Heat pump water heaters (HPWH) are not boilers and cannot be sized the same way. Boilers are sized to output peak hourly hot water demand with a small tank to buffer starts and stops.
- Heat pump costs scale with output capacity
- Penalty for oversizing significantly more expensive than boilers
- Design goal: use smallest units possible
HPWHs are most efficient when able to run for 16-20 hours per day, with long run intervals to prevent accelerated compressor wear. To allow HPWHs to operate in ideal conditions, storage becomes critical in system design, being sized to cover peak usage conditions.
Daily usage of a building becomes the primary factor in sizing HPWH systems to ensure the heat pumps can recover the daily hot water demand in 16-20 hours. Tanks are sized by considering building type and their usage patterns to ensure enough hot water is in reserve for peak hourly demands.
Heat Pump + Storage Draw = Building Usage
Heat pumps are on and output is plumbed in parallel with storage tanks to supply building
Condition 2: Building draw equal to heat pump output
Heat Pump = Building Usage
Heat pumps are on and no hot water is being drawn from tanks.
Condition 3: Building draw less than heat-pump output
Heat Pump = Building Usage + Tank Recovery
Heat pumps are on and output supplies building draw and recovers tanks.