What are the symptoms of a faulty heat exchanger in a boiler?

Author: Jillian Runolfsson  |  Last update: Thursday, December 18, 2025

Six Signs Your Furnace Heat Exchanger May be Cracked
  • Strange smells. A malfunctioning heat exchanger will often create an unpleasant and strong odor that smells similar to formaldehyde.
  • Soot Build-up. ...
  • Corrosion & Cracks. ...
  • Change in the flame appearance. ...
  • Audible Sounds. ...
  • Presence of Carbon Monoxide.

How do I know if my boiler heat exchanger is bad?

Signs of a bad heat exchanger in a gas furnace include strange noises, a yellow or flickering burner flame, increased fuel bills, carbon monoxide detector alarms, and unusual odors, such as a burning smell or the scent of exhaust fumes.

How much does it cost to replace a heat exchanger on a boiler?

Your boiler's heat exchanger is one of its most critical components. Ensuring it functions efficiently can prevent costly repairs or replacements. A new heat exchanger can cost between £400 and £600, but if your boiler is still under warranty, you may be able to get it replaced for free.

What happens when a heat exchanger goes bad?

If the heat exchanger rusts through or cracks, flue gases, including deadly carbon monoxide could leak into the home resulting in illness and possibly death of the occupants. Carbon monoxide poisoning can produce headaches, flu like symptoms, and even death.

How do I know when my heat exchanger needs replacing?

The only signs that you have a damaged heat exchanger are no heat during the winter, reoccurring flu-like symptoms, or a beeping carbon monoxide detector.

My Boiler issues. Vaillant 837 Blocked Main Heat Exchanger

How often should a boiler heat exchanger be cleaned?

Boiler service

The gas burner should be removed (or the inspection plate) to inspect the heat exchanger and electrodes. These can be cleaned and the heat exchanger washed, cleaning the heat exchanger on an annual service helps massively to increase the efficiency and reliability of the boiler.

Can you run a furnace with a bad heat exchanger?

Rule #1: If your Heat Exchanger has been deemed cracked by a HVAC professional you should NOT be running it. Unfortunately, some owners will attempt to run their furnace even when a cracked heat exchanger has been detected.

What is the average cost to replace a heat exchanger?

The cost of replacing a heat exchanger ranges from $2,000 to $3,500. Replacing your furnace's heat exchanger would be a good route to take if your furnace is fairly new. However, it is not worth spending your money on replacing your heat exchanger when it is near the end of its life span, which is ten years or more.

How to test a heat exchanger?

The tracer gas test - all openings in the heat exchanger are sealed, a methane tracer gas is injected into the heat exchanger, and a combustible gas leak detector passed around the outside looking for areas where the tracer gas is escaping.

What is the most expensive part of a boiler?

Generally, the most expensive part of a gas boiler is the heat exchanger, as it plays the main role in transferring heat from burning gas to circulating water for heating purposes. Heat exchangers can become damaged over time, leading to failures that are expensive to replace.

Why do boiler heat exchangers fail?

Household Chemicals – Believe it or not, corrosion can form on the heat exchanger. Things like hair spray, fabric softeners, and bleach can enter the air stream, pass the filter, and attach to the heat exchanger for good. Over time, the chemicals bore a hole through the metal and cause it to fail.

How much is a heat exchanger for an ideal boiler?

£189.99. £134.90. The Ideal Logic Heat Exchanger is a reliable and efficient component for central heating systems that require hot water.

Can you run a boiler with a cracked heat exchanger?

In most cases, furnaces or boilers with a cracked heat exchanger are completely replaced. This is because the cost to replace the heat exchanger alone can exceed the cost of upgrading to a new unit. Furthermore, a cracked heat exchanger can allow Carbon Monoxide to spread into your home.

How to inspect a boiler heat exchanger?

With a boiler, the heat exchanger will not be visible for inspection. As you examine the burner area note if there is any evidence of leaking from the heat exchanger onto this area. In many cases there would be signs of rusting, corrosion, flaking metal, or water seepage and even dripping water.

Is it worth replacing a boiler heat exchanger?

Unfortunately, a heat exchanger is not cheap to fix and it may be better to get a new boiler replacement in the long term.

What is one of the signs of a bad heat exchanger?

Immediately call an HVAC professional to assess your furnace if you notice any symptoms of a bad heat exchanger:
  • Orange, yellow, or flickering burner flames.
  • Pungent smells.
  • Rattling, popping, or banging sounds.
  • Carbon monoxide detection.
  • Corroded or damaged components.

Should I replace the heat exchanger or buy a new furnace?

Furnaces have an average lifespan of around 10 years, and most exchangers tend to go out around this time. If your furnace is close to this 10-year mark, a new furnace is likely a better investment. If your furnace is relatively new, a new heat exchanger may be worth it to avoid early furnace replacement.

How many hours does it take to replace a heat exchanger?

Because replacing a heat exchanger can take up to eight hours or longer, it represents the bulk of the cost. If your warranty is expired, and you would need to pay both parts and labor for a new heat exchanger, look further into whether a full unit replacement would be more advantageous for you.

What causes a heat exchanger to go bad?

Clogged filters reduce the air flow that result in development of cracks inside the system. Other reasons for reduced air flow are the dirty fan blades, dirty duct work, and obstructed air vents. All of these factors can cause major damage to the heat exchanger.

Can I replace a heat exchanger myself?

And remember, trying to replace a heat exchanger without professional assistance can pose potential hazards such as carbon monoxide poisoning and other safety risks.

What can a technician do to repair the heat exchanger?

Repairing a heat exchanger involves welding the cracks or applying heat-resistant epoxy. However, due to the critical role the heat exchanger plays in a furnace, ensuring the integrity and safety of the repair is paramount.

What happens when a boiler heat exchanger fails?

Without it, your boiler would fail to heat your water - which, is kind of important. Unfortunately, when the heat exchanger is faulty, a boiler can fail at its job (like the British Government during a pandemic), leading to cold or lukewarm water.

How much is labor to replace a heat exchanger?

How Much Does Labor Cost to Replace a Heat Exchanger? Generally, labor to replace a heat exchanger costs $400-$600. The job can take 5-8 hours at a cost of $75-$125 per technician. Furnaces operate the same way regardless of if they use natural gas, propane or oil.

Can you clean a heat exchanger without removing it?

Cleaning-In-Place (CIP) equipment can clean plate heat exchangers without disassembly. CIP is a combination of time, temperature and concentration. CIP provides both chemical and mechanical cleaning to the heat exchanger.

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