Indirect Water Heater Not Heating or Running Out Fast

Does the tank call, and does the boiler answer?

The tank’s call. On the Alliance SL, U.S. Boiler says the thermostat’s light is green when powered, blinks while it calls for heat, and stays solid when it is satisfied. Weil-McLain’s Aqua Plus control can show its setting, or OFF.

What should happen next. Weil-McLain’s checklist asks three things during the tank’s call: does the boiler’s pump start, does the zone valve open, and does the boiler fire? U.S. Boiler says the thermostat calls when the tank falls below its setting, so run the hot water until the tank calls, then watch for each one.

If one does not. Peerless’s table gives a boiler that does not run, with the main service switch and fused disconnect to check. For a pump that does not run, it says check its power supply and switching relay, and for a zone valve that does not open, its power supply. Utica adds checking the fuses and resetting a tripped breaker, and HTP says a zone valve that will not open can be opened by hand or replaced. Each part has its own guide: a boiler not working, a boiler circulator pump not working and a zone valve not working.

The tank thermostat setting

Where it starts. U.S. Boiler sets the Alliance SL’s thermostat to 120 degrees at the factory, Utica sets the H2O’s to 125, and Weil-McLain gives 120 as a typical starting point on the Aqua Plus. Peerless sets the Partner’s thermostat to its lowest at the factory, and says the boiler’s limit must be at least 20 degrees above the tank setting.

Too low. HTP, Peerless and Utica all list a tank thermostat set too low for no hot water or not enough, and Weil-McLain says a setting too low may not give the boiler the chance to heat the whole tank. Test the water at the nearest tap, as Weil-McLain and U.S. Boiler both say.

Raising it safely. Weil-McLain says a mixing valve should be fitted on an Aqua Plus set at 125 degrees or higher, and U.S. Boiler says not to go above 140 on the Alliance SL without a properly installed mixing valve. Peerless says the Consumer Product Safety Commission and some states recommend settings of 130 degrees or less, and that going higher increases the scald risk. More on settings: water heater temperature.

A failed thermostat. Peerless and Utica list a faulty tank thermostat, replaced. Weil-McLain’s test for one needs a meter on its terminals, a technician’s job.

The boiler's temperature and priority

The boiler set too low. HTP lists a boiler control set too low for no hot water and a boiler temperature too low for not enough, both fixed by raising it. Weil-McLain’s Aqua Plus manual says, if possible, raise the boiler’s operating temperature to 180 or 200 degrees, and U.S. Boiler says the boiler’s high limit must be set no higher than 200 for the Alliance SL.

Priority. Peerless and Utica list an undersized boiler with no priority to hot water, fixed by rewiring for priority. HTP’s sign of it is the heat and the tank coming on together, fixed by checking the wiring or setting the tank as priority over the heating. On Weil-McLain’s Ultra Series 4 boiler, with the tank wired as priority 1, its control turns the space heating off while it heats the tank by default.

After the summer. Weil-McLain says to keep a minimal temperature in the boiler outside the heating season, as a cold boiler after a long standby may absorb the tank’s stored heat at the start of a call.

More hot water than the tank can give

Peak use. Peerless and Utica list a peak draw bigger than the tank holds: work out the peak use and compare it with the tank’s volume. HTP lists hot tubs, spas and multiple showers as high demand, and a boiler or tank sized too small, and Utica’s answer to too much demand is reducing it or a larger boiler and or water heater.

Time the shower. Weil-McLain asks whether a tank was sized for 2.0 gallon a minute shower heads when the real heads use 5.0, and says use a bucket and a stopwatch to find out. Time how long the shower takes to fill the bucket to a gallon mark. Weil-McLain turns gallons a minute into gallons an hour by multiplying by 60.

What changed. Weil-McLain says a remodeled bathroom with a new whirlpool tub will change the demand a great deal. For demand that is too great, HTP’s fix is a mixing valve and or a flow restricting valve, with the tank and boiler temperatures raised.

Air, scale and a cold loop

Air. HTP and Peerless list air trapped in the loop from the boiler to the tank, fixed by purging it. Weil-McLain’s Aqua Plus manual says an air bound water heater or boiler will not circulate the water properly, so heat is not passed to the tank.

Scale on the coil. Utica says that if the boiler, pump and thermostat all work and the boiler cycles on its high limit several times before the tank is satisfied, the coil may have a coating of sediment or scale, and the coil is cleaned. Utica also lists a boiler that cycles more than 5 times a day in summer, from heavy demand, a faulty thermostat, the boiler’s high limit set too low, or scale on the coil. Weil-McLain’s Aqua Plus manual says a dirty boiler system can leave deposits on the tank’s coil that insulate it, and it cleans the boiler system the boiler maker’s way.

Heat drawn back out. Weil-McLain’s Aqua Plus manual says lab tests showed boiler piping can act as a thermal siphon during long standby and draw heat out of the tank, so it puts a zone valve, check valve or heat trap close to the tank and insulates the piping.

A broken dip tube. On its Gold and Ultra Plus tanks, Weil-McLain says a broken or melted dip tube sends the cold water across the top of the tank and short cycles the heater.

Too hot, or the relief valve opening

Too hot. Peerless lists a tank thermostat set too high, lowered, then wrong plumbing or wiring, or a faulty thermostat. Weil-McLain says water too hot is usually stacking: it typically happens with small draws, leaving layers in the tank with the hottest at the top. Water far too hot is the emergency in the rules above.

The relief valve. HTP lists three causes of its T and P valve opening: the tank too hot, water expanding as it heats, which an expansion tank deals with, and the supply pressure too high. Utica says never try to repair the valve, and that one that discharges again and again can be thermal expansion or a faulty valve. More in a water heater relief valve leaking.