When a water heater is connected to a closed system, an expansion tank must be installed to prevent excessive pressure in the water. To help understand what this is all about, please read on.
OPEN SYSTEMS
When a water heater fires up, the water in the tank expands. When this happens, where does the water go? Right back out the cold water inlet, all the way back to the water supply coming into the house. The municipal water supply for the house acts as a gigantic expansion tank… and nobody notices. This is illustrated in the diagram to the right.
CLOSED SYSTEMS
What happens when a one-way valve, or check valve, gets installed on the water supply piping for the house? The water has nowhere to go. As the water heater heats the water, it expands, which builds up pressure. When the pressure builds up enough, the temperature and pressure relief valve on the water heater simply does its job. It relieves the excess pressure by leaking a little bit of water.
This doesn’t happen every time that a pressure reducing valve or check valve is installed, but there may be other problems that show up in the house, such as the toilet fill valves randomly re-filling toilets, or faucets chronically dripping.
In Texas, it’s rare for a check valve to be installed on the water supply line for the house, but it’s fairly common to have a pressure regulator installed. The problem that these regulators can create is that they will act as a check valve; they’ll allow water into the home, but they won’t allow water back out. This creates what is called a ‘closed system’.
EXPANSION TANKS
When a closed system exists on the water distribution piping in a home, an expansion tank needs to be installed somewhere on the plumbing system. This is a fairly simple and straightforward fix; an expansion tank uses an air bladder which is compressible which gives the water somewhere to go when it expands, and the temperature and pressure relief valve on the water heater will stop causing problems.
REQUIRED OR JUST A GOOD IDEA?
As of 2018, the International Plumbing Code 607.3 requires an expansion control device whenever a check valve or pressure reducing valve is used in the water supply.
Where a storage water heater is supplied with cold water that passes through a check valve, pressure reducing valve or backflow preventer, a thermal expansion tank shall be connected to the water heater cold water supply pipe at a point that is downstream of all check valves, pressure reducing valves and backflow preventers. Thermal expansiontanks shall be sized in accordance with the tank manufacturer's instructions and shall be sized such that the pressurein the water distribution system shall not exceed that required by Section 604.8.