TM
Installation & Service Manual
11 Operating information
General
How the water heater operates
The Shield water heater combines an advanced stainless steel heat exchanger, electronic control module, and tank that allows fully condensing operation. The blower pulls in air and pushes flue products out of the heat exchanger and flue piping. The control module regulates blower speed to control the water heater firing rate. The gas valve senses the amount of air flowing into the water heater and allows only the right amount of gas to flow. An internal pump circulates the water between the tank and heater and mixes the incoming water to provide maximum efficiency.
Temperature control
Modulation
The Shield water heater is capable of modulating its firing rate from a minimum of 20% to a maximum of 100%. The firing rate is dictated by the hot water draw and various other temperature limitations.
Night setback
The controller may be programmed to reduce the tank set point during a certain time each day. A start and stop time can be programmed for each day of the week.
Flame current support
To prevent nuisance shutdowns when the water heater is firing at minimum rates, the control will increase fan speed when the flame signal drops too low.
Protection features
Outlet temperature, flue temperature, and temperature rise limiting
The heat exchanger outlet temperature is monitored by the heat exchanger outlet temperature sensor. When the heat exchanger outlet temperature exceeds 190°F, the unit will reduce the fan speed. If the heat exchanger outlet water temperature exceeds 198°F the control will shut the unit down until it cools off.
The control module monitors the flue temperature by a sensor located in the flue exhaust. If the flue temperature exceeds 215°F the control will reduce the maximum fan speed. If the flue temperature exceeds 240°F the control will shut the unit down. The unit will restart automatically once the flue temperature drops 25°F and the minimum off time has expired.
The control monitors the temperature difference between the heat exchanger inlet and the heat exchanger outlet sensors. If this difference exceeds 25°F the control will reduce the fan speed. If the temperature difference exceeds 30°F the control will shut the unit down. The unit will restart automatically once the temperature difference has dropped below 25°F and the minimum off time has expired.
Freeze protection
DO NOT install the water heater in a room likely to freeze.
The following integral feature of the electronic control module provides some protection for the water heater.
• The electronic control module provides
•Below 45°F, the internal pump operates constantly.
•Below 37°F, the burner turns on.
•The burner and pump will turn off if water heater water temperature rises above 45°F.
CAUTION This feature of the electronic control module does not eliminate the possibility of freezing.
The installation must still use recognized design, installation and maintenance practice to prevent freeze potential for the water heater.
Monitor external limits
Connections are provided on the connection board for external limits. The electronic control module will shut off the burner and inhibit relighting whenever an external limit opens.
Runtime and alarm outputs
The water heater provides dry contacts for indicating when the water heater is running, and when it is unable to operate.
Error logging
The control will hold in memory the last 10 faults. The date and time of the occurrence will be recorded as well. Only the 10 most current occurrences will be held in memory.
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