Hey there, Boiler Bosses 🔥 — Savvy Mavi here!
If you’ve ever noticed gurgling pipes, pressure problems, or mystery leaks near your boiler, don’t go blaming the whole system just yet. The real troublemaker might be that unsung hero—or ticking time bomb—called the expansion tank.
Let’s get into what this little tank actually does, how it helps your Weil-McLain hot water boiler system run smooth, and most importantly, how to spot the signs that it’s failing (so you can avoid a full-blown flood situation 🫣).
What Is a Boiler Expansion Tank, Anyway?
Think of your expansion tank like a pressure cushion. Your boiler heats water, and heated water expands—basic science, right? But your pipes and boiler can’t stretch to hold that extra volume. Enter the expansion tank!
It’s a small metal tank, often located above or beside the boiler, and it’s half air, half water (with a diaphragm separating the two). As the water heats and expands, the excess is pushed into the tank to prevent pressure buildup.
Without this humble little tank, the rising pressure could trigger your relief valve, damage your pipes, or even make your boiler shut down. That’s why the expansion tank is essential to a balanced, happy heating system.
Want a deeper dive into how hydronic heating works overall? Check out this Energy Saver guide from the U.S. Department of Energy.
Types of Expansion Tanks
Not all tanks are created equal. Here are the two most common types you’ll run into:
1. Steel Expansion Tanks
The old-school kind. These are just steel cylinders filled partially with air. No diaphragm. Over time, the air dissolves into the water, and you have to drain them manually. Not very 2025, if you ask me.
2. Diaphragm Expansion Tanks
More modern and reliable. These contain a rubber diaphragm that keeps air and water separated, so you don’t need to recharge the tank. They’re compact, efficient, and far easier to maintain.
If you're installing or upgrading your system, chances are you're using a diaphragm model—and Weil-McLain boilers play nicely with them.
Signs Your Expansion Tank Might Be Failing 🚨
Expansion tanks aren’t immortal. In fact, they usually last 5 to 10 years, depending on the quality of water, pressure settings, and how well they’re sized.
Here are some common warning signs to watch for:
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Your boiler pressure gauge is swinging wildly or stays too high
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The pressure relief valve keeps opening (hello, puddle on the floor!)
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You hear clunking, banging, or gurgling in your radiators
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The tank feels heavy and waterlogged
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You tap the tank and it doesn’t have that “empty on one end” echo
Want a visual guide? This YouTube tutorial by RealHouseDIY walks you through how to identify and troubleshoot common expansion tank issues.
How to Test an Expansion Tank (Without Freaking Out)
Alright, DIYers—if you’re feeling brave, here’s how you can check whether your tank is doing its job.
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Turn off the power to the boiler.
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Let the system cool down. (Hot water = burns = no thank you.)
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Locate the Schrader valve (it looks like a tire valve) on the tank.
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Check the pressure with a standard tire pressure gauge.
You’re aiming for 12–15 psi on a residential system. If it’s way off—or water sprays out of the valve when you press it—💀 it’s time for a new tank.
Still unsure? This great article from Steve Jenkins offers a step-by-step walkthrough with illustrations.
How to Replace a Boiler Expansion Tank
Here’s the short version of what to expect if you (or your HVAC tech) need to swap in a new tank:
Tools You’ll Need:
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Wrenches
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Thread seal tape
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Pipe dope
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New diaphragm expansion tank (sized correctly!)
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Bucket or towels
Steps:
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Shut off the water supply and power to the boiler.
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Drain the water from the boiler.
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Remove the old tank (watch for water drips).
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Wrap the new tank threads with seal tape.
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Install it snugly, reconnect the water, and repressurize the system.
If you're not handy, call a licensed tech. And if your system is under warranty, double-check first—installing it yourself may void your coverage.
Need to size the right tank? Try this handy calculator from Watts. (They basically invented modern expansion tanks, so they know what’s up.)
What Happens If You Ignore a Failing Expansion Tank? 🧨
I’ll keep it real—ignoring a failing tank can have expensive consequences:
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Leaking pressure relief valves
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Corrosion inside your boiler
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Frequent system shutdowns
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Major repairs or full boiler replacement
In other words: that $80–$200 expansion tank could save you thousands.
Why Your Expansion Tank MUST Be Properly Sized
You can’t just slap on any tank and hope for the best. Too small, and it’ll overflow. Too big, and it won’t regulate pressure correctly.
Your tank must be sized based on:
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System volume (how much water is in the loop)
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Operating pressure
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Temperature rise
For help sizing it to your Weil-McLain boiler, refer to this sizing chart from SupplyHouse.
When to Call a Pro 🔧
Look, I’m all for getting your hands dirty, but if you’re dealing with any of these, phone a friend (a licensed HVAC technician):
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Your boiler keeps tripping the relief valve
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You don’t know the correct pressure for your system
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You’ve never drained a boiler before
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Your boiler is making spooky sounds 👻
Regular maintenance—including checking your expansion tank—is key to keeping that cozy heat flowing.
Final Thoughts from Mavi 💬
Boiler expansion tanks may be small, but their role is huge—kind of like the bassist in a rock band. If the pressure in your system is out of whack, don’t wait until it blows. Test it, replace it, or call someone who can.
And if you’re upgrading or troubleshooting your Weil-McLain boiler system, your next best click is right here 👉
https://thefurnaceoutlet.com/collections/weil
Is your boiler leaking and your not sure what to do about it? Visit my guide: Boiler Leaks Be Gone.
Because staying warm and dry shouldn’t feel like rocket science. Just good vibes and good valves. 🛁🔥
Catch you next time, boiler buddies!
— Savvy Mavi
