System design considerations
Some system design and control principles from chilled-water systems transfer well when applied to chiller-heater systems. But, there also are some important considerations that designers must consider regarding equipment sizing, buffer tank location, the need for supplemental heat in some climates and defrost management.
Reliability –The consequences of poor heating system performance are potentially more significant than a cooling system failure. Engineers need to factor into system design the potential for a 50-year weather event. Ascend® air-to-water heat pumps can deliver 140o F fluid temperature at 55o F outdoor air temperature (OAT) and are capable of heating down to 0oF while delivering 100o F fluid temperature.
Because air-to-water heat pumps have operating limits that become more restrictive as temperatures drop, engineers need to ensure they factor into their plans a redundant and reliable backup heating strategy to accommodate the potential for extremely low OAT. Comprehensive chiller-heater solutions can be optionally configured to allow for supplemental or dual-fuel heating when necessary, with more system configuration options such as dedicated heat recovery, free cooling and diurnal energy storage.
Flexibility – Heat pumps are designed to serve two systems with different expectations: cooling and heating. A cooling system for example may be designed for 10o F-12o F degree delta T while a traditional heating system might have been designed for a 20o F-30o F degree delta T. The system must be able to accommodate both needs. A Comprehensive Chiller-Heater System can be configured with two (or more) heat pumps – one for heating and one for cooling – with the proper system volume to match heating and cooling loads in real time.
Outdoor air temperature – Heat pump capacity and maximum supply fluid temperature are reduced as the OAT drops, and the system has outdoor air temperature operating limits. Equipment sizing is affected by these temperature conditions, the dual heating-cooling role and the availability of supplemental heat sources. It’s important to determine equipment sizing that fits temperature, flows and redundancy requirements to meet your year-round cooling and heating needs.
Defrost mode – An air-to-water heat pump will occasionally operate in defrost mode to ensure reliable heat exchange with ambient air. While the system is in defrost mode, heating from the hydronic system is interrupted. This interruption can be mitigated through equipment sizing, the use of a buffer tank and/or the use of a supplemental boiler. A proper mitigation strategy can minimize these disruptions.
Cold climates – A Comprehensive Chiller-Heater System featuring air-to-water heat pumps can also be configured for colder climates through the use of phase change energy storage and water-to-water heat pumps. The concept is to allow the air-to-water heat pump to only bring in supplemental energy into the building when the outdoor air conditions are suitable for operation, for better efficiency, and for better heating capacity. The energy brought in is stored in the phase change of water. Later, either when outdoor air conditions are unsuitable, energy prices are high, or when loads are higher, the water-to-water heat pump removes the stored energy and pumps it to a higher grade of heat for distribution. This system can be paired with other sources of low-grade heat such as solar thermal, wastewater systems, and airside energy recovery.
Another solution for cold climates is to use supplemental heat. This can take the form of fossil fuel-based boilers to be used only in extreme temperatures, auxiliary electric-hydronic heating in the form of an electric boiler, electric resistance heat in air-handling elements or electric radiant panels in the space.
Design support and expertise to simplify the task
Electrification brings new kinds of system components and system design requirements into the mix for consulting and specifying engineers. Trane has a full complement of resources to support design, equipment specification and selection to ensure a Comprehensive Chiller-Heater System featuring the Ascend® heat pump meets your application needs. Contact a Trane for more information.
Our Application Guide provides extensive information about system codes and standards, system and unit sizing, configuration and optional system components. We also offer system layouts and control sequences, and water-volume and carbon calculator tools.
There are also many system design and analysis tools from Trane to select the equipment you need on PC, tablet or smartphone, plus system design resources, system analysis tools and various calculators, including LEED compliance.