Tankless Coil Leak FAQs

Q&A: How to Repair Leaks at a Tankless Coil

Tankless Coil Leak Evaluation FAQs

The coil is leaking; can I turn off water to it and then just leave it alone?

The coil "Gasket" has a whole lot of 'calcium' build-up, I was thinking the gasket needs to be replaced, but it would be a pain in the A#$ just to access it.

There was an 'auxiliary' valve in line with the 'mixer' which brought more hot water to the mixer. Would this added (pressurized) water cause some 'back pressure' on the 'auxiliary' valve causing the water to 'bleed' out of the gasket? The 'mixing' valve also was leaking with calcium (white crud).

The reason I'm asking is if the 'auxiliary' valve is in the 90 degree (off) position, then less pressure (hot water) goes to the mixing valve & less backpressure to the coil gasket; therefore no leak. I really DON'T want to "F' with this, me or pay someone else too!

I'm hoping it will run just fine as long as this 'auxiliary' valve is left in the off position. Any thoughts would be greatly appreciated. Thanks, Mark On 2022-04-01 by Mark Rist -

Reply by Inspectapedia Com Moderator (mod) - Probably not: Evaluate the risk of a catastrophe before taking apart leaky parts on a boiler but also compare the risks of loss of heat or flooding

@Mark Rist,

I don't think that the auxiliary valve position will fix the corroded leaky mating joint between the tankless coil and the heating boiler, but

Do take a close look to see exactly where the leak or leaks are occurring as that's an important detail in deciding how to proceed.

And report on the extent of corrosion - its seriousness - and on the degree of rust at the tankless coil mounting bolts.

Watch out: I agree completely that taking apart any old plumbing component risks opening a can of worms, as you may find that a nut or bolt snaps off or that threaded parts are so corroded that re-assembly is impossible, leading to need for an expensive replacement.

Just yesterday we discussed a leak at an elbow that in my opinon ought not even be in place, on a 1990s gas fired boiler in northern Minnesota. The TPR valve needs replacement. The valve is mounted on a 90 degree elbow on the boiler where usually the TPR is mounted directly to the boiler.

The tech said they NEVER are willing to touch such parts, for just the reasons we've given. They get blamed for what was a pre-existing condition but note that touching the part results in a boiler replacement and a furious customer.

On the other hand, and here's the rub, leaving the leak un-attended will ultimately destroy the boiler.

See our photo below of a steam boiler from a New York home - it was completely destroyed and its cast-iron literally rusted through, afrter the homeowner ignored boiler leaks for years. I'll attach a photo below.

So in our OPINION here's the critical decision you and your service tech would make:

1. Is the existing corrosion and rust so bad that there is a significant chance that trying to disassemble the parts will lead to a total failure and need to replace the whole boiler?

If not, give it a shot. With appropriate and due warnings.

Details about the terribly rusted boiler whose sections are shown above are given

at BOILER LEAKS CORROSION STAINS

When do we let the boiler alone and ignore the leak ? What's the cost? What are the risks?

2. If the parts are so corroded that a replacement of the whole boiler might be required, what's the cost of ignoring the problem until we see water on the floor?

Answer, as was experienced by the homeowner of the boiler we show in our photos.

2.a. The leaky corroded heating boiler is most-likely to fail when it's working the hardest, hottest, under longest periods of highest pressure: during the peak of the heating season on the coldest days of the year.

That's what this homeowner experienced. The result was a boiler that was so leaky and deteriorated what when the homeowner called for emergency service the heating tech took one quick look at the boiler and shut it down for safety, worrying that the whole unit could explode or burst or flood the home's basement. Deeming the unit unsafe he red-tagged it.

The homeowner was then without heat for several days until the heating service company could locate, obtain, and bring a new boiler to the site for a replacement job.

2.b.There was a further risk that the home's basement could be flooded by leakage - mitigated if the service company or homeowner were able to turn off the water supply to the now shut-down and red-tagged boiler.

There were additional costs as well. Because the boiler had to be replaced on an "emergency" basis, the boiler and its installation cost more than it might had it been replaced at a more-opportune time, such as mid-summer.

Our photo, above showed how the cast-iron sides of the boiler were rusted through.

This boiler was beyond repair by the time the homeowner called for repair service.

Sometimes a leaky gasket at a tankless coil can be replaced by removing the coil, cleaning the surfaces, and installing a new gasket + sealer.

But if you see thick exfoliating rust and corrosion, even before there's water on the floor, there is are some critical questions to answer:

- where is the rust? Only on the part to be removed or also on the mating surfaces of the boiler?

- how bad is the rust? Is it surface rust - the parts can probably be removed successfully, or is there thick exfoliating flaking rust - the parts may meet with disaster on trying to execute the repair

- what are the surrounding conditions? Is it very cold? Can the building be without heat for a time? What are the potential costs and risks of a more-serious leak or of a loss of heat that may occur if the repair is deferred?

- would it be better to wait for warmer weather to perform the repair?

Bottom line: if you can fix a leak in time to avoid destroying the boiler, it's worth giving that a shot.

 

Tankless coil leaks into boiler: is thecoil repairable?

My boilermate! I think it is the coil leaking and backfeeding my boiler causing high pressure in the boiler.

I can hear cold feed pipe running into the boiler mate. When I shut the ball valves it seems to stop.
Is it worth trying to fix it,,(new coil) or get a new one? On 2015-04-28 by doug

Reply by (mod) - low chance of successful repair of corroded tankless coil

Doug:

Because of the nature of corrosion in coils, the chances that a leaky one is repairable in this circumstance are close to nil.

Add the fact that the labor cost to remove and replace the coil is significant and the reasonable conclusion is: replace the tankless coil.

Boiler water feed keeps running - tankless coil problem?

I have a pressure problem and I can hear the water feed to the boiler mate running. The boiler blew out the Pressure valve. Is it possible to replace the coil inside? or get a new one? On 2015-04-28 by Anonymous

Reply by (mod) - check the water feeder valve for malfunction

Watch out: leaks at the pressure relief valve risk serious safety hazards (risk of a boiler explosion) if the valve becomes clogged with minerals.

What you describe sounds like a faulty water feeder valve not a tankless coil leak.

Boiler internal water pressure won't exceed about 30 psi - that's when the relief valve opens.

So it would be unusual for the boiler to leak INTO the building water supply system and almost impossible for it to do so continuously unless you were leaving a hot water faucet open somewhere in the buidling.

Do I need a new tankless coil or a whole new boiler?

I would like to chage a new tankless coil, but one plumber recommended me to buy a new boiler.

He told me that the boiler itself is weak and might have small cracks,as evidenced by the observation of about two cups of water from underneath the boiler and wetted insulator there, during cooling of the syetem after power had been shut down.

So I do not know whether the plumber is right or there are other explanations. In other words, whether it is worth to change a new tankless coil. Could you help explain? Thank you very much! On 2014-05-23 by Frank 05/23/14 -

Reply by (mod) -

Frank if the old coil leaked into the boiler that would spill the relief valve and put water on the floor.

Really? If there are other boiler leaks in the boiler itself, ask the plumber to show them to us.

If there are not such leaks then you are getting advice that's safe for the plumber but dangerous for your wallet.

I'd agree that if a boiler is badly rusted or cracked it needs replacement but if the leak is only the relief valve, replacing the whole boiler is a bit extreme: like replacing your car when it gets a flat tire. Only Scrooge McDuck would do that.

Signs of a leaky tankless coil: water dripping from boiler relief valve: is my boiler shot?

I have a 31-yeay oil boiler (tankless water heater), which had water dripping from the pressure relief one week ago.

The pressure and temperature reading was as usual as 30 psi and 180 degree. After I shut down the power, there was no water dripping from the pressure relief anymore. But with temperature decrease to room temperature a few hours later, I noticed about two cups of water spread the floor from underneath the boiler bottom where the insulator became wet.

Changing the pressure relief valve and feed water regulator did not eliminate water dripping from the pressure relief. It seems that the issue is related to the coil inside the boiler.

My question is why cooling the boiler system has caused leaking of about two cups of water from underneath the boiler bottom where the insulator became wet. One explanation is attributed to compaction with cooling of the gasket to the problematic coil.

The other explanation is that the boiler itself is weak and might have small cracks.

Is there any way to differentiate these two root causes and are there other explanations? Thank you very much! On 2014-05-23 y Frank 05/23/14

Reply by (mod) -

30 psi is not normal it's high which says the valve was working as it should.

Look for a leak from tankless coil into the boiler. If the tankless coil is leaking house water supphy INTO the boiler that could be causing the relief valve to leak.

If the boiler pressure is over 30 psi and its temperature is under 200degF I suspect that's the problem.

Replace the leaky tankless coil.

Tankless Coil Repair FAQs

How to fix a broken-off tankless coil mounting bolt

2020/06/28 mac said:

I twisted off middle bolt front of New Yorker furnace domestic water coil to replace a gasket. What to do?

This Q&A were posted originally

at TANKLESS COILS

Moderator reply: Options for repair of a broken-off Tankless Coil Mounting Bolt

Mac

Leaky Tankless Coil Repair: How to extract a broken-off bolt at the Tankless Coil

Don't feel bad, you're not the first to break off a bolt. Good news is that there's no problem we're likely to have on a mechanical system that nobody has faced before, and usually somebody has figured out a great solution.

There are several options to salvage the situation.

1. Use Vise-Grip Pliers to Turn the Broken-off Bolt

IF all of the other bolts can be removed and you can remove the tankless coil assembly,
THEN

IF you can get a good bite on the now protruding bolt with a Vise Grip® pliers,

THEN

AFTER you soak the fitting with Liquid Wrench for 1 few hours or longer

THEN see if you can simply turn out the bolt.

2. Tap to Rotate the Broken Bolt

IF there is not enough protruding bolt to get a grip on it and turn it out, you can

Try the Liquid Wrench treatment followed by carefully tapping the face of the bolt on an oblique angle to see if you can get it to turn out (tap counter-clockwise or to the left.

3. Use a Bolt or Screw Extractor

You can step out, wearing your COVID face mask, to a local hardware store where you buy a spiral flute extractor set also referred to as a screw or bolt extractor set.

You'll need a hard carbide tipped drill bit set of a diameter that's about half that of the diameter of the bolt and a screw extractor or - basically a very hard steel coarse-reverse-threaded or "fluted" device (shown above, manufactured by Irwin / Hanson, also produced by other manufacturers.

You will note that you can buy an individual bolt or screw extractor of the right size, sold packaged together with the appropriate high speed drill bit.

Drill a hole in the center of the bolt, then tap in the screw extractor and turning it counterclockwise, back out the bolt.

(Pre soaking with a liquid wrench type product can help the extraction)

4. Drill out the broken bolt and re-thread the bolt hole

In the worst case, I've had to drill out the entire bolt, using a succession of carbide-tipped drill bits and drilling slowly - you should see curls of steel coming off of the bit.

Watch out: If you run the drill at high speed all you do is overheat the bolt and bit and dull the bit quickly. Press hard and drill slowlhy.

Use a succession of bits, starting at the *center* of the bolt, until you've drilled out to just short of cutting into the original threads of the opening.

Then you'll use a tapping tool to chase out the remnants of the old bolt and clean up the threads.

If, as I've done, you over-drill the hole so that it's too big for threading-in an original-sized coil mounting bolt, then you can often recover by drilling and tapping the hole for the next-size larger bolt.

Watch out: before going to all this trouble, inspect the coil mounting area on the boiler.

If it's so badly rusted that a new coil isn't going to seal properly then (although you could hire a welder to try repairing the area) it's probably time to either abandon the tankless coil and seal up the coil opening entirely OR it's new boiler time.

So before starting all this take a look at

TANKLESS COIL / HOT WATER COIL LEAKS

Irwin/Hanson and other manufacturers sell screw and bolt extractor kits as well as smaller sets that provide the drill bit and screw extractor in individual sizes, as I illustrate here.

• Liquid Wrench, RSC Chemical Co., 600 Radiator Road Indian Trail, NC 28079 USA Tel: (866-973-6247) Website: https://www.liquidwrench.com - note that regular lubricating oil is not nearly so helpful in loosening rusted parts as is a penetrating oil like this one.

Can a pinhole leak in a tankless coil be repaired with solder?

Can a leaking coil (no corrosion) be repaired by applying the same non-lead flux applied to connect lengths of tubing?

Thanks On 2016-12-01 by Isaac

Reply by (mod) -

Isaac you can sometimes repair a small pinhole leak in copper piping by soldering.

I'd want to understand the cause of the leak to be sure the repair is appropriate.

For example you may need to test your water supply for corrosivity and if the water has a high corrosivity index, install a water treatment system.

Can I plug the water inlet and outlet on the tankless coil mounting plate and leave it in place?

Thanks for the diagnosis procedure, which helped me to find the dripping issue related to leaky tankless coil.

Now, my question is: I want to completely abandon the tankless coil water heater, and install a stand-alone water heater (new heat pump type), Is it possible to plug the water inlet and outlet on the mounting plate (without removing the tankless coil inside the burner water? On 2014-12-26 by Marlon

Reply by (mod): yes if it's not leaking

Yes, you can cut the cold-in and hot-out pipes at the coil and cap them, then reroute those pipes to and from your new water heater.

Watch out: but if the tankless coil plate or bolts are leaky you'll need to repair or replace that coil plate to avoid boiler damage.

See details at TANKLESS COIL ABANDONMENT or CONVERSION

Plumber suggested remove and plug leaky temperature/pressure relief valve

Hello we had our boiler cleaned about 3 months ago. Soon after cleaning, we noticed water dripping from a relief pipe. We have two? One for the heat and one for hot water?

Plummer said it was not required. He also noticed that temp was set too high and he lowered it to normal. The pipe is still leaking, and is worse when heat is not on. Should we just remove this pipe and plug it as the plummer is suggesting? Boiler is working great mormally - On 2014-11-24 by Anonymous -

Reply by (mod) - Do not do that - you risk a disastrous boiler explosion

Anon,

Watch out: I cannot agree with removing a temperature /pressure relief valve. Those devices are there to prevent a potentially catastrophic system BLEVE explosion that could even injure or kill people in or near the buildng.

If a needed relief valve is leaking the valve should be replaced, and if the new valve leaks the cause needs to be found and fixed.

Since you are asking this question in an article about tankless coil sim guessing your two safety valves are on different equipment or piping or tank systems.

See relief valve leak guidance in detail at https://inspectapedia.com/plumbing/TP_Valve_Leaks.php

Fix leaks at the top of the tankless coil faceplate

I have a Burnham oil fired boiler V-14 A-T. It's about 30 years old.

\ I'm leaking water from the top of the tankless coil face plate. If I am able to remove the plate can I get a gasket for this if so do I use any other sealant with the gasket.

Also, I might want to install a separate electric hot water heater. I read some of the information you have on this. Is there an aquastat I can replace mine with that will just serve for heat and not hot water. On 2014-01-29 by Bill -

Reply by (mod) -

Bill,

I would not try this repair if it is bitterly cold outside as you could end up with the boiler shut down for a time or even destroyed - it depends on how severe the tankless coil leak is, how corrosive is the water, and how long it has been leaking.

IF visual inspection makes you think that the boiler is not severely corroded then it may be possible to disconnect the pipes, remove the coil, pick up a replacement tankless coil gasket from your plumbing and heating supplier, seal the new gasket and bolt the coil face plate back in place.

Check the repair for leaks and check again at full boiler pressure and check again in a day - then watch it every few days too since a subtle leak may take time to make itself visible.

A separate topic is abandoning the tankless coil, installing an indirect fired water heater as another zone connected to the boiler - a solution that gives more hot water and may be more efficient.

In the More Reading links we have inks to articles that describe each of those options.

by Bill

Thanks for the information. From just looking at the faceplate the rust doesn't seem too bad but I know I won't know the extent of the rust until I remove the plate. I will wait for the heating season to be over before I try this. Thanks again for getting back to me

by (mod) -

Bill that sounds sensible. I'd like to see sharp photos of the coil faceplate too.

Home Depot "experts" said that repairing a tankless coil leak is impossible - but I used radiator heavy duty stop-leak from an auto parts store

I went to home Depot asking if there is any way to fix my boiler coil’s water leak. They looked at me like an idiot. No way can you fix the coil, they said. You must change it.

Then I thought of how to fix it the cheapest way. It did cost me $5.00 to fix it.
Here is what you do:

1- Shut off the boiler electrical switch and Close all valves around the boiler.

2- Drain the boiler from any water left.

3- Fill the coil (From any possible water inlet to the boiler) with Radiator Heavy Duty Stop Leak (Find it in any auto supply store).

4- Add about one liter warm water to the same inlet.

5- Switch on the boiler and let it run till the thermostat shut the fire off. The higher temp setting the better result you’ll get.

6-Shut off the boilers electrical switch and Let it cool down for 20 minutes.

7- Repeat the same twice more.

8- Now open the valves and let the boiler filled with water.

9-Some water may come out through expanding the cold water when it is heated for the first time only but the coil will be sealed for many years to come.

Repeat it after few years when it happened again.

You can use the same to stop auto radiator small leaks.

Sorry plummers. Enjoy it home owners.... :)

Talal Mikhail 4/1/12

Reply: Watch out: Bad repair - replace a leaky tankless coil; in your patch you risk poisoning building occupants; don't try oatmeal in the tankless coil either.

Thanks for this interesting, if questionable repair idea for leaky tankless coils. It needs some clarification.

Leaks into the boiler: a tankless coil may develop a leak in the finned copper tubing that is inserted into the heating boiler interior. Such a leak will send household water supply into the boiler, causing excessive pressures in the boiler, perhaps showing up as a leaky boiler pressure / temperature relief valve, or as abnormally high pressure on the boiler pressure gauge. This is the sort of leak the reader below is describing.

We are not sure what the effect may be on the heating boiler, circulator pumps, zone valves, relief valve, of the passage of any stop leak compound into those components through the leaky coil before the leak stops - those may be harmful. Auto radiator stop leak is not designed for this circumstance

. It might work but it leaves me a little worried.

The second type of tankless coil leak, and the one illustrated on the page above, is a leak of heating boiler water out of the boiler around the tankless coil mounting plate or bolts.

This leak, as you can see in our photos, destroys the boiler if it is not found soon enough, and destroys the tankless coil mounting plate - an integral part of the assembly.

That's why the Home Depot rep was dead right when he said "replace the coil" - she or he was thinking of this common leak. Auto radiator "stop-leak" products won't fix this problem, and in fact running a stopleak product through the coil is irrelevant because that's not where the leak is found.

Old timers used to run oatmeal through the car radiator, and there is a similar lore of magic leak fixes that were applied to cast iron boiler leaks.

But for a corroded, leaky tankless coil around its mounts, you have to catch the leak early, remove the coil, clean up the mating surfaces, install a new gasket and reassemble. If you wait long enough, it's new boiler time.

Tankless Coil Leak Cause Diagnosis FAQs

What caused us to have to replace our tankless coil three times?

I have a Rheem tankless hot water heater. Rheem has replaced my heater 3 times, I am on my 4th heater in seven years! It is now malfunctioning, same problem,

Leaks! It is now out of warrenty. Rheem tells me they've never had problems like mine, none of the 3 plumbers, rheem certified, I've had out here to look over the years has ever heard of one leaking.

Mine begins to leak from the back of the unit, no one has dissected an old one to find out why, of course. Once it begins to leak, within a week it will short out the fuse near the fan and not come back on. this time I will have to purchase a new unit myself. Looking for suggestions.

It was suggested that my water is too hard, the last plumber tested it to be a 7, hard, but not out of range.

Suggested too much use, but we operate within range, top of range but within range.

Now plumber thinks another chemical, in our water is causing breakdown, of what no one knows. we are at a loss. supposedly installation is correct. We flush unit with water every 6 months or year.? On 2016-04-19 by Diane Sparks

Reply by (mod) - Test these causes of tankless coil leaks

Diane:

I'm impressed at Rheem's backing of their product. If it's true that the leak problems are unusual - and it sounds that way - I'd sure want to know why before installing ANY new water heater. I'd sure want to do what you suggest: dissect the water heater to look closely at what exactly failed and to try to figure out why.

Examples - I AM MAKING THESE UP as SPECULATION

- uneven mounting on a wall or uneven torquing of mounting bolts stressing the heater components

- hard water forming scale deposits causing uneven heating of the heater's heat exchanger

- aggressive water chemistry (corrosive) - test your water supply for corrosivity and if the water has a high corrosivity index, install a water treatment system.

- excessive use?

As it's easy to do so, I'd have your water tested for hardness, corrosivity, and ask the lab if water in your area has been found to have anything else that's unusual that they would add to the testing.

If you can find a decent forensic expert who can cut an old heater apart - or maybe Rheem would agree to do it - if scale or uneven mounting are at fault those causes should be self-evident on examination.

Rheem may be afraid of bad PR - you'd have to convince your rep of goodwill and trust. Or they may have already looked: did your old heaters go back to Rheem?

Tankless coil leaks around the bolts

My furnace guy cleaned the surface and the plate to get a good seal. I don't have a round coil it's rectangle. Just think it's strange to be leaking around the bolts. I know over tightening is not good either. It didn't leak until the coil was pulled out to be flushed because I had no hot water pressure. Have hot water pressure now.

Thank you for your prompt replies. Wonder if putting plumbers tape on the bolts would stop them from leaking? I guess eventually some day I'm going to have to replace my boiler, but need one to heat the hot water and garage too. On 2016-01-06 by Robin Thompson

Reply by (mod) - diagnose cause of leaks at coil mounting bolts

Robin:

Some tankless coils are indeed mounted through a rectangular plate, though the coil inside is still usually a round system; a few use a horizontal "back and forth" tube design; all work in a similar fashion.

I'm not sure why anyone would remove a coil for flushing it; in my experience the plumber connects fittings to the coil in- and out- lines by adding stop and drain valves if they're not already present, then she uses a pony pump, de-scaling liquid, and a plastic bucket to cycle the de-scaler thorugh the coil, ending by flushing it with clean water.

The bolts on a coil would not be expected to pass all the way through the boiler face into the boiler itself but perhaps on your boiler they do. So indeed one might try this:

turn off heat

let the boiler cool

drain pressure off of the boiler but don't empty it

start removing one coil bolt: as it loosens if you see water coming out of the hole, stop, turn it back in, and drain enough water from the boiler so that the water flow at the bolt(s) stops.

Remove the bolt: inspect for stripped, damaged threads; if the bolts are stripped more troublesome boiler repairs are needed (or your plumber will argue for replacement);

if the bolts are in good shape the hole into which they screw may be OK too.

You can try teflon tape or a pipe sealant that tolerates heat and high pressure on the bolt threads; turn the bolt back into place. Re-fill the boiler, turn it on and look for leaks;

I know it's a lot more trouble but I'd be tempted to try one bolt before fooling with all of them.

Actually I still suspect a bad gasket or bad coil mounting surface at the boiler. If the section of gasket between the bolt hole and the inner circumference of the gasket is not in good shape that's where boiler water might leak into the bolt hole.

If I'd hired a plumber to de-scale a tankless coil that was not leaking at all before the repair, and if it leaked afterwards, I'd expect the plumber to return and finish the job by repairing the leaks. I'd understand if he or she argued that the boiler and coil were old and in poor shape and that they were unavoidably damaged during removal of the coil - that could happen, though I still don't get why the coil was removed.

Will a new coil stop leaks at coil mounting bolts?

I have a Burnham boiler and it's 25 yrs old. Would replacing the heat coil stop the leaking around the bolts?

Maybe the heat coil plate is damaged somehow or warped? Expensive fix though, but cheaper than buying a new boiler. It's not a big leak and I keep a pan under the boiler to try to catch what leaks, but some still ends up on the floor from run off. Just don't want it to cause more damage to the boiler if that would happen. On 2016-01-05 by Robin Thompson

On 2016-01-06 by (mod) - understand the nature of long-standing leaks at the tankless coil mounting plate

Robin

Usually the round coil plate is quite thick and not likely to be warped such that it can't be bolted properly.

More likely is that leaks around the coil face plate corroded the plate itself or corroded the mating surface on the boiler itself. If the boiler face is badly corroded it may be very difficult to get a good seal even when a new coil is installed. The way to find the problem is to turn off and drain the boiler, remove the coil face plate, and examine the mating surfaces.

Coil leaks at bolts even after new gasket

The heat coil was removed and cleaned out due to it being clogged. New gasket installed. Leaked around gasket.

Guy came back, tightened bolts and still leaked. Put new gasket and sealer on. Worked for about a month then started leaking again around the bolts. I tightened them just enough to stop the leaking. This worked for a while and now it's leaking again around the bolts. Why? On 2016-01-05 by Robin Thompson -

Reply by (mod) -

Robin:

I don't know why - with so little actual information about conditions at your specific boiler, but some examples of persistent leaking at a coil, presuming the leaks are occurring around the outer perimeter of the coil mounting plate and not at the pipe connections, include

• Improper gasket or poor gasket installation
• Omission of gasket seale
• Badly-corroded mounting face on the boiler itself. If the surface is very irregular the new gasket and sealer may still be unable to stop leaks and
  
  Watch out: If the prior coil leaked for a long time the boiler could itself have been damaged beyond economical repair.