Apollo Water Heater Troubleshooting
The Short Answer: Apollo is not one machine. HydroHeat uses a household tank as the boiler for an air-handler coil. Apollo MVC is a mixing valve on someone else’s heater. Compact Apollo-badged electrics are 120- or 240-volt storage. Read the plate. A wet jacket is a replacement. Do not paste a Rheem flash chart onto a coil pump.
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Search Intent & The Real Problem
The invoice, the air handler, or a compact tank says Apollo. Showers died, or the air handler blows cold while the tank is hot. Forums invent a unique “Apollo 50-gallon.” I see three jobs: HydroHeat (pump + coil + a real water heater), an Apollo/Conbraco mixing valve on a Bradford White or Rheem, and a compact electric sold under the Apollo name. Wrong board, wrong element, a pump air-lock blamed on the tank. Same rule as an Airstream door: the rating plate picks the parts.
What other guides get wrong
They write one “Apollo water heater ritual” for a mixing valve, a hydronic coil, and a mini-tank. They tell you to raise the tank to 140°F to fix a mixer stuck on cold — that scalds when the cartridge later fails open. They skip the air-handler pump, so a hot 50-gallon gets condemned. And they still describe cracking a gas union — do not purge it.
Step-by-Step Troubleshooting
Step 1: Name the Apollo — coil, mixer, or tank
| What you are looking at | What failed when it is “cold” | Path |
|---|---|---|
| Air handler with a hot-water coil, pump, and a separate tank (HydroHeat / hydro-air) | Pump, air lock, coil, or the actual heater | Split house-heat vs DHW |
| Brass mixing valve stamped Apollo / Conbraco on the hot outlet | Mixer, pressure imbalance, tank actually fine | Step 4 |
| Compact electric storage, two doors or a 120 V plug | Breaker, ECO, element, thermostat | Step 3 |
| Gas tank with an Apollo sticker on a store-brand jacket | Standing pilot or LED valve — treat as the OEM | American-class hub if it is AO Smith hardware |
| Jacket wet, rust at the seam | Tank shell | Bottom leak |
- What to look for: Rating plate, pump, coil piping, mixing valve on the domestic hot. Photograph it. “Apollo” on a work order does not pick a 4500 W element.
- Required tools: Flashlight, phone camera.
Step 2: HydroHeat — is the tank hot, or is the coil dead?
- What to look for: Tank jacket hot, showers hot, air handler blowing cold = pump seized, air in the hydronic loop, dirty filter, or a relay that never starts the circulator. Tank cold, rooms cold, showers cold = the water heater is down (breaker, pilot, lockout). Tank hot, showers lukewarm, rooms hot = mixer or recirc, not the coil. Both coil pipes hot and the air still cold = airflow, not water.
- Required tools: Contact thermometer on coil supply vs return. Filter pulled on the air handler.
Purging a HydroHeat coil is a hydronic procedure at the bleed, not a domestic faucet. Do not dump boiler additive into potable. Cousin physics: tankless coil, Amtrol indirect.
Step 3: Compact electric Apollo — breaker, fill, ohms
- What to look for: Double-pole fully ON. Tank full. 1-reset ECO that returns is a grounded element or a browned thermostat — thermostat burn-up, reset loops. Wires lifted, power verified dead: a healthy 4500 W / 240 V element reads ~12–13 Ω. Infinite = open. Screw-to-tank continuity = grounded. 120 V mini-tanks read much higher ohms; use the wattage on that plate. A breaker that will not reset or trips after a few minutes is a short, not a bigger breaker. Holding a tripped breaker ON is how panels burn.
- Required tools: Multimeter, element wrench. No meter = hire it.
Never energize an empty electric tank. Point-of-use cousins: Powerstar, Lochinvar electric.
Step 4: Apollo mixing valve — lukewarm vs scald
- What to look for: 120°F at the tank, 105°F at the shower = mixer or crossover, not a dead heater. Pressure differential over about 30 psi hot vs cold makes some MVC valves hunt. Hot backing into the cold line is a missing check. Scald at one bath, tepid at another = cartridge, not “turn the tank up.” Same fixture logic as apartment not hot enough and Bradford White lukewarm.
- Required tools: Thermometer at tank-out vs mixed-out.
Do not exceed 120°F at fixtures without a working mixer. Legionella control at the tank and scald control at the tap are different jobs.
Step 5: Gas, T&P, and a tank that is actually done
- What to look for: Standing-pilot HydroHeat tanks die on thermocouple millivolts (~20–30 mV) or a packed FVIR screen — will not stay lit. Electronic valves: count that sticker, not a Rheem 5-flash. T&P dribble: static over ~80 psi or a waterlogged expansion tank — expansion full, T&P on a new heater. Typical relief 150 psi / 210°F. Never cap it. Water in the burner tray: shut gas; that tank is scrap. Connector drip is not the shell — connector.
- Required tools: Paper towel, hose-bib gauge. Manometer work is licensed: NG ~3.5–7 in. w.c. / LP ~10–11 in. w.c. — confirm the rating plate.
Need it off? Shutoff.
When DIY Ends: Calling a Licensed Pro
You may read the plate, feel whether the tank is hot, check a pump LED, measure mixed vs tank temperature, and ohm elements with power verified dead. Stop at gas fittings, a wet 240-volt bucket, hydronic purging you cannot do at a dedicated bleed, and any leaking shell. A licensed plumber or HVAC tech splits HydroHeat from DHW, gauges manifold pressure, and replaces the part that actually failed — the pump, the mixer, or the tank.
Frequently Asked Questions
Q: Why is my Apollo water heater not working? A: Either the tank is down (breaker, ECO, pilot, lockout), a HydroHeat pump or coil is air-bound, or an Apollo mixing valve is set or failed cold. Read the rating plate before you order parts.
Q: What is an Apollo HydroHeat system? A: A forced-air handler that uses a household water heater as its heat source. Hot tank plus cold air is usually the circulator or coil. Cold tank is the water heater itself.
Q: Can I turn an Apollo mixer up instead of fixing the heater? A: Not as a diagnosis. If the tank cannot make 120°F at the outlet, fix the tank. If it can, and the shower cannot, fix the mixer or a crossover. Cranking to 140°F scalds when the valve fails open.