HVACR Systems: Mid-Winter Maintenance
By: Gary B. Xavier
The winter season varies, of course, from region to region. In some climates, the temperature variance from winter to summer may be slight, but in others it is extreme.
Just as heating equipment varies widely based on need, the maintenance of such equipment varies widely as well. But all residential, commercial, and industrial HVACR equipment needs – and will benefit from – a program of preventive maintenance that encompasses the start, middle, and end of the operating season.
Furnaces versus boilers
The word furnace is often used to denote a residential forced-air heating system, and rightfully so. But a furnace can be any place where a fuel is burned, thus a boiler has a furnace (often called the firebox or combustion chamber) where the fuel is consumed. Likewise, commercial buildings may have rooftop cooling and heating equipment using forced-air as a heat-transfer medium, thus those units have a furnace as well.
Three things about air
When a fuel is burned, three components are necessary: fuel, oxygen, and a source of ignition. Since the atmosphere is approximately 20.9% oxygen, air is used in the combustion process to supply the necessary oxygen for burning the fuel.
Air, though, may also be used as a heat-transfer medium, so in forced-air heating systems, air is not only used for combustion but for heat transfer as well. Boilers use either steam or water as a heat-transfer medium, but once the heat is moved to the radiators or heat-exchangers, air then completes the heat transfer process to heat the building.
In water boiler systems, air entrained in the water causes three problems: loss of heat transfer, as air has a lower coefficient of heat transfer than water; accelerated corrosion, as air contains oxygen, one of three components of corrosion; and water hammer, a pounding or knocking of the piping in the system, which can lead to costly and dangerous piping failures.
Check combustion air regularly
A serious problem will occur if insufficient air is provided for combustion, leading to incomplete combustion and carbon monoxide in the flue gases rising to a dangerous level. During the heating season, be aware of signs of negative air pressure in the mechanical room. A simple way to check is by paying attention to the doors leading in and out of the room. A door difficult to push open can be a sign of lower pressure in the mechanical room than there is outside the room, meaning the burner is starving for air.
On average, burners require approximately 10 cubic feet per minute (CFM) of combustion air for every boiler horsepower (BHP). This equates to a direct outside opening in the mechanical room wall of 1 square inch (in2) for every 4,000 Btu (British thermal unit) per hour of burner input.
Carbon monoxide (CO) detectors should be placed in all mechanical rooms and occupied spaces. Because carbon monoxide is heavier than air, the detectors should be placed to signal a build-up of CO before it reaches a deadly level at a height above the floor where it can be inhaled by a person sleeping.
Clean or replace air filters
When air is used as the heat transfer medium, air filters provide a level of purification for air entering and moving through the building. Air filters should be cleaned or replaced during the heating season, as determined by the system parameters.
As the need for air purification has increased dramatically in the past several years, filters with a higher Minimum Efficiency Rating Value (MERV) are often being installed. These denser filters trap smaller particles, thus becoming occluded much more rapidly.
A visual inspection of high-density filters is seldom a satisfactory means of determining filter efficiency, so the pressure drop across the filter should be measured and compared to the filter manufacturer’s specifications. A Magnehelic gauge or manometer is used to determine the pressure differential.
Water hammer is never normal
Attention should always be given to the sounds made by a heating or cooling system, and water hammer – that banging sound as water moves through the piping of a hydronic heating system – is never normal.
Piping systems are not designed to tolerate water hammer. During the heating season, pay close attention to any noises heard, especially during start-up of the system. If water hammer occurs, investigate, determine the cause, and correct the problem.
Water hammer is never normal!
Watch fuel consumption
It’s easy to neglect heating equipment when everything appears to be running normally and keeping the building warm. But fuel consumption should be monitored, as an increase or decrease in fuel use without a matching change in building load can indicate issues with the burner before they become a cause of a catastrophic failure. Using ‘degree-days’ data as provided by the fuel supplier will help in determining the load as the weather changes during the winter.
If the fuel consumption appears to be out of line, a complete burner check-up and retuning is indicated.
Watch water consumption
Boilers, both steam and hot water (hydronic) systems, require make-up water to provide for losses. Commercial and industrial boiler systems should have a water meter on the make-up line, and readings should be taken regularly. If water consumption increases while the load remains constant, additional water is being lost from the system.
In hydronic systems, this typically indicates a leak; in steam systems it could be a leak of boiler water, steam or condensate, failed steam traps, or excessive blowdown (blowoff) from the boiler.
In all systems, excessive water consumption should be investigated immediately.
Preventive and predictive winter maintenance
Required maintenance will vary widely by the type, size, and fuel of the heating system, but building owners and operators should always follow the equipment manufacturer’s recommendations for maintenance of the system.
While major equipment service is typically performed in the off-season, routine preventive and/or predictive maintenance may be required during the heating season to keep the system performing properly.
At a very minimum, a daily walk-through of the mechanical room should be performed, paying close attention to the sounds emanating from the furnace, boiler, and burner. Any sound that seems unusual should be immediately investigated.
Listen to the burner as it starts, runs, and shuts down. Loud or booming noises on ignition may indicate delayed ignition or excess fuel in the combustion chamber, both of which are a serious and dangerous occurrence.
Listen to the air moving through the ducts, and the water or steam moving through the piping system. Loud air handler noise can indicate fans that are dirty or out-of-balance, dampers stuck or out of adjustment, or filter failure. The sound of water moving through a hydronic system is indicative of air in the system, which left unchecked can cause loss of heat transfer, corrosion, and water hammer. Air vents that are not functioning properly are often the culprit.
Water hammer in steam systems can be deadly and should be investigated immediately. Common causes are both steam in the condensate lines, and condensate in the steam lines. Either situation requires immediate attention.
Observation is the key
Regardless of the heating system in use, observation – a routine look and listen – is the key to maintaining the system to provide reliable service throughout the heating season.