A common phrase that I like to often use is “heat pumps aren’t boilers.” This quote captures a critical point. Heat pumps can perform almost all tasks that a boiler can, but that doesn’t mean they are the same things. For instance, in my home, a five-ton air-to-water heat pump handles heating, cooling, and soon, domestic hot water. It actually does not just serve the role of a boiler, but in my case, it is my old furnace that was taken out.
Understanding the differences in piping between boilers and hydronic heat pumps is essential, though it shouldn’t be a source of intimidation. The market is seeing innovative monobloc air-to-water heat pumps, placing plumbers and hydronic specialists in a prime position to excel with the right distribution and manufacturer partners. Heat pumps require different piping approaches compared to boilers, but mastering this is not overly complex.
Hydronic heat pumps
To go back to the basics, when it comes to hydronic heat pumps, there are two types — air-to-water and water-to-water (geothermal). These can be installed in similar ways and viewing them under the collective term of “hydronic heat pumps” rather than “heat pump boilers” aids in setting up our industry for success. Heat pumps are not boilers, and we really should not call it a heat pump boiler or people will wrongly think we can apply traditional installation methods that may result in issues.
To this end, the Canadian Institute of Plumbing and Heating (CIPH) is developing new air-to-water training modules, while the International Ground Source Heat Pump Association (IGSHPA) focuses on water-to-water, aiming for a universal training program for hydronic heat pumps.
DHW with heat pumps
Using heat pumps for domestic hot water is feasible, though they differ from boilers in rapid heating capabilities. A typical residential hydronic heat pump has about half the output of a standard residential boiler. This difference necessitates a unique design approach. It’s akin to comparing a Toyota Corolla to a Porsche in terms of acceleration – both reach the same speed, but the time they take to get there differs. Think of heat pumps for domestic hot water like the Toyota and the boiler as the Porsche. Both will accomplish the end goal, but one will do it very efficiently.
The market is introducing new domestic hot water technologies ideal for hydronic heat pump applications, potentially enabling performance akin to tankless systems but powered easily by a hydronic heat pump with some potential limitations for large dump loads. I will write about and share photos and videos of testing in our live fire lab in the coming issues.
Importance of buffer tanks
Buffer tanks can be vital in hydronic heat pump setups (and boilers, for that matter) to ensure consistent system performance and prevent issues like high-head pressure lockouts. Some hydronic heat pump systems require up to five-GPM per ton with a maximum Delta T of 5 F to 8 F.
Buffer tanks play a key role in sustaining heat pump run times, maximizing performance and decoupling loads, allowing for different Delta T’s on the primary/secondary circuits. However, buffer tank necessity depends on system characteristics, such as modulation rates. Aligning heat generation with dispersion rates is vital to avoid idle periods and mitigate anti-short cycling issues.
Short cycling can drastically affect a heat pump’s coefficient of performance (COP) by reducing efficiency, causing uneven temperature control, and increasing energy usage. It also leads to more frequent start-ups, which can cause excessive wear and tear on components. This additional wear and tear can shorten the lifespan of the heat pump. A buffer tank is not just about thermal storage, regardless of fuel source. It allows you to optimize operational cycles and consider system configurations.
Efficiency and COP
The efficiency of hydronic heat pumps heavily depends on flow rates and temperatures. Although they generally offer better efficiency than boilers, designing with incorrect water temperatures can diminish this advantage, incurring higher operating costs.
In our live fire lab, we tested various hydronic heat pumps at different operating temperatures. For example, running the Enertech five-ton unit at a leaving load temperature of 113 F significantly improves the COP compared to running it at 131 F. This illustrates the importance of design in optimizing system efficiency. With this in mind, it is important to recognize that emitter choice can also greatly impact performance.
Piping and system design
Designing around hydronic heat pumps requires an acknowledgment of their temperature output limitations. Most North American models currently use refrigerants like R-410 and are transitioning to R-454. This limits the temperature going out to the system at or below 140 F.
There are new units arriving from Europe with R-32 and R-290 A2L refrigerants that can produce 150 F to 160 F water, but it is important to note that just because it can produce those temperatures doesn’t mean it should. After all, we are designing for efficiency and comfort.
Thermal storage batteries
Water’s limitations in storing latent energy led us to explore phase change thermal batteries, which operate at temperatures as low as 118 F. The technical mechanism of PCM batteries lies in their ability to absorb a significant amount of heat as they melt.
This heat is stored as latent energy and can be retained without a significant rise in temperature. When the PCM solidifies, it releases this stored heat. This process of storing heat in the form of latent energy rather than sensible energy allows for more efficient energy storage within a smaller temperature range compared to traditional water-based systems.
This technology, still emerging in North America, has demonstrated the potential to reduce gas consumption by up to 50 per cent in gas boiler applications. Its application in hydronic heat pumps and the possibility of charging with ultra-low off-peak rates opens exciting opportunities for energy efficiency and sustainability in the hydronic industry.
Challenges
There is a big concern in the industry over the hesitation among skilled HVAC technicians to adopt these new technologies. It is critical to dispel these fears. For instance, monobloc air-to-water heat pumps are straightforward to install, requiring only water lines to be run from the exterior to the interior.
Their operation is not drastically different from boilers, and with the right design partners and brand support, the transition can be seamless.