Boiler Expansion Tank Problems and Baseboard Fin Care: A Guide to Hot Water Heat

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What a boiler expansion tank does

If your home has hot water heat, with baseboards, radiators or in-floor tubing, there is a small tank near the boiler that does an important job. A boiler expansion tank gives heated water somewhere to go. When it fails, the pressure in the system climbs and the relief valve may start dripping or discharging. Along with the tank, the fins on hot water baseboards are a common item we look at during inspections.

This guide explains how expansion tanks and baseboard fins work, the signs of trouble, what these findings mean in an inspection report, and how they are fixed and maintained.

Why expansion tanks matter in a hot water system

A hot water heating system is a closed loop. Water expands as it heats, and because the loop is sealed, that extra volume has to go somewhere. The expansion tank holds a cushion of air, usually separated from the water by a diaphragm, that compresses as the water expands and pushes back as it cools.

Amtrol, a major expansion tank maker, ships its standard hydronic tanks with an air charge of 12 psig unless the label says otherwise, and says the charge should match the minimum system pressure at the tank. The U.S. Department of Energy lists checking that the tank is filled with air, not water, as part of routine maintenance for hot water systems.

You will find hot water heat in many older homes around Fargo-Moorhead, and in newer homes and garages with in-floor heat. Our heating season is long, so a problem that goes unnoticed in October can turn into a no-heat call in January.

Signs of a waterlogged, leaking or rusty expansion tank

Waterlogged. A tank is waterlogged when it has lost its air cushion and filled with water. Diaphragm tanks can slowly lose their air charge over time, and older steel tanks without a diaphragm can lose air into the system water. Bell & Gossett, a long-time hydronic manufacturer, lists a waterlogged or undercharged expansion tank as a common reason relief valves discharge. Signs include:

  • The pressure on the boiler gauge rising sharply as the system heats up.
  • The relief valve dripping or discharging water.
  • Water near the boiler with no other obvious source.

Leaking. Water dripping from the tank body, the air valve or the connection to the piping. A leak from the air valve can mean the diaphragm inside has failed.

Rusty. Rust on the tank shell or fittings. Amtrol warns that pressurized tanks can corrode and weaken over time, and that a tank that shows signs of corrosion should be looked at by a qualified professional. Rust can start from a slow drip from piping or fittings above the tank.

Do not try to adjust, recharge or tap on a corroded tank yourself. Amtrol specifically advises against re-pressurizing a tank that is corroded or damaged.

What it means in an inspection report

When we find an expansion tank that appears waterlogged, is leaking, or shows rust during an inspection, we recommend having it evaluated by a qualified heating contractor and repaired or replaced as needed. Expansion tanks are relatively inexpensive parts. The concern is what a failed tank can lead to: a relief valve that keeps discharging, water damage near the boiler, extra wear on the system, and a boiler that shuts down when you need it most.

If the relief valve is discharging, call a heating contractor. Never cap or plug a relief valve. It is a safety device.

Baseboard fins: why they matter

Hot water baseboards heat a room by convection. Water flows through a copper pipe lined with thin aluminum fins. The fins heat the air around them, which rises out of the top of the cover and draws cooler air in at the bottom.

When fins are bent, crushed, missing or packed with dust and pet hair, less air moves across them and the baseboard puts out less heat. Fins get damaged by vacuums, furniture, kids and pets, especially when the front cover is loose or missing.

When we see damaged, dirty or missing fins during an inspection, we recommend having the fins cleaned, straightened or repaired so the baseboard can heat the room as designed.

How these problems are fixed

  1. Have the expansion tank checked. A heating contractor can isolate and drain the tank and test its air charge with the system cool. Amtrol notes the charge can only be properly set at room temperature. A tank that will not hold its charge, leaks or is rusted is usually replaced, and it should be sized correctly for the system.
  2. Fix the cause of rust. If a fitting or valve above the tank is dripping, have it repaired so the new tank does not rust the same way. Amtrol calls for a drip pan connected to a drain where a leak could cause damage.
  3. Check the rest of the system. The DOE recommends that a heating professional test the pressure relief valve and high-limit control, inspect the expansion tank and clean the heat exchanger as part of regular hot water system maintenance.
  4. Clean and straighten baseboard fins. Natural Resources Canada recommends vacuuming baseboard units each autumn and gently straightening bent fins with a plastic comb. Badly damaged or missing sections of fin tube can be replaced by a heating contractor.
  5. Replace missing or damaged covers. Covers protect the fins and help direct airflow.

How to keep a hot water system working well

  • Schedule regular service. Natural Resources Canada recommends servicing natural gas and propane boilers at least every two years and oil-fired boilers every year. Follow your boiler manufacturer’s schedule if it calls for more frequent service.
  • Glance at the gauge. Learn what your boiler’s pressure normally reads cold and when running. A big jump is worth a service call.
  • Watch for drips. Check around the boiler, relief valve and expansion tank a few times each heating season. Water sensors can help, as we explain in Why You Should Put Water Sensors in Your Home.
  • Keep baseboards clear. Vacuum the fins each fall and keep furniture and drapes from blocking airflow.
  • Have working CO alarms. The DOE advises every home to have a working carbon monoxide alarm. See The Importance of Smoke and Carbon Monoxide Detectors.
  • Protect against freezing. A boiler that shuts down in a cold snap can lead to frozen pipes. Our post How to Prevent Frozen Pipes in North Dakota has more tips.

A Nordic Fall Home Checkup is a good way to have the heating system and other key items looked at before winter.

Boiler expansion tanks and baseboard fins: common questions

How do I know if my boiler expansion tank is bad?

Common signs are a relief valve that drips or discharges, pressure that climbs sharply as the boiler heats, and leaks or rust on the tank. A heating contractor can confirm by checking the tank’s air charge with the system cool.

Can I recharge an expansion tank myself?

It is best left to a heating contractor. The tank has to be isolated and drained, and the charge set at room temperature to match the system. Amtrol advises against re-pressurizing a tank that is corroded or damaged.

How long does an expansion tank last?

It varies with water quality, how the system was installed and whether the tank has been leaking. Rather than going by age, have the tank checked whenever the boiler is serviced.

Is a dripping relief valve dangerous?

It is a warning sign. The relief valve protects the boiler from too much pressure, and a waterlogged expansion tank is a common cause of discharge. Have the system checked, and never cap or plug the valve.

Do bent baseboard fins really matter?

Yes. The fins are what transfer heat from the pipe to the air. A few bent fins will not make a big difference, but long sections of crushed, dirty or missing fins reduce heat to that room.

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