When Does Your Water Heater Need a Thermal Expansion Tank?

August 7th, 2019

Standing in front of a water heater, if a heating engineer suggests the installation of a Thermal Expansion Tank, it would be nice not to feel the need to ask questions. You know precisely what he’s talking about, so you’re aware of the implications and ready to make an informed decision. At any rate, the engineer is saying the water heater tank needs overflow protection. Strange, shouldn’t the water expand safely through the system?

Open vs. Closed Heating System Dynamics

Ordinarily, that’s what happens inside an open water heating plumbing. If the volume of the hot water increases, which it will, then that extra volume pushes its way back to the mains supply line. Alternatively, it’s stored in an open tank that’s located at the top of the installation. That’s just how open system water heating reacts. The water heats up, the fluid volume expands, and the excess water has somewhere to go. However, closed water heating systems, also known as closed-loop systems, use special check valves. Water enters the pipes but can’t exit. Using this configuration, leaks and pressure drop issues are minimized. Moreover, there’s less chance of air getting into the pipes, but there’s a problem which occurs because there’s nowhere for any excess, over-heated tank water to go anymore.

Installing a Thermal Expansion Tank

Often referred to as a Water Heater Expansion Tank, these fittings really do look like smaller copies of the primary hot water tank. Installed on a closed hot water supply system, if the main tank does produce an overflow, then it’ll collect inside the expansion vessel. Otherwise, the backflow prevention valve stops the overflow from discharging. Stuck in the closed-loop plumbing, the excess volume causes a dangerous pressure rise, which is discharged when a pressure safety valve trips. At the end of the day, your Thermal Expansion Tank will be fitted to prevent closed-loop system stress from overworking the tank and all of its associated plumbing. Not sure if you already have an expansion tank fitted? Look for that diminutive vessel just above the water heater.

In an open system, there’s an overflow tank, which is probably located in the attic. Excess pressures, as caused by heat-incurred volumetric variations, always have somewhere to go during a peak pressure fluctuation. Fitted with a one-way valve, closed-loop plumbing is different. If the pipe pressure is at capacity and the tank volume is still increasing, where’s the overflow going to go? Not to worry, based on the pressure and volume capacity of a specified water heater, a heating engineer will fit a thermal expansion tank to deal with any and all overflow conditions.

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