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Heat-Timer® Corporation

PLL Controller is A Robust Solution to Multiple Pump Management

We all know the importance of rotating our tires. On most vehicles, tire wear is uneven front to back, so manufacturers recommend tire rotation at every oil change. Tire rotation enables all four tires to wear evenly, so they can all be replaced at once when they are worn.

Unfortunately, even with proper maintenance, a tire will still fail occasionally due to a stray nail or a winter pothole. That is why most cars still carry a spare tire that can be swapped in for the flat tire, preventing you from being stranded far from home late at night.

It turns out that a similar situation exists for building managers looking to the health and efficiency of their hydronic heating systems. Much like our tire rotation analology, failure to rotate the hydronic load across the system can lead to early pump failure and unnecessary replacement expenses.

What is a Pump Sequence Control?

A pump lead-lag (PLL) controller (also called a pump sequence controller) can balance the run time of multiple boiler pumps to minimize the number of scheduled and unscheduled service calls.  It can also automatically manage a backup or standby pump, which like a spare tire, will be ready to use in case of a pump failure. Unlike a spare tire, which requires a roadside repair to install, the PLL will switch over to the backup pump automatically, with no interruption in heating system availability.

A simple system managed by a PLL consists of two or three pumps. Each pump is sufficient to meet the heating system demand. The PLL controller will balance the run time between the two pumps, as much as doubling the preventive maintenance interval of the pumps.  An example of a simple 3 pump system is shown below:

In this system, the Heat-Timer PLL Control is shown with a 3 pump single boiler configuration. This system is often referred to as a 3 pump, 1 call, 1 flow piping. Named because there is only one place where hot water is called to the heating system and only 1 flow from the boiler to the pumps. In the above configuration the Heat-Timer PLL controls the pumps, the flow switch, and monitors the input of “proof of flow” sensors. With these capabilities and more, many configurations of boilers, pumps and number of calls and flows are possible. For instance, in a two boiler system, the Heat-Timer® PLL can manage three pumps; two primary pumps, one for each boiler, and a back-up pump that can automatically connect to either boiler via PLL controlled solenoid valves. The PLL monitors the flow to each boiler.  If the flow to one boiler is interrupted, then the PLL will open the solenoid valve between the backup pump and the boiler with the failed pump. The backup pump will then provide uninterrupted flow to the boiler until the primary pump can be replaced.

Our Tire Rotation Analogy Extended

Honestly, who would remember to rotate their tires at the scheduled interval if the dealer didn’t nag you?

Fortunately for building managers, a PLL control operates completely independently. It would be as though your tires rotated themselves every 10,000 miles. Heat-Timer’s PLL control for instance offers two distinct modes of rotation, either time based rotation or alternating demand rotation which activates a different pump each time a call for hot water is initiated.

Sometimes no matter what you do, you get a flat tire. Likewise if one of your pumps should fail, you need to know immediately. The Heat-Timer®  PLL control continuously monitors pump flow rate to the boiler or boilers. If the on-call or primary pump fails the PLL can sense the interruption in flow in less than a minute and immediately failover to the remaining pump or the backup pump.  An alarm is set locally on the panel and can be sent to alert building management of the pump failure. Because of the automatic failover, there is no interruption to building heat and to the comfort of the occupants and no weekend or holiday emergency call.

Having a spare tire is useless if it is flat when you need it. When was the last time you checked the pressure in your spare tire?  Having a back-up pump that won’t start when needed is equally useless.

A building manager does not need to worry about this because the Heat-Timer® PLL can prevent having a “flat spare”. Our controllers have an “exercise” feature that will periodically run standby pumps for a short time to break up rust and sediment and lubricate pump seals. Periodic exercising of the back-up pump will ensure that it is ready to run when called on.

Heat-Timer® PLL Controller is A Robust Solution to Multiple Pump Management

For the building HVAC manager, adding a second or third feedwater pump controlled by a Heat-Timer® lead-lag controller is an inexpensive way to increase hydronic heating system reliability, while reducing expensive emergency service calls and preventive maintenance intervals. What used to be a late night emergency becomes a minor problem that can be dealt with at a convenient time.