Tank Heat Loss and Thermal Stratification Basics
Tank behaviour - heat loss and thermal stratification - has a direct effect on heat pump water heater performance and this comes through in the AS/NZS 4234 modelling, impacting final certificate values.
Thermal stratification
Thermal stratification occurs when water in a tank forms temperature layers. Hotter water remains near the top of the tank and colder water remains near the bottom.
A well-stratified tank has a narrower transition zone, or thermocline between hot and cold water. A poorly stratified tank is more mixed.
Factors that impact thermal stratification
Different HPWH systems will show dramatically differently thermal profiles depending on several factors:
- Refrigerant - CO2 systems involve highly stratified tanks with distinct hot/cold layers, whereas R290 systems require multi-pass heating which results in a mixed tank
- Tank port positions - the position of the HP return port especially can impact stratification and mixing in the zone around where hot water returns from the heat pump
- Demand water flow rates - the faster the flow rate of cold water entering the system and hot water leaving, the more disruption to stratification in the tank.
- Standby time - as a tank sits idle, heat transfer between hot and cold layers gradually results in a mixed tank
Why stratification matters
Stratification affects heat pump inlet water temperature, delivered hot water temperature, electric boost operation, minimum delivery temperature in AS/NZS 4234 modelling, annual energy consumption, and certificate outcomes.
Heat pumps often perform better when the inlet water to the heat pump is cooler. A well-designed separate system can return hot water to the top of the tank while drawing cooler water from the bottom, which can support better stratification.
What disrupts stratification
Stratification can be reduced by high flow rates, poor inlet diffuser design, return water entering at the wrong height, heat being added too quickly or too broadly, sensor positions that do not match the control strategy, and plumbing layouts that promote mixing.
Integral systems can behave differently from separate systems because the tank may be heated across a larger surface area.
Tank heat loss
Tank heat loss is the standing heat lost from stored hot water to the surrounding environment.
Tank heat loss is primarily impacted by thickness and thermal conductivity of tank insulation material, and also how well ports and penetrations are insulated. Tank heat loss scales up with exposed surface area.
How tank heat loss is evidenced
| Tank type | Typical evidence |
|---|---|
| Residential storage tank | AS/NZS 4692.1 test report |
| Commercial tank up to 700 L | AS/NZS 4692.1 test report |
| Commercial tank greater than 700 L | Calculation using tank geometry and insulation properties based on AS/NZS 4234:2021 Appendix E |
For large tanks, the drawing should state internal dimensions, insulation thickness, insulation material, and enough geometry to calculate heat loss through the wall, top, and bottom.