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Improved Boiler Plant Control

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  rev. 2008-11-25        

Existing Conditions

The existing boiler plant is under <pneumatic> <electric> <manual> control that is very basic and does not work well.  With this type of control, the supply water temperature is higher than necessary, and pumps run longer than necessary to cover all possible expected operating conditions.  <Also, there is a continuous water flow through the boilers, resulting in high standby losses.>

These controls are subject to tampering when they fail to perform well, and cannot be set up with the energy-saving strategies available with a modern digital controller or building automation system.

Retrofit Conditions

We suggest replacing existing controls with a 'unitary' automation controller.  This type of controller is a standalone unit with input and output capability and a small LCD display and keypad.  It would be programmed at the time of installation according to energy-conserving strategies specified by the design engineer.  The building operator would be able to use the keypad to change basic settings such as occupancy schedules and temperature setpoints.  Some of these controllers are members of product families that can be networked together into facility-wide DDC (Direct Digital Control) systems.  DDC systems can include many safety and convenience features such as full graphic terminal operation, remote access via phone line, dial out of alarms, security monitoring, and trending of energy and equipment data.

For the purpose of this study, we are recommending the most cost-effective approach, which is the installation of small local controllers intended to save energy.  The decision to connect several units together into a single network can be deferred until clear operating benefits are identified.

For this unit, we recommend the following strategies:

•  Indoor / outdoor reset of supply water temperature, with sample room temperature adjustment.

•  Operator adjustment of reset curve endpoints and room sensor authority.

•  Night setback of sampled space temperature or supply water temperature and optimized morning warmup.

•  Weekly lead / lag rotation of boilers.

•  Heating pump rotation.

•  Summer shutdown of boilers and pumps, with daily pump exercise to prevent seizing.

•  Holiday scheduling.

•  Auxiliary output for scheduling associated exhaust fans.

•  Auxiliary output for scheduling outdoor security lights.

•  Auxiliary output for scheduling domestic hot water recirculation pump.

•  Text alarm messages displayed at local terminal.

Further Benefits

Reduction in short cycling will extend equipment life.

Application Details

Install an automatic boiler sequencing controller designed for the installation layout, Tekmar or equivalent.  This will eliminate the need for continuous operator attention and will ensure best appliance efficiency under all conditions.

The new controller will require accurate setup to ensure that it is properly applied to the situation.  Go through the installation manual careful and consider all of the features and options, rather than simply using the default settings from the factory.

Issues and Concerns

Controls must be set up to avoid "short cycling", which is overly rapid "on" and "off" firing cycles.  Just like with a car, a certain amount of heat and fuel are wasted each time a boiler is started.  In addition, short cycling puts stress on boiler components and can reduce life expectancy.

Immediately after new controls are installed and set up, a period of observation and adjustment is usually needed to avoid short cycling under the full range of load conditions.

Quite often, short cycling is due to design deficiencies (e.g. the available boiler stages are too big for the thermal mass of the heating system), however controls can minimize the problem.

If short cycling continues to be a problem even after controls are set up and adjusted, have a service technician check and correct these conditions as necessary:

Wrong temperature sensor location.
Inadequate water flow due to pipe blockage or malfunctioning pump.
Gas input (manifold pressure) too high.


Conforms to recommendations in "Energy and Water Efficiency in Multi-Unit Residential Buildings", Canada Mortgage and Housing Corp.



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