Expansion tank details (C) D FriedmanHow to Drain a Heating Boiler Expansion Tank
How to service the expansion tank (compression tank) on hot water heating systems

  • EXPANSION TANK DRAIN & AIR RE-CHARGE - CONTENTS: how to replace the air lost from a steel, bladderless heating boiler compression / expansion tank. This procedure addresses expansion tank waterlogging & fixes relief valve leaks traced to that problem.
  • POST a QUESTION or READ FAQs about heating system expansion tanks: their function, size, location, maintenance, and need for draining (on some models)
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Hot water heating system expansion tank / compression tank drain procedure to re-charge lost air:

This article describes the steps in draining of a steel compression or expansion tank used on hot water system heating boilers - hydronic heat. Draining water out of a waterlogged expansion tank is a common heating boiler service procedure used to replace a lost air charge. If this step is not taken when needed the waterlogged compression / expansion tank will cause boiler leaks and improper, even unsafe boiler operation.

In this article series we provide a heating system expansion tank / compression tank Troubleshooting & Repair Guide that will address just about any problem traced to this heating system component.

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How to Drain & Re-Charge the Air Needed in a Heating Boiler Expansion Tank, step by step

Bladderless expansion tank on gas fired heating  boiler (C) HHPeriodic draining of non-bladder boiler expansion tanks:

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.

Newer internal-bladder type expansion tanks don't need draining but might need an additional air charge, as we explain at EXPANSION TANK DIAGNOSIS

As we detail below in another photo, the green arrow indicates the expansion tank drain valve and the blue arrow marks the isolating valve that gets shut off first.

So if no one has ever drained the expansion tank, or not for years, chances are it's ready and waiting for that attention. Or as we've explained above, it's past due for draining.

How to determine when the expansion tank needs drainage

A traditional rule of thumb practiced by old heating service technicians (including me - DF) was simply to always drain all of the water out of the bladderless-expansion tank at every annual service.

If a call-back customer complaint led to a need to drain the expansion / compression tank more often than once a year we figured something else was wrong and needed to be diagnosed and fixed. At that point we'd check to see if the expansion tank were waterlogged. If not we'd look for a different problem such as LEAKS into the BOILER CAUSE RELIEF VALVE LEAKS

If the heating boiler expansion 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.

Watch out: don't go pushing the expansion tank all over the place. Just a tiny test push is sufficient. If you wiggle the tank around too much you're asking for a pipe leak. Also some expansion tanks are strapped tightly against the ceiling so you can't move them anyway.

Watch out: modern internal-bladder type expansion tanks such as AMTROL's EXTROLl®, the FILL-TROL® Diaphragm-Type Expansion Tank do not need periodic draining - those tanks use an internal bladder. If your Amtrol expansion tank is waterlogged there is a problem to correct. It might be a slow loss of the tank's air charge over several years, fixed simply by adding air. IF the tank has a pinhole leak or a ruptured bladder, the tank probably needs replacement.

See EXPANSION TANK DIAGNOSIS if you missed that step
see EXPANSION TANK PRESSURE ADJUSTMENT if you need to adjust the pressure in a bladder-type expansion tank.

So Now, Finally, Let's Drain that Heating Boiler Expansion Tank

Expansion tank details (C) D Friedman

Can't Find the Expansion Tank?

This article section has been moved to FIND the EXPANSION TANK

Many expansion tanks use a special drain valve that permits air to flow into the tank as water is drained out.

Watch out: this hose idea is convenient and is recommended by some folks who lack experience with expansion tanks.

But the hose only works if a special air-admittance valve is installed on your expansion tank. If your tank just has a regular boiler drain valve installed, the hose thing will be very difficult to work - in that case plan some trips with a bucket held under the opened tank drain valve instead.

Expansion tank drain valve (C) Daniel Friedman

The expansion tanks we show with our arrows in the photos above had a simple boiler drain. We open the drain and let it spill into a bucket. That lets air bubble back up into the expansion tank - something that would take forever if we had a hose hooked up here. This can be a tedious process. Some expansion tank drain valves include an extra feature, an air inlet that allows air to enter the expansion tank through an air inlet on the drain valve at the same time that water is draining out of the expansion tank.

Watch out, the expansion tank water could be hot, and is usually a bit smelly and dirty.

Expansion tank isolating valve (C) Daniel Friedman

Our photo above shows an expansion tank isolating valve. Below it and to its left you can see the top of a B&G water feed valve that sends water into the heating boiler.

Below: The boiler drain valve on an oil fired heating boiler is located low on the boiler at the bottom of the boiler's water jacket.

Boiler drain valve on an oil fired Weil Mclain heating boiler (C) Daniel Friedman


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