Zoning Hydronic Systems with ECM Technology
by John Fantauzzi

When designing hydronic systems, heating and/or cooling, we are often faced with a system that will require control of various zones to provide efficient performance and comfortable space for the building occupants. Prior to ECM (electronically commutated motor) technology, we were faced with a choice of zone pumps or zone valves. Each approach has benefits and limitations. When determining which method to use, one must consider the characteristics of each approach.

Zone Pumps:
This method uses two or more pumps to provide each zone with controlled circulation. It is
required that a self-closing check valve be used on each zone in this type of system. Without the check valve, water can flow through the inactive zones and cause hot water to enter and heat those zones even though there is no call for heat. This can occur by the active pump pulling water from the other zones or by hot water thermosiphoning through the inactive zones.

Since individual zones only require a portion of the total system load, the circulators on each zone may be smaller than using one larger circulator for the entire system. However, consideration must be made to ensure that proper flow exists in the boiler regardless of the number of zones active. This may require a primary/secondary piping arrangement with a primary circulator providing sufficient boiler flow.

With multiple pumps, the failure of a single zone pump is less critical than the failure of a single system pump. While the individual zone may be without heat, the entire system is not “off line”. It is important that the capacities of the multi-zone pumps be matched for flow and head. The use of a high-head and a low-head pump in the same distribution system may cause problems if both are operating at the same time.

The high-head pump could overcome the capability of the low-head pump and prevent heat from circulating though the low-head pump’s zone. Also, it is important to match the output of the pump to the zone’s requirement. Placing a zone pump on a single loop, such as a bathroom circuit, can cause very fast flow within the circuit leading to lower delta-T, potential noise, and possible premature erosion of copper or brass fittings and piping.

Using multiple pumps makes it easier to balance the zones. Since the zones are in parallel and being serviced by their own pump, balancing valves can be used to match the head resistance in each zone. When properly balanced, the head resistance in each zone will not change regardless of the number of zones active. Multi-zone pumps are best utilized in systems with large zones.

Zone Valves:
Zone valve systems have performance characteristics that are significantly different from zone pump systems. As the room thermostat calls for heat, the zone valve opens and the end switch in the zone valve signals the boiler and circulator to turn on. When the heat call is satisfied, the zone valve closes thereby preventing further flow into the zone. Check valves are therefore not required with zone valve systems.

However, there is a considerable difference in the flow characteristics between zone valve and zone pump systems. Zone valve systems use a single system circulator to service the entire system. Therefore, the circulator must be sized to provide the total system load when all the zones are open.

When only one zone is open, the system circulator is still providing maximum output, which can exceed the design flow rate through the active zone. This increased flow rate may lead to noise within the zone and premature wear of the valve seat. This can also lead to increased power consumption since a large circulator is being used for only a portion of the system load. Since only one circulator is servicing the system, a circulator failure will disable the entire system.

Balancing with zone valves is more difficult than with zone pumps. When one zone is open, the system is operating as though it is one circuit. When a second zone opens, the flow divides between the two zones according to the resistance in each zone. When a third zone opens, the flow divides again. Each circuit on each zone must be balanced. Then each zone must be balanced to ensure proper flow when all zones are open

As the differential pressure rises there can be noise, premature valve wear, damage due to hammering, and valve malfunction if the valve seat is forced open. This zone “behavior” necessitates the use of a pressure differential bypass to help equalize the system pressure drop through various open/closed zones.

The pressure differential bypass operates according to the adjustment of the bypass setting. As the zone valves open and close, the system differential pressure can increase or decrease as pump flow goes down or up respectively. The pressure differential bypass valve is a control valve that opens and closes automatically to maintain a near constant differential pressure across the supply and return piping.

Zone valves can effectively be used in systems with smaller zones containing few circuits. Using a system pump with a flat pump curve will help the system performance. Since the system pump head changes very little with increases or decreases in flow, the overall effect is a relatively stable flow rate as zones open and close.

ECM Technology
Using ECM circulators eliminates the problems associated with the two methods described above.  ECM circulators automatically adjust the output to match the system requirements. By adjusting the circulator’s head setting to the maximum system requirement, the ECM circulator will monitor the system status and adjust its speed up or down depending on the number of zones open.

This technology eliminates the issues of using constant speed zone pumps or zone valves. Since the ECM circulator adjusts to system requirements, noise and erosion issues are eliminated and proper system/zone flow is assured. Also, when using zone valves, the pressure differential bypass valve is no longer required.

Since the flow is automatically adjusted to the system status, components are not subjected to high velocity and pressure and can achieve a longer system life. With constant speed pumps, many zone valves have failed due to high head pressure on the valve seats as they close against increasing pump head as the pump follows the traditional pump curve. As the flow requirements are less, the pump head is higher. This is backwards to the system requirement. With ECM circulators, this issue is eliminated. As the flow requirements decrease, the head decreases also.

Another benefit of ECM technology is reduced energy consumption to achieve the same system performance. ECM circulators can save up to 80% of the wattage normally consumed by constant speed pumps while achieving improved system performance. By reducing the speed of the pump, the wattage is reduced significantly. Instead of a constant speed pump operating near its maximum wattage regardless of the system requirements, ECM circulators are constantly adjusting their power consumption based on system status.

Additionally, by reducing the speed of the circulator, bearing life is increased resulting in extended operating life of the ECM circulator and reduced maintenance costs compared to traditional constant speed circulators.

Zone valve systems tend to be lower in cost than zone pump systems since control wiring is low voltage. Since ECM technology eliminates the pressure issues imposed by constant speed pumps, there is no longer an issue with valve failure due to excessive head pressure reducing maintenance costs. Therefore, the expected life of the zone valve is improved.

Since ECM circulators operate at minimum flow and head when all zones are closed, they effectively idle with minimum wattage draw when there is no call for heat. These circulators can “dead head” with no damage to the circulator. This characteristic allows for reduction in control wiring as there is no need to switch the circulator on and off using pump relays or zone valve end switches.

In the past, the zoning choice usually came down to preference. While both zone pump and zone valve options can provide good zoning control, the ECM technology improves on the system performance and yields much better system efficiencies. It makes good practice to develop a system approach when installing high efficiency equipment. When taking advantage of the new modulating/condensing boilers, it is good practice to utilize pumping components that will improve on the boiler’s performance. Matching the flow rate to the system requirements improves boiler performance and allowing proper flow and temperature drop in the zones improves boiler efficiency while reducing operating wattage for the circulator(s).