The AO Smith BTH-400A Water Heater

The AO Smith BTH-400A water heater is a member of the Cyclone Mxi modulating high efficiency commercial gas water heater family.

First, a quick note on how the AO Smith BTH-400A Water Heater achieves the remarkable Thermal Efficiency of 95%. By utilizing a Top mounted, Down-Fired, forced draft pre-mix burner and a Helical Heat Exchanger which is strategically positioned in the center of the Water Heater Tank, the swirling hot gases are forced thru the Heat Exchanger and radiate heat to the surrounding water. The Spiral Heat Exchanger keeps the hot combustion gases in the tank longer which creates a longer heat transfer cycle.

Modulation

The BTH-400A Water Heater is capable of modulating it’s firing rate. The on board Central Control Board (CCB) monitors several sensors and regulates the firing frequency when there is a call for hot water. It monitors the water temperature Set Point and the water temperature Differential Point. Example: The water temperature set point is 160 degrees F. That is the target water temperature. The water temperature Differential Point is the number degrees you allow the water temperature to drop before the unit starts another heat cycle. Let’s say you have set the water temperature Differential to 150 degrees F. You have a 10-degree F difference. The water heater will fire at 150 degrees and the heating cycle will continue until the water temperature Set Point of 160 degrees F is reached, at which time the heating cycle will cease. The BTH-400A water heater can be set to a maximum of 20-degree differential.

Benefits of the Cyclone Mxi Modulating water heaters:

  1. Higher Thermal Efficiency. Modulation increases the amount of time the hot combustions gases are in the heat exchanger. This results in fuel savings.
  2. Longer water heater tank life. Modulation decreases the firing rates and heating cycles thereby extending the life of the tank.

Benefits of the Cyclone Mxi Modulating water heaters:

  1. Higher Thermal Efficiency. Modulation increases the amount of time the hot combustions gases are in the heat exchanger. This results in fuel savings.
  2. Longer water heater tank life. Modulation decreases the firing rates and heating cycles thereby extending the life of the tank.

Features of the AO Smith Bth-300A water heater:

  • 119 Gallon tank
  • 399,900 BTU input per Hour
  • Recovery capacity at a 100-degree rise is 460 Gallons Per Hour
  • 75-3/4” tall x 33-1/8” diameter.
  • The side water inlet connection is 1-1/2” NPT.
  • The top water outlet connection is 1-1/2” NPT.
  • The Gas inlet is 1-1/2” NPT.
  • ¾” NPT Recirculation Return
  • The AO Smith BTH-400A Water Heater is available in either Natural Gas or Propane.
  • 3-Year Limited Tank and 1-Year Limited Parts Warranty
  • Electrical characteristics: 120 volts AC, 60-HZ
  • ENERGY STAR Qualified
  • Advanced Diagnostics with large LCD Display. It utilizes a Central Control Board (CCB) to set, monitor and regulate all functions of the water heater. It also displays the run history of the water heater.
  • Temperature control from 90 degrees F to 180 degrees F.
  • iCOMM Compatible: Can be monitored from remote locations.
  • Standard model certified from sea level to 10,100’ elevation.
  • Maximum Hydrostatic Working Pressure: 160 psi
  • Utilizes two Non-Sacrificial Powered Anode Rod
  • Easy to Remove Top Cover for access to serviceable parts.
  • 0” installation clearances on sides and rear, 1-1/2” installation clearance for top.
  • Hand-hole cleanout allows for easy interior tank cleaning.
  • Approved for installation on combustible floor.

Mechanical Venting

  • Conventional Power Venting or direct Venting
  • Vents Vertically or thru a sidewall
  • Vents with PVC Schedule 40 Intake and exhaust pipe.
  • Also approved for CPVC Schedule 40, Polypropylene and AL29-4C stainless Steel.
  • Direct-Vent intake and exhaust can terminate separately outside building or through a single opening using Concentric Vent Kit.
  • CSA certified and ASME rated T&P Relief Valve
  • Complies with SCAQMD Rule 1146.2 and other Air Quality Management District’s low-NOx Emissions

Sequence of Operation of the AO Smith Cyclone Mxi Commercial Gas Water Heaters.

  • Step 1. The CCB monitors two Temperature Probes to determine existing water temperature and compares that finding to the Set Point and the Differential. If the water temperature has reached the differential setting, a heating cycle will be initiated. Once the Set Point water temperature has been reached the heating cycle is terminated. But before the heating cycle is initiated, the position of 4 pressure switches will be checked by the CCB.

What do I mean by “Position”? Pressure switches come in either a normally open or normally closed position. All pressure switches have a diaphragm that either opens or closes an electrical switch when the activation pressure is met. So if you have a Normally Open pressure switch when the activation pressure is applied, the pressure switch will close. So let’s take a look at the 4 pressure switches the Central Control Board checks in order.

  • Step 2. Low Gas Pressure Switch

The Low Gas Pressure Switch, which is Normally Open, is the first pressure switch the CCB checks. When the minimum gas pressure is applied this switch closes. The minimum gas pressure for a particular model of AO Smith commercial gas water heater is listed on the data plate. If the gas pressure drops below the minimum listed gas pressure, a Low Pressure Gas Error Message will appear on the LCD screen. If the actual gas pressure is correct you may need to replace the pressure switch. This switch should remain closed for the life of the heater.

The Minimum Natural Gas Supply Pressure is 4.8” W.C.
The Minimum Propane Supply Pressure is 8.5”.
The next pressure switch to be checked by the CCB is the:

Step 3. Blocked Inlet Switch

The Blocked Inlet Switch is Normally Closed that will open if the negative pressure in the inlet pipe is too high.  The next pressure switch to be checked is the:

Step 4: Blocked Exhaust Pressure Switch

The Blocked Exhaust Pressure Switch is Normally Closed that will open only if the positive pressure in the exhaust pipe is too high.
Note: All 3 of these Pressure Switches should stay closed.
The next and final pressure switch is the:

Step 5: Blower Prover Pressure Switch

The Blower Prover Pressure Switch is Normally Open until it senses increased air pressure from the blower housing. Once power is supplied to the blower, you will see a symbol of a fan blade on the LCD screen. If the switch closes you will see a check mark which “proves” the blower is working. Note: If the CCB senses the Blower Prover Switch is Closed before the blower is powered up, you will receive a Blower Prover Failure Message on the LCD display. If the Blower Prover Switch remains in the Open position, the LCD screen will display a “Blower Prover Open” message. You will probably need to replace the pressure switch.

Note: If you have the series 300 or newer BTH-400A water heater, the Blower Prover Switch has been replaced with a Speed Reference Feedback that tells the Central Control Board if the Blower is operating.

Step 6. Hot Surface Igniter

If the Low Gas Pressure Switch, the Blocked Inlet Pressure switch, the Blocked Exhaust pressure switch and the Blower Prover Pressure Switch all close, the next step is to power the Hot Surface Igniter. As current is supplied to the Hot Surface Igniter, you will see a lightning bolt on the LCD display. If the igniter is good you will have an amp draw of approximately 2 amps or higher and a check mark will appear next to the lightning bolt.

Note: If you have an earlier series dating before the 300 series AO Smith Cyclone Mxi water heaters, you will have a Hot Surface Igniter. Due to its fragile nature, AO Smith replaced the Hot Surface Igniter with the Spark Igniter in the newer models, which is very similar to the spark plug in a car. The control system energizes the Spark Ignition control with 120 VAC during the ignition period. High Voltage then energizes the Spark Igniter and ignites the air/gas mixture.

If all the above steps are successful, the next step is…

Step7. Opening the Gas Valve

Displayed on the LCD will be a gas valve symbol which will turn from the off position to the on position. The Flame Sensor must detect ignition within 3 seconds or the gas valve will close and the blower motor will continue to run for 30 seconds to purge any unburnt gases. If this continues for 3 cycles, then you will have displayed an Ignition Failure Message.

You now have Air, Fuel and Spark and the initiation of the heating cycle of the water heater. The Heating Cycle will continue until the Set Point has been reached, then the blower will run for 30 seconds to purge any un-burnt gases and the unit will shut off. 

AO Smith has created excellent resources including a 15-minute video on the sequence of operation for the Cyclone Mxi 300 series water heaters and a 20-minute video showing how to use the control display at here.

To Order a natural gas AO Smith BTH-400A water heater: Stock Item

To Order Propane Gas (LP) AO Smith BTH-400A water heater: Special order, please allow 3-4 weeks.

Includes Free Standard Shipping! (On Water Heaters shipped within the Mainland Continental United States)

To view or purchase the BTH-250A Commercial Water Heater in either Natural Gas or Propane or view all our AO Smith Commercial Electric Water Heaters, AO Smith Commercial Gas Water Heaters and AO Smith Parts or Rheem Commercial Electric Water Heaters or Rheem Commercial Gas Water Heaters, please visit Wallingford Sales or the Water Heater Store.

I wish you much success!
“Keeping you in hot water…so you stay out of hot water!”
Doug Wallingford
Wallingford Sales
Water Heater Store