The advent of sophisticated control features integrated into residential condensing boilers offers both opportunities and challenges to hydronic system designers. Features such as ‘smart’ modulation and outdoor reset control allow systems to adjust to changing loads, and provide enhanced comfort and fuel economy. Designers of multi-temperature systems face particular challenges, though, because the conventional methods of controlling temperature can either isolate some zones from the features of the boiler controller, or require
additional controls which duplicate those features and add expense to the project. The Passive Injection Loop is a simple piping arrangement which provides for multi-temperature system operation, and works well with the two-pipe configurations illustrated in installation manuals published for many condensing boilers. It is similar to pumped injection loops in that it sends small amounts of high temperature water into a zone, but it does not include a dedicated pump, relying instead on differential pressure generated by the system pump. In this sense it is “passive” rather than “active.”

Strategies for creating reduced-temperature zones on a single boiler system often include a thermostatic device. Consider thermostatic mixing valves. These are set to provide a specific “mixed” temperature to a zone using a combination of higher temperature water from the boiler and cooler recirculated water from the zone itself. The “mixed” temperature is typically selected to meet the heat requirements of the zone during worst case conditions. The thermostatic mixing valve is unable to sense or respond to situations when loads are lower, however, such as during milder weather. Suppose, for example, a mixing valve is set to provide 130 degree F. water to an underfloor radiant heating zone on the coldest design days. It will send that same temperature to the zone on mild days as well, even if the boiler resets its own outlet temperature down from, say 170 degrees to 140 degrees. In this case, the radiant heating zone does not benefit from the boiler’s outdoor reset feature. Any gains in comfort or fuel savings will simply not apply to that zone.

Injection mixing loops can respond to load changes, but require additional controllers of their own. These can range from sophisticated and expensive control panels to less expensive “smart” components, including control valves and circulating pumps with integral sensors and controls. While the expense of some newer controllers is often within reach of smaller budgets, there is still an added layer of complexity when multiple controllers are acting on a single system. It is wise for hydronic system designers to consider how the system and its functions will appear to a service technician a few years in the future. Tracing the functions and effects of multiple controllers may be difficult, or at least time consuming, and many good controllers are either disconnected or under-utilized in the field because they defeated the technician sent to service them. The system owner ultimately pays the bill and may be better served by a simpler design.

To incorporate a Passive Injection Loop, a designer of a multi-temperature system may begin with the installation instructions provided by a condensing boiler manufacturer. These consistently show piping schematics based on a two-pipe distribution system, with elements of primary-secondary piping. The boilers are generally shown with dedicated circulators installed in a boiler loop which connects to the main system via a pair of closely-spaced tees. The boiler circulator ensures proper circulation through the boiler itself, and the primary-secondary nature of the connection to the system prevents changes in zone valve or circulator operation from altering boiler flow. The distribution system itself may be divided into zones controlled by electric valves or by circulating pumps. In either case, the point where the boiler outlet connection connects to the main system will see the highest water temperatures, and the air separator and expansion tank assembly should be installed as close to this point as possible, followed by whatever system pumps are to be used.

The Passive Injection Loop incorporates a complete zone circuit, with its own circulator, piping and heat emitters, and is connected to the supply and return mains by dedicated small diameter supply and return piping. Key to the Passive Injection Loop concept is that the small diameter injection piping connects on one side to the supply and return mains, respectively, and on the other side to the zone circuit via a pair of closely-spaced tees. Pressure differential generated by the main system pump stimulates flow through the small diameter supply and return piping, while the primary-secondary connection to the zone circuit provides the hydraulic disconnect to allow it to operate independently. For multiple-zone systems, a differential pressure bypass valve is a good way of maintaining consistent differentials between the supply and return legs of each injection loop. This in turn will maintain a consistent flowrate through each loop for all combinations of operating and non-operating zones. A simple calculation will show the designer how much high temperature flow is required to maintain the required zone temperature, and an adjustable flow-regulating valve in the small diameter return line allows temperature control. As with pumped injection loops, the amounts of flow required are often very small—fractions of a gallon per minute—and so pipes and valves may be sized accordingly.

Note that a designer may decide to use electric zone valves in the injection piping to ensure positive shutoff of the high temperature water and prevent zone overheating. An argument could be made that if high temperature water were to continue to flow into the zone circuit even without a call for heat, the zone could not transfer that heat unless its circulator were operating. This reflects the hydraulic decoupling effect of the closely spaced tees which connect the injection piping to the zone circuit. Any flow into the circuit would have the option of flowing through the length of the circuit itself, or simply “short-cycling” back into the return leg of the injection piping. A flow-control valve installed in the zone circuit would further ensure that unneeded high temperature flow simply returns to the return main via the injection piping. If electric zone valves are to be used, an additional relay will be needed since both the pump and the zone valve must be energized.

When the system is to be started, the installer must first purge all air from the both the main and zone loops. For purging, it is best to keep the control valves open fully. Once the system is free of air, the installer should set the flow regulating valves in the injection piping to about 50% open. Note that properly sizing piping and valves makes the system easier to adjust; guard against the tendency to oversize components. With all pumps running and any electric zone valves open, adjust the flow regulating valves as necessary so that the flowrate through the injection piping matches the calculated injection flow.

As the boiler comes up to temperature, it will be possible to verify that the injection flowrates are producing the desired temperature in the zones, and to make any adjustments as needed. The zone circulators will govern the injection flow requirement; if the flowrate through the zone is higher than expected, then the regulating valves in the injection piping can be adjusted accordingly. Radiant heating zones often benefit from a slightly high flowrate through the zone because the temperature differential decreases, yielding a more
consistent floor temperature.

A Passive Injection Loop offers hydronic system designers a means of providing multiple supply water temperatures from a single heat source using a single system controller. It is as simple in its operation as a manual bypass valve, yet can be adjusted with some precision. And when used with a differential pressure bypass valve, a Passive Injection Loop will perform predictably in multiple zone applications.

Jack Guilfoyle
25.Feb06