Water heater troubleshooting: Leaks, poor heating & more
When a residential or commercial water heater breaks down, your customers need a fast, permanent fix. While basic checks cover the obvious stuff, master plumbers use a step-by-step water heater troubleshooting process to find complex system failures.
This guide breaks down how to diagnose the four most common problems you'll encounter when troubleshooting a water heater: leaks, noises, smells, and no hot water.
Before you begin
Before you get too deep into troubleshooting, the very first step is to look closely at the data plate on the water heater. You need to check the serial number to determine the exact age of the unit and verify its current warranty status. Checking this crucial information upfront dictates your entire approach — whether you will bill the customer for parts, file a manufacturer warranty claim, or recommend a full system replacement if the unit is past its prime. Once the warranty and age parameters are established, you can safely proceed with diagnostic testing.
Water heater not heating
Water heater leaking
Water heater making unusual noises
Water heater smells like sulfur
1. Water heater not heating
When a water heater stops heating, you need to systematically check either the power supply (for electric units) or the gas supply (for gas units).
Checking voltage and elements
For comprehensive electric water heater troubleshooting, don't just rely on a simple voltage stick. Use a digital multimeter to see exactly what is going on:
Check the voltage: Measure the incoming power at the top thermostat. If the meter reads 240V but the heater isn't working, turn off the power and test the individual heating elements.
Test the resistance: Disconnect the wires from the heating element. A working 4,500W element should read around 12.8 ohms. You can figure this out using Ohm's Law:
Resistance = (Voltage x Voltage) / Wattage
Note: If your meter reads "OL" or infinity, the internal wire is broken, and the element needs to be replaced.
- Check for a short circuit: Test for continuity between each element terminal and the metal tank itself. If you get a reading, the element has cracked open and is shorting out against the tank, which is a major hazard.
Checking pilots and pressures
In the event of a gas or tankless water heater not heating, focus on the safety pilot and the gas supply:
Test the thermocouple or thermopile: Use your multimeter (set to millivolts) to test the safety sensor while the pilot flame is hitting it. A good single thermocouple should read 20 to 30 mV. A larger thermopile should read 650 to 850 mV under a load. If the readings are lower, the safety magnet inside the gas valve won't stay open.
Check gas pressure: Hook up a manometer to the gas valve. Check the incoming pressure while the gas is off (it should be 5 to 7 inches of water column for natural gas, 11-13 inches for propane). Watch the gauge when the burner lights up; if the pressure drops drastically, you have a blocked gas line or a bad regulator upstream.
2. Water heater leaking
If you are called out for a water heater leaking, a puddle at the bottom of a heater doesn't always mean the tank is ruined. You need to figure out if the tank itself is cracked or if a smaller part is leaking.
Pressure build-up and thermal expansion
High city water pressure or a closed plumbing system can cause the Temperature and Pressure (T&P) relief valve to drip occasionally.
Screw a water pressure gauge onto the tank's drain valve.
Watch the pressure while the water heats up. If the pressure climbs toward 150 PSI as the water expands, the T&P valve is just doing its job.
The fix: The issue is thermal expansion. You need to install an expansion tank or recharge the existing one.
Condensation and loose connections
High-efficiency condensation: High-efficiency gas water heaters naturally create condensation. If the internal drain trap or hose is plugged or cracked, water will puddle under the heater, looking exactly like a tank leak.
Pipe connections: Use an inspection mirror to check the cold water inlet and hot water outlet connections at the top of the tank. Slow, hidden leaks at these joints can run down behind the outer jacket, making it look like the tank itself is leaking from the bottom.
Smart leak detectors and shut-off valves
Whether the puddle is from a simple loose fitting or an early sign of tank failure, a leak can quickly cause thousands of dollars in property damage. Use this diagnostic visit as an opportunity to offer your client proactive protection. Recommending a smart leak detector or an automatic shut-off valve is a high-value upsell that gives homeowners peace of mind. These systems send instant alerts to a smartphone the moment moisture is detected, and advanced automatic shut-off valves can completely isolate the water supply line to stop a catastrophic flood before it starts.
3. Water heater making unusual noises
Loud noises coming from a water heater are usually caused by buildup or restricted water flow.
Popping
In hard water areas, lime and calcium settle to the bottom of the tank, forming a thick layer of crust over the lower element.
Water gets trapped underneath this crust. When the burner turns on, the trapped water instantly boils into steam and collapses, creating a loud popping or crackling noise as it breaks through the sediment.
Hissing
A high-pitched hissing sound during heavy water use means the water is trying to force its way through a tight spot, which is often caused by hard limescale buildup narrowing the pipes.
Use a mechanic's stethoscope along the pipes to find the noise.
Look for partially closed valves, broken dip tubes, or heavy buildup inside the nipples at the top of the tank. These restrictions act like a nozzle and speed up the water — causing loud vibrations.
The fix: Give the tank a high-velocity power flush. If the buildup is too hard, use a professional descaler to dissolve the crust before flushing it out.
4. Water heater smells like sulfur
A distinct smell coming from the hot water taps are usually caused by a chemical reaction inside the tank. While the reaction is largely harmless, the bacteria growing inside the tank produces a foul, rotten-egg odor that can be quite unpleasant.
Bacteria and anode rods
- Find the source: Test both the cold and hot water. If the smell is only coming from the hot water, the inside of the water heater is the problem. The standard magnesium anode rod (installed to prevent tank rust) releases electrons as it breaks down. This creates a perfect environment for the bacteria to thrive and produce sulfur gas.
The fix: Pull out the old anode rod. If the water quality is poor, replace it with a zinc or aluminum rod (zinc naturally kills bacteria) — or install an electric powered anode rod, which protects the tank without causing odors.
Navigating the "Repair vs. Replace" conversation
When severe failures are found, you need to discuss repair vs. replacement options with the homeowner. Replacing the unit is more cost-effective if:
The tank is leaking: A cracked internal tank cannot be fixed; it requires a full replacement.
The 50% Rule: If the total cost of repair parts and labor exceeds 50% of a new installation, a replacement offers better long-term value.
Age of the unit: Standard tank systems last 8 to 12 years. If the heater is past its 10th year, investing in major repairs is often wasted money, as other components are likely near failure.
Framing the choice around safety, utility bill savings from high-efficiency models, and new warranty protections helps homeowners see when replacement is the smarter investment.
Testing for common water heater problems
| Problem | Likely Cause | Best Test | Tools Needed |
|---|---|---|---|
| No Hot Water (Electric) | Broken element or bad thermostat | Test for continuity and resistance (in ohms) across element screws | Digital Multimeter |
| No Hot Water (Gas or Tankless) | Low gas pressure or weak pilot/sensor signal | Test millivolts at the sensor; check gas pressure lines | Digital Manometer & Multimeter |
| T&P Valve Dripping | Too much pressure / thermal expansion | Watch for pressure spikes while the water is actively heating | Pressure Gauge |
| Popping Noise | Thick mineral buildup at the bottom | Perform a high-velocity power flush | Descaling Pump Kit |
| Rotten Egg Smell | Bacteria reacting with magnesium anode rod | Verify the smell is only in hot water; inspect the rod | Zinc or Aluminum Anode Rod |
How to perform a high-velocity power flush
When dealing with heavy mineral buildup or loud popping noises, a standard gravity drain won't cut it. Master plumbers use a high-velocity power flush to forcefully break up and clear out crystallized lime and calcium from the bottom of the tank.
Follow this professional procedure to safely restore system efficiency:
1. Cut the power and gas supply
For electric water heaters, shut off the dedicated breaker at the main electrical panel. For gas units, turn the gas control valve knob to the "PILOT" or "OFF" position.
Pro tip: Never flush a water heater while the elements or burners are active. Heating an empty or low-tank unit will instantly destroy the elements or crack the tank's glass lining.
2. Shut off the cold water supply
Locate the cold water shut-off valve at the top of the water heater and turn it completely off to isolate the tank from the home's water system.
3. Relieve the pressure
Go to a sink and turn on the hot water tap. Leave it running. This introduces air into the plumbing, allowing the water heater to drain.
4. Attach a hose
Hook up a heavy-duty, high-temp utility hose to the drain valve located at the bottom of the heater. Run the other end of the hose to a safe drainage location, such as a floor drain, sump pump basin, or outdoors.
5. Open a hot water faucet to break the vacuum
Open a hot water tap somewhere in the building (ideally on an upper floor) or gently lift the lever on the T&P relief valve. This introduces air into the plumbing system, breaking the vacuum and allowing the tank to drain smoothly without sputtering.
6. Open the drain valve and perform the power flush
Slowly open the water heater's drain valve. Once the initial volume of water finishes draining out, perform the high-velocity power flush to clear the heavy sediment:
- Keep the drain valve wide open.
- Quickly flip the cold water supply valve fully open for 15 to 20 seconds, then shut it off again.
- Repeat this blast cycle 4 to 5 times.
- The sudden, high-pressure bursts of incoming cold water hit the bottom of the tank, violently agitating the settled scale and forcing it out through the hose. Continue until the discharge water runs completely clear.
7. Close up and refill the system
Once the water runs clear, close the bottom drain valve tightly and disconnect your hose. Turn the cold water supply valve back on completely to refill the tank. Keep the hot water faucet inside the house open until water flows out in a steady stream with no air pockets—this ensures the tank is 100% full.
8. Restore system power
Only after the air is completely purged from the lines and the tank is entirely full should you turn the electrical breaker back on or relight the gas pilot flame.
Water heater troubleshooting FAQs
Why does a heating element show the right resistance but still fail to work?
This means the element is shorted to ground. The internal wire has broken and is touching the outer metal sleeve. While a test across the two screws might look normal, the electrical current is escaping into the water and the tank itself. Always test from each screw to the metal tank to check for a short.
What causes a tankless water heater to shut down and display an error code?
Most tankless systems shut down due to either a lack of airflow or mineral buildup inside the heat exchanger. If the air intake or exhaust vent is blocked, the unit will throw a code and turn off for safety. Similarly, if hard water scale builds up inside the narrow heating tubes, the system will overheat and shut down to prevent damage.
Can a broken dip tube cause a lack of hot water without leaking?
Yes. If the cold-water dip tube breaks off near the top of the tank, incoming cold water mixes right away with the hot water leaving the tank. This ruins the natural layering of hot water at the top, sending lukewarm water to the house even though the burners or elements are working perfectly.
Partner with Ferguson for commercial-grade tools and parts
Getting the job done right requires the right tools and exact replacement parts. Ferguson supports your business with a massive inventory of troubleshooting gear and OEM parts to get your customers back up and running fast.
- Precision diagnostic tools: Equip your team with rugged digital multimeters, dual-port manometers, and high-accuracy pressure gauges from top brands.
- OEM & universal parts: Get fast access to replacement elements, commercial thermostats, gas valves, and pilot assemblies that match original manufacturer specs.
- Water quality fixes: Fix water quality issues for good with our stock of zinc-aluminum anodes, powered anode systems, and professional descaling solutions.
Log in to your ferguson.com account today for pro pricing, live inventory tracking, and easy ordering built for professional contractors.