How to Diagnose & Repair Clogged Hot Water Piping or a Clogged Tankless Coil
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Clogged tankless coils: here we explain how to correct clogging at the tankless coil in order to improve the hot water pressure, quantity, flow from a tankless coil used for making domestic hot water. We include a simple diagnostic procedure to find out if your bad hot water pressure is due to clogging in the hot water piping or in the tankless coil itself. If you need to improve the hot water temperature and flow from a tankless coil system that is not clogged, see Tankless Coil for Hot Water Increase.
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How to Check for and Correct Clogged Piping at a Hot Water Heater Tank or Clogged Tankless Coil to Improve Hot Water Pressure or Flow
Hot Water Heater Tank Clogging Diagnosis & Repair - Tankless Coil De-Liming Procedure Details
Thanks to Carson Dunlop, a Toronto Home Inspection Firm and Home Inspection Educator, for permission to use sketches shown in this article. For an explanation of the role of boiler temperature in the rapid formation of lime and scale clogging in tankless coils used for domestic hot water production, see Water Heater Scale Prevention.
Readers should also see Water pipe clog diagnosis and Water pipe clog repair guide.
Also see WATER PRESSURE LOSS DIAGNOSIS & REPAIR in a building and see MUNICIPAL WATER PRESSURE DIAGNOSIS or WELL WATER PRESSURE DIAGNOSIS where we describe the effects of clogged piping on water flow and offer remedies for this problem. If only hot water pressure and flow are poor, also see CLOGGED SUPPLY PIPES, HOT WATER.
Corrosion, debris, dip tube or anode problems in the water tank:
Accumulated debris in a water heater, and debris from a corroded or disintegrating hot water tank dip tube or hot water tank sacrificial anode can also block the hot water outlet opening, resulting in low hot water pressure in a building. See ANODES & DIP TUBES on WATER HEATERS
Clogged hot water supply piping:
Water supply pipes may be clogged at the water heater or at other locations, from minerals, soil, or other debris. See Water pipe clog diagnosis and WATER HEATER SCALE - De-Liming Procedure for diagnosis and repair procedures.
Tankless Coil Clogging Diagnosis & Repair - Removing Lime & Scale from a Tankless Coil Water Heater - also for Side-Arm Coils
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 Hot water pressure and hot water flow in a building may be poor even where a mixing valve has been installed. This condition could be due to poor overall building water pressure or due to clogged pipes.
If cold water pressure is good and hot water pressure is poor, the problem is not an overall building water pressure issue. There is a problem with the hot water system that needs to be found and corrected.
If hot water pressure is strong when the water is first turned on but flow quickly falls off to a weak hot water stream, it's likely that the hot water piping (or tankless coil) are clogged.
Often this repair involves using acid to try to remove minerals that are clogging hot water system piping or a tankless coil, or the tankless coil may need replacement.
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Abandoning a tankless coil altogether: When we bought our house the tankless coil was almost totally clogged with minerals, and we figured that it wouldn't provide enough hot water anyway, so we abandoned it. In the photo at left you can see the round black tankless coil face, with two holes - where we removed the cold-water-in and hot-water-out pipes and simply abandoned the tankless coil.
A separate water heater was installed in our utility area. (We'd have preferred an indirect-fired water heater which is discussed later in this article but that's not what we got.)
Guide to Acid Flushing of Clogged Tankless Coils or Clogged Hot Water Piping
Some plumbers will attempt to clean out water supply piping that has become mineral-clogged. The general procedure is the following:
- First, confirm the diagnosis that the problem is clogged piping, not a clogged strainer, valve, or improper pipe connections (such as use of too much solder on copper piping connections). Some types of pipe or tankless coil clogging such as due to sediment, rust, or mechanical damage will not be corrected by an acid flush. See WATER PRESSURE LOSS DIAGNOSIS & REPAIR and Water pipe clog repair guide for more general water pressure diagnosis guidance.
- Install shutoff valves and drain valves at each end of the water supply piping (or right at the tankless coil inlet and outlet piping)
- Shut off water supply to and then drain the piping or tankless coil to be cleaned
- Connect the two drain fittings (at the two ends of the copper piping or tankless coil) to hoses (often washing machine hoses are used) as follows:
- One drain hose empties into a bucket
- The other drain hose is connected to a "pony pump" capable of pumping acids
- Prepare the acid flush mix: The bucket receives a few gallons of a cleaning acid selected by the plumber (such as muriatic or dilute sulfuric acid)
CAUTION: Acids can cause dangerous burns or loss of vision: appropriate hand, eye, and skin protection are needed as well as protection against acid spills.
- The acid is pumped cycling through the piping or tankless coil to be cleaned.
- The acid is flushed from the piping or tankless coil using fresh water
- The hoses and acid are removed and the drain valves closed.
- The plumbing water supply to the cleaned section of piping or tankless coil is turned back on
- The plumbing system is flushed to be sure no acid remains in the piping
Does acid flushing of water piping or tankless coils work?
Often an acid flush will indeed restore or improve water pressure and flow through previously-clogged building piping. But the process has these limitations:
- Etching speeds new clogging: Some plumbers have told us that the acid flush process also leaves the piping interior "etched" - this roughened surface may result in mineral-clogging recurring sooner than it did in the first place.
- Acids can cause leaks in previously marginal piping that is already thin and corroded
- Acids may leak out and cause damage if piping is not in good condition
- Clogging will recur: If the cause of the original pipe clogging is not corrected, it will recur. For example, hard water that results in mineral clogging of pipes or clogging of a tankless coil can and should be treated with a water softener.
A water softener won't remove clogging in pipes but it will help prevent its occurrence or recurrence - see WATER SOFTENERS & CONDITIONERS for a discussion of how to install and adjust a water softener to avoid clogging of hot water pipes and tankless coils while still protecting the septic system from excessive water or salt damage.
The pile of tankless coils shown at the top of this page is probably an example of terrible recurrent mineral clogging of tankless coils at the building where we took this photo.
- Working with acid is dangerous. Some plumbers will therefore not provide this service.
Krappe described using an acid-resistant pump and tubing connected to fittings to circulate an acid-wash through scale-clogged copper piping back in 1940. Plumbers still use this method today. The apparatus must be designed to let the scale particles and gases escape. Plumbers may simply use an acid-proof pony pump and fittings to cycle the acid wash using a plastic bucket as the receiving sump.
Watch out: this is a dangerous procedure risking acid burns or other hazards. Don't try this procedure if you are not trained and equipped to do so.
But we have been informed by some plumbers that using acids to clean a tankless coil leave the coil internal surfaces etched and that the result can be faster scale formation than previously
See TANKLESS COILS for an explanation of how these water heaters work and why they clog up and how to stop clogging up the coil.
See Water pipe clog repair guide for a discussion of loss of water pressure due to clogged piping or clogged tankless coils.
At Larger Diameter Water Supply Piping we discuss the benefits of using larger diameter water supply piping both to improve water pressure and flow and also to delay the clogging of pipes due to minerals or rust.
How do we Prevent Tankless Coil Clogging Again in the Future?
See Water Heater Scale Prevention
Field Report: Two Extra Steps to Improve the Effectiveness of Tankless Coil De-Liming Procedure
Reader comment:
Your information on how to remove scale in a tankless coil to improve hot water pressure and flow led to me doing this task myself. I am very mechanically inclined and rather technologically proficient but I still wanted to get an understanding of the processes.
I had a pump that would do the job but I was concerned about using an acid, even diluted.
I turned off the valves separating the tankless coil in my Burnham oil burner and luckily had tees in place already. All I had to do was get fittings for the garden hose adapters and screw them in place, I then connected the hoses to the pump and one to drain freely into my 5 gallon pail filled with about 2 gallons of water.
I then added a 28 ounce container of Lime-A-Way and started pumping. The water was nearly non-existent when I started and the pumps thermal overload went off giving me a break.
I did this pump on-thermal off routine about three times, each time letting the solution sit in the tankless coils for approximately a half hour. I was getting almost no improvement. I then decided to reverse the flow thinking what could be the harm. The pump started and the bursts of gray water were getting more and more frequent. After three more thermal overloads I was running freely. Success is at hand!
But wait! All this bragging of success has its downside. As best as I can remember this issue isn't covered anywhere on your site.
Flush Debris from Faucet Strainers & Valves after De-Liming Water Pipes or a Tankless Coil
What I found is that after the water flow had returned to normal I still had low flow after a few bursts in my faucets upstairs. I then removed the aerators and strainers from the faucets and all flow had finally returned!
It might be a good idea to add a step to your water pipe or tankless coil de-liming or de-scaling process to remove the strainers and aerators from their faucets before turning them all on to rinse the tankless coil and system.
[See Water pipe clog diagnosis and Water pipe clog repair guide for details about diagnosing and clearing clogs in pipes, faucet strainers, and plumbing valves, including debris flushing - Ed.]
Bernie Daraz, Newtown, CT
Watch out: using Lime-A-Way, EasyPaks®, or Jelmar CLR® ("stain removing" cleaners designed to remove rust, calcium or lime) and similar cleaners to remove the white mineral crust on exposed metal surfaces such as a stainless steel kitchen sink can leave a black stain that is difficult to remove. If you already have this problem, try using a nonabrasive cleaner designed for cleaning stainless steel surfaces, such as ScotchBrite™.
How do we Improve the Hot Water Volume & Quantity?
As we discussed beginning in the previous section of this article, there are several different hot water problems:
- Poor hot water flow, or bad hot water pressure, which is usually a piping or clogging problem or an overall building water pressure problem
- Poor hot water quantity: not enough hot water, or running out of hot water very quickly. Insufficient hot water quantity may be due to the way hot water is being made (a water heater that is too small, a tankless coil on a new small heating boiler, or an instantaneous water heater that is being asked to supply hot water too rapidly.
- Hot water temperature that is not hot enough: water temperature is just too cool. Insufficient hot water temperature may be due simply to the setting of a temperature control on a water heater or mixing valve, or it may be due to flowing water too fast through a tankless coil. Hot water that is not hot enough can be caused by quite a list of problems, some of which are easy to correct. See Temperature of Hot Water is Too Low for steps to correct water that just won't get hot.
- See HOT WATER IMPROVEMENTS and HOT WATER QUANTITY IMPROVEMENT for detailed advice on how to get more hot water pressure, flow, quantity, temperature.
Articles Describing Steps to Increase Hot Water Quantity
The articles listed below offer more details about steps one can take to increase hot water quantity, pressure, and flow in a building.
- ALTERNATIVE HOT WATER SOURCES - besides electric, gas, oil, solar, indirect, & range boilers, are alternatives
- ANODES & DIP TUBES on WATER HEATERS discusses hot water tank sacrificial anodes and dip tubes whose problems can produce both sulphur smells and even reduced hot water pressure and flow.
- Anti-Scald Valves & Hot Water Quantity - improve safety and meter hot water more slowly - makes it last longer
- CLOGGED PIPING & Hot Water Flow - diagnose and correct poor hot water pressure and flow due to pipe clogging
- ELECTRIC WATER HEATERS - suggestions for inspection, diagnosis, repair
- Extra Tanks to Increase Hot Water- pre-heat your hot water absorbing ambient building heat or increase hot water quantity with cascaded, staged, multiple water heaters, possibly using more than one energy source
- GAS FIRED WATER HEATERS - suggestions for inspection, diagnosis, repair
- High Efficiency Water Heaters - if replacing a water heater, consider a model that saves money in operating cost
- HOT WATER DELIVERY SPEED UP - hot water pump and loop systems for instant hot water in large buildings
- HOT WATER QUANTITY IMPROVEMENT - lead article on improving the quantity of hot water in buildings
- INDIRECT FIRED WATER HEATERS - allow a larger tank of hot water, heated efficiently by a separate heating boiler
- Insulate Hot Water Piping - worth doing where piping is accessible, especially on piping running through cool areas
- Insulate Hot Water Tank? - probably not worth doing except in special cases; important safety warnings
- Larger Diameter Water Supply Piping - worth doing if replacing clogged pipes or building new construction
- OIL FIRED WATER HEATERS - fast recovery water heaters, suggestions for inspection, diagnosis, repair
- MIXING / ANTI-SCALD VALVES - can not only avoid scalding hot water dangers, but can meter hot water usage to give a longer bathing shower time
- RANGE BOILERS - a separate water heater tank warmed by cycling water through a separate heating boiler
- SOLAR HOT WATER HEATERS - lower energy cost, effective hot water source in sunny climates
- Tankless Coil Hot Water Increase - various tricks can significantly improve the safety and water quantity available
- Temperature of Hot Water is Too Low - where to look for problems when the "hot water" is not hot enough
- WATER HEATER SCALE - De-Liming Procedure explains how mineral scale build-up in a water heater (or in building water piping) can result in both poor hot water quantity and
- Water Heater Scale Prevention explains how to prevent lime and scale formation in water heating equipment
- WATER PRESSURE LOSS DIAGNOSIS & REPAIR - overall guide to improving building water pressure
- WATER SOFTENERS & CONDITIONERS - how to install and adjust a water softener to avoid hot water piping or tankless coil clogging due to hard water and minerals
The topics discussed in this article address improving hot water flow and improving hot water total quantity.
A Comparison of Alternative Hot Water Heaters & Sources
The following articles discuss alternative ways to produce domestic hot water for washing and bathing.
The characteristics of various water heaters such as life expectancy, cost, safety, and capacity are discussed at WATER HEATER PROBLEM DIAGNOSIS
WATER HEATER PROPERTIES
Ways to improve total hot water quantity, pressure, temperature and flow are discussed beginning at HOT WATER IMPROVEMENTS and continuing at HOT WATER QUANTITY IMPROVEMENT.
Frequently Asked Questions (FAQs)
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- WATER SOFTENERS & CONDITIONERS for a discussion of how to install and use a water softener to avoid mineral clogging of the hot water supply piping and equipment
- Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
- 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:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
- Arlene Puentes, an ASHI member and a licensed home inspector in Kingston, NY, and has served on ASHI national committees as well as HVASHI Chapter President. Ms. Puentes can be contacted at ap@octoberhome.com
- Thanks to reader Bernie Daraz, Newtown, CT for discussing additional steps to improve the results of tankless coil scale removal, November 2010.
- Lime-A-Way, Jelmar CLR-CL-12 Cleaner, Vanish DRK cleaner, Simple Green Lime Scale Remover, Eco Friendly lime and scale remover [links to buy at Amazon.com] and other plumbing de-liming and de-scaling products mentioned in this article are examples of products used to remove calcium, lime, other minerals, and in some cases rust or other clogs or scaling in piping and on plumbing fixtures. Other products are available from your plumbing supplier.
- 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
- "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.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
- Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
er- Plumbing Diagnosis & Repair: Water supply, drainage, septic systems, water testing, water contamination, defective plumbing materials & products.
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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