Guide to Hot Water Heater Tank Lime & Scale Prevention

Water heater noise troubleshooting, diagnosis, causes & repair
- How to stop & prevent water heater rumbling, clanking, cracking, or other hot water heater noises
- Scale formation in water heaters or geysers, and methods of prevention
- Rate of deposition of lime and scale in plumbing systems
- Water heaters: how to inspect, test, adjust, repair, and choose among types of hot water heaters
- Water heater synonyms: geyser (non-potable water), calorifier (potable water)
- Water heater tank synonyms: hot water tank, water heater tank, calorifier tank, geyser tank, water kettle, water cauldron, water pot, water copper, domestic hot water, or DHW.
NOISE, WATER HEATER - separate article
Water Heater Flush Procedure - separate article
WATER HEATER SCALE - De-Liming Procedure - separate article
Water Heater Scale Prevention
WATER SOFTENERS & CONDITIONERS - separate article
Questions & Answers about how to prevent water heater scale & lime formation
References

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How to stop scale and lime formation in water heaters: here we explain how to Prevent Future Scale Deposits in Water Heater Tanks or on Electric Water Heater Electrodes - Scale formation in water heaters and methods of prevention. 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.

Step 3: How to Prevent Future Scale Deposits in Water Heater Tanks or on Electric Water Heater Electrodes - Scale formation in water heaters and methods of prevention

At WATER HEATER SCALE - De-Liming Procedure we discussed how to remove existing scale and line from a water heater.

Here we continue with water heater de-liming Step 3: advice on how to prevent scale and lime formation in or on water heating equipment.

Lime and scale in a hot water tank, 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 (and easier to prevent in) 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.

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

Rate of scale formation in water heaters (C) Daniel Friedman

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 chart at left (and our table below) both show the rate of water heater scale and lime formation as a function of water heater temperature. - Purdue Bulletin 74 - 1940. This important research was the first laboratory verification of scale formation and the first laboratory confirmation of the role played by water temperature in water heater scale formation.

Automatic gas-fired water heaters operating in the typical water heater temperature range - 110 degF to 180 degF - were the focus of the 1940 study. Previous studies of scale formation had been performed with respect to steam boilers, where temperatures of the hot water were generally at boiling or higher. Surface smoothness and water velocity effects were also considered, and finally, the study considered the effect of scale formation on the thermal efficiency of water heaters.
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. The Purdue University bulletin cited above 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.

Pounds of Mineral Deposit in a Water Heater / Year
Water Temperature	Gallons per Day	Pounds of Lime
120 degF	300	0.85
120 degF	600	1.00

140 degF	300	2.00
140 degF	600	6.00

160degF	300	6.00
160 degF	600	18.00
Notes: Pounds of lime are estimated by interpolation from graph Safety Warning: water temperatures above 120 deg.F. are scalding. If your water heater is going to be set at higher temperatures not only will you see a faster deposition of lime in the tank, for safety you should install a mixing valve at the water heater outlet or at every plumbing fixture that uses hot water.

Thanks to researcher Robyn Goldstein for the full citation of Scale formation in water heaters and methods of prevention", Purdue Bulletin No. 74.

Surprisingly, while scale formation in water heaters was found to be rapid at higher temperatures, the water heater thermal efficiency was not as significantly effected as the researchers anticipated. It was also interesting that scale formation in the heater was not uniform - because temperatures across a water heater and its parts are themselves not uniform.

What is important about this data is the dramatic reduction in scale and lime formation in water heaters when the temperature was reduced from 175 degF. down to 140 degF. Considering that hot water is dangerously scalding even at 140 degF, only where scale formation is not a problem (low mineral content in the water source) and where a cold water mixing valve has been installed for safety, would it be reasonable to operate a residential water heater at high temperatures.

Back in 1940 Krappe reported that unlike high temperature residential heating boilers (heating water to 180-200 degF) where lime and scale formed rapidly in a tankless coil (TANKLESS COILS) , in side-arm type water heaters ( Range Boiler Water Heaters
Side Arm Coil Water Heaters) attached to boilers, (and in separate water heaters heated to high temperatures), gas fired water heaters fitted with temperature controls that kept water below 140 degF. were often found to run for 8 years or longer with little or no scale formation problems.

The role of high temperature in water heater lime and scale formation also explains the rapid clogging of tankless coilsused for domestic hot water production. Residential heating boilers regularly operate at temperatures between 180 degF. and 200 degF., causing rapid scale formation where hard water is passing through the tankless coil. (See Clogged Tankless Coil & Hot Water Flow.)

Types of Water Heaters & Scale Formation

Krappe listed the following types of water heating devices in order of their tendency to "give service difficulty from scale formation"

Furnace coils - tankless coils
Water heaters not equipped with temperature limit controls
Manual side-arm type water heaters
Automatic side-arm type water heaters
Instantaneous, or continuous-flow-type water heaters without a temperature limiting thermostat
Instantaneous, or continuous-flow-type water heaters with a temperature limiting thermostat
Automatic water heaters with internally heated tanks
Automatic water heaters with externally heated tanks

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
If your water supply provides hard water, install a water softener on the incoming water supply. See WATER SOFTENERS & CONDITIONERS for details.
Reduce hot water quantity use. This can mean conservation at the tap or installation of a flow restricter 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 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
Threshold treatment (not recommended for residential systems): the 1940 tests experimenters observed that adding a small amount of hexametaphosphate solution (corrosive to iron and steel in concentration) to the incoming water, rather than a softening treatment, could prevent precipitation of mineral scale, and could also dissolve modest amounts of existing scale. A hexametaphosphate feeder system was tested experimentally for commercial water heating systems where in the 1940's it was already in use in some commercial environments such as restaurant water softeners.

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 & CONDITIONERS for suggestions about dealing with hard water in buildings.

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

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John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: johncranor@verizon.net
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
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Thanks to researcher Robyn Goldstein, Rush Manager, Information Express, 650-812-3585, email: service@ieonline.com,for researching the Purdue bulletin No. 74 original citation 03/2010 . Information Express supplies "... institutional document delivery needs - A full service company specializing in the fast, accurate and competitively priced delivery of published articles for all industries"
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
"Scale formation in water heaters and methods of prevention", Krappe, Justus Maximilian, Engineering experiment station. Gas engineering bulletin; no. 6; Research series; no. 74; On cover: Engineering bulletin, Purdue university. Vol. xxiv, no. 3a. June, 1940 (Layfayette Indiana) commonly referred to in some references as "Purdue University Bulletin No. 74" - thanks to researcher Robyn Goldstein for the full citation. LCCN: 40028844 & OCLC: 1038544 - Water analysis, water softening, hot-water supply. 27pages. You can obtain this document through your local library. (full copy file at InspectAPedia 3/31/2010) Purdue B074 can be hard to locate online.
Also Bradford White Corporation (a manufacturer of water heaters) has published excerpts from that document, available at Purdue_B074_BradfordW.pdf
Other sources of information on calculating the rate of lime deposition from hard and hot water:
- Wilkes University Center for Environmental Quality Environmental Engineering and Earth Sciences Hardwater, Water Hardness "Hard Water Hardness Calcium Magnesium Water Corrosion Mineral Scale" http://www.water-research.net/hardness.htm
- Chemical Engineering, Joseph D. Hagerty Editor, McGraw-Hill, 1989, ISBN 0685270831, 9780685270837
  "To calculate the amount of material that will be deposited in 1000 gal. of water per day, take the number of grains per gallon of each as shown by the ..." also difficult to obtain except as a used copy.

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