Excessive water temperature is usually the result of stacking within the water
heater. Stacking is the occurrence of various water temperatures layering within
the water heater with the hottest water in the uppermost layer. This layering or
stacking effect typically occurs during small draws of hot water (typically less
than 25% of the storage capacity) which are long enough to create a call for heat
on the thermostat, but are short enough not to deplete the stored energy within
the tank. Excessive stacking can occur when frequent short to moderate draws
are taken in quick succession. During this type of situation, the temperature of
the domestic water can approach the temperature of the boiler water.
REMEMBER: All water heaters (direct and indirect) will stack.
Reducing stacking within the tank (Non-Warranty)
- Reduce the boiler operating temperature to 160ºF - 170ºF. This will limit the maximum
domestic outlet water temperature during high stacking water usage.
- Raise the thermostat sensing bulb higher in the drywell. This will reduce the frequency of
thermostat calls for heat during small draws of hot water. However, it will reduce the
quantity of available hot water during a deep draw by delaying the call for heat to the
boiler.
- Reverse the boiler side piping. The older installations have the boiler supply at the top of
the water heater and return at the bottom. Reversing the piping, supply at the bottom and
return at the top will:
a) Result in lower tank stacking by having the hottest boiler water closer to the cooler
incoming domestic water.
b) Reduce the effect of “thermal overshoot” after the thermostat call for heat has been
satisfied.
c) Provide a more uniform hot water delivery temperature during moderate to deep
draws (25% to 100% of the tank’s storage capacity). Reversing the boiler piping will
not affect the performance of the water heater.
Install thermostatic mixing valve
- Installation of a thermostatic mixing valve will provide an uniform delivery temperature
with minimal regard to water usage.
Recirculation Loop
- Installation of a properly sized recirculation loop not only provides prompt delivery of hot
water, but it provides circulation and mixing of the water within the tank.
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