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Guide to Heating Boiler Expansion Tanks
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Here we discuss Heating System Boiler Expansion Tanks, Waterlogged tanks, How to Drain an Expansion Tank and we provide a heating system expsansion tan k Troubleshooting & Repair Guide.

This website answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs. The photo at page top shows a modern Extrol(R) bladder-type heating system expansion tank. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2010 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Heating Boiler Expansion Tanks

Heating boiler expansion tanks are installed to absorb the initial pressure that occurs when the heating system warms up. Air molecules entrained in water inside the heatin boiler itself as well as in the heating system piping, baseboards, or radiators, expands and thus cause an initial pressure increase in the heating system.

If we do not provide some way to absorb this initial pressure increase, it is possible that the heating system's internal pressure would exceed 30 psi - the typical point at which a heating boiler pressure/temperature relief valve will open to spill excess pressure. If the relief valve is forced open in this manner the heating system will first lose water each time a heating cycle starts by heating up the boiler.

Then the heating system will take in makeup water (through the automatic water feed valve) each time the system cools down. The result would be recurrent loss and then inflow of water through the boiler, increasing the risk of system corrosion as well as wasting water and possibly causing other damage or operating problems.

When the heating boiler and system are cool, the expansion tank will contain mostly air. As the heating system warms up and air entrained in the water raises system pressure, the increased pressure forces some of the heating system water into the expansion tank, thus permitting the tank to absorb the initial increase in system pressure.

Naturally if the heating system is not operting properly and the boiler temperature or pressure continue to rise above acceptable levels, and since the expansion tank is a fixed volume which cannot absorb an unlimited heating system pressure increase, eventually the relief valve should also open to spill excess pressure.

Signs of Trouble with a Heating Boiler Expansion Tank

This means that if you see dripping at the pressure/temperature relief valve on a heating boiler, one thing to check is whether or not the expansion tank is working properly. We discuss relief valve leaks caused by expansion tank problems just below.

Expansion tanks on hot water heating systems can be divided roughly into two groups: older type bladderless heating system expansion tanks and newer type bladder-type expansion tanks such as those sold by Extrol(R).

Some heating system expansion tanks need periodic draining

Older heating system expansion tanks such as the one shown in this photo need periodic service: because air in the expansion tank can become absorbed into the heating water over time, eventually the expansion tank can become waterlogged. A waterlogged heating system expansion tank will be unable to absorb the initial heating system pressure increase - it stops working. This can lead to spillage at the boiler relief valve each time the heating system warms up. Spillage at the relief valve is potentially dangerous: eventually minerals in the water can clog a leaky relief valve, causing it to stop leaking - which might look ok but this means that the relief valve has become clogged - the boiler is operating without this critical safety device and an explosion could occur. We can determine when the expansion tank needs drainage: if the tank is heavy (try pushing it up or tapping on it) or if the relief valve is leaking, we probably need to drain the tank and let air return to it. Many expansion tanks use a special drain valve that permits air to flow into the tank as water is drained out. We drain the expansion tank simply using a garden hose connected to its drain valve. But watch out, the draining water could be hot.

Newer expansion tanks using an internal bladder should not need service

Newer type heating system expansion tanks that use an internal bladder keep their water and air separated. These tanks should not need service. If a bladder-type expansion tank has become waterlogged it's because the bladder has ruptured and the tank needs repair or replacement.

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