WSHP System Design Part 2: Introducing Outside Air into a Space

By Chad Edmondson

Now that you know how to determine the amount of outside air needed for a given space, it’s time to think about how to bring the air into a space that is conditioned by a WSHP.

There are two main ways to bring outside air into the space. Outside air can be introduced into the space either by direct duct into the space itself or by ducting into the return air duct that sits between the return air from the space and the WSHP. If the outside air is unconditioned and comes straight from the outdoors then the WSHP should be sized for the space load and the outdoor air load, including the latent heat load needed to dehumidify. Introducing unconditioned outside air directly into the room is not a recommended method.

If you decide to bring the outside air to the WSHP then this will require the WSHP fan to run continuously while the room is occupied. If you bring the outside air directly into the space the WSHP fan runs only when needed. However, if outdoor air is brought directly into the room, additional duct work and diffusers may be required to introduce the air. You also should take care that the air is not introduced into the room at a location that is in close proximity to occupants, as direct exposure to this unconditioned air may be uncomfortable.

Regardless of which method you choose, the WSHP will have to be sized for a dehumidification/cooling design day. The chart below gives an example of what those design parameters might look like in the South, in particular the mixed dry bulb (DB) and wet bulb (WB) of the outdoor and return air. (Table 1)

Table 1

Notice that the indoor humidity is at a reasonably comfortable level when the system is fully loaded assuming it was sized to handle the latent load. Problems start to occur when the system is at part load on a day that is warm and rainy. Let’s look at how the humidity of the supply air to the heat pump starts to creep up on such a day.

At 8:00 AM, the outdoor air is at 68°F DB with a wet 66°F WB. This is the air that we will be bringing into the building as people come to work. Notice that the thermostat is set at 74°F and our mixed air temperature to the WSHP is also 74.2°F. The WSHP is not yet operating and already the supply air humidity is up to 53% RH.

By 10:00 AM the outdoor air is 72°DB at 72°WB. It is raining outside and the outdoor air is fully saturated.

Our mixed air temperature is 74.6°F DB with a 65.7°F WB. The humidity of the supply air to the WSHP is now up to 63% RH and our thermostat is still set at 74°F. Because the thermostat only sees the sensible load of the building, the WSHP only comes on for a short time before it achieves the 74°F setpoint. The indoor humidity is now very uncomfortable and the only choice we have to dehumidify is to overcool the room. Either way, occupants are going to be uncomfortable.

By lunch time the outdoor air is 74°F DB at 72°F WB.

Our mixed air temperature is now 74.8°F DB at 68.4°F WB. Our mixed air to the WSHP is now extremely moist with a humidity of 73% RH! Since the WSHP is still only slightly loaded with our 74°F thermostat setting, it will not be running long enough to dehumidify the space. Our occupants are now extremely uncomfortable and the room feels very muggy.

Table 2

Table 2 summarizes the conditions in our example as the day progresses. We are bringing in 72°F outdoor air that has a humidity close to 100% RH. There is no external heat load outside so the WSHP turns off. As we bring more outdoor air into the building with the heat pump turned off, we are unable to dehumidify the space.

This happens to be a fairly common weather day during the shoulder seasons here in the Southeast. These part load, warm, rainy days are why we often need to condition the outdoor air before we bring it into the room.

How do we do that? Stay tuned for Part 3!