LARGER VIEW of a heating boiler expansion tankExpansion Tank / Compression Tank Diagnosis
How to troubleshoot the expansion tank on hot water heating systems

  • EXPANSION TANK DIAGNOSIS - CONTENTS: Heating System Boiler Expansion Tanks, Waterlogged tanks, 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
  • POST a QUESTION or READ FAQs about heating system expansion tanks: their function, size, location, maintenance, and need for draining (on some models)
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Hot water heating system expansion tank / compression tank troubleshooting guide:We provide a heating system expansion tank Troubleshooting & Repair Guide.

This article series provides a heating system expansion tank (compression tank) troubleshooting & repair guide.

Green links show where you are. © Copyright 2017, All Rights Reserved.

Signs of Trouble with a Heating Boiler Expansion Tank

LARGER VIEW of a heating boiler expansion tankExpansion tanks on hot water heating systems can be divided roughly into two groups:

[Click to enlarge any image]

  1. Older type bladderless heating system expansion tanks (atmospheric expansion tanks) and
  2. Newer type internal bladder-type expansion tanks such as those sold by Amtrol Inc.

The behavior, maintenance, and sources of leaks or trouble are not identical with these two different types of compression / expansion tanks on boilers.

A modern internal bladder type expansion tank like this Amtrol Extrol tank (photo at left) is generally trouble free, and it's unusual for these tanks to fail. The Ex-Trol® is properly referred to as a pressurized diaphragm-type expansion tank.

Both older non-bladder type expansion tanks and even modern (and much smaller) internal bladder expansion tanks can get in trouble.

But on occasion the internal bladder in one of these tanks could rupture or develop a leak. If that happens, because the tank is not field-repairable, the "fix" is to install a new expansion tank.

But far more often, it is older non-bladder type expansion tanks on boilers, like the one below) that become waterlogged.

What Happens if an Expansion Tank Becomes Waterlogged?

When the expansion tank is no longer able 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.

Watch out: spillage at the pressure temperature 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.

Atmospheric expansion tank on hydronic boiler (C) Daniel Friedman Two Harbors MN

Shown above, an older style ceiling mounted atmospheric expansion tank on a hydronic heating boiler in Two Harbors, MN. The drain valve shown at the left end of this expansion tank is a special valve that makes it easier for replacement air to enter the tank when the tank's water is being drained - a standard procedure during annual heating boiler maintenance. We describe that procedure separeately at EXPANSION TANK DRAIN & AIR RE-CHARGE .

[Click to enlarge any image]

Trouble signs that could be traced to a water-logged boiler expansion tank include:


How to Diagnose an Internal Bladder-Type Boiler Expansion Tank Needs an Increased Air Charge or Needs Replacement

Newer type heating system expansion tanks that use an internal bladder keep their water and air separated. These tanks should not need frequent service but might need a small additional air charge every few years, as we explained

But there are at least three possible conditions for which to check an internal bladder type expansion tank for trouble.

  1. Internal bladder type expansion tank air permeation loss: If the expansion tank has not been functioning properly but on removal you don't find that there is water trapped inside the tank bladder (as described above), check the tank's air pressure.

    If the air pressure is below the factory-set pressure, or below the pressure that you previously set when the tank was installed, the tank may have lost its air charge over a period of several years. This tank may be perfectly serviceable if re-charged to proper setting. That's why we put this possibility first in the list.
  2. Rapid air charge loss at the internal bladder expansion tank: When a bladder-type expansion tank has become waterlogged over a short interval it's most likely because the bladder has ruptured and the tank needs replacement.

    If the bladder has burst you will probably find that a little depression of the pin in the tank's air valve will send water squirting out of the valve - a sure sign that the tank bladder is burst and the tank needs replacement.
  3. Pinhole leak failures in an internal bladder expansion tank: But what if no water comes out of the tank. There may still be a pinhole leak of water into the air space that has begun to consume air space in the tank.

    If the tank has been removed and drained through its point of connection to the heating system, and you then shake it or feel that it has gained weight over a new empty tank of the same size and model, and if you (possilbly) hear water or feel water sloshing around inside the tank bladder, then the pressure tank had a pinhole leak, water is in the bladder, and the tank needs replacement.

Additional Diagnostic Tests for a Waterlogged Heating Boiler Expansion Tank

Airtrol expansion tank compression tank Bell & Gossett (adapted by InspectApedia)The following tips are from Bell & Gossett's installation manual for pressure reducing valves and from their Airtrol Compression Tank [expansion tank] System Installation Manual [4][5]. Our illustration at left shows the typical location of a traditional conventional, non-bladder-type heating boiler expansion tank, and is adapted from that B&G manual.

[Click to enlarge any image]

Article Series Contents


Continue reading at EXPANSION TANK DIAGNOSTIC FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see LEAKS into the BOILER CAUSE RELIEF VALVE LEAKS for other causes of leaks into a boiler that can cause TPR valve discharge

Suggested citation for this web page

EXPANSION TANK DIAGNOSIS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman