LARGER VIEW of a heating boiler expansion tankGuide to Heating Boiler Expansion Tanks
How to troubleshoot, drain, or service & repair the expansion tank (compression tank) on hot water heating systems

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Hot water heating system expansion tank / compression tank installation, troubleshooting, repair:

How to identify, inspect, install, repair, or service heating boiler expansion tanks: here we explain the function of expansion tanks on hot water (hydronic) heating systems.

We discuss what happens if the expansion tank becomes waterlogged, how to drain a waterlogged expansion tank, and what sorts of expansion tanks, like the one shown at page top, should never become waterlogged.

We describe where the expansion tank may be found (sometimes in the attic!) and we illustrate different sorts of expansion tanks used over the history of hydronic heating in buildings. 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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Heating Boiler Expansion Tanks - Troubleshoot, Understand, Fix

Photograph of a heating boiler expansion tank The photo at page top shows a modern Extrol(R) bladder-type heating system expansion tank. At left over our client's head is a reddish tan steel heating boiler expansion tank (or compression tank in some texts).

Here discussion includes: How to Drain an Expansion Tank - Troubleshooting & Repair Guide: Diagnostic Tests for Waterlogged Heating Boiler Expansion Tank - How to troubleshoot an internal-bladder type expansion tank or How to troubleshoot a traditional bladderless expansion tank or compression tank. HOW TO diagnose trouble with internal bladder Extrol type expansion tanks - What goes wrong with bladderless expansion tanks - waterlogging & its causes

[Click to enlarge any image]

What is a Heating Boiler Expansion Tank & How Do Expansion Tanks Work

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

That pressure is increase is absorbed by allowing water from the heating system to squash or compress a reservoir of air inside the expansion tank - which is why some heating experts call our expansion tank a "compression tank".

Inside of a working expansion tank is a reserve space of air. In fact, when the heating boiler and system are cool, the traditional bladderless boiler expansion tank will contain mostly air.

Or in newer smaller internal bladder type expansion/compression tanks an internal rubber bladder keeps air separated from the heating system's water - avoiding rapid loss of air by absorption into the heating systems' water.

Steel expansion tank in a Two Harbors Minnesota Home (C) Daniel FriedmanAs the heating system warms up and as air entrained in the water raises system pressure, the increased pressure forces some of the heating system water into the expansion tank, squashing its air-charge into a smaller volume, thus permitting the tank to absorb the initial increase in system pressure.

Technical note: the water and air inside the expansion tank will always share the same pressure, but since water is not (very) compressible and air is, the increase in pressure will squeeze the air down to a smaller volume. That's why some heating system water can pass into the expansion tank.

If heating system pressures and/or temperatures exceed normal (for a residential heating boiler that would be a pressure above 30 psi or a temperature much above 200 °F) then the expansion tank is dismissed from duty and the pressure/temperature relief valve on the boiler will open to provide emergency relief.

What Happens if the Expansion / Compression Tank is Not Working?

The loss of air cushion in the expansion / compression tank means that at each heating-on cycle the system pressure can be too high, causing the temperature/pressure relief valve to spill.
See RELIEF VALVE LEAKS for a catalog of causes of TPR valve leaks.

Watch out: if you see water dripping or squirting out of the temperature/pressure relief valve on any vessel, such as a hydronic heating boiler, the situation is unsafe. The valve may be doing its job of relieving excessive pressure or temperature, but there is a risk that the valve becomes clogged by mineral deposits or crud left by the escaping water. A clogged TPR valve is no longer protecting the system from an explosion - BLEVE EXPLOSIONS

A second problem with older expansion/compression tanks - those that do not use an internal bladder to keep water and air separate, is the loss of air out of the compression tank and into the heating system water. We won't like the result.

Why do we care about keeping air out of the heating system water?

Air in the hot water heating system can cause loss of heat: air in baseboards radiators keeps hot water out - they don't get hot. And air in hot water heating system piping can simply keep hot water from ever reaching the radiators or baseboards.


See HOT WATER PRESSURE EXPANSION RATE if you're feeling curious about the physics involved in the pressure increase as we heat up water,

How & Why do Expansion Tanks Get Waterlogged?

Expansion tank details (C) D FriedmanLoss of Air Charge in a Bladderless Steel Traditional Heating System Expansion Tank

In older hot water heating boiler expansion tanks that do not have an internal bladder to keep air and water separate, over time air in the expansion tank may become absorbed into the heating water. Air may also find its way via the heating water to other air bleeders or vents in the system.

As air is lost and water just stays in the expansion tank, the space and pressure cushion provided by air in the expansion tank gets too small, or is lost entirely. There is no more air cushion to absorb initial pressure increases in the heating system during each heating boiler on-cycle.

Loss of Air Charge in an Internal-Bladder-Type Heating System Expansion Tank

In a modern internal-bladder type expansion tank use on heating systems it is not usually necessary to add air nor to change the tank pressure. There are however several sources of loss of the air charge and causes of water logging in an internal bladder-type expansion tank:

Article Series Contents

Expansion Tank Sizing Procedures

LARGER VIEW of a heating boiler expansion tank

This article has moved to EXPANSION TANK SIZING GUIDE

Amtrol's Adapted Extrol®, the Fill-Trol® Diaphragm-Type Expansion Tank

This article section has been moved: see AMTROL's EXTROL®, the FILL-TROL®

Signs of Trouble with a Heating Boiler Expansion Tank

This content has moved to EXPANSION TANK DIAGNOSIS

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

This topic has moved to EXPANSION TANK DRAIN & AIR RE-CHARGE

Can't Find the Expansion Tank?

This article section has been moved to FIND the EXPANSION TANK

Other Leaks Into Boilers Can Cause Over-Pressure in the Boiler & Relief Valve Leaks

Reader Question: I drained the expansion tank but the system pressure is still too high; high radiators in the house are cold & the relief valve spills

We moved this discussion to LEAKS into the BOILER CAUSE RELIEF VALVE LEAKS

Pressure Settings for Expansion Tanks


Service Procedure Using a Pump Damages Expansion Tank Internal Bladders

This discussion has moved to BOILER DRAIN DAMAGES EXPANSION TANK

See HOT WATER TANKS, INDIRECT FIRED for details about that type of domestic hot water heater.


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