WSHP System Design Part 3: Options for Conditioning Outside Air
/By Chad Edmondson
Outdoor air typically requires conditioning before it is brought into a space. By “condition,” we primarily mean dehumidification, a common requirement here in the Southeast. Ideally the conditioned air should be drier than the air in the space. This usually involves overcooling the air, so some amount of reheat is often required, especially during part-load conditions.
There are three basic options for conditioning outdoor air before delivering it to the space. Each has its own advantages and disadvantages to consider with respect to the particular application.
Option 1: Dedicated Outdoor Air System
Dedicated outdoor air systems (DOAS) are the most common choice for conditioning outside air before it is introduced to the heat pump or the space. DOAS units provide conditioned outdoor air to many types of HVAC systems, not just WSHPs, so they are quite common on commercial size systems. These units include both a cooling coil for dehumidification and a reheat coil that uses recovered heat from the refrigeration circuit to heat the air to the desired dry-bulb temperature.
DOAS units are popular because they can remove large amounts of moisture from the air while delivering “neutral” air that has approximately the same dry-bulb temperature as the air in the space. As a result, the WSHP need only be sized for the sensible load, which may lower the cost of the overall installation, depending on the number of heat pumps in the system. Sometimes, the DOAS can supply even cooler air, thus relieving the WSHP of some of the sensible load. This can work as long as no reheat is required.
Option 2: Energy Recovery Unit
Energy recovery units (ERU) condition outdoor air via a desiccant media wheel or other devices that can exchange moisture and BTUs with the building exhaust air. These units have a low operating cost and are often used in combination with a smaller DOAS to help keep costs down. ERUs do not typically have enough dehumidification capacity on their own to manage the latent load of a building, especially on warm, rainy days. Therefore, an ERU is typically “part” of a solution rather than a solution in and of itself. In addition, ERU components (wheel, filters, motors) require regular inspection and maintenance to remain efficient and operable long-term.
Option 3: WSHP with Dehumidification and Built-in Reheat
Standard WSHPs do not typically provide sufficient dehumidification on part-load cooling days, especially if the outdoor humidity is very high. The units tend to run for very short periods, satisfying the sensible load long before adequate dehumidification is achieved. However, some WSHP manufacturers now offer their equipment with integral dehumidification and reheat, allowing owners to avoid the cost of a DOAS and/or ERU. A WSHP with a dehumidification/reheat option can often provide cost-effective dehumidification, depending on the type of reheat that is used.
Reheat options for WSHPs include hot gas, electric, or hot water. Regardless of the heat source, outdoor air is overcooled for the purpose of dehumidification and then reheated to the thermostat setting.
• Hot gas reheat works by passing the hot discharge gas from the compressor through a separate air coil to warm the dehumidified outdoor air to a suitable space temperature. Unfortunately, these systems rely on complex controls to operate and contain multiple valves and complex piping. In addition, the amount of reheat tends to fluctuate so supply air temperature and humidity tend to be inconsistent.
• Electric reheat coils provide more reliably conditioned air but increase installation costs due to their higher electrical requirements and loads. Electric reheat also yields higher operating costs.
• Hot water reheat is the best option in terms of simplicity, efficiency and reliability. WSHPs with a hot water reheat option include an internal pump that circulates warm condenser water through the hydronic reheat coil during the “cooling with reheat” mode of operation, warming the dehumidified supply air before it enters the space. The amount of reheat that occurs is dependent on the setpoint and how far from setpoint the supply air temperature is. A modulating mixing valve and a proportional controller maintains precise control of water through the reheat coil, so space temperature remains consistent and comfortable at all times, regardless of outdoor conditions.
Figure 1. internal schematic of a WSHP with hot water reheat.
Next up we will look at the water side design and the major equipment that you will find there.
