HRM-V™ System Control

HRM-V heat pipes can be selected with control valves that are installed in the liquid lines of each circuit. HPT provides a control panel that receives power (120VAC) and one or more analog signals from the Building Automation System (BAS) – both 0-10VDC and 4-20mA can be used. When fully open the valve does not restrict refrigerant flow and as the valve closes it restricts refrigerant flow with progressively larger pressure drop until it is fully closed, when there is no refrigerant flow and therefore no energy recovery.


For best control the valves can be operated using individual stages that cascade, one valve after the other, (a 6 row system could have three stages) or, for more economy, valves can be grouped.


An alternative to using valves is bypass dampers, where air is routed around the heat pipe coil to reduce the amount of energy recovery. These are not provided by HPT. A bypass damper alone is insufficient to provide a full control range. A more complete solution is full face and bypass dampers that block air from flowing though the coil whilst offering an alternate flow path. If dampers are applied in one airstream only, and if frosting is a concern, the dampers should be installed into the supply side.


The Dynamic Seasonal Offset (DSO) option is used when coils are installed level and the user wants optimized performance for both summer and winter conditions. Partial face dampers (provided by HPT) are installed in the top and bottom of each heat pipe module and direct air through the modules in a way that always places the condenser higher than the evaporator, so gravity promotes better refrigerant flow. The dampers are linked together so a single signal (115VAC supply) operates them either open or closed and, when power is removed, a spring return changes them to normally closed or normally open respectively. For example, when there is no signal power the DSO defaults to optimize for heating, when most recovery is achieved in the year in the majority of North America. With no power applied, the upper and lower supply dampers are open and closed respectively and the upper and lower exhaust dampers are closed and open respectively (see Figure 5). When the outside air temperature rises above the return air temperature, the damper positions should change to optimize for cooling.


Table 1 shows the different control scenarios seen through the year as outside air temperature changes.

Outside Air Condition

Control Type



Very Cold

(e.g. <0°F)

Frost Control

Optimized for Heating

Maintain an Exhaust Air leaving temperature ≥36°F

Below Supply Air set point

(e.g. <55°F)

Modulate Heating

Maintain Supply Air at set point or if not enough recovery, full performance


(e.g. ≥55°F and <75°F)


No recovery wanted

Summer cooling

(e.g. ≥75°F)

Full Recovery

Optimized for Cooling

Allow full heat pipe performance


Table 1 – Control Types for Various Outside Air conditions assuming a Supply Air Set Point of 55°F and 75°F Return Air