Tankless Water Heaters and Thermal Expansion Tanks

Thermal expansion tanks are typically installed in conjunction with storage water heaters, but what are the requirements for tankless water heaters and thermal expansion tanks? This scenario is sometimes overlooked. Let’s look at the specifics.

Section 607.3 of the 2012 International Plumbing Code (IPC) provides the requirements for controlling pressure from thermal expansion. The requirements contained in paragraphs 607. 3 and 607.3.1 do not specifically distinguish between thermal expansion from a storage water heater, a tankless water heater, or other system equipment. Paragraph 607.3.2 does specifically require a device for controlling pressure “when there is a backflow prevention device, check valve or other device installed on the water supply system utilizing storage water heating equipment such that thermal expansion causes an increase in pressure.”

The IPC does not specifically require a thermal expansion tank or other device be installed with a tankless water heater. However, if a storage tank is used in conjunction with a tankless water heater in a closed system, which is sometimes the case with recirculation, then a means of controlling thermal expansion must be provided.

Here is a thorough explanation provided in the 2012 IPC Code Commentary.

Note that this section (607.3.2) identifies only “storage water heating equipment” as the generator of hot water. While this appears to imply that thermal expansion-induced pressure increases are only created by storage tank water heaters, this is not the case. A question that is frequently asked is, “Does a water distribution system with tankless water heater require control of pressure increases caused by thermal expansion?” The answer is provided in the following two examples.

System A: Water distribution system with a tankless, on-demand water heater (electric or gas) without a storage tank. In this system, water is heated only when a hot water outlet is open. Because the water distribution system is now an “open” system (i.e., a hot water outlet is open), any thermal expansion is relieved through the open hot water outlet and no pressure increase can occur. When the outlet is closed, the system becomes a closed system but, since the flow has ceased, the tankless water heater stops heating the water. Therefore, methods for controlling pressure increases due to thermal expansion are not required.

System B: Water distribution system with tankless, on-demand water heater (electric or gas) with a storage tank. The water service line has a backflow preventer (such as a dual check valve at the water meter). In this system, an unfired hot water storage tank may be required to provide for hot water recirculation to lessen the “wait time” for hot water to arrive at any fixture. Or the storage tank might be required to provide for large simultaneous hot water demands (that the tankless water heater cannot produce without the storage). For either reason, as the hot water storage tank cools off, a circulation pump is switched on to cause flow out of the storage tank, through the tankless water heater and back into the storage tank until the water in the storage tank is at an acceptable temperature level. Because there will be times where no hot water outlet is opened during the time that the tankless water heater is heating water, the water distribution system is a “closed” system Therefore, a means for controlling pressure increases due to the thermal expansion must be provided.

When there is a closed system with a heating source, there is a potential for pressure increases from thermal expansion. This is the case with tankless water heaters installed with a recirculation loop and a storage tank. Therefore, a device, such as a thermal expansion tank, must be provided to control the potential pressure increase. The pressure relief valve on a water heater cannot be used as the device to control the pressure.

Note: The Uniform Plumbing Code requires thermal expansion tanks if there is a check valve, pressure regulator, or backflow preventer installed in the main supply line, regardless of the type of water heater. So the adopted municipal code must be determined to ascertain whether a thermal expansion tank is required.

Are there other situations that you have encountered where thermal expansion tanks are needed with tankless water heaters?