Detroit Radiant Products Company QTD Series - Manual (Page 39)

QTD Series
39
5.0 Maintenance • Troubleshooting Guide
If heater does not enter high fire mode, check the following:
Is there 24VAC across
the red wire on the
relay board and ground
on the circuit board?
Check for 24VAC across the
COM and HIGH on the 24VAC
terminal. Is there 24VAC?
No
Yes
Repair or replace faulty
wiring or thermostat
Replace
relay.
Replace
gas valve.
Yes
No
NOTE: To confirm the heater is not in high-fire mode, check the manifold pressure (3.5 inches W.C. natural gas or 10
inches W.C. propane gas). If the indicator light is not illuminated, it is faulty and should be replaced. If the manifold
pressure ranges from 2.3 to 2.8 inches W.C. natural gas (model dependent) and 6.0 to 7.5 inches W.C. propane gas
(model dependent), the heater is in low fire mode and the troubleshooting steps described below should be followed.
Check to make sure gas pressure is
within minimum and maximum inputs as
indicated on the heater’s rating plate.
Is gas pressure OK?
Check to make sure
gas pressure is within
minimum and maximum
inputs, as indicated
Yes Replace gas valve.
No
Correct problem.
Is the heater properly grounded?
Is the heater’s polarity correct?
No
Yes
With microampmeter, check DC
ampperage at flame rods.
Is it 1.0 microamps? (See illustration)
No
Check to make sure both flame
sensor wires are OK and then
replace circuit board.
One or both of the flame sensing rods are faulty or flame is
weak. Check to make sure heater is operating at proper gas
pressure as indicated on the heater’s rating plate.
If needed, replace one or both flame sensing rods.
Yes
Pressure switch may be faulty or there
is a restriction in the exhaust. Remove
restriction or replace pressure switch.
Yes
No
Correct
problem.