Introduction to Water Source Heat Pump Systems Part 1: What We Love About Them

By Chad Edmondson

Water source heat pumps (WSHP) have become a go-to solution for many commercial buildings -- and with good reason. You would be hard-pressed find a heating and cooling system that offers better overall efficiency, flexibility and lifecycle costs.

WSHP systems are relatively simple in concept and design. WSHPs operate very similarly to commonly used residential heat pumps which reject and absorb heat from the outside air. However, instead of exchanging BTUs with the air, WSHPs use water which is more efficient. More on that later.

In a typical commercial building, there will be multiple WSHP units, each serving a particular zone which is independently controlled. All the heat pumps are tied to a central hydronic loop, which acts as a water storage battery for BTUs. The heat pumps add and remove heat as needed to keep their individual zones satisfied. The hydronic loop is typically connected to a cooling tower for rejecting excess heat from the loop and a boiler to add heat when the loop is running short. The basic concept is wonderfully simple and straight forward.

Flexible Installation and Operation

Part of the appeal of WSHP systems is that they can provide heating and cooling simultaneously. In fact, most WSHP systems provide heating and cooling at the same time throughout much of the year. Taking the winter as an example, the interior parts of building typically stay much warmer, and may even require cooling, while the perimeter will require heating. You can exchange BTUs between these units ,utilizing the building water loop instead of wasting them. This is what makes a WSHP system so efficient!

Individual WSHP units can be placed above drop ceilings, in mechanical rooms or even in closets within the corresponding zone, often resulting in a significant reduction in ductwork. Of course, each unit is individually controlled for the given zone with the option of having all the units globally controlled by a central BAS. Another advantage to the individual zones and control is if a WSHP goes down, only that zone will be affected whereas in a typical HVAC system it would result in multi-zone or total system failure.

A WSHP system utilizes a hydronic loop, one of the most efficient ways to move thermal energy around a building. For example, a 2-inch water pipe can carry the same amount of cooling as a 24-inch air duct while requiring up to 90% less transport energy, not to mention taking up less space. The hydronic loop of a WSHP system can even serve other types of equipment such as domestic hot water heat pumps, dedicated outside air units, and even water-cooled variable refrigerant (VRF) units. A WSHP system is also very adaptable to expansion.

Low Lifecycle Costs

Owners of WSHP systems will typically be rewarded with low lifecycle costs. The lifespan of WSHPs are generally quite long, comparable to a packaged chiller; and the installed costs are about the same as a packaged rooftop unit.

Operationally speaking, WSHPs are among the most energy-efficient HVAC appliances that are available to commercial buildings. In fact, ASHRAE imposes a higher efficiency standard on WSHPs (13.0 EER (cooling) and 3.7 COP (heating) than it does on similar sized air-cooled equipment and VRF systems. This is due to the efficient heat transfer of water, as well as the BTU storage capability of the hydronic loop. As a point of reference, a 2,000 gallon water loop with a 50°F to 90°F temperature range can store over 650,000 BTUs! That is a BIG battery!

Environmentally friendly

One final perk is the very small amount of refrigerant that each unit uses for the refrigerant cycle, making WSHPs an environmentally friendly choice. A 2-ton unit only has about 43 ounces of refrigerant. This small refrigerant requirement also helps when adhering to ASHRAE Refrigerant Code Standards 15 and 34 which address refrigerant concentration limits for a given area. Furthermore, the refrigeration circuit in each unit is self-contained, so if a leak should occur, it will not impact the other zones or equipment.

That is a quick overview of everything there is to love about WSHP systems. Next up we’ll explore the applications that most favor this solution.