valent Innovent Air Source Heat Pump User Guide
- June 1, 2024
- VALENT
Table of Contents
PRODUCT APPLICATION GUIDE | AIR SOURCE HEAT PUMPS
ACHIEVING CONSISTENT SPACE COMFORT WITH AIR SOURCE HEAT PUMPS
Innovent Air Source Heat Pump
As the availability and use of air source heat pumps continues to grow, it is critical to make design choices that optimize HVAC system performance and reliability. Depending on the climate and unit features, air source heat pumps can be challenged while operating in heating mode. This guide outlines some of the key considerations when designing an air source heat pump unit to avoid wide swings in discharge air temperatures that create uncomfortable spaces for occupants.
TEMPER INCOMING AIR WITH ENERGY RECOVERY
Some areas are beginning to require electric heat vs. gas in new and renovated
buildings regardless of climate. While air source heat pumps are an all-
electric solution, extreme temperatures may cause the unit compressors to shut
down, or fail to start, when using high amounts of outdoor air. To protect the
compressors and ensure each compressor can start within its operating
envelope, the air leaving the indoor coil must be at or above 30 °F.
Therefore, it is highly recommended that a recirculation damper be available
for unoccupied and “unit cold start” periods to ensure 30 °F air is present at
the indoor coil for the initialization of the lead compressor. A recirculation
damper allows ASHP units to utilize recirculated air as needed to achieve the
refrigeration pressures required to start the lead compressor.
If a recirculation damper is not available, an enthalpy wheel and/or preheat
coil may be able to deliver the necessary air temperatures during these
periods. Optimizing the unit design efficiency by pairing energy recovery with
a high outdoor air ASHP can help avoid this low-temperature issue and reduce
energy consumption. Energy recovery devices temper incoming air using
previously heated return/exhaust air, thus ensuring the incoming air is warm
enough when it reaches the indoor coil.
Energy recovery can be accomplished with several different technologies.
However, some air source heat pump units may not include the optimal energy
recovery device. Choosing the right energy recovery technology can extend the
applications where an air source heat pump is both efficient and effective.
Energy recovery options that offer both sensible and latent recovery are the
most effective choices. Enthalpy wheels are highly efficient but have moving
parts that require maintenance. If maintenance is not done on a routine basis,
the wheels can fail and thus yield no system benefit. Alternatively, an
enthalpy core device is a solution that has no moving parts and will typically
meet ASHRAE 90.1 standards for efficiency. Manufacturers that offer several
energy recovery options will enable each customer to choose an energy recovery
technology that best fits their application.
ENERGY RECOVERY COMPARISON
| ENTHALPY WHEEL | CATEGORY | ENTHALPY CORE |
|---|---|---|
| Polymer or Aluminum | Heat Transfer Medium | Fiber or Polymer |
| 70-85% | Sensible Performance | 60-70% |
| 65-80% | Latent Performance | 35-50% |
| Moderate | Internal Pressure Drop | High |
| Low | Leakage and Cross Contamination | Very Low |
| Moderate, rotates | Maintenance | Low, no moving parts |
UNDERSTAND DEFROST SEQUENCES AND THEIR IMPACT
Despite the benefits of providing an all-electric heating source, air-source
heat pumps have an inherent application challenge where frost can accumulate
on the outdoor (evaporating) coil in heating mode. Frost accumulation degrades
the heating performance of the system and requires a defrost cycle to remove
the frost and allow the coil to operate at maximum effectiveness. An effective
defrost control sequence helps to mitigate potential frost issues on the
outdoor coil and keep the compressors operating.
The ultimate goal is to reduce the frequency and duration of defrost cycles.
When a refrigeration circuit is in defrost mode, the heat of the compressor is
redirected away from distributing warm air to the space and instead toward the
outdoor coil to melt the frost. To keep refrigeration circuits from going into
defrost mode unnecessarily, the defrost sequence needs to be initiated based
on the right data. Some HVAC unit manufacturers use outdoor air temperature as
that trigger. However, the outdoor air dewpoint is more indicative of the
possibility of forming frost. Cool (i.e. 40 °F), moist air on the outdoor coil
of an ASHP unit can quickly create frost. Conversely, a cold (0 °F) day, with
little to no moisture in the air, may not create frost conditions.
DEFROST MODE
VALENT/INNOVENT PRODUCT APPLICATION DESIGN GUIDE | AIR SOURCE HEAT PUMPS
Triggering defrost based on only the ambient temperature may cause refrigeration circuits to go into defrost mode regardless of whether the outdoor coil is truly frosted or not. Using additional data from the unit, such as compressor suction pressure and outdoor air dewpoint, is the best way to determine when there is frost on the outdoor coil and defrost mode is therefore required.
SECONDARY HEAT PROVIDES RELIABLE HEATING PERFORMANCE
Ensuring supply air meets the setpoint is a critical consideration when
designing any HVAC unit and especially when designing an air source heat pump
unit. While heat pumps are highly efficient, there are limitations to their
cooling and heating capacity.
Generating enough heat to satisfy building needs can be challenging with air
source heat pumps, which usually cannot operate below certain incoming air
temperatures. While energy recovery devices help precondition entering air,
additional heat is often needed to achieve proper space temperatures.
Secondary heating options allow owners to get the most out of the air handling
unit, even in extreme temperatures. The use of secondary heat ensures
consistent control over supply air temperatures. Secondary heat may be offered
as an electric heater, hot water coil, or even a gas furnace. There are two
different types of secondary heat:
- Supplemental Heat can run at the same time as the air source heat pump compressors.
- Back-up Heat is a source of heat that only kicks in when the air source heat pump compressors are locked out.
By combining an air source heat pump with secondary heat, the unit can generally meet the required space conditions, keeping occupants comfortable.
OPTIMIZING AIR SOURCE HEAT PUMPS
Designing a commercial air source heat pump HVAC unit to create consistent, comfortable spaces for occupants doesn’t have to be challenging. However, it is important to understand the technology and its nuances to design the most efficient ASHP unit. By using energy recovery, understanding how and when the unit will activate defrost mode, and making the appropriate secondary heat selection, you can effectively, and efficiently, utilize air source heat pumps in many applications.
OUTDOOR AIR EXPERTS
ROBUST DESIGNS | DEDICATED SUPPORT
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Air Source Heat Pump Heating App Guide 042524
References
Read User Manual Online (PDF format)
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