Gary Perry of Altecnic explores the advantages of using hydronic-based systems for heating future homes that are targeted to achieve much lower energy use
There are many ways to provide space heating, cooling and domestic water heating in all-electric net zero buildings. They range from separate systems for each load, to integrated approaches that leverage energy recovery and energy management to minimise consumption and coordinate the needs of building occupants with the real time demand on the systems.
COMFORT FOR THE END USER
Firstly, looking at thermal comfort, history has shown that approaches to space heating and cooling that require sacrifices in comfort to achieve high energy-efficiency targets or absolute minimum energy use, usually fail to gain significant market share. The lesson is that end-user comfort continues to be one of the most important underlying factors in establishing and maintaining a market for building energy systems.
Comfort is established when the conditions surrounding the body allow metabolic heat production to be dissipated at the same rate it is generated. Some degree of discomfort is experienced when these two rates of heat transfer are not balanced.
Several types of hydronic distribution systems simultaneously influence air temperature, temperature stratification and surface temperatures of rooms in ways that enhance human physiological comfort. Air-based delivery systems have less influence on interior surface temperatures and can create undesirable drafts or air temperature stratification. As such, they are not as well matched to human comfort needs.
Acoustic comfort is also increasingly important – people want their home to be a quiet refuge from the pace and noise of modern life. They don’t want to hear sounds emanating from their heating and cooling systems. Properly designed and installed hydronic systems using radiant panels or panel radiators can operate with virtually no detectable sound within occupied spaces. The sound produced by the source equipment, such as the compressor in a heat pump, is either outside the building or it can be acoustically isolated within a designated plant room.
Professionals who design low energy and net zero buildings apply scrutiny when selecting the source equipment that supplies space heating, cooling and domestic hot water. They often limit selections to state-of-the-art devices with the highest available thermal efficiencies.
While this approach is certainly relevant and logical, it is also incomplete. The energy used by the source equipment, be it a boiler, heat pump or chiller, is only part of the total energy used by the system. Regardless of how heating energy or cooling effect is generated, additional energy is needed to distribute that thermal energy within a building. Treating this ‘distribution energy’ as insignificant or inconsequential is a serious oversight in the design process, especially when the objective is to create buildings that minimise energy use.
The energy required to distribute heat produced by any heat source, or the cooling effect generated by any cooling source, should always be considered when designing a heating or cooling system for a low energy or net zero building. Systems that use a significant amount of energy to move heat from where it is produced to where it is needed in the building are undesirable, even if the thermal energy is produced at high efficiency by the source equipment.
Distribution energy is an even more important consideration for cooling systems. Every watt of electrical energy used to move the cooling effect through a building is a watt added to the building’s sensible cooling load.
Designers should also consider that air-to-air heat pumps typically require higher air flow rates per unit of heat delivery compared to fossil-fuel furnaces, and thus their distribution power requirement is higher, assuming the same blower motor technology in each device.
The higher the distribution efficiency, the lower the operating cost of the distribution system per unit of heat delivered. For example, consider a ‘traditional’ hydronic heating system
that uses four small circulators, each operating on 75 watts input power, and collectively delivering 100,000 Btu/hr to the building. Assuming that all circulators are operating under design load conditions, the distribution efficiency would be 333.3 (Btu/hr)/watts.
A contemporary ‘homerun’ hydronic distribution system is well-suited for use in low energy homes, as it uses a high-efficiency variable-speed pressure-regulated circulator to create flow between the buffer tank and eight individually regulated panel radiators. Under design load conditions, the water leaving the buffer tank is maintained at 120°F, well within the operating range of an air-to-water or water-to-water heat pump, giving flexibility for designers as well as ensuring efficiency is maintained.
A SYSTEM DESIGNED TO LAST
Many professionals who plan buildings or HVAC systems are being asked to incorporate ‘resilience’ into their designs. The objective is to create buildings and systems that are reliable, long-lasting, adaptable and easy to repair when necessary.
For example, most of the components used in a properly designed, installed and maintained hydronic distribution system will last for many decades. They will outlast the system’s initial
heat source or cooling source, and perhaps even its second or third heat source or cooling source. Put simply, properly executed hydronic systems are long-term investments rather than ‘throw away’ technology. Contrast this with the typical service life of many modern appliances, such as refrigerators, washing machines and microwaves, some of which will not even last 10 years under normal service. Portions of those discarded appliances will inevitably end up in landfills.
Professionals who plan low-energy and net-zero buildings, or place emphasis on decarbonisation, environmentally conscious design, and resiliency, should carefully consider these benefits associated with hydronic heating and cooling systems.
Gary Perry is managing director at Altecnic