Precedent 6 - 10 Ton Modulating Heat Info
Sequence of Operation from Service Facts:
During heating operation, the control will monitor the space temperature and space heating setpoint and with a PI control algorithm determine if active heating capacity is required. Once active heating is required, the control will go through an ignition sequence to start the heater. As the space temperature deviates from the space heating setpoint, the unit controller will create an active discharge air heating setpoint that the modulating heat output will be controlled to meet. The control modulating heat output operational range is determined by active supply airflow measurements. Once space temperature conditions are satisfied, the control will go through a shutdown sequence to properly shut down the heater.
On single-zone VAV units, the supply fan will operate at minimum speed until the control determines that a discharge air temperature greater than the user selected discharge air heating maximum setpoint (DA Heat on the RTOM) is required. At this point, the fan speed will increase as necessary up to 100% to meet the space demand. On multi-zone VAV units, the control will monitor the duct static pressure and duct static pressure setpoint to determine the supply fan speed with a PI control algorithm.
Sequence of Operation from the Reliatel Diagnostic Manual:
When there is a call for heat, ReliaTel™ will send a command to the ignition module to fire and open the modulating valve to 100% (10.0Vdc) while also commanding the gas inducer motor to High speed and controlling the fan speed to normal algorithm control.
After operating at this condition for the 60 second startup period, the modulating heat control will be released to normal algorithm control and the valve will begin modulating in the Modulating Gas Heat Output Operating Range between the unit preferred Min-Fire voltage and the unit Max-Fire voltage.
The Modulating Gas Heat Output Operating Range is determined by the Supply Airflow measurement.
To keep the heat exchangers from getting too hot or cold, the supply fan airflow is calculated by measuring the differential pressure between the fresh air section and a piezo-electric transducer / airflow ring (0”-5” iwc Kavlico transducer) on the supply fan discharge orifice.
The resulting airflow will then be standardized to get CFM/Ton and used to determine the Modulating Gas Heat Output Operating Range for the valve operation according to the charts below.
Gas Pressure:
The minimum gas supply pressure for modulating models should be 5.5" w.c. Manifold pressure for modulating heaters is 3.5" w.c. at high fire, 0.7" w.c. Low
Transducer Troubleshooting:
RTVM Connections…Green Wire – J8-6, Black Wire – J8-7, Red Wire – J4-3
With the supply fan off and the unit powered up:
- Measure DC voltage from the green wire to the red wire at the transducer. It should be 5 VDC. If not, power is not being sent to the transducer.
- Measure DC voltage from the green wire to the black wire at the transducer. It should be 0.25VDC. If not, unplug both air tubes from the transducer. If 0.25VDC is now seen, a tube is clogged. If 0.25VDC is still not seen, there could be a short in the wiring or the transducer may have failed.
- With the tubes connected to the transducer and the supply fan running, read the DC voltage from the green wire to the black wire at the RTVM, as the pressure increases, the voltage increases.
Gas Heat Diagnostics:
- Supply Air Flow Sensor Failure - Heat will be discontinued
- Gas Heat Supply Airflow Low Limit - If the Supply Airflow is less than 175CFM/Ton for more than 5 minutes (the 5 minute timer will only reset for loss of active heating call or if the airflow increases above 175CFM/Ton) - Heat will be discontinued, Manual Lockout Diagnostic will be set.
- 6 ton Low limit 1050 CFM’s – approximately .35 VDC
- 7 ton Low Limit is 1225 CFM’s – approximately .40 VDC
- 8.5 ton Low Limit is 1487 CFM’s – approximately .45 VDC
- 10 ton Low Limit is 1750 CFM’s – approximately .60 VDC
- Refer to the Transducer Voltage / CFM Chart on Page 2 for associated voltages.
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.