This webinar focused on the role of system water volume in ensuring stable and reliable operation of air source heat pump (ASHP) systems, particularly during defrost events in heating mode. The discussion emphasized that water volume is not a secondary detail, but a core design parameter driven by thermodynamics, compressor behavior, defrost strategy, and hydronic system configuration.
The ASHP systems discussed include both two‑pipe reversible and four‑pipe multifunction units capable of simultaneous heating and cooling with heat recovery. The units are monoblock, air‑cooled packages with four compressors per machine arranged across two independent refrigerant circuits. A key system capability highlighted throughout the presentation is the ability to defrost each refrigerant circuit independently, rather than defrosting the entire machine at once. Individual unit capacities range from 30 to 135 tons, and systems can be staged and coordinated up to 32 units, enabling total system capacities on the order of 4,000 tons. While this scalability was noted, the technical focus remained on how defrost behavior and system water volume interact regardless of system size.
Modern ASHPs discussed in the webinar use liquid‑injected compressors, which significantly improve low‑ambient heating performance. With this technology, the units can:
- Produce approximately 140°F supply water down to 14°F ambient
- Continue operating to –4°F ambient while still delivering ~130°F water
This represents a major improvement over earlier generations of ASHPs with non‑injected (low‑lift) compressors, which experience a steep drop‑off in available supply temperature as outdoor temperatures fall. With liquid injection, ASHP heating performance approaches that of condensing boilers, making them viable for a wider range of heating applications. Despite this capability, the webinar emphasized that most systems are not sized for extreme low‑ambient conditions. Instead, they are typically sized for 5°F to 14°F ambient, with supplemental heating (gas or electric) covering the coldest hours. In climates like Denver, outdoor temperatures between 5°F and 55°F account for roughly 57% of annual hours, which is where the majority of energy and carbon savings occur.
As discussed by the presenter, in heating mode, ASHPs extract heat from outdoor air by circulating refrigerant through an outdoor coil at a temperature below ambient air temperature. When the coil surface temperature falls below the dew point, moisture condenses. If the surface is also below freezing, this moisture freezes onto the coil, forming frost. Counterintuitively, the worst frosting conditions typically occur between 35°F and 40°F ambient, not at the coldest temperatures. At these conditions, the air still contains significant moisture, while coil surface temperatures are low enough to cause freezing. To remove frost, ASHPs use reverse‑cycle defrost, temporarily switching the unit into cooling mode for several minutes. During this time, heat is pulled from the water loop to warm the outdoor coil and melt the frost.
A critical technical point emphasized in the webinar is that defrost creates a net cooling effect on the hydronic system. Two factors drive this:
- Heating capacity decreases as outdoor temperature drops
- Cooling (heat rejection) capacity increases at lower outdoor temperatures
This is similar to how an air‑cooled chiller becomes more efficient at lower ambient temperatures. When a refrigerant circuit enters defrost at low ambient conditions, it can extract more heat than its nominal rating. Using a 135‑ton unit as an example:
- Each circuit is nominally ~65 tons
- During defrost at ~23°F ambient, the defrosting circuit can extract ~75 tons of heat
- At the same time, the heating circuit may only deliver ~52 tons of heat
Even with independent defrost, the system experiences ~23 tons of net cooling for the 3–4 minute defrost period.
Independent circuit defrost significantly reduces system disturbance, but it does not eliminate it. If both circuits were to defrost simultaneously, the system could experience ~160 tons of cooling, which would cause a severe water temperature drop. In multi‑unit systems, the effect compounds further. For example, if four units were to defrost concurrently, the system could experience 60 tons or more of net heat removal, even with independent circuit defrost. This behavior explains why adequate system water volume is mandatory, not optional.
Water volume provides thermal inertia, allowing the system to absorb temporary energy imbalances without excessive supply temperature fluctuations. The required water volume depends on:
- Net cooling rate during defrost
- Duration of defrost
- Heat added by non‑defrosting units
- Heat removed by building loads (air handlers, fan coils, etc.)
- Acceptable maximum temperature drop
Building loads continue to draw heat regardless of system temperature, further increasing the need for sufficient stored thermal energy.
To simplify design decisions, system water volume is grouped into temperature stability levels:
- Low stability:
- Maximum temperature drop ≈ 15°F
- Suitable for most comfort heating applications
- ~7–8 gallons per ton
- Medium stability:
- Maximum temperature drop ≈ 10°F
- Tighter temperature control
- ~10–12 gallons per ton
- High stability:
- Very small temperature drop
- 20+ gallons per ton
- Rarely practical due to space, weight, and cost
High‑stability designs are generally avoided because the water volume required becomes excessive.
The webinar clarified that “water volume” is not synonymous with “buffer tank.” What matters is the total connected water volume available to the heat pumps during defrost.
- In variable primary systems, water volume in piping contributes meaningfully and can reduce buffer tank size.
- In primary‑secondary or decoupled systems, only the primary‑side volume is reliably available during defrost, often necessitating a larger buffer tank.
System topology therefore directly affects minimum water volume requirements. Water volume decisions must be made early in the design process, as buffer tanks affect the mechanical room space, structural loading, and project cost. Late identification of water volume requirements often leads to redesign or compromised performance.