How to Diagnose & Repair Clogged Hot Water Piping or a Clogged Tankless Coil
Lime scale can quickly clog a tankless coil. How to de-clog & prevent future scale clogged pipes or heaters?

Diagnose the Cause of Weak Hot or Cold Water Pressure

Before starting on a project to de-clog or de-scale building water supply piping, let's be sure that we are solving the right problem by making a reliable hot water pressure or both hot and cold water pressure diagnosis.

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.

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

Tankless Coil Clogging Diagnosis & Repair - Removing Lime & Scale from a Tankless Coil Water Heater - also for Side-Arm Coils

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.

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.)

For more detailed help in diagnosing the cause of poor water pressure or flow rate due to clogged pipes or clogged tankless coil

see CLOGGED SUPPLY PIPES, DIAGNOSIS

How to Acid Flush Clogged Tankless Coils or Clogged Hot Water Piping

Given first, just below, is the general procedure for using an acid flush to clear clogged water pipes or tankless coils.

Some plumbers will attempt to clean out water supply piping that has become mineral-clogged.

Photo: lime scale fragments collected from a water heater. Similar hard limescale forms inside of hot water pipes and particularly in tankless coils used to produce domestic hot water.

[Click to enlarge any image]

1. 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 for more general water pressure diagnosis guidance.
2. Install shutoff valves and drain valves 
   
   at each end of the water supply piping (or right at the tankless coil inlet and outlet piping)
3. Shut off water supply 
   
   to and then drain the piping or tankless coil to be cleaned
4. 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:
   1. One drain hose empties into a bucket
   2. The other drain hose is connected to a "pony pump" capable of pumping acids
5. 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.
6. The acid is pumped 
   
   cycling through the piping or tankless coil to be cleaned.
7. The acid is flushed 
   
   from the piping or tankless coil using fresh water
8. The hoses and acid are removed 
   
   and the drain valves closed.
9. The plumbing water supply 
   
   to the cleaned section of piping or tankless coil is turned back on
10. The plumbing system is flushed 
    
    to be sure no acid remains in the piping
    
    Watch out: Remove & clean faucet strainers & shower heads.
    
    for any piping system flush-out to be successful you will need to remove faucet strainers, shower heads, or any other devices with small openings through which your water supply flows.
    
    Otherwise, debris loosened by the pipe cleaning procedure will clog those small orifices and flow at fixtures will probably be worse than before.

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:

1. 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.
2. Acids can cause leaks
   
   in previously marginal piping that is already thin and corroded
3. Acids may leak out 
   
   and cause damage if piping is not in good condition
4. 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.
5. 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 rec

eiving 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 for a discussion of loss of water pressure due to clogged piping or clogged tankless coils.

At LARGER DIAMETER WATER SUPPLY PIPES 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.

Plumbing Scale Removal: Compressed air followed by acid cleaner

We received this un-solicited promotional information from Worp in October 2020. The company says that their portable de-scaling mahine uses a combination of compressed air and a pump to "strip off" mineral scale from the interior of almost any device: hot water pipes, tankless coils, water heaters, industrial equipment. Following their air treatment HCL (Hydrochloric acid) cleaner is used as a final step.

The briefcase device is the pulse generator, and you'll notice that the de-scaling system requires a separate source of compressed air.

We would like to see field reports from people who've tried this system.

• Worp, PORTABLE SCALE REMOVER [PDF] (2020) Worp Ogawa Kazutoshi7-19 Yokotacho, Kashi-ku, Shizuoka CityTel: 090-1234-1875 Email: worp@nifty.com
  
  Excerpt: The air pulse generator unit intermittently shoots a mass of air into the piping. The fluid supply unit, on the other hand, puts fluid into the piping. When a mass of air is shot, it instantly becomes a large air bubble.
  
  The fluid is pushed by air bubbles and advances at a speed according to the air pressure.
  
  The fluid collides with the scale adhering to the piping and is stripped off. Peeled scale flows out with the fluid from the outlet.As the series is repeated over and over again, the scale will gradually decrease. Only 0.3-0.7 Mpa [43.5 - 102 psi] compressed air is required as a power. The shoulder straps and handbag handles are also useful for moving. Not only for water but also for hydrochloric acid solution is available.
  
  From the company's Email: (excerpting and paraphrasing)
  
  Worp is a Japanese manufacturer of plumbing scale removers. This time we will introduce an inexpensive type. Its power is only compressed air.
  
  The purpose of the Worp cleaner is to remove scale layers such as calcium and iron rust adhering to the pipe inner wall.
  
  By switching the air pulse generator unit, pulsed compressed air is supplied from the air pulse generator unit. On the other hand, the pump of the fluid unit starts operation by the output of the air pulse generator unit.
  
  The fluid and air pulse merge at the cheese part and hit the plumbing scale. As a result, the scale is removed.

Field Report: Two Extra Steps to Improve the Effectiveness of Tankless Coil De-Liming Procedure

Reader comment: use Lime-A-Way or similar products to remove lime scale in p;ipes

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 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™.

Research & Products for Cleaning / Preventing Pipes Clogged with Rust or Scale

• Also see the RECOMMENDED ARTICLES at the end of this page.
• Chaturvedi, Shalini, and Pragnesh N. Dave. "Removal of iron for safe drinking water." Desalination 303 (2012): 1-11.
• Cho, Hyunwook, Jong-Sang Youn, Inhwan Oh, Yong-Won Jung, and Ki-Joon Jeon. "A new air-washing method to clean fabric filters clogged with submicron fume particles: A pilot-scale study." Journal of Hazardous Materials 383 (2020): 121186.
  Abstract:
  This study investigates a new air-washing cleaning system that directly injects compressed air on the filter surface for filter regeneration in a fabric filter (FF) dust collector.
  
  A pilot-scale FF is designed to test the new system and to compare it with the conventional pulse-jet cleaning system with regard to filter clogging by fume particles. A pleated filter with a filtration area of 2.4 m2 is installed in the FF and a thermal steel spraying gun is used to supply the fume particles.
  
  Pressure drop and particle emission concentration are monitored to examine the effect of the new system on filter regeneration and collection efficiency.
  
  The results show that the air-washing cleaning is effective for filter regeneration, as it allows the FF to operate stably for a long time, whereas the pulse-jet cleaning fails to achieve filter regeneration, resulting in a continuously increasing pressure drop.
  
  In addition, air-washing cleaning shows better performance on collection efficiency than the pulse-jet cleaning method, as it reduces the outlet particulate matter concentration to less than half that of the pulse-jet cleaning.
• Hong, C. K. and Choi, M. S., 1999, A Study of CaSO₄ Scale Formation and Heat Transfer ina Vertical Tube Evaporator, The Korean Society of Mechanical Engineers, Vol. 23, No. 11, pp. 1363-1370.
• Kang, Hyung Seok, Won Suk Yang, Young Il Kim, Sean Hay Kim, and Dong Hee Choi. "A Study on the Performance of Pipe Scale Cleaner using Natural Organic Acid." Korean Journal of Air-Conditioning and Refrigeration Engineering 29, no. 10 (2017): 530-537.
  천연 유기산을 이용한 배관 스케일 세정제 성능에 관한 연구
  Abstract
  
  Scales generated inside pipes cause negative effects on heat transfer performance, pressure loss and flow rate due to increased thermal resistance and reduced flow cross-sectional area.
  
  If these scales are not prevented or eliminated, thermal-fluid performance of the facilities can be deteriorated, or in extreme cases, accidents such as explosion due to overheating can occur. There are two ways to remove the scales, physically and chemically.
  
  Removing the scales physically needs specific machines which are expensive, and removing them chemically may provoke corrosion or shorten the age of the facilities. In this study, an eco-friendly pipe scale cleaner using natural organic acid is developed by applying the concept of a limestone cave generation.
  
  The manufactured scale cleaner is applied to remove the scales in industrial, water heating and urinal pipes.
  
  The results show that this cleaner removes scales more effectively and safely compared to existing scale treatments. Scale removal efficiencies of this work is 1.2~10.7 times for industrial pipes and 1.8~15.5 times for boiler water heating pipes higher than those of conventional cleaners.

• Navien, PeakFlowE ANTI-SCALE SYSTEM [PDF] Model #GXXX001725 PeakFlowE Chemical-Free, Salt-Free Scale Prevention
  
  Excerpt: The PeakFlowE Anti-Scale System provides protection from scale formation on internal plumbing surfaces.
  
  The PeakFlowE system is a single cartridge-based system that must be installed on a cold water line prior to a single residential water heating device (water heater or tankless water heater).
  
  The PeakFlowE system is built with the finest and most advanced materials and each system is quality inspected and pressure tested prior to shipment.
  
  With proper installation and routine maintenance, you will have years of trouble-free operation. Please refer to this manual when performing routine filter changes. The instructions make periodic maintenance quick and easy and ensure you will receive maximum benefit from your system.
• Sohaili, Johan, Hon Siau Shi, Noorul Hassan Zardari, Noraziah Ahmad, and Shantha Kumari Muniyandi. "Removal of scale deposition on pipe walls by using magnetic field treatment and the effects of magnetic strength." Journal of Cleaner Production 139 (2016): 1393-1399.
  Abstract
  
  Scale deposition on pipe walls is prevalent in water distribution systems and is difficult to remove. Commonly, chemical treatment is applied to remove the hard scale. However, it is detrimental to users' health and causes adverse environmental impacts.
  
  The need for clean water production for potable and other applications is essential. This study shows the application of magnetic water treatment as a safe and effective method for scale removal in water purification.
  
  Permanent magnets were installed in the designed treatment devices that consisted of pipes with scale deposition and the effect of magnetic field on scale reduction was monitored.
  
  The scale removal efficiency was evaluated based on calcium concentration in the outlet, after passing through the magnetic field. Magnetic strength was varied between 0.1 T to 0.4 T to investigate its effect on scale reduction.
  
  The morphology were analyzed by field emission scanning electron microscope. It was found that magnetic field enhanced scale removal from pipe walls by 46.7%. With respect to the increasing the magnetic field strength to 0.4 T, the efficiency of removal also increased to 30%.
  
  Possible mechanisms involved in the magnetic treatment that affects scale reduction such as the effect of magneto-hydrodynamics and magnetically modified hydration are discussed. Magnetic technology is a simple, cost-effective and environmentally friendly treatment approach for clean water production with significant scale removal efficiency.
  
  Magnetic treatment can be used either as a stand-alone technology or in water purification systems.

• Worp, CLOGGED PIPE SHUTTLE CLEANER CATALOGUE [PDF] Op. Cit. The Shuttle cleaner is used for scale removal in cooling water piping systems.
• Worp, PIPE DE-SCALING using AIR PULSE GENERATOR MANUAL [PDF] an alternative to acid cleaners, Worp, Sales Kazutoshi Ogawa, 7-19 Yokota-Cho, Aoi-Ku, Shizuoka-City, Japan, Tel: 090-1234-1875 Email: worp@nifty.com - contributed by Worp, 2022/03/05
  
  According to the company the Worp pipe de-scaling process uses only air pressure (and water) to remove rust and scale from piping systems.

• Worp, PORTABLE SCALE REMOVER [PDF] (2020) Worp Ogawa Kazutoshi7-19 Yokotacho, Kashi-ku, Shizuoka CityTel: 090-1234-1875 Email: worp@nifty.com
  
• Zhang, L., Wu, P., Zhu, D. and Zheng, C., 2017. Effect of pulsating pressure on labyrinth emitter clogging. Irrigation Science, 35(4), pp.267-274.
  
  Abstract
  Emitter clogging has a direct impact on the performance and service life of a drip irrigation system. A study of labyrinth emitter clogging was conducted to determine the effects of pulsating versus constant pressure on average emitter flow rate (𝑞⎯⎯⎯), the Christiansen uniformity coefficient (Cu), and the location of clogged emitte
  
  rs. Under constant pressure, emitters in this study began to be clogged after the 8th irrigation, whereby 15 emitters eventually became clogged; the average emitter flow rate decreased to 75% of the original emitter flow rate (𝑞0) after the 15th irrigation, and Cu decreased rapidly by 68.2%.
  
  For pulsating pressure, emitters began to be clogged after the 14th irrigation, only 5 emitters became clogged, and 𝑞⎯⎯⎯ only decreased to 85.3% of 𝑞0, and Cu declined by onl
  
  y 22.6%. These results demonstrate that the anti-clogging performance of emitters with pulsating pressure is better than with constant pressure. Emitter clogging at a constant pressure generally occurred at the inlet and front of labyrinth channels, mainly due to deposition of sediment particles.
  
  Emitter clogging with pulsating pressure is due to other impurities (e.g., debris of Teflon tape) besides sediment particles plugging the labyrinth channel. Pulsating pressure is recommended for use in drip irrigation to prevent emitter clogging.
  
• Zollinger, W. Thor, and Frank Carney. Pipeline blockage unplugging and locating equipment. No. INEEL/CON-03-01459. Idaho National Lab.(INL), Idaho Falls, ID (United States), 2004.
  
  Abstract:
  This paper describes the development of a pulsed hydraulic system, specifically designed to unblock plugged piping. It uses the differences between the resonant vibrations of the fluid column and pipe walls to separate the blockage from the pipe wall, break it up, and clear the line.
  
  Using resonant frequencies, the system can stay below the design pressure of the system, preventing pipe failures from occurring, which is a major concern with DOE radioactive waste transfer lines.