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Hot water heating renewal series: plant monitoring and control

 In the Canadian climate, heating is the biggest energy use is most buildings. And most of our heating plants are based on water heated by boilers. This series of articles discusses fundamentals of hot water heating plants. This month’s article is on plant monitoring and control.


April 17, 2009
By Scott Martin

Proper control of your heating/cooling plants is essential in keeping your tenants happy and your utility costs down. Because the plants are designed to meet extreme weather conditions, they are seldom running at full capacity.

The heating plant was sized to meet the heating requirements of the coldest day in January. Typically, safety factors and redundancy are included in the sizing method, resulting in a boiler capacity that is often twice as large as needed. The chilled water plant was sized for the hottest, most humid day in July, with similar safety factors.

Fortunately, it is only the coldest day in January once a year. The rest of the time, the boilers and pumps are larger than needed. As a result, the equipment must be properly controlled to keep your tenants comfortable and to minimize utility costs. Otherwise, it would be like driving your car full throttle without brakes.

Basic Control
The simplest heating/cooling plant control consists of a combination of manual control of pumps and the use of simple mechanical/electrical devices called aquastats to control boilers and chillers.

This type of control is not very precise, resulting in overheating, overcooling and frequent cycling of equipment.  These imprecise controls result in poor tenant comfort and wasted energy. Windows are often opened in the middle of winter to compensate. (Riding the throttle and brakes at the same time.)

Microprocessor Unitary Control
More complicated unitary controllers are microprocessor based. They use sophisticated strategies to control the entire heating or cooling plant instead of just individual pieces of equipment. A higher-end controller will properly stage multiple boilers, start/stop pumps, regulate domestic hot water heating, etc. These controllers have much of the control functionality of a full DDC (Direct Digital Control) system, but at a fraction of the cost.

Direct Digital Control (DDC)
Direct Digital Controls are commonplace in commercial buildings and are making strong headway into the multi-residential market.  Although a DDC system is more expensive than other control methods, there are significant advantages. 

The main advantage is that DDC systems can control any type of common equipment or plant and make decisions based on numerous inputs.  For example, if your boiler plant supplies heating to the suites, the pool, the domestic hot water system and the ramp heating, the controls can monitor these systems and supply the lowest possible water temperature required to meet the heating requirements.

Another big advantage of DDC systems is that they are graphical in nature. Instead of going to the mechanical room, finding an error code and looking it up in a generic owners manual, a DDC system gives a graphic display of the system on your computer screen with indications of real-time operating conditions such as temperatures, pressures, valve positions, status of boilers, etc. This information can be viewed from the Superintendent’s office, across town or across the country.  And operating adjustments (schedules, temperature setpoints) can be made the same way. Newer systems allow you to view and adjust equipment through a normal web browser.  

A third advantage for DDC systems is that they can be set up to send notifications or “Alarms” to property managers or directly to maintenance contractors.  Contractors can then diagnose and fix problems, often before any tenants notice.

As DDC prices continue to drop, the advantages of reduced utility costs, remote monitoring and adjustment, and the ability to send alarms will result in further penetration in markets (like multi-residential) where uptake has been limited up to now.

Scott Martin, P.Eng., a partner with Efficiency Engineering Inc. in Cambridge, specializes in energy efficiency and control of central plants.  He can be reached at smartin@ee-solutions.com.

To read the first article in this series, on boiler efficiency, click here.