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 How to Diagnose, Evaluate & Repair Leaks at a Tankless Coil on a Heating Boiler
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Tankless hot water coil leaks, corrosion, & repair procedures: This article explains leaks at Tankless Coils on heating boilers: a guide diagnosis, evaluation, & repair of leaks at tankless coils used for making hot water.
Leaks can occur at a tankless coil where it is mounted to the top or side of a heating boiler.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
How to Find, Diagnose & Fix Leaks at Tankless Coils
 We introduce tankless coils as a source for domestic hot water, and we explain how they work at TANKLESS COILS. We discuss how to improve hot water quantity at HOT WATER IMPROVEMENT. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Leaks at tankless coils on heating boilers can seep away for a long time before anyone notices. But if you wait too long, the boiler may be beyond repair.
Leaks at or around tankless coils on the heating boiler that leak out of the coil or boiler
Boiler leaks out at a tankless coil mounting plate or at the pipe fittings that pass through the coil mounting plate are a common defect on heating boilers, both steel and cast iron units. You may not see water (the boiler is hot, after all) but you'll see mineral deposits and crud or even crud and rust build-up at the leak source and below it.
Caught and repaired early these leaks at a tankless coil can be repaired without serious damage to the heating boiler. Left alone these same leaks, particularly at the tankless coil mounting plate can completely destroy the heating boiler. Deciding how badly a heating boiler has been damaged by leaks is tricky, controversial, and important since both safety and big cost concerns are involved.
- At the tankless coil face tapping for mounting a temperature sensor (bottom center pipe shown in this photo, leading to a boiler combination control).
- At the tankless coil face tapping for the incoming cold water or outgoing hot water pipe (the two copper pipes shown in this photo)
- At the tankless coil mounting plate where it is bolted to the face of the heating boiler (the round and rusty steel face shown in this photo).
A leak at any of these points will not normally show up as water on the boiler or on the floor around the boiler.
That's because the boiler is normally hot all of the time. Water leaking at any of these locations will rapidly evaporate.
Leaks from a tankless coil into the heating boiler will raise boiler pressure
 Watch out for this second and more tricky to spot tankless coil leak. Corrosive water, usage, repeated acid cleanout of a mineral-clogged tankless coil, or other wear and tear can cause the finned copper tubing of the tankless coil to leak water at house pressure into the interior of the heating boiler itself.
Take a look at that pile of tankless coils in our photo (left). When we found all three of these near the heating boiler in a home we knew that there had been a history of trouble with leaky tankless coils. You can see in the photo (click to enlarge) that there is virtually no rust or corrosion around the coil mounting plate.
You would never spot this leak by just looking at the boiler externally. But each of the three coils has some funny-looking brass-colored discoloration around the finned tubing near the mounting plate, in the first three coil turns. That's just where you might find thin coil tubing and perforations due to one of the causes we list below.
Causes of internal leaks in a tankless coil - leaks into the boiler
- Building water supply that has a high corrosive index
- Building water supply high in minerals, leading to clogging of the tankless coil, combined with repeated acid washings to try to clear the blocked coil
- (Rare) mechanical damage to the coil itself during shipment, storage, or installation
How to spot and diagnose a tankless coil that is leaking into the heating boiler
- You may see a dripping or actually spilling of boiler water at the boiler's temperature/pressure relief valve. Now more than one problem can cause the relief valve to spill (see RELIEF VALVES - TP Valves on Boilers ). But if the cause is due to a perforated tankless coil you will also see
Watch out: a dripping pressure/temperature relief valve on any tank or vessel means that a competent diagnosis and repair are needed to avoid the potential of dangerous BLEVE EXPLOSIONS
- Abnormally high pressure inside the heating boiler. Typically a residential heating boiler runs between 12 psi cold and under 30 psi hot. More than one thing can cause abnormal boiler pressure, such as setting the temperature higher than 200 degF or a control that is malfunctioning, or a water feeder / pressure reducer that is failing, or someone manually over-feeding the boiler. But if the problem is caused by a tankless coil leak into the heater you can often prove that cause by:
- Turn off the boiler, let it cool down to room temperature, and then drain water out of the boiler drain sufficient to drop the boiler to its usual cold operating pressure. For taller homes the cold pressure may be higher than 12 psi. For this test it doesn't matter where you stop as long as the cold boiler pressure is set to below the incoming building water supply pressure.
- Now turn off water supply that feeds the boiler. This will rule out a bad water feeder/pressure reducer valve.
- Leave the boiler off overnight, or longer if you can. Obviously you can't do this procedure if weather is so cold you can't leave the heat off for a while. At the end of your observation period, 12-24 hours, or sooner if it occurs, look for increasing pressure inside the boiler. With the boiler off and its own water supply turned off, if you see boiler pressure increasing you can bet the problem is most likely a leaky tankless coil. (Typical building water supply pressure is 40-75 psi, higher than the normal internal operating pressure of a residential heating boiler. That's high enough to force water out of a perforated tankless coil and into the boiler itself.)
- When you turn the boiler back on, don't forget to turn on its own water supply as well.
We continue below with photographs and details about finding and evaluating and fixing leaks out of the coil.
What evidence of leaks will we observe at a boiler with leaks at the tankless coil?
White or other mineral salts left behind from evaporating leaky water, at and below the point of leakage, often staining the face of the boiler, such as we show in the photograph above. Even without close inspection one can observe white leak stains below the round black tankless coil plate and running down the face of the boiler in this installation.
 Rust or corrosion at the point of leak, such as the flaking exfoliating and badly rusted tankless coil we show in our photograph at left. .
A leak that has produced rust like this might mean that the tankless coil unit needs to be replaced, or worse, that the boiler has been so damaged by rust that it is beyond repair.
In the next section of this article we discuss how to evaluate leaks on heating boilers at a tankless coil
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How to Evaluate a Leak at a Tankless Coil on a Heating Boiler
Young leaks at a tankless coil can be repaired
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 If a leak is found in time, that is, before severe rusting and exfoliating or flaking rust have occurred, it should be possible to disassemble, clean, seal, and re-assemble the parts.
In our photo at left we'd probably call this a "middle-aged" leak rather than a "young leak" because the white mineral salt deposit you see below the tankless coil on the face of the boiler is pretty thick. But it's possible that this tankless coil leak can be repaired. A closer inspection is needed.
Leaks at piping connections on the tankless coil itself can be repaired without removing the whole coil assembly from the boiler.
Leaks at the tankless coil mounting plate will require that the boiler be shut down, drained, and water piping connections cut or opened at the coil so that the coil assembly can be removed. If inspection shows that the coil face and boiler mating face are un-damaged, the mounting gasket is replaced, sealed, and the coil reassembled to the boiler.
This may seem an odious task, since it means you'll need to shut down the boiler and also domestic hot water, and boiler water and piping water may need to be drained or partly drained in order to make this repair.
But you should make this repair as soon as possible.
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Old leaks at a tankless coil may require boiler replacement
If a leak is not found in time, or if no one was willing to repair a young leak, the cost is likely to be very high.
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The white deposits on the face of this tankless coil appear to originate at a pipe connection at the coil face (top center of the photo); but notice that second leak trace at the bottom of the tankless coil mounting plate and to the right of the larger white leak stain?
This tankless coil appears to also be leaking at its gasket.
Our next photos show how prolonged leaks at the coil face plate mounting gasket can destroy a heating boiler.
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Photo Guide to Severe Tankless Coil Leak & Rust Damage
Leaks at piping connections through the face plate of the tankless coil, if they have progressed for a long time, will have caused so much corrosion that disassembly and repair of the coil is impossible. The good news is that leaks at this location only destroy the tankless coil itself and are less likely to destroy the whole heating boiler. The coil will need to be removed and replaced.
Leaks at the tankless coil mounting plate, if they have progressed for a long time, will have caused damage to the coil mounting plate.
But much worse, the mounting surface on the boiler will also be damaged. If the boiler surface has been damaged it may be impossible to mount a replacement tankless coil without continuing leakage.
It is technically possible to perform a repair to such a boiler by welding on a new coil mounting surface, but the welder is not going to be very interested in performing such a small but time consuming repair, and knowing that the alternative to her welding service is a whole boiler replacement, the price for the welding job may be rather high.
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Which Tankless Coil Leaks & Boiler Leaks Can Be Repaired and When is the Boiler Ruined?
Rusty tankless coil face plate and boiler surface: Look closely at the coil mounting plate and the boiler mating surface to see just where the rust and exfoliating have occurred.
In our photo at left, this boiler, in Wappingers Falls, NY, was beyond repair. Leaks at the tankless coil had badly rusted the coil face plate, the boiler face, and even the boiler jacket.
In most cases a leak at the coil mounting plate, left unattended, will have badly rusted both surfaces and the boiler is likely to be beyond repair.
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Rusty tankless coil face plate, possibly salvageable: But in some cases the rust and flaking may be occurring on the outer face of the tankless coil mounting plate, and there is a chance that the coil can be removed and replaced. In the photo above we show here we see severe flaking exfoliating rust on the outer face of the tankless coil face plate.
In this photo we can see that the inner mating surface of the boiler, the surface to which the tankless coil face plate has to mate, looks as if it might be ok. In this case it seems worth trying to remove the tankless coil to see just what the situation really is.
Replacement of the coil and saving the boiler might be possible in this case.
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Rusty tankless coil bolts & Studs: Of course a related problem is that the coil mounting bolts and studs may be so badly rusted that they will be broken during removal of the coil. It's possible to drill, tap, and restore broken coil mounting studs, but lots of work and cost are involved.
That's why your plumber will be reluctant to promise that a boiler can be saved even when it appears worth making an attempt to do so.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- BOILER LEAKS CORROSION STAINS for a discussion of leaks on heating boilers.
- 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:
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