Although hot water boilers are relatively simple technologies, they are often not controlled well presenting plenty of opportunities for optimisation.
Designing hydronic heating stages by switching on/off boilers in systems with multiple boilers will save energy and limit potential wear and tear while retaining comfort.
Using controls to obtain optimum performance from multiple boilers requires an understanding of staging control and the individual boiler characteristics.
The benefits of multiple boiler staging controls includes:
- staging of boiler modules to meet load demand
- automatic rotation of boiler modules to ensure equal run time
- PID control logic for precise control of supply water temperature.
For non-condensing boiler systems, checks should be carried out to ensure the reset control strategy does not lower return water temperature to the point where flue gas condensation is triggered.
Using air-to-fuel ratio control equipment such as oxygen trim control ensures a more complete combustion to reduce energy consumption by up to two per cent.
The stack gas temperature should be measured under normal operating conditions after servicing and cleaning and this optimal temperature regularly compared with the stack temperature.
Major variations can indicate a drop in efficiency and a need to adjust air-to-fuel ratios. It is estimated that there is a one per cent efficiency loss with every 5°C increase in stack temperature above the optimal temperature.
Boiler efficiency can be improved by installing heat recovery equipment such as economisers or recuperators. An economiser is an air-to-liquid heat exchanger that recovers heat from the flue gases to pre-heat boiler feed water.
A recuperator is an air-to-air heat exchanger that is used to recover heat from flue gases to pre-heat combustion air. VSDs can be retrofitted to combustion air fans to continually match the load of the boiler.
Running more boilers at lower loads (i.e. the optimal load for the specific plant) can provide better efficiency for condensing boilers.
Multiple boilers also provide an inherent backup in the event of a boiler failure. Variable primary pumping should be supplied when possible as it provides lower initial and operating costs.
This should also include two-way, two-position control valves at each boiler to prevent flow to a boiler when not in use.
Where condensing boilers are installed, it is important to ensure that the HHW return is below 55°C as often as possible to ensure that optimum efficiency is achieved.
Where condensing boilers are used with conventional boilers, it is important to configure the condensing boiler as the lead boiler and to ensure that when conventional boilers are used for backup and/or additional capacity, the HHW return temperature is greater than 55°C to prevent boiler back-end corrosion.
Source: HVAC Optimisation Guide which is published by the NSW Office of Environment and Heritage in partnership with
AIRAH.