The recent outbreak of legionnaires’ disease in New York City has brought renewed attention to the inherent risks from legionella bacteria in HVAC and plumbing systems. There are several things that plumbing engineers and designers can do to minimize the risk of legionella colonizing and growing in plumbing systems.
Selection and Installation of Thermal Expansion Tanks
Using the calculated acceptance volume and total volume for the tank, select an expansion tank from the manufacturer’s data that meets these requirements.
Be aware that in most states, unfired pressure vessels larger than a certain volume must have an ASME stamp. Check your state’s requirement for this limit. If the expansion tank you need is larger than this limitation, it must be constructed in accordance with ASME Section VIII requirements. ASME rated vessels are much higher in cost than non-ASME vessels. Installing multiple smaller tanks, which are below the volume limitation for ASME tanks, is also an option. Most tank manufacturers offer both ASME and non-ASME thermal expansion tanks.
How To Size Thermal Expansion Tanks For Hot Water Systems
Referring to sizing tables in an expansion tank manufacturer’s catalog is the easiest method, although not the best method, for sizing thermal expansion tanks. Sizing tables provided by the manufacturers, give the tank model number and size based on the water heater’s volume and the system supply pressure. These tables, however, are based on three important assumptions that designers need to be aware of.
First, for most manufacturers, the tables are based on a maximum allowable line pressure of 150 psi. This is the maximum allowable working pressure of most water heaters and thermal expansion tanks. It is also the setting of the water heater relief valve. In other words, the thermal expansion tank, if selected by the tables, could accommodate the thermal expansion up to a system pressure of 150 psi, which is the relieving point for the water heater relief valve. Remember that the purpose of the thermal expansion tank is to avoid the relief valve from relieving. Sizing the thermal expansion tank for a maximum allowable pressure of 135 psi, which is 10% below the relief valve set point, is a better alternative.
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.
When heated, water expands. We know that. When water is heated in a water heater, the water expands, if possible. But what happens when water is heated in a water heater and there is no hot water usage?
This can occur after a period of showering/bathing during peak times followed by a period of no hot water usage. For example, evenings are a time when there is hot water usage for showering followed by a period of no usage while people are sleeping during the night.
During peak usage times, hot water stored in the water heater is distributed to the end users, and cold water fills the water heater. The cold water in the tank is then heated, but when there is no subsequent hot water usage, the heated water has no room for expansion. If there is no place for the expanded water to go, the pressure in the system increases significantly. This leads to one of two possible scenarios:
If you stay in this business long enough, you see a bit of everything. Notice the water heater’s T&P valve in the above photograph. The T&P valve discharges vertically up. What’s wrong with that?
There are many specific code requirements (13 to be exact) for the discharge piping from water heater T&P valves (Ref: 2012 International Plumbing Code 504.6). This installation violates requirements 8 and 9, which are:
8. Not be trapped.
9. Be installed so as to flow by gravity.
Obviously, with this incorrect installation, the discharge cannot flow by gravity. Also, water can fill and remain in the discharge pipe. Over time, stagnant water that remains in the discharge pipe can corrode the pipe and cause it to fail.
This is just a reminder. During field inspections, check the discharge pipe to verify code compliance.