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If it is desired to control the heat pipes off or throttle back the heat transfer, this can be achieved through optional control valves or bypass dampers for the HRM-V™ system.

Modulating Control Valves - A control system with proportional valves for each circuit is available to control capacity. Advanced stepper modulating valves that manage the internal refrigerant flow and draw no power except when adjusting are available for frost protection and economizer operation. For heat recovery during the heating season, the leaving temperature of the exhaust air stream may be lowered to near freezing or below freezing.  Such conditions normally result in the formation of frost on the exhaust side of the heat pipe with partial or complete blockage of the air stream.  To remedy this situation, control valves can be installed in the liquid lines of each circuit. With all control valves open, the heat pipe system will operate at full capacity. With all valves closed no heat transfer takes place. The valves are equipped with over multiple steps each to modulate performance anywhere in between fully opened and fully closed positions.  Closing valves in freeze protection mode ensures that the leaving temperature of the exhaust air stream is kept warm enough to prevent frost formation. As the leaving air (exhaust) temperature drops below 36°F, the valves should begin to modulate closed. The control valves must be controlled by the Building Automation System. The BAS can monitor the temperatures in and out of the coil and determine when to start modulating the valves. Control valves can also be used for capacity control, when outside air is suitable for direct cooling and will not need to be heated by the heat pipes.


HRM-V Wiring Diagram

Figure 7 – Example of 3 circuit, 3 valve, wiring diagram

Face and Bypass Dampers – Face and bypass dampers (provided and installed by others) can also be used for capacity, economizer or frost control.  Bypassing the air around one end of the coil reduces the amount of air flow through that end of the coil, thereby reducing the heat that will be absorbed and transferred to the opposite side. In frost protection mode opening the bypass damper will reduce the heat transfer to a rate that will not cause frost to form while still transferring as much heat as possible. By monitoring the leaving air temperature from the exhaust heat recovery coil, frost can be prevented. Again, as the leaving air temperature drops below 36°F, the damper should modulate open and be fully open at 35 °F. The damper actuator must be controlled by the Building Automation System in the building. The BAS can monitor the temperatures in and out of the coil and determine when to start opening the bypass. Heat transfer can be variably controlled with this method, thus modulating performance under economizer conditions or completely bypassing supply air to prevent frosting in extreme cold conditions. If space is available, dampers are recommended for economizer operation because bypassing air can provide hours of free conditioning in moderate weather and helps save on air pressure drop. Economizer operation is optional.


Control Strategy by Season

Control of the split passive heat pipe heat recovery system shall be as dictated by needs such as frost control, and to perform energy transfer to fit the requirements of the specifications. The design engineer is responsible for providing the set points for operation. Controls are designed to allow the Building Automation System to interface and control the heat pipe circuits as required

Summer Control
During summer operation, all of the control valves can be opened whenever the outdoor air temperature or mixed air temperature, depending on design, rises above the temperature of the exhaust air stream. Typically, maximum heat pipe pre-cooling will be needed for summer recovery.

Winter Control
There are two common control strategies for winter operation with HRM-V™ split passive heat pipe systems. A simple approach is to control the stages of the system with outdoor air temperature, having a set point for each circuit. The second approach is to use the leaving air temperature of the air handler. This approach is more complex, but provides more precise control. For outdoor air temperature control, it will be necessary to estimate the outdoor air temperature at which the first call for heating in the building will occur. This temperature is typically around 50°F, but could be higher or lower depending on the construction of the building and the heat load. For example, with a 2 row circuit design using 50 °F as a starting point, and 70°F exhaust, each stage on the heat recovery system will produce about 3-4° F of pre-heat. With a three circuit system, 3 set points will be needed. The first stage would need to come on at 50°F, the second at 47° F, and the third at 44°F. These set points can be adjusted during the first winter of operation to provide better accuracy. Another method of control is to use the leaving air temperature set point. As the temperature reaches and goes below set point, the three stages of heat recovery can be used as the first stages of heat. Again, each stage will add 3-4°F to the leaving air temperature, so a dead band and/or time delay, will have to be entered appropriately for each stage to prevent short cycling.

Frost Control
When outdoor temperatures approach 0°F, enough heat will be transferred from the exhaust coil to the supply coil so that frost formation may occur on the exhaust coil. The point at which this begins depends on the system, location, and the air flow through each coil. To control frost, the downstream temperature on the exhaust coil must be monitored. As the temperature of the exhaust coil approaches freezing the temperature of the air leaving the exhaust coil will follow by about 4°F. Valves should begin to close one circuit at a time. For example, with a three circuit system, at 36° F leaving the exhaust coil, the valve on circuit 1 should begin to modulate closed. If the leaving temperature again reaches 35 °F, the valve on circuit 2 should begin to modulate closed and so on, until all valves are closed.