Photograph of a multi function combination control on a heating boiler 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?

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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

Abandoned tankless coilHot Water Heater Tank Clogging Diagnosis & Repair - Tankless Coil De-Liming Procedure Details

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.


[Click to enlarge any image]

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.


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

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.)

Guide to Acid Flushing of Clogged Tankless Coils or Clogged Hot Water Piping

Lime scale fragments from a water heater (C) Daniel FriedmanSome plumbers will attempt to clean out water supply piping that has become mineral-clogged.

Photo at left: 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]

The general procedure is the following:

  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

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 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 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.

How do we Prevent Tankless Coil Clogging Again in the Future?


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.


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™.

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:

  1. Poor hot water flow, or bad hot water pressure, which is usually a piping or clogging problem or an overall building water pressure problem
  2. 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.
  3. 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.

     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.

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 and at WATER HEATER COMPARISONS, PROPERTIES

Ways to improve total hot water quantity, pressure, temperature and flow are discussed beginning at HOT WATER IMPROVEMENTS

Thanks to Carson Dunlop Associates, 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


Continue reading at TANKLESS WATER HEATER INSTALLATION REQUIREMENTS, COSTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

If only hot water pressure and flow are poor, also see HOT WATER PRESSURE IMPROVEMENT


If you need to improve the hot water temperature and flow from a tankless coil system that is not clogged, see TANKLESS COIL HOT WATER INCREASE.

Readers should also see WATER PIPE CLOG DIAGNOSIS



Or see TANKLESS COILS - home

Suggested citation for this web page

CLOGGED TANKLESS COIL or PIPES, LIME SCALE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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