What are hydronic heating systems?
This measure applies to hydronic heating systems in which heat energy is supplied to the hot water loop in the building for various applications. The mechanism of heating is founded in the refrigeration cycle where the liquid refrigerant absorbs heat from the outdoor ambient environment (air or water depending on the system) evaporating the refrigerant into a gas vapor. With the use of a compressor, the refrigerant is transformed into a high-pressure, high-temperature vapor and circulates to the inside coil. Heat exchangers will absorb the heat from the refrigerant and transfer it to the hot water loop to be distributed throughout the building. With the hot temperature absorbed, the refrigerant has transformed into a high temperature but slightly cooler, high-pressure liquid and is cycled to the exterior coil. Before reaching the exterior coil, the refrigerant will pass through an expansion valve converting the refrigerant to a low-pressure, low-temperature liquid providing the ability for higher heating capacity and is cycled to the exterior coil to repeat the cycle.
Heat pumps can traditionally operate in reverse using a reversing valve to provide cooling to the building, essentially acting as a traditional air conditioning unit, but the focus of this measure is for heating mode to evaluate the potential gas offset from using the heat pump in comparison to traditional gas system such as a boiler or furnace. In addition, for cooler climates the heat pump may have the ability to operate in defrost mode resulting in periodic heating interruptions. Equipment sizing, a buffer tank and/or a supplemental boiler can all be used as strategies to mitigate heating interruptions.
Air-to-water heat pump
The air-to-water heat pump will utilize the refrigeration cycle described above using the outdoor ambient air as the source for heating. The air-source will operate at design conditions until outside temperatures lower around freezing temperatures (will vary depending on system, refrigerant, manufacturer etc.) and in cold climates the air can drop well below freezing further derating the efficiency and thus overall effectiveness of the system. To prevent heating disruptions in such conditions it is typical for the building to utilize a backup system to compensate, which will increase operational costs.
Water-to-water heat pump
A water-to-water heat pump system uses water as the heat source (or heat rejection in cooling mode). The water loop consists of a glycol solution, which has higher density and heat transfer properties than pure water, leading the “water source” pumps to consume more energy in comparison to the fan of an air-source system. This is in addition to traditionally higher installation costs. In cooler conditions the efficiency of the system will diminish as the water source temperatures decrease but carry an inherent advantage as water temperature typically stabilizes at or above freezing in cold climates. This is in comparison to air temperatures in cold climates that could experience air temperatures below freezing and may not require a backup system depending on conditions and demand.
What are the benefits?
- The primary focus of this measure is decarbonization of the building site by utilizing an efficient and electric solution for heating the hot water loop in the building.
- Equipment can be used for space heating, heat recovery, simultaneous heating and cooling and process loads.
- Energy efficiency with higher performance and lower energy use.
- Expandable designs available for incremental size capacity, redundancy, and future load requirements.
- Flexibility in capacity for small and large commercial applications.
- Full commissioning and FDD ensure the system are operating to optimize maximum efficiency and to ensure consistent performance during equipment lifecycle.
- May reduce installation and maintenance cost compared to a conventional chiller plant.
- May reduce mechanical room requirements and installation area required for mechanical equipment.
What are the challenges/constraints?
- Equipment efficiency may be derated for projects with required high hot water temperatures (typically greater than 120˚F).
- Equipment performance optimization may require additional design, commissioning, and controls.
- Initial cost impact, especially for a water-to-water system
- Limited operating conditions at lower ambient or water temperatures.
What are the qualifications for inducements?
- Air-to-water or water-to-water heat pump systems for heating, heat recovery or simultaneous heating and cooling applications.
- The system must be designed with documented criteria to define optimum operating conditions based on project-specific conditions.
- The system must be fully commissioned and operating at optimized design conditions.
- Fault Detection Diagnostics (FDD) to ensure ongoing optimized performance are strongly recommended but not required.
Hydronic heating systems have emerged as a leading technology in the decarbonization of building applications while maintaining reliability and providing additional advantages in comparison to a traditional gas heating system. This includes versatility in the ability to operate in several configurations and integration with other systems, increased flexibility in the space required for installation serving a wider range of buildings and reduced energy consumption and maintenance costs yielding savings well into the future.