Boiler Short-Cycling After a Load Change
A low-pressure hot-water boiler with a rated output of 300 kW serves a building whose heating load has fallen after a wing was vacated. The boiler now fires for about 4 minutes and rests for about 6 minutes, repeating through the day. Supply water reaches 82 C within the firing period and the return is around 68 C. The boiler has a fixed high-limit aquastat set at 85 C and a low-limit set at 70 C. The pump runs continuously. The building manager reports the plant room is noisy and the gas bill has risen despite lower occupancy. A contractor proposes raising the high limit to 90 C to lengthen firing periods.
1. Diagnose the cause of the short cycling and evaluate the contractor's proposal.
Read the worked answer
The symptom pattern points to the boiler being oversized for the reduced load combined with a control band that is too narrow. With a 300 kW output and a much smaller connected load, the water in the primary loop heats quickly; the aquastat band between 70 C and 85 C is only 15 K, so the boiler reaches the high limit in about 4 minutes and then waits while the loop cools. Continuous pumping spreads that heat into the system, which is why the return sits at 68 C and the cycle repeats.
Raising the high limit to 90 C is the wrong fix. It widens the band slightly but pushes the boiler closer to its safety limit, increases standing losses and the risk of nuisance high-limit trips, and does nothing about the mismatch between output and load. It may lengthen firing marginally while making the plant less efficient and less safe.
The real causes are capacity mismatch and control strategy. The correct direction is to reduce effective output, for example by staging or sequencing with other boilers, fitting a modulating burner, or isolating the boiler from the vacant wing so the connected load and the boiler match. The control band should be reviewed against the boiler manufacturer's minimum cycle guidance, not widened arbitrarily.
2. State what data you would collect to confirm the diagnosis and how you would verify the fix.
Read the worked answer
Collect the boiler's rated output and the actual connected load after the wing was isolated, ideally from a heat-load estimate or from measured flow and temperature difference across the boiler. Log supply and return temperatures and burner on/off times over at least a full day at part load, so the 4-minute/6-minute pattern is quantified rather than anecdotal. Record the aquastat settings and confirm the pump is truly continuous. Check gas consumption against degree days before and after the occupancy change, so the cost rise is separated from weather.
Confirm the diagnosis by showing that the firing period ends at the high limit rather than at a load-driven signal, and that the cycle rate is well above the manufacturer's recommended maximum starts per hour. If the boiler has a minimum firing rate or a minimum return temperature requirement, check those too, since short cycling can also cause flue-gas condensation and corrosion in a boiler not designed for it.
Verify the fix by re-logging after the change: cycle frequency should fall, firing periods should lengthen at part load, and the return temperature should stay within the manufacturer's range. Compare gas use over a comparable period and confirm the plant room noise has reduced. Safety limits must remain at their designed values throughout.