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Guide to Hot Water Heater Noise Diagnosis, & Water Heater Tank Scale Removal InspectAPedia®  -     Water heater noise troubleshooting & repair How to diagnose and cure water heater rumbling, clanking, cracking, or other hot water heater noises Advice and photo-guide to de-liming and de-scaling hot water heaters How to remove silt or debris from the hot water heater tank Water heaters: how to inspect, test, adjust, repair, and choose among types of hot water heaters

The articles at this website will answer most questions about diagnosing and curing noisy domestic water heaters and about the procedure to remove mineral deposits, lime, water scale, silicates, sulfates, aluminates, or silt and sand from a water heater tank. For the broad topic of controlling plumbing noises in buildings see Sound Control for Plumbing.

Lime caused by hard water as well as silicates, sulfates, and similar materials form water scale that coats the bottom of a hot water tank or the surface of the electrodes in an electric water heater, interfering with proper water heater operation, causing water heater noises, reduction in hot water quantity, increased water heater operating costs, and a shorter water heater life. Silt and soil debris can also collect in a water heater tank where it can cause similar problems even if the water supply is not hard.

Silt and debris can be easily removed from a hot water tank by periodic flushing. In fact if your water heater is quite noisy, it will probably be easier to remove un-wanted mineral deposits from the water tank than it will be later if you wait until the heater is no longer making much noise (because the mineral layer has solidified). Install a sediment filter on the incoming water supply to avoid this problem in the first place.

But lime or water scale are more difficult to remove from a water heater: manually scraping the tank bottom through the drain opening or use of a de-limer chemical will be needed. Below we describe these procedures. Readers should also see our discussion of mineral clogging of water piping, water heaters, and tankless coils, organized at HOT WATER IMPROVEMENTS. Other gas fired water heater noises are discussed at Gas Flame & Noise Defects.

This website contains links to in-depth articles on inspecting, testing, and repairing problems residential hot water heaters of all types, including their parts, controls, and alternative sources for hot water as well as tips for improving hot water temperature, hot water pressure, and hot water quantity. Reproduction of this web page electronically at other websites is prohibited. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. The sketch at page top is courtesy of Carson Dunlop.

Noise Diagnosis & Repair Guide for Residential Hot Water Heaters

If you hear loud popping, cracking, or clanking noises from your hot water heater when it is operating, the cause might be mineral deposits which have accumulated on the bottom of the water heater tank interior surface. If your water heater is an electric unit, these same mineral deposits can cause a hissing noise when the heating electrodes are operating.

If your water supply is hard (see DETECT HARD WATER), the minerals in the water tend to precipitate out at the hottest place in the plumbing system. Often this means that the water heater itself will accumulate solid precipitate of minerals.

Our photograph (left) shows our measurement of the thickness of mineral scale removed from the bottom of a six-year-old gas-fired water heater - more than 1/2 cm in thickness! The "lime" scale that forms in water heater tanks is composed principally of calcium and magnesium carbonate.

Hard Water and Water Heater Noises

The hard water mineral deposits tend to collect as a hard skin on the bottom of the heater, interfering with heat transfer, increasing operating cost, and over longer periods, possibly weakening the bottom of the water heater itself.

Mineral deposits also accumulate on the electrodes of an electric water heater, eventually leading to the failure of the heating electrode. If you never remove the scale from a water heater tank the clanking or cracking noise heard at the water heater during operation may eventually cease when the scale solidifies, no longer trapping hot water and interfering with its movement upwards in the water tank.

However the development of a thick lime or scale layer in the bottom of a water heater also reduces the water heater tank life.

• Water heater manufacturers recommend flushing the heater monthly to remove silt, and where water is hard, regular deliming of the water tank - steps which in our experience we see that almost no one does. With experience you may discover that flushing is required less often (or more often) in your area.
• On occasion when a heater fails (or in rare multi-defect cases, explodes), the leak and failure occur at the bottom of the tank, leading to a BLEVE explosion and a water tank that becomes a rocket.

Why Should You Remove the Scale Deposits, Silicates, and Silt from Your Water Heater Tank?

Our photo (left) shows chunks of water heater scale on the ground after removal from the water heater.

Noisy water heater operation: Water heater noises, as we described above, may be the first thing people notice when the water tank scale layer has become thick enough to interfere with good water heater operation. But A.O. Smith and other water heater manufacturers cite additional reasons why removing water heater scale is important. Clean the water heater tank to reduce water heater noise.

High water heater costs & longer water heater "on" time: you can reduce the cost of making hot water with your oil, gas, or electric-fired water heater by removing scale from the tank bottom. The scale layer on the tank bottom (or on an electric water heater's electrodes) interferes with heat transfer into the hot water. This means that the water heater will have to operate longer (and you will spend more on hot water costs) than necessary to re-heat incoming water. Clean your water heater tank to reduce your water heating cost by improving heat transfer into the water and correspondingly shortening the water heater "on" cycle time.

Hot water quantity has diminished: by slowing heat transfer into the hot water in the water tank, the apparent hot water quantity will also be reduced, because incoming cold water is not heated as rapidly when occupants are drawing hot water out of the tank. Thus the incoming cold more quickly dilutes and cools the hot water that remains in the tank during use. Clean the water heater tank to increase hot water quantity.

Extend the water heater life: because scale on the water heater tank bottom insulates the tank bottom from the water inside the tank, the bottom of an oil or gas fired water heater will become hotter as the scale layer gets thicker. These increased temperatures weaken the water heater tank bottom and shorten the life of the water heater. Similarly, scale deposits on the electrodes in an electric hot water heater shorten the life of the electrodes. Clean the hot water tank regularly to extend its life.

Protect water heater warranty: if a water heater tank fails the manufacturer may not honor the water heater's warranty if the failure is due to the accumulation of excessive scale inside of the water heater tank. Water heater warranties typically indicate that the tank is warranted against leaks due to rust, corrosion, or chemical action of the water but the warranty will exclude a tank failure due to scale.

How Often to Remove Scale Deposits from a Water Heater

When the lime thickness reaches the hot water tank drain, there will be about one inch of mineral deposit on the tank bottom. If the water tank has a cleanout opening, when the lime thickness reaches that opening the mineral deposit thickness will be about 2". When you have observed the length of time required for lime to accumulate in your water heater you can set the appropriate maintenance schedule.

How to Flush the Hot Water Tank to Remove Silt & Debris

Safety Warnings: don't tackle this water heater cleanout project on a Sunday night when you can't call a plumber or buy a replacement part. Water heaters, their heating source (oil or gas burner or electricity or solar hot water), and particularly their relief valves include critical safety components. Do not modify or remove relief valves, chimney connections, draft hoods, etc. as you may create dangerous conditions.

Our photo (left) shows scale deposits removed from a water heater tank. Here are the steps the plumber followed to get this mineral debris out of the tank bottom:

1. Turn off all electrical and or gas power to the water heater and let it cool.
   
   This means turning off electricity that controls the water heater oil burner or electric heating elements, or for gas fired water heaters, turning off the gas supply to the device.
   
   
2. Turn off water supply into the water heater.
   
   This normally means turn off the cold water valve (cold water isolation valve) that feeds water into the heater.
   
   Sometimes on an older system the shutoff for the inlet to the water heater may not fully stop incoming water. In that case it will be necessary to turn off the building water supply during the tank draining and cleaning procedure.
3. Drain the water heater - using a common garden hose routed to outdoors or to an indoor building drain lower than the water heater outlet.
   
   Our photo (left) shows a typical water heater tank drain on a gas-fired water heater.
   
   
4. Open a nearby faucet, or as some manufacturers recommend, remove the pressure/temperature relief valve from the water heater tank to let air into the tank and speed the draining process.
   
   Removing the relief valve may also let combustible hydrogen gases formed during deliming (if you use a deliming chemical) vent out of the tank. Soak the relief valve in delimer or in vinegar if its sensor or other components are coated with mineral deposits, or simply replace the valve with a new one (to be extra safe) of the same rating.
   
   When the water heater tank has been fully drained, remove the drain valve and inspect it for clogging or damage - clean or replace the drain valve as needed.
   
5. Flush the water heater by opening the cold water tank shutoff valve, flushing cold water into the water tank intermittently. To flush a water heater tank you open the water heater tank valve and let water run until it flows clean. If water does not flow out of the water heater drain valve the valve may have become clogged with mineral or silt debris.
   
   As we suggested above and as we explain in more detail below, removing the water heater drain valve entirely may permit the valve and the tank opening to be cleaned more effectively.
6. If mostly silt is what is flushed out of your water heater and no significant scale is observed, when the flush-water runs clean, you can complete the cleaning procedure and return the water heater to operation. If you see chunks of scale, or if you can loosen scale by probing the water tank bottom with a wire, you'll need to either mechanically remove the scale by scraping or by using a delimer - steps we discuss below at How to Remove Scale Deposits from a Water Heater Tank - Scraping the Bottom of the Hot Water Tank
7. If you removed the pressure/temperature relief valve, inspect the valve for cleanliness and proper operation. The test lever should open and close the valve smoothly and the temperature probe should be undamaged. If there is the slightest doubt about the condition of the valve, or if there is corrosion indicating that the valve was previously leaking, it should be replaced with a new one of the proper rating for your water heater. Replace the safety valve and its extension tube.
8. Close the water heater tank drain
9. Open the cold water supply valve to slowly refill the water tank. Refilling slowly helps avoid stirring up any debris left on the tank bottom. Stirred debris in the water tank could clog a shutoff valve or faucet in the building.
10. Do not turn on the water heater yet: be sure that the water tank has filled and that all trapped air has been run out of the system. Let hot water run from a nearby plumbing fixture for at least two minutes after you observe that no more air appears to be coming out of the piping. (We remove the faucet strainer in case some debris is also coming through the piping at this time, replacing the strainer when we turn off the water.).
11. Now you can turn on the water heater and allow it to re-heat the water in the hot water tank.
12. Check again for leaks anywhere in the system: at the water heater relief valve, at the water tank drain, at the water heater cold line shutoff valve, and at the tank itself. If leaks are found you'll need to shut down the system, let it cool, remove water pressure and water, and repair the leak.

Additional tips on draining a water heater tank can be found at ELECTRIC WATER HEATERS

How to Let Air Into the Hot Water Tank to Speed the Tank Draining Process

Here we discuss using or removing temperature and pressure relief valves on residential water heaters to let air into the hot water tank during draining for cleaning or other purposes. For more details about water heater safety valves see Relief Valves - Water Heaters and for a discussion of temperature and pressure relief valves used as safety devices on heating boilers please see Relief Valves - TP Valves on Boilers.

It is possible to speed the heater draining and also to ease the flushing procedure if you open a hot water tap nearby so that you can let air into the heater as water leaves it Some plumbers open the water heater pressure/temperature relief valve for this air-in purpose - but there is risk that you'll later be unable to get the valve to shut fully again - debris can clog the valve seat - sometimes we can stop a relief valve from dripping by tapping on the valve lift rod that protrudes through the valve lift lever Other plumbers simply remove the relief valve entirely - this is the process recommended by some water heater manufactures such as A.O. Smith. Removing the relief valve makes it easy to inspect this critical safety component itself, and it's easy to clean or replace the safety valve at that time. When replacing the relief valve use an approved teflon tape or pipe sealant and work neatly and with care so that there will be no leaks at this location. Safety Warning - a damaged, improperly-selected model, or modified pressure/temperature relief valve is dangerous and could lead to a serious BLEVE explosion should the water heater later overheat.

How to Remove Scale Deposits from a Water Heater Tank - Scraping the Bottom of the Hot Water Tank

Safety Warning: never turn on a water heater's heat source before the hot water tank has been re-filled. Otherwise you may damage the heater or create a dangerous condition.

Be sure the water heater pressure and temperature relief valve is properly installed and is the proper type. See Relief Valves - TP Valves.

How to Use Scale Removers to Clean a Hot Water Tank

There are products such as UN-Lime® (a professional non-muriatic food-grade phosphoric acid based chemical de-limer intended for removing scale from water heaters, recommended by A.O. Smith, available in 1 gal. or 5 gal. containers) or possibly other acid-based products that can dissolve mineral or lime deposits in a hot water tank.

Our opinion (feedback is invited) is that using a de-scaling chemical other than one intended for water heaters or other than one specifically recommended by the water heater manufacturer would be a potentially dangerous approach for a water heater for several reasons:

• Acid etching of a water heater tank may cause hidden damage and weakening of the tank itself
• One is exposed to possible acid burns during cleanout, given the sloppy way we need to drain at the tank drain valve opening
• There is a risk of leaving acid in the water system if you do not flush it adequately

We suggest suggest dislodging as much water heater tank bottom crud as possible, but also taking a gentle approach so as to avoid even the slightest possibility of damage to the tank.

If you elect to use a deliming chemical in your hot water tank be sure to follow the manufacturer's instructions, including these general steps:

• Safety Warning: Use of any de-liming product that comes in contact with the sacrificial anode of a water heater can lead to production of an explosive gas. Some large (125g and more) vertical commercial water heaters include a sacrificial anode in the tank bottom. (Sacrificial anodes are intended to reduce tank corrosion.)
• Turn off and drain the water heater tank as described above
• Use a standpipe (simple 3/4" N.P.T. plastic PVC or CPBC Sch. 80 plumbing fittings & teflon tape or pipe dope including a short nipple, ell or tee, and standpipe of sufficient height to provide a way to pour in the delimer at the hot water tank drain opening) or a delimer installation kit provided by the manufacturer to place the deliming solution only in the bottom of the tank. The "kit" may simply be a connector that joins the container of de-limer by a hose to the water tank drain valve so that by lifting the container higher than the tank drain valve, de-limer flows into the water tank; the delimer later is drained out of the water tank back into the delimer container.
• Do not smoke or have sparks or open flame nearby during deliming - explosive or flammable gases may be produced.
• Do not let delimer chemicals come in contact with skin, eyes, etc - dangerous burns can result. Delimers should be treated as dangerous chemicals. Use safety goggles, gloves, etc.
• Use the proper quantity of deliming solution - which depends on your water heater size. A table of quantity versus tank size is provided by the water heater manufacturer or the de-liming product manufacturer. For example, a 30 gallon water heater uses 2 gallons of delimer and a 30-66 gallon water heater can use 3 gallons of de-limer.
• Flush the hot water tank: After the delimer has been used and drained from the water tank, you'll need to flush the tank with clean water - typically for ten minutes. (See the instructions provided by the delimer manufacturer).
• Return the hot water heater to operation: bleeding air, replacing relief valve, etc: Continue with the steps we described in detail above

Delimer disposal note: if the delimer is draining into a sump pit be sure to run enough water to cycle the sump pump several times to flush that component out as well - leaving acid and debris from the hot water cleanout in the sump pump could damage it.

How to Remove Lime and Mineral Scale from an Electric Water Heater's Heating Electrodes

Electric water heaters use one or two electrodes inserted into the bottom and top of the hot water tank. Controlled by a thermostat that senses water temperature inside the hot water tank, the electrodes are turned on to heat water inside the water tank.

On some electric water heater models (Rheem for example) the top heating unit is called the "Booster" and operates only during periods of high demand for hot water. Wiring and controls on the water heater may on some models ensure that only one of the heating elements operates at a time.

Our photo (left) shows a small electric water heater with two heating electrodes - one is located behind each of the dark access panels on the front of the heater.

• Turn off all electrical power to the water heater and drain the hot water tank as we've described in detail above.
• Remove the upper and lower water heating electrodes and inspect each for lime or scale build-up (or other visible damage)
• Clean the scale from the water tank heating electrodes. Soaking the electrodes in vinegar may be sufficient, or you'll need to use a professional de-liming product.
• Replace the electric water heater electrodes - you may want to test each electrode for proper function before replacing it, or if the electrode is visibly damaged, simply buy and install a new one of the same wattage and length.
• Re-fill the hot water heater tank slowly, as we described above, including the steps to remove air from the system before electricity to the water heater is turned back on.
• Never turn on an electric water heater that has not been filled with water with all air removed from the system - doing otherwise is likely to burn-out the heating electrode(s) and could be unsafe.
• Check the hot water tank for leaks and repair any that are found before turning on the water heater.

How to Avoid Scale Deposits in Water Heater Tanks or on Electric Water Heater Electrodes

Lowering Hot Water Heater Temperature to Reduce the Rate of Lime & Scale Formation in a Water Heater Tank

The rate of deposition of scale inside of a water heater tank depends on

• The hardness of the incoming cold water. See MEASURE WATER HARDNESS for details about how water hardness is measured.
• The temperature to which the water is being heated. Higher water temperatures cause minerals to be deposited faster than lower water temperatures.
• The water quantity used: the more water that flows through the water heater (in gallons of use per day, for example), the larger is the quantity of minerals that are available to be deposited in the water tank.

Water heated to higher temperatures deposits lime much faster than at lower temperatures. A Purdue University bulletin charts the relationship between water temperature, total gallons of hot water used per day, and the annual quantity of lime deposition in a plumbing system. Our table below shows data for typical residential use of a hot water heater. At larger volumes of daily use the same relationship holds but the total quantity of lime deposited is much greater.

Other Steps to Avoid Lime, Scale, and Silt Deposits in a Hot Water Tank

• Lower the water heater temperature, as we discussed just above at Lowering Water Heater Temperature
• Reduce hot water quantity use. This can mean conservation at the tap or installation of a flow restrictor on the hot water line. See HOT WATER IMPROVEMENTS for alternative ways to increase hot water quantity and flow without increasing hot water temperature.
• If your water supply provides hard water, install a water soften err on the incoming water supply. See WATER SOFTENERS for details.
• If your water supply provides water containing silt and debris, install a filter on the incoming water supply. See WATER TESTING GUIDE for advice about detecting impurities and contaminants in drinking water.
• Inspect the water tank anode rod - and replace the rod when it has become deteriorated. The anode rod protects the water heater tank from corrosion. After the warranty period of your water heater, inspect the anode rod annually until it needs replacement.
  
  After that you'll know about how long an anode rod typically lasts in your water heater. If your water supply comes from surface water in a part of the country where there is acid rainfall, the low pH of the water will speed the deterioration of the anode rod - it will need to be inspected and replaced more often. See Water Heater Sacrificial Anode for details.

How Mineral Deposits and Heating May Weaken a Water Heater Tank Bottom, Contributing to BLEVE Explosion

Water heater tank explosions are rare thanks to the widespread requirement for and use of pressure and temperature relief safety valves. The photo (left) shows a hole in the roof made when a water heater exploded, becoming a rocket that passed up through the building.

But if the safety valve has been damaged, modified, or even omitted (as we saw on our neighbor's water heater), that condition, combined with overheating can cause a water tank to explode, creating a BLEVE - boiling liquid vapor explosion that releases tremendous force and causing extreme damage to a building.

Because repeated heating of the water tank bottom may combine with other conditions (such as corrosion or excessive heating due to mineral deposits on the tank bottom) to produce a weak water heater tank bottom, that is the part more likely to fail in an overheat and overpressure condition.

A failure at the water tank bottom may explain why a BLEVE can produce a water tank explosion that behaves like a rocket, sending the water tank skyrocketing up through a building.

Cures for Hard Water that Clogs Hot Water Tanks, Tankless Coils, and Plumbing Pipes

See WATER SOFTENERS for suggestions about dealing with hard water in buildings.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

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Technical Reviewers & References

Search InspectAPedia

• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
• Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Pounds of lime deposited vs. temperature and hot water usage: see Purdue Bulletin #74 (also provided by A.O. Smith in the article below). Purdue's chart shows the number of pounds of lime deposited per year as a function of the water usage in gallons per day, with an assumed 10 grains of water hardness.
• "When, Why, and How to Remove Water Scale from Tank Type Glass-Lined Water Heaters (for non glass-lined tanks, consult water heater manufacturer)" PDF provided by A.O. Smith Water Products Company - hotwater.com/lit/training/4800r9.pdf 800-433-2545 - 01/07/2009.
• A.O. Smith's Form No. 4800 Rev. 8 Why? When? & How? /UN•LIME Specific Deliming Instructions for use with Up-N-Down Transfer Kit for Tank Type Heaters. (Normally supplied in UN-LIME shipping cartons), Supersedes Form Nos. 4800 Rev. 7 and 4813-100.
• A.O. Smith's Form No. 4778* All about Deliming Coil-Type/Tube-Type Commercial Water Heaters and Hydronic Boilers *Normally supplied when ordering Part No. 4930 Motorized Deliming Pump Kit
• Rheem Electric Water Heater "Owners Guide and Installation Instructions", (Australian models) rheem.com.au/images/pdf/owners_dom-elec_121996C_0707.pdf
• Purdue University Bulletin No. 74 - Purdue B074 can be hard to locate online, but Bradford White Corporation (a manufacturer of water heaters) has published excerpts from that document, available at Purdue_B074_BradfordW.pdf

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