Air Source Heat Pump (ASHP) systems are regularly being implemented throughout buildings today via the following products:
While there are many things that a design engineer must consider for the successful implementation of an ASHP system, one aspect that is often considered as an afterthought is the defrost process.
As any of the ASHP systems noted above extract heat from the ambient air, the ambient air experiences a cooling effect. When the entering ambient air is humid, condensation will start forming on the outdoor coil and if the refrigerant temperature inside the coil is below 0oC, this condensate will freeze. This is a big concern in cold climates because a large portion of our heating season is spent with ideal conditions for frost/ice to build-up on the outdoor coils. After a period of time, the ice on the outdoor coil will restrict air flow and cause instability in the refrigeration circuit if it is not removed regularly.
For this reason, just about any ASHP system being installed in Ontario and Quebec requires an automatic defrost sequence to maintain reliable heating operation. The primary intent of the defrost sequence, common to all ASHP manufacturers, is to consistently remove all ice from the outdoor coil as quickly as is reasonably possible so that the unit can return to normal heating operation with minimal impact to the building and its occupants. However, this defrost sequence can come in a variety of different forms which range in complexity and sophistication.
The volume of water which is generated from a defrost sequence is rarely considered and usually more than one might expect. Two questions that come to mind are:
Firstly, it is always recommended to raise the equipment above the anticipated level of snow accumulation to prevent the mechanical components from being submerged in ice, snow, or water. Next, there are a variety of ways to approach the management of this condensate, but below is a summary of the various solutions that I have encountered and some additional considerations to be aware of when evaluating each of these for your specific application.
|
Option #1 |
|
|
Description: |
Considerations: |
|
Collect the condensate into a factory installed heat traced condensate pan to be transported via field condensate piping to a safe location inside the building to be discharged. This seems to be the standard for most manufacturers. |
- Site coordination is required to ensure that all exterior condensate piping is adequately heat traced. - Who powers the heat tracing? - Can the heat tracing be on an emergency power supply? - How do we detect if the heat tracing has failed? With the condensate pan immediately below the coils, there is a significant risk of ice damage if heat tracing is disabled during cold weather. - Can the heat tracing be easily replaced in the future? - How many heat-traced condensate lines do you need? - Is there an indoor mechanical room close to the drain? |
|
Option #2 |
|
|
Description: |
Considerations: |
|
Do nothing! Essentially, when the unit defrosts, the condensate will drain away from the coil and land on the roof/ground below the unit. |
- This solution may result in an ice pad developing all around the unit, which could be considered a hazard to service personnel. Has the roof structure been designed to accommodate the volume of water/ice that will accumulate under the ASHP? |
|
OPTION #3 |
|
|
Description: |
Considerations: |
|
Specify an ASHP unit without drain pans and allow the condensate to naturally fall to the roof/ground below. Install a full perimeter expanded steel service platform around the unit. |
- A service platform may be expensive but will help to ensure a safe working environment for maintenance personnel. - Has the roof structure been designed to accommodate the volume of water/ice that will accumulate under the ASHP? |
|
Option #4 |
|
|
Description: |
Considerations: |
|
Install heat tracing at roof level below the entire unit with a heat traced roof drain and build a water damn around the perimeter. |
- Most comprehensive and reliable solution. - Most expensive and requires multidisciplinary coordination. - Is not easily adapted to an on-grade installation. - By allowing the condensate to immediately fall away from the ASHP coil, there is less risk of ice damage to outdoor coils in the event the heat traced area is disabled for any reason. - Will usually make sense to use electric heat tracing under the unit. |
Depending on the limitations of each application, one of the above options may make the most sense. For successful implementation of an ASHP unit, the design process should include a thorough analysis of the product being specified, the arrangement of the outdoor coils, the defrost sequence being implemented and the final location of the unit. The most important thing is to not ignore or forget about this important aspect.