How to properly size and select a whole-house tankless water heater
1.Ensure 240V or 208V is available
. Most whole-house electric tankless units use this voltage and while majority of homes in the USA have 240V, some may not. Also consider that most electric whole-house TWH (Tankless Water Heaters) draw anywhere from 50 to 150 Amps and would need from 100A - 300A service.
2. Determine groundwater temperature
. Avoid the common mistake of using an average or even summertime groundwater temperature for fixtures which are also used during winter. While measuring cold water temperature in winter is best, there are other ways, such as through a water utility company servicing the area, or as a last resort, with the help of a groundwater temperature map supplied by water heater manufacturer. While the latter is not exact, it can provide a general reading, but we advise caution and to select a lower range on the map when in doubt.
To emphasize the importance of this aspect, consider that the same electric TWH unit would provide twice as much hot water with 70-75°F (think Florida, south Texas and California) ground water, as it would in northern climates with 40-45°F water (all northern states along Canada border, from Oregon to New York).
3. Determine combined flow rate of fixtures
. This is among the most essential, yet often overlooked steps. Not all the fixtures need to be taken into account - only the ones which are expected to be used at the same time, often during the peak use (~7-8am and 7-8pm in the USA on average, according to EIA). The most used fixtures during these hours are showers and kitchen/lavatory faucets, representing about 30% and 25% of overall daily use, respectively. Thus, a family of 3 is likely to use a shower and a faucet at the same time, while families with 4-5 individuals are likely to use two showers and a faucet simultaneously. For most fixtures, flow rate is indicated on it’s body of found in the manufacturer specifications. If unavailable, the flow rate is easily determined by the time it takes to fill a common 1-gallon milk or juice jug. For instance, if a fixture fills the jug full in 60 seconds at full flow rate, it is rated as 1 GPM (Gallons Per Minute). At 45 seconds fill time, it would be rated at 1.5 GPM and so on.
4. Calculate delta T (or simply, temperature rise)
. This is the temperature differential between the incoming groundwater temperature and the pre-set temperature on the electric water heater unit (it should be noted that nearly all whole-house electric TWH’s have a temperature dial allowing to select desired hot water temperature). For instance, a 55°F incoming water would need to be heated by 65°F to become 120°F, thus producing a delta T of 65.
5. Match the Delta T (°F) & Flow Rate (GPM) with manufacturer’s sizing chart.
. When in between two models, we always advise a larger one, since the price difference is often negligible compared to the frustration of under-sized and under-performing unit and it’s possible replacement, as well as costs associated with it.