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Indirect fired water heater hot water coil leaks:
What is an "indirect" fired water heater coil leak and how can leaks at the hot water tank cause the heating boiler to be placed at over-pressure causing in turn leaks at the boilers temperature & pressure relief valve.
How do we detect this condition, why is it dangerous, and how is it repaired?
Internal Leaks in an Indirect Water Heater's Hot Water Heating Coil
An indirect water heater is a hot water tank system whose water is heated by circulating water from a separate building heating boiler thorugh a heat exchanger coil inserted into the hot water tank. This turns the concept of tankess coils inside out.
Thanks to Carson Dunlop, a Toronto Home Inspection Firm and Home Inspection Educator, for permission to use sketches shown in this article.
In an indirect water heater it is boiler water (not building potable water) circulating from and back to the heating boiler that heats a volume of water stored in the hot water tank.
See INDIRECT FIRED WATER HEATERS for details.
Our page top photo shows an Alliance indirect hot water heating system from Burnham. Look around the oil-fired hydronic heating boiler in the left of the photograph. This boiler is providing hot water that circulates through the heating coil inside the Alliance indirect water heater.
We found water around the floor of the heating boiler not at the hot water tank itself. What was going on? Leaks at the TPR valve were wetting the floor around the boiler. Several problems can cause pressure/temperature relief valve leaks. One of them is an internal or hidden leak in the indirect fired water heaters coil that resides inside of the hot water tank.
You can't see this leak but you can see its effects.
Building water supply pressures pressures inside the hot water tank, calorifier, cylinder or geyser (depending on where you live) are normally higher than pressures inside the heating boiler.
Therefore a leak in the heat exchanger coil used in an indirect water heater will normally send building water supply water (potable water) into the heating boiler and its piping system, entering at the coil leak and passing into the piping that connects the coil inlet and outlet to the heating boiler.
Thus pressures in the heating boiler will increase. This will show up as higher or abnormally high pressures on the heating boiler's pressure gauge, and the leakage will ultimately cause dripping or discharge at the boiler's temperature and pressure relief valve or TPR valve.
See RELIEF VALVE LEAKS
Watch out: under normal conditions, because building water supply pressures are above boiler pressures, an internal leak in the indirect water heater's heat exchanger coil will cause boiler pressures to rise. But there can be exceptions in the direction of water leakage, as we explain
at TANKLESS COIL LEAK DIRECTION IN or OUT
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