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Field Test Best Practices: A Resource for Practical Residential Building Science

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Gas Tankless Water Heater


A gas tankless water heater shown
with an expansion tank
Credit: Behrat, Wikimedia Commons

Gas tankless water heaters use natural gas combustion to deliver hot water when it is needed in the house, rather than maintaining a tank of hot water. Tank type water heaters need to supply enough energy to heat the tank of water and to compensate for heat transfer losses through the tank. Tankless water heaters do not have tank losses and so can be more efficient. Gas tankless water heaters have larger gas burners, needed to quickly heat water from the mains water temperature to the desired delivery temperature at the required flow rate. Gas tankless water heaters can sometimes have trouble with low flow rates. Very low flow rates will not trigger the burner to come on. Also, tankless water heater, both gas and electric, can create a "cold water sandwich." After one person in the house is done with their demand for hot water, the tankless water heater will turn off. When the next person asks for hot water, there may still be hot water in the pipes. The tankless water heater cannot immediately start heating water, so there will be a small amount of cold water in the lines, followed by newly heated hot water. This is not a problem in a tank style water heater, since demand for hot water will always be supplied from a preheated tank.



Recommended measurements for field tests involving gas tankless water heaters: