WSHP System Control and Optimization Part 4: Why Waterside Economizing Might Not Improve Efficiency

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By Chad Edmondson

Just about every major water source heat pump (WSHP) manufacturer now offers equipment with an integral waterside economizer, no doubt to give engineers a straight forward path to meeting the following ASHRAE 90.1-2013 requirement for air or waterside economizing:

However, while the economizer requirement applies to just about every climate zone in the US, a closer read of the standard shows it is very likely not required on the majority of multi-zone commercial WSHP systems. If the cooling efficiency of the WSHP unit meets or exceeds the efficiency improvements requirements shown in ASHRAE Table 6.3.2, an economizer is not required.

Currently there is very limited information about what efficiency gains an economizer might yield over the efficiency of the WSHP system itself. In fact, it may actually increase long term operational cost, not to mention system complexity. Fortunately, ASHRAE 90.1 includes exceptions within the standard that give engineers plenty of room to prioritize the efficiency of the overall system over the singular strategy of an economizer. But before we get to that, let’s explore what makes these two energy saving strategies potentially incompatible.

Water Temperature Requirements: Economizer versus operating in the WSHP Dead Band

The point of conflict between typical WSHP operation and WSHP economizer operation is likely to occur when it is cold outside, but part of the building still needs cooling.

If the system has been designed to operate in accordance with ASHRAE 90.1-2013 Section 6.5.1.2, then the fluid economizer would provide up to 100% of the expected system cooling load whenever the outdoor temperature drops to 50°F dry bulb/45°F wet bulb or below. In order to provide full cooling, the economizer in each unit may require supply water temperature to be as low as 45°F. Meeting this requirement means resetting the WSHP loop temperature below the preferred dead band temperature range (typically 50°F to 90°F) of system until cooling is no longer required. This sidelines the most valuable asset of a WSHP system, its battery capacity.

If we reset a WSHP loop to 45°F, then the heat pumps that are in heating mode become less efficient. Simultaneous heating and cooling is the key to WSHP system efficiency. This is what allows us to maximize the battery capacity of the water loop while minimizing the use of the cooling tower and boiler.

The efficiency penalty for resetting the loop temperature to 45°F can be significant. If the coefficient of performance (COP) for operating a WSHP at 70°F is 5.9, the COP for the same system drops to 3.6 at 45°F. This could actually interfere with ASHRAE 90.1-2013 compliance since Section 6.5.1.5 Economizer Heating System Impact states that the HVAC system design and economizer control shall be such that economizer operation does not increase the building heating energy use during normal operation.

Here are a few other things to consider before including waterside economizers in your WSHP system design:

  • The addition of a waterside economizer coil increases the pressure drop through each zone. (Note: ASHRAE 90.1-2013 Section 6.5.1.5 Maximum Pressure Drop states that the water-to-water heat exchanger used as part of the economizer shall be less than 15’ or an additional secondary loop must be created with its own recirculation pump.)

  • The economizer coil also creates additional airside pressure drop through the unit. This pressure drop will exist whether we are operating in economizer mode or not.

  • Operating in economizer mode means operating the cooling tower to make 45°F water, regardless of the season. This gets complicated (and more expensive) in winter since cold air holds less moisture. Approach and wet bulb temperature become critically important for effective operation of the cooling tower in winter. And remember, under normal WSHP operating conditions, you typically would not have to operating the cooling tower at all in the winter as long as the loop stays within dead band.

  • What is leaving water temperature off WSHP in heating mode? If we bring 45°F to a WSHP in heating mode the leaving water temperature could be in the 30’s.

  • Do you need a glycol system to keep the system from tripping out due to low water temperature?

  • What is your operational sequence? Again, this gets tricky in winter. Remember, the economizer is expected to provide partial cooling even when mechanical cooling is in operation.

How to Eliminate the Economizer Requirement If It Doesn’t Make Sense

As we stated early, you do have options when it comes to meeting ASHRAE 90.1-2013 without having to implement waterside economizing in your design.

In our part of the country, Climate Zones 3a and 4a (North Carolina, South Carolina, Tennessee, Georgia, and Alabama), the cooling efficiency of the system must be improved over code by 27% or 42% respectively to avoid the economizer requirement. We’ve created an easy cheat sheet to help you determine if the WSHP equipment you select for your application will allow you to meet these efficiency improvements in these zones. Click here to download the document for Increased Efficiency Requirements for Elimination of Waterside Economizers in Zones 3A and 4A.

You can also model the system using the Energy Cost Budget Method described in ASHRAE 90.1-2013 to compare the overall energy of the system you are designing with and without an economizer, regardless of what type of heating and cooling system you are using. This can be found in Section 11.1.1 Energy Cost Budget Method Scope and will let you evaluate whether an economizer would be more efficient for your application.

As discussed, each application should be considered individually to see if a water side economizer makes sense. Factors such as building type, load profile, occupancy and location need to be considered. A detailed list of exceptions can be found ASHRAE 90.1-2013 Section 6.5.1 and should be consulted.

Remember a WSHP system is naturally an economizer by design. This is because you can turn off the mechanical heating and cooling equipment while operating within the dead band and run off your hydronic battery. The larger the dead band the larger your battery.

Next up we will look at tips to help you design the best water source heat pump system for you application.