How do I know what size tankless water heater I need?
Tankless water heater sizing depends on your home's peak flow rate (GPM), which is calculated by adding up the flow rates of all appliances and showers you might use at the same time. Undersizing the unit will leave you with a shortage of hot water, while matching it correctly to your household demand ensures you do not fall short during peak usage.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Researching flow rate requirements independently and consulting plumbers for recommendations.
- 2Installing a traditional thermal expansion tank
- 3Relying on a thermal expansion control valve as an alternative
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“Looking at the model currently places it at a max of 6.8 gpm which by my research is way short by what is generally recommended. By what I listed above we should be around the 9 gpm closer to 10, is the recommended model by the plumber satisfactory or is he incorrect?”source ↗
Where this came up
People with this problem also raised
- 2Using a tankless water heater for radiant floor heat
- 3Will half-inch pipes cause pressure drops in a second bathroom?
- 2Can I replace a hot water heater with a direct swap if ports match?
- 21Water heater going hot and cold with weird refill noises
- 7Will my DIY water heater installation pass inspection?
- 6Why does a new water heater require so many infrastructure upgrades?