An under sink water heater or what is commonly referred to as a point-of-use water heater provides hot water for sinks, eliminating the need to wait for hot water from a central water heating system. As useful as they are, these compact water heaters have specific installation requirements that must be followed for proper and safe operation. One key requirement is whether or not the under sink water heater needs an expansion tank or vessel.
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Quick Answer
Most under sink water heaters do require an expansion tank or vessel to be installed. The expansion tank absorbs the increased water pressure and volume that results from heating cold water in the closed system of an under sink water heater. Without this expansion capacity, the increased pressure could damage pipes, joints, and even the water heater itself.
Why an Expansion Tank is Needed
When cold water is heated in a closed system like an under sink water heater, the water expands in volume and takes up more space. With no place for this expanded water to go, pressure builds up. The expansion tank provides space for the heated water to expand into without creating excessive pressure.
Specifically, the expansion tank contains a bladder or diaphragm that compresses as the heated water expands, absorbing the pressure increase. Tanks are sized to handle the total volume increase from fully heating the amount of water in the system.
By giving the expanded water somewhere to go, the risk of damaging or rupturing pipes, fittings, seals, or the water heater itself is greatly reduced. The expansion tank regulates pressure so it stays within safe levels.
Signs of Inadequate Expansion Capacity
Some key signs that an under sink water heater needs more expansion capacity include:
- Dripping relief valve, indicating excessive pressure buildup
- Popping or banging noises from pipes and joints
- Leaking connections and fittings
- Premature failure of parts like valves or seals
- Difficulty maintaining stable water temperature
These issues often arise gradually over time as more heated water cycles through the system. Adding an appropriately sized expansion tank can resolve them.
Sizing the Expansion Tank
Choosing the right expansion tank size is important so it can handle the maximum volume of expanded water. The main factors in sizing are:
- Total water volume – The amount of water contained in the under sink water heater system when full. The tank must be able to absorb the expansion of this full volume when heated.
- Water heater temperature rise – The difference between the incoming cold water temperature and the heated water temperature. The greater the temperature rise, the more expansion has to be absorbed.
- System pressure – The standard pressure maintained in the system by the water pressure reducing valve. Higher pressure requires a larger expansion tank.
Using these key factors, the required expansion tank size can be calculated. Tank suppliers also provide sizing calculators to help choose the right model.
As a rule of thumb, the expansion tank size should be around 10% of the total water volume in the system. But precise sizing using the temperature rise and system pressure is recommended.
Expansion Tank Sizing Example
Here is an example to illustrate selecting an appropriate expansion tank size:
| Total water volume | 6 gallons |
| Temperature rise | 60°F (from 50°F to 110°F) |
| System pressure | 150 PSI |
Using a sizing calculator, the recommended expansion tank size for these parameters is a 2.1 gallon tank. This should be adequate to absorb the pressure and volume increase from heating the 6 gallons of water by 60°F.
Installation Tips
Properly installing the expansion tank is critical for it to work correctly. Here are some key tips:
- Install on the incoming cold water line according to manufacturer instructions
- Position horizontally and level for proper bladder/diaphragm orientation
- Allow sufficient clearance around tank for service access
- Connect drain line if supplied to avoid waterlogging
- Support tank weight appropriately without straining pipe connections
- Insulate tank to minimize condensation and heat loss
Also be sure to pre-charge the tank to the system pressure. Adjust the tank’s air fill valve to match the setting of the water pressure reducing valve. This allows the bladder/diaphragm to compress properly.
Typical Expansion Tank Installation
Here is a diagram of a typical under sink water heater installation with an expansion tank:
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The expansion tank is installed on the cold water supply line leading to the water heater. A suitable drain line and supports are also used. This allows the tank to absorb and regulate the water expansion when heated by the under sink unit.
Alternative Options
In some cases, alternatives to an expansion tank may be used with an under sink water heater:
Thermal Expansion Valves
This type of valve opens to release water and relieve pressure when the temperature and pressure rise to a preset level. They provide some expansion capability but are not as effective or controlled as a properly sized expansion tank.
Lengths of Flexible Hose
Short flexible connector hoses on the water lines leading to/from the water heater can absorb some expansion. But they provide minimal capacity compared to expansion tanks.
Water Heaters with Built-In Expansion
Some under sink water heaters have built-in expandable bladders or chambers to handle water expansion. In these cases, an external expansion tank may not be required.
Open Systems
Connecting the water heater discharge to an open drain allows expanded water to simply overflow. But this wastes energy and water.
While these alternatives may work in some situations, properly sized expansion tanks are generally recommended for under sink water heaters to ensure safe pressures and temperatures.
Conclusion
In summary, the majority of under sink water heaters require installation of an expansion tank or vessel to safely handle the pressure and volume increase that occurs when heating water. Choosing the proper expansion tank size based on the water volume, temperature rise, and system pressure is critical to prevent damage and ensure proper operation. Following manufacturer guidelines for appropriate installation is also important. With the right expansion capacity in place, an under sink water heater can provide efficient local delivery of hot water for years of service.
Some key points to remember:
- Heating water causes it to expand, increasing pressure in a closed system
- The expansion tank absorbs this pressure increase and added volume
- Adequate sizing based on water volume, temperature rise, and pressure is needed
- Alternative options like valves and flexible hoses have limited effectiveness
- Improper expansion capacity can lead to leaks, damage, and water heater failure
By understanding the need for and properly installing an expansion tank, under sink water heater systems can operate safely for extended periods. This allows them to deliver the convenience and efficiency of on-demand hot water at the point of use.