Weil-McLain Indirect Water Heater Problems
On this page (7 sections)
The Short Answer: A Weil-McLain Aqua Plus has no burner of its own; boiler water flowing through a coil heats the tank. When showers go cold but the boiler works, the boiler is usually not being asked to heat the tank (thermostat or wiring), boiler water is not reaching the coil (pump, zone valve, closed valve, or air), or a mixing valve is limiting the temperature.
How the system works
The Aqua Plus has a tank thermostat (or a sensor, on some boiler controls) mounted in a drywell on the side inspection plate. When the tank cools, the thermostat signals the boiler, which runs a circulator or opens a zone valve to send hot boiler water through the coil. Many boilers also give the water heater priority over home heating so the tank recovers quickly.
So a “water heater problem” can actually sit in four places: the tank thermostat, the boiler control, the circulator or zone valve, or the piping between them.
Narrow it down
| What you notice | Likely cause |
|---|---|
| Home heating works, tank never gets hot, boiler does not respond when hot water is used | Tank thermostat, wiring, or boiler’s domestic hot water setting |
| Boiler fires for hot water, but both coil pipes at the tank stay cool | Circulator, zone valve, or a closed isolation valve |
| Both coil pipes hot, tank water still cool | Air in the coil or poor heat transfer |
| Tank outlet is hot, taps lukewarm | Mixing valve set low or failed |
| Hot water runs out quickly | Boiler water temperature set low, recirculation line stripping heat, or demand above the tank’s capacity |
| No hot water and no home heat | Boiler problem, not the tank |
What you can check safely
- Check that the boiler has power and is not showing a fault on its display.
- Look at the tank thermostat dial on the Aqua Plus inspection plate cover and note its setting. Do not remove the cover.
- When hot water has been used and the tank should be calling, carefully feel the two boiler pipes at the tank (they can be very hot). Note whether they get warm.
- If you have a mixing valve on the hot outlet, check whether its setting was changed.
- Check that the boiler’s hot-water or “DHW” setting has not been turned off on its control, using the boiler owner’s manual.
Do not drain or open the boiler loop. Draining and refilling a hydronic system requires purging air and restoring pressure and treatment, which is heating-technician work.
What the technician will check
A heating technician will confirm the tank thermostat closes and signals the boiler, check the boiler’s domestic hot water settings and priority, verify the circulator or zone valve operates and its end switch works, purge air from the coil, and compare supply and return temperatures across the coil. They will also check the anode rod, which Weil-McLain says must be inspected at least annually (SB-1401 (opens in a new tab)).
The anode matters even when the tank is stainless
The Aqua Plus is a stainless steel tank, but Weil-McLain fits a magnesium anode to protect the weld joints. Its service bulletin warns that if the anode is fully depleted, corrosion can start on the thermostat drywell on the inspection plate and eventually cause a leak, and that this is not covered by warranty. If the tank is leaking, see Weil-McLain water heater leaking.
A rotten-egg smell in the hot water is usually the magnesium anode reacting with bacteria in the water, not a gas problem; see water heater smells like a dead animal.
Similar systems
Other indirect tanks work the same way, though their controls and parts differ: Buderus indirect problems and SuperStor indirect problems. If your hot water comes from a coil inside the boiler itself with no separate tank, see tankless coil water heater problems.
Frequently Asked Questions
Q: Why does my house heat work but I have no hot water? A: The boiler is fine, so the issue is the hot-water side: the tank thermostat not calling, the boiler’s hot-water setting, the circulator or zone valve for the tank, or air in the coil.
Q: Why does my hot water run out fast in winter? A: If the boiler gives the tank priority, it should recover quickly. If priority is disabled, or the boiler water temperature is set low, the tank recovers slowly while the house heating also demands heat.
Q: How often should the Aqua Plus anode be checked? A: Weil-McLain’s bulletin says at least annually. It also notes that very aggressive water erodes the anode rapidly, and that in that case the water should be tested against the limits in the Aqua Plus manual.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- Weil-McLain service bulletin SB-1401: Aqua Plus water heater maintenance, anode inspection (opens in a new tab)All Weil-McLain Aqua Plus indirect-fired water heaters.
- A. O. Smith technical bulletin 45: thermal expansion (opens in a new tab)Background on pressure rise in a closed water system. Pressure testing and repairs are professional work.
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