Boiler Installation Pitfalls Part 2: Avoid These Critical Venting Mistakes!
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All indoor gas-fired boilers require exhaust vent to remove the harmful by-products of combustion for the safety and longevity of the system. When a boiler is installed indoors, venting (or a louvered air supply) is also required for clean air delivery to the combustion chamber. Boiler venting requirements can vary by brand, application, and code, so it is critically important to consult the manufacturer’s installation and operation manual (IOM), as well as local code requirements.
The following list is intended as a quick overview of what to avoid when venting gas-fired boilers.
Mistake #1 - Improper Vent Selection. Boilers are vented with a range of materials, including PVC, CPVC, stainless steel, and polypropylene. Vent materials are generally selected based on the boiler category, as well as the boiler manufacturer’s specifications.
Boiler categories differentiate boilers based on their operational temperatures and any expectation for condensing of flue gases, as well as whether the boilers operate at positive or negative vent pressures. (See Table 1). Condensation and pressure are the primary concerns when it comes to selecting a vent material.
All gas-fired boilers fall into one (or possibly two) of four categories: Category I, Category II, Category III, or Category IV. Knowing a boiler’s category is critical for proper vent selection, but engineers must also consult the manufacturer’s IOM. No two boilers are the same and manufacturers typically specify what vent materials and brands meet the criteria for each model they build.
table 1 * Type B vents are vents suitable only for listed, draft-hood-equipped, gas-fired appliances, including most domestic heating and hot water systems. Type L vents are designed for venting approved oil-fired and natural-gas appliances that produce draft hood flue gases that do not exceed 570°F or 926°F for 10 minutes in an over-fire situation. The minimum clearance from combustible materials is generally 3 inches. Type L vents should not be used to vent coal- or wood-fired appliances.
Mistake #2 - Insufficient Combustion Air. When there isn’t enough combustion air a boiler will burn too much fuel during combustion, a condition that is known as “rich combustion burn.” Rich fuel combustion derates the efficiency of your boiler. You also risk starving the boiler of air which will cause nuisance lockouts and operational issues. Combustion air should be considered before sizing the boiler exhaust system to ensure proper delivery.
Mistake #3 - Too Many Elbows and/or Long Vent Runs. This creates extra static pressure through the exhaust vent which interferes with the complete removal of flue products. It’s best to minimize the number of fittings and flue pipe length, as both can impact the performance of the boiler given certain variables. Excessive fittings and length also prevent sufficient mixing of fresh combustion air and fuel, which can result in rich combustion and inefficient operation.
Mistake #4 - Improperly Sized Vent. The outlet pressure ranges of a boiler or water heater play a vital role in exhaust system sizing. A properly sized vent helps ensure the appliance stays within the appropriate range. Oversized vent pipe on a boiler could create too much draft, leading to ignition failure, flame failure, and other potential operational issues. An undersized vent on a positive pressure system creates too much backpressure on the boiler and will result in rich fuel combustion and derate the boiler.
Other variables, including variations in outside air temperature and boiler modulation, should also be factored in when sizing exhaust vent.
Mistake #5 - Poorly Supported Vent Pipe. It is very important to consider proper support in all specialized venting applications. The support provided by the manufacturer should be included as part of the UL-listed system and must be installed per the manufacturer’s instructions to guarantee proper operation.
The support should be able to withstand the thermal expansion and contraction of higher temperatures systems; otherwise, undue pressure will be placed on the joints. In condensing applications, poorly supported vent pipes will start to sag, causing condensate to pool in low spots. This leads to corrosion and ultimately failure of the vent pipe. To avoid this, the vent pipe should be pitched back to the condensing boiler and be fully supported to prevent sagging.
Mistake #6 - Contaminated Combustion Air. Ideally, combustion air should come from the outside, either by direct vent or into the space where the boiler is installed. If the air is vented into the space rather than directly to the boiler, it is important there no contaminants are in the space. Some examples of contaminants are listed below. If there’s a chance that contaminants might be introduced into the space then install the boiler elsewhere or remove the sources of contamination from the room. If neither of these options is possible, the combustion air should be vented directly into the boiler. Also, make sure exhaust fumes from the boiler aren’t recirculated into the combustion air.
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
Cement 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
Mistake #7 – Negative Boiler Room Pressure. The boiler room should always be pressurized to ensure proper firing and venting of the boiler. Negative pressure in the boiler room is a code violation. It is frequently caused by the presence of an exhaust fan in the space.
Mistake # 8 – CommonVenting Multiple Boilers. Generally speaking, it is best to avoid common venting in boiler applications if possible because doing so can create safety risks and derate the boiler efficiency. Common vent systems can cause exhaust gases to be pushed back through an unfired appliance. If a common vented system is exposed to too much natural draft, heat that is supposed to be retained at the appliance, can be pushed up the stack.
If you are considering common venting multiple boilers, be sure to check the IOM or with the manufacturer to confirm that common venting is permitted on the boilers in question. There are situations where common vented systems may be the best or only solution. In this case, be sure to consult with the boiler manufacturer and partner with an industry venting expert who understands how to design a properly functioning common vent system.
Mistake #9 – Failure to Meet Code. Nothing puts a damper on a project like a red tag from an inspector. To avoid this unwanted sight, make sure you read and understand all applicable codes as well as the IOM for the boiler you are installing.
Boiler venting is a complex and often confusing process—so much so that we are dedicating our next blog series to that very subject. Stay tuned!
