Direction of tankless hot water coil leaks into vs. out of a heating boiler or indirect water heater tank.

This article explains conditions under which the direction of a hot water coil leak can reverse from its expected behavior and how that condition may be detected. We explain that an indirect or hidden coil leak usually increases boiler temperature but when building water supply pressure is lost the leak can reverse and may contaminate the building potable water piping or water heater.

Leaks can occur at a tankless coil where it is mounted to the top or side of a heating boiler, at fittings connecting hot and cold water piping to the coil, and more subtle leaks can occur inside the boiler - leading to excessive boiler pressure and relief valve leaks.

Directions of Water Leakage: Tankless Coil or Indirect Water Heater Coil Leaks Effect on Building Water Supply Piping, Contamination, Safety

Reader Question: Can a tankless coil leak be repaired using StopLeak type products?

17 March 2015 Karl R. said: TPR valve leaking, flooded the basement, replaced pressure reducer & TPR valve

... two weeks ago ... the pressure relief valve on the boiler was triggered, pretty much flooding my basement. After careful perusal, I determined that the Taco 329-3 pressure reducing valve @ the cold-water supply was leaking and needed to be replaced - along with the Watts 374A pressure relief valve. I then took the opportunity to replace every single worm-style control and purge valve with new equivalent ball valves, as most of them were leaking to some extent as well.

However, this did not solve the boiler issues, as the new pressure relief valve triggered almost immediately after firing the boiler up. (pressure @ the gauge read 42 PSI). After further trouble-shooting, I determined that the (Amtrol Mod 30 4.4 gal.) expansion tank had also failed, as did the Amtrol Mod 700-30 float-type air vent located above the air scoop where the expansion tank mounted. I double-checked the pressure charge on the new tank just in case - 13 PSI.

It was at this point I had discovered the Amtrol boilermate had been excluded from the system by various valves being shut off (although the Honeywell control valve to the unit was still operating and functional). After resolving the indirect-fired Amtrol hot water tank issue by re-introducing the tank into the system, I thought I had all problems resolved.

Fired the boiler up, pressure was steady @ 19. temperature at 190. Following your advice, I reduced the Hi-temp on the aquastat to 145, lo = 125, Diff =15 for the summer months. Everything seemed to be functioning correctly, pressure and temps good...yet the floor still was wet.

Upon further inspection, it seems the boiler drain petcock valve had developed a leak, and could not be fully shut off. Well, I've replaced almost every other valve, might as well replace this one...and so I did with another 3/4" ball valve. Fired the boiler up confident that all problems are now resolved.

TP valve still leaking

Evidently, I spoke too soon...This morning, I go down to the basement to check that everything is ok, and I find the pressure relief valve had kinda triggered.......a very slow drip, and upon checking the pressure gauge, I find the pressure had raised to 38 PSI at a temperature of 145.

My question is....the temperature of the boiler is not enough to generate that kind of pressure...what could possibly be wrong ? It might be relevant to mention that I had left the cold water supply valve on in order to ensure a constant availability of top-off water through the Taco pressure-reducing valve.....a valve that I have since shut off, as I suspect that the new taco valve might be defective....It's the only possible explanation I can think of for the excessive system pressure....any ideas ?

Karl R. said:
Yano...the answer to the [above] query was in your fine wiki site, and it might explain why the Amtrol indirect-fired water tank was cut out of the system in the first place....it's quite likely the coil in that tank has developed a leak...forcing the pressure to equalize between the street-pressure of the domestic hot water and the closed low-pressure boiler system, correct ? That said, and if that's the case....is that coil conducive to repair (Soldering, brazing, etc) ? Or, failing that, I've seen some articles about a form of 'stop-leak' introduced into the system.....are these viable options ?

This question and the reader's comments were originally posted

at AQUASTAT HI LO DIFF SETTINGS FAQs

Reply: which way do tankless & indirect water heater coils leak

Leaks out of heat exchanging coils can be tricky to first understand and second to track down.

It helps to understand the different pressures involved.

Inside a hydronic heating boiler the operating pressure range is usually lower than building water system's pressure.

A residential boiler operates between about 12 psi cold and maybe 20, certainly under 30 psi hot.

Building residential water supply systems, even considering modest pump and well system, typically operate between about 20 psi and 40 to 70 psi.

So usually a leaky coil in the heating boiler (a tankless coil) or a leaky coil inside of the indirect fired water heater (in the water tank) will send water FROM the building water supply TO the boiler interior, raising boiler pressures abnormally and leading to a temperature/pressure relief valve TPR valve leak as well as possibly abnormally high pressure readings on the boiler pressure gauge. This is an unsafe condition since TPR valve leaks can lead to TPR valve clogs that can lead to a BLEVE - explosion.

There are a few odd cases in which building water supply system pressure can however be less than boiler pressures, such as when there is a power loss and the pump and well lose prime or when a municipal water system has a service outage. I

In these cases higher pressures in the boiler can push unsanitary or even chemically-treated water out of the boiler and into the building water supply-system, or even back out into the municipal supply system piping, contaminating everyone's water supply. Backflow preventers on the boiler water feed line are intended to protect against this hazard.

However, there is no backflow preventer normally installed on a tankless coil nor on an indirect water heater heat exchanger coil that can assure that boiler water can't leak backwards out into the water supply (from a tankless coil) or into the potable water inside of an indirect-fired water heater in these cases. They are unlikely to occur since we need a combination of multiple faults.

Why repairs to a leaky tankless coil or indirect water heater coil may not be feasible

About repairing a leaky coil by soldering or stop-leak products, and ignoring questions of toxicity of the leak repair product or possible system and control clogging and safety worries, still:

I would not consider that approach. In my opnion soldering a leak in a heat exchanger coil either in a boiler or in an indirect water heater,

1. Is not a durable repair since the coil leaked from corrosion which means that perforation elsewhere is likely soon and the whole coil is too fragile anyway
2. Will cost about as much as simply replacing the coil since most of the cost is in the labor to drain the boiler (or indirect water heater tank), cut plumbing fittings, remove the old coil, and replace the new one.

...

Continue reading  at EVIDENCE of TANKLESS INTERNAL COIL LEAK INTO BOILER / WATER TANK or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• TANKLESS COIL / HOT WATER COIL LEAKS
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• TANKLESS COIL EXTERNAL LEAK EVALUATION
• TANKLESS COIL RUST TYPES: SUPERFICIAL vs EXFOLIATION
• TANKLESS COIL INTERNAL LEAKS INTO the BOILER
• INDIRECT WATER HEATER COIL LEAKS INTO the BOILER
• TANKLESS COIL LEAK DIRECTION IN or OUT
• EVIDENCE of TANKLESS INTERNAL COIL LEAK INTO BOILER / WATER TANK
• REPAIR OPTIONS for TANKLESS COIL LEAKS

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TANKLESS COIL LEAK DIRECTION IN or OUT at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to WATER HEATERS

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