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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
Hot 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.
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
Some 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:
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.
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 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
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:
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.
EXTRA TANKS 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
SOLAR WATER HEATERS using solar collectors, an indoor water tank, pump and controls, using minimal "on-grid" energy
TANKLESS COILS - using a coil in the heating boiler or in an indirect heated water tank to produce domestic hot water
TANKLESS COIL HOT WATER INCREASE a heat exchanging coil immersed inside of a heating boiler heats provides (somewhat limited) hot water. various tricks can significantly improve the safety and water quantity available
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time to use an acid wash in my tankless coil?
(Apr 29, 2014) john popa said:
rusty water coming from hot water in bathtub that is never used. Also from another sink. All found during home inspection. We have found nothing.Inspector suggested could be a issue w/heating coil. We think it is from non use. Acid wash? Suggestions?
Don't do anything yet.
If the home has galvanized iron pipes and water sat in piping without flowing for some time, rusty water would be no surprise and should flush out with use.
An acid wash has other risks including toxicity - not something to undertake lightly.
And tankless coil tubing is copper, not likely to be a source of red rusty water.
As long as you are not seeing the same discoloration elsewhere, the unused pipe theory holds water.
I took this photo of reddish brown water running into a bath tub
under just the same conditions as you describe.
There may be other implications: galvanized pipes in an older home mean new piping is in your future.
Question: no hot water from our geyser
(June 6, 2014) Anonymous said:
After municipal stoppage of water to our home we are unable to get any hot. water flow from our geyser even though the cold water tap is fine
. Can you assist with a solution
Are you seeing no water flow at all or water flows but is not at all hot?
If the former look for a debris-clogged valve or pipe elbow and try flushing the piping system
If the latter the problem is with the geyser itself not the water stoppage
(Aug 15, 2014) D. Laws. Near Bedford, Beds. said:
My 36 year old house has an indirect hot water cylinder that I suspect contains a huge amount of limescale, as I live in area with very hard water! Due to size of the airing cupboard, I'd guess the plumbing problems would be huge, as there is very little room to get at the pipework surrounding the cylinder? I can figure out how to dissolve the limescale OK, but as the hot water flows out of the cylinder at the top, I'd guess that any solid residues will stay in the cylinder, due to gravity? I might remove the immersion heater and work through the hole somehow? Am I going to run into problems afterwards, if I use chemicals here and it might thereafter build up again quite quickly? Perhaps I should consider a follow-on yearly descale, to keep on top of it afterwards? Being a pensioner, I don't have the funds, to get a new cylinder fitted!
You're correct that scale or lime will tpically remain in and on the bottom of the hot water cylinder. We've removed such scale by removing the heater's bottom drain, draining out all of the water, and then GENTLY probing and breaking up loose scale into fragments that would flow out through the drain opening. We then flushed the heater by briefly sending bursts of cold water in at the heater inlet.
If scale is absolutely solid on a hot water cylinder interior surface, only by using a de-scaling agent are we likely to get that out. If you use chemicals you'll want to be sure the cylinder is very thoroughly flushed to avoid drinking or bathing in something harmful.
Once cleaning the cylinder, (for U.S. & Canadian readers - a hot water tank) the most effective long term fix for hard water high in mineral content is to install a water softener. You may hesitate at that expense, but consider that a longer-lasting solution is likely to be less costly and less disruptive than ruining the building piping and heater with scale.
5 Feb 2015 Robin said:
I have had to replace my heating coil, for my hot water, on my oil furnace 3 times in the last 3 years. It is now at a point where my hot water again is just trickling, I'm assuming due to the coil being clogged again. The heating coil on there was just replaced about a year ago. This is getting extremely costly and I want to know how I can fix this problem once and for all?
In reference to my question below, the house is only about 8-9 years old and went unoccupied for 2 of those years.
I would first have the water tested for hardness and for aggressiveness or corrosivity. Based on those results you may want to install a water treatment system to process all of your home's incoming water to avoid these problems.
An alternative worth considering is to abandon the tankless coil approach to making hot water completely by installing a separate water heater. An option to consider is the installation of an indirect-fired water heater. See
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
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Thanks to reader Bernie Daraz, Newtown, CT for discussing additional steps to improve the results of tankless coil scale removal, November 2010.
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.
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