This bank of four boiler modules is operating at part load and infra-red imaging confirms that the left-hand module, which is shut down, is not losing heat to atmosphere thanks to an automatic flue damper which prevents cold air being drawn through it.

The other three modules are sharing a relatively light load by running at low output. This tends to incur less loss than operating one or two units at high fire, because the exhaust temperature is lower at reduced burner output.