listed product made from non-corrosive material is required to prevent flue leakage. Non-corrosive materials are required to withstand the acidic nature of condensed flue gases. Stainless steel or polypropylene are generally suitable for Category IV boilers. However, some boiler manufacturers may specifically require stainless steel for their boilers.

When boiler venting with plastic and polypropylene alternative materials, it is important to consider the temperature thresholds of the pipe in comparison to the actual exhaust gas temperatures the system will experience. Be sure the exhaust gas temperatures are not going to exceed the listed thresholds by the pipe manufacturer. It’s also important to take thermal expansion into account.

As an example, CPVC expands by 4″ in a 100′ run at 100°F. The expansion can exert additional pressure on the fittings and may contract back to their original state, leading to the risks of dips and valleys in horizontal runs.

For UL 1738 products for Category II and IV operation, consideration around leakage is very important to guarantee the system will not leak back into the building space. There is the potential for gallons of condensate to be running through the system at any point. Inferior products and designs will almost always be a concern for leakage.

It is important to specify products that have been tested to no less than 20″ w.c. to help eliminate the potential of leakage after installation. This ensures specification around high-quality products and manufacturers while providing a wide range of reliable options.

Mistake #2: Insufficient Combustion Air

When the combustion air supply is inefficient, a gas-fired boiler will burn excessive fuel during combustion, resulting in a condition known as “rich combustion burn.” This significantly reduces boiler efficiency. From a boiler venting standpoint, it’s important to understand the amount of combustion air supplied to the appliance and consider it when sizing exhaust systems.

Mistake #3: Excessive Elbows and/or Long Vent Runs

The presence of numerous elbows and long vent runs can lead to increased static pressure in the exhaust vent, impeding the thorough removal of flue products. To guarantee optimal boiler venting performance, it is advisable to minimize the number of fittings and the overall flue length during the design phase. This approach helps mitigate potential variables that could negatively impact boiler performance. Additionally, it promotes the proper mixing of fresh combustion air and fuel, which is essential for achieving efficient and effective combustion. Failure to address this issue can result in rich combustion burn and a subsequent decrease in boiler efficiency.

Mistake #4: Improperly Sized Vents

When sizing boiler and water heater exhaust systems, it is important to understand the role of the given outlet pressure ranges of the appliance manufacturer. Sizing the system regarding the outlet pressure safeguards that the appliance always operates within the designated parameters. It’s important to note that Category III outlet pressure ranges typically have larger tolerances than Category IV appliances, making the latter more sensitive to sizing.

Using an oversized vent pipe on a boiler can create an excessive draft, leading to ignition failure, flame failure and other operational issues. Conversely, using an undersized vent on a positive pressure system creates excessive backpressure on the boiler, resulting in inefficient fuel combustion and reduced boiler performance. This issue is particularly concerning for Category IV systems due to their high turndown rates.

Many variables, including differences in outside air temperatures, modulation and maintenance of appliances, can further complicate the sizing process, even for seemingly straightforward systems. By considering those aspects, the vent system can be properly sized, promoting optimal performance and appliance efficiency.

Mistake #5: Inadequate Vent Pipe Support

Proper support is of utmost importance in all specialized venting applications, particularly boiler venting. The manufacturer’s support system, which should be included in the UL listed system, must be installed per the manufacturer’s instructions for proper operation.

In applications involving higher temperatures, thermal expansion is important to consider. Without adequate support, the expansion and contraction of the vent pipe can exert unnecessary pressure on the joints.

For condensing applications, poorly supported vent pipes can sag, causing condensate to accumulate in low spots. This leads to corrosion and eventual failure of the vent pipe. To prevent such issues, you must ensure that the vent pipe is pitched back towards the condensing boiler and is fully supported.

By prioritizing proper support and following the manufacturer’s instructions, potential problems related to vent pipe sagging, corrosion and failure can be mitigated.

Mistake #6: Contaminated Combustion Air

Ideally, combustion air should come from the outside, either by a direct vent or into the space where the boiler is installed. If air is vented into the space rather than directly to the boiler, it’s critical to ensure contaminants are not present. Common contaminants include:

• Spray cans containing chlorocarbons and fluorocarbons
• Chlorinated waxes and cleaners
• Chlorine-based swimming pool chemicals
• Permanent wave solutions
• Calcium chlorides used for melting ice
• Sodium chlorides used for softening water
• Refrigerants or refrigerant leaks
• Paint or varnish removers
• Hydrochloric acid and muriatic acid
• Cements and glues
• Antistatic fabric softeners used in clothes dryers
• Chlorine-type bleaches, detergents and cleaning solvents found in laundry rooms
• Adhesives used to fasten building products and other similar products

If contamination is possible, consider relocating the boiler or eliminating the sources of contamination. If neither of those is feasible, ensure the combustion air is vented directly into the boiler and avoid recirculating exhaust fumes or other sources of contamination. This also goes for other exhaust sources such as other gas-fired appliances, laundry exhaust, etc. Keeping the combustion air free from contaminants is essential for optimal boiler performance and safety.

Mistake #7: Negative Boiler Room Pressure

The boiler room should always be pressurized to guarantee proper firing and venting of the boiler. Negative pressure in the boiler room is a code violation and is frequently caused by an exhaust fan somewhere in the space.

Mistake #8: Common Venting Multiple Boilers

Some manufacturers allow common venting of boilers and water heaters to a single vent stack, while some don’t. If a common vented solution is being considered, it’s important to refer to the appliance’s IOM and reach out to the manufacturer for guidance. Common venting is not always allowed, but there are cases where it will be necessary for the most effective design. Working with experts who specialize in common manifold systems is important.

Two main considerations arise with common venting appliances:

• Building Safety. It is crucial to prevent the backflow of exhaust gases into unfired appliances, particularly in Category IV applications with tight outlet pressure ranges.
• Efficiency: Sizing the vent pipe to maintain a slight positive pressure within the outlet pressure range ensures optimal boiler performance. Depending on system sizing, draft control measures like fans and dampers might be necessary.

When considering venting solutions, it’s recommended to approach the process in close collaboration with a trusted venting specialist like Schebler. Confirming that your engineered vent system meets both IOM and code requirements is critical. By engaging with trusted experts, you ensure not only cost savings on materials and installation but the assurance of a compliant and optimally performing vent system.

Mistake #9: Failure to Meet Code

There’s nothing more discouraging than receiving a red tag from an inspector. To prevent this setback, read and comprehend all relevant codes, and review the boiler’s IOM. This cannot be overstated. By ensuring compliance with codes and following the manufacturer’s guidelines, you can avoid those unwelcome surprises and keep projects on track.

In conclusion, proper boiler venting is paramount to ensure safety, efficiency and code adherence. By avoiding critical mistakes, installers can mitigate risks and assure optimal performance. Consulting manufacturer guidelines, adhering to local codes, and partnering with venting experts can help avoid setbacks, improve system longevity and achieve successful boiler installations.