A boiler that performed adequately through last heating season may not perform adequately through this one. The difference is not always the boiler’s age or its mechanical condition. Sometimes it is the load. As outdoor temperatures drop further and hold there longer than they did in previous seasons, the boiler plant operates closer to its design limits for extended periods rather than for the brief peak load days that a moderate winter produces. Equipment that has enough capacity to meet an average winter load may not have enough capacity to meet a cold winter load, and the degradation in components that was manageable at partial load becomes visible when the plant is running at or near its rated output for days at a time.

This article covers what happens to aging commercial boiler performance as the heating season progresses into cold weather and what facility managers should be watching for as temperatures drop.

Capacity degradation at high load

A boiler that has accumulated heat transfer surface fouling operates with a reduced effective heat transfer area. At low to moderate firing rates, the boiler can compensate by firing for longer periods. At high firing rates, the boiler hits its effective capacity ceiling faster and is less able to maintain building temperature when the outdoor temperature drops and the heating demand increases.

A building that was comfortable at outdoor temperatures above 20 degrees Fahrenheit may become uncomfortable at outdoor temperatures below 10 degrees not because the boiler has failed but because the available capacity at high firing rates has been reduced by fouling to the point where it can no longer meet the peak load. The symptom looks like inadequate boiler capacity. The cause is often addressable through cleaning and water treatment rather than replacement, if the fouling is caught before it has been compressing output for multiple seasons.

The test is straightforward. If the building temperature holds at design setpoint on a 20-degree day but cannot hold it on a 5-degree day with the boiler firing continuously, the boiler is either undersized for the current load profile or is delivering less than its rated capacity at high firing rates. Distinguishing between the two requires a combustion analysis and a heat transfer surface assessment.

Combustion drift at high firing rates

Combustion control systems that are correctly calibrated at low and moderate firing rates may not be correctly calibrated at high firing rates. Modulating burner control systems set their fuel-to-air ratio through a combination of characterized positioning curves for the fuel valve and the combustion air damper. If the characterization is not accurate at the high end of the modulation range, the burner may be running lean or rich at full fire in ways that are not visible at the partial-load conditions where most seasonal tune-ups are performed.

A lean combustion condition at full fire reduces the heat release rate below what the fuel input would produce at the correct fuel-to-air ratio, which reduces boiler output at the high firing rates where output is most needed. A rich combustion condition at full fire produces carbon monoxide and unburned fuel, which is both an efficiency loss and a safety concern.

Pre-season combustion analysis should include measurements at the full-fire condition, not just at the intermediate firing rates where most of the heating season operation occurs. A tune-up that only verifies combustion at partial load has not verified the combustion condition that determines whether the building stays warm on the coldest nights.

Safety control performance under sustained load

Safety controls that have not been tested under sustained high-load conditions may behave differently than they do during periodic test cycles at low load. Low water cutoffs that trip intermittently under high evaporation rates in steam systems, pressure relief valves that weep at high operating pressures that the system rarely reaches in moderate weather, and high-limit controls that trip during high firing rate conditions because the operating control setpoint is too close to the high-limit setpoint are all failure modes that appear under sustained cold weather load.

For steam systems, the evaporation rate at full load is significantly higher than at partial load. A makeup water system and low water cutoff that functions correctly at partial load may trip the low water cutoff at full load if the makeup water flow rate is insufficient to keep pace with the evaporation rate. Confirming the makeup water capacity against the boiler’s maximum evaporation rate is part of cold weather preparation that a moderate-weather pre-season inspection may not cover.

Flue system performance at high firing rates

The breeching, stack, and flue gas path that functions correctly at partial load may not provide adequate draft at full fire. Draft requirements increase with firing rate, and a breeching or stack that is marginally adequate at design conditions may produce insufficient draft at high firing rates, which increases backpressure at the boiler flue gas outlet and affects combustion quality.

For systems where the breeching was sized for a previous boiler with different flue gas outlet conditions, or where the breeching has accumulated deposits that restrict the flue gas path, cold weather high-load operation is when the problem becomes visible. Draft measurements under full-fire conditions, compared against the manufacturer’s draft requirements for the boiler at maximum firing rate, identify whether the flue system is adequate for cold weather operation.

What to do when cold weather performance is inadequate

A boiler plant that cannot maintain building temperature during the coldest weather is communicating something specific. The question is whether the diagnosis is inadequate capacity, degraded performance from a correctable condition, or a mismatch between the current building load and the boiler plant’s design parameters.

Inadequate capacity that has been consistent since the boiler was installed points toward a sizing problem or a load increase since the original installation. Inadequate capacity that has developed over time points toward performance degradation. The repair versus replace decision described in our earlier article on boiler end of life indicators is the framework for determining which of these is driving the problem.

For GP Energy Products service customers across Pennsylvania, New Jersey, Delaware, and Maryland who are experiencing cold weather performance issues with their boiler plant, reach out and we will start with a diagnostic assessment rather than a replacement quote. The assessment tells you which of these is the right conversation to be having. For the pump systems serving the boiler plant, the Merion Pump Company team handles pump system assessment and emergency service. Visit merionpump.com for more.

References
1. ASME CSD-1. Controls and Safety Devices for Automatically Fired Boilers. Covers safety control testing requirements including performance under sustained load conditions. asme.org
2. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Boilers. Covers boiler capacity assessment, combustion analysis at high firing rates, and cold weather performance evaluation. ashrae.org
3. American Boiler Manufacturers Association. Boiler Room Guide. Covers boiler performance diagnosis and cold weather operational requirements. abma.memberclicks.net
4. NFPA 85. Boiler and Combustion Systems Hazards Code. Covers combustion safety requirements applicable to high-load boiler operation during cold weather conditions. nfpa.org