Every year about this time I receive calls about older packaged roof top units that the heat will not shut off even when the thermostat wires are removed. This occurs mostly on the Sunline roof top units and is usually a result of the unit cycling on limit because no one has changed the filters since back in the fall (along with a few other reasons).
In all of the older Sunline units, there is an automatic reset 3-wire limit switch. This switch has a set of normally closed and a set of normally open contacts in the switch (the unit may or may not have a manual reset high limit depending on age). The way the switch functions is the “W” call comes into the switch and is normally closed to the wire going to the ignition control. When the limit “trips”, these contacts open and shut the unit down.
But, also on the switch is a set of normally open contacts and an “R” from the transformer comes into this third terminal.. When the limit trips, it should open the normally closed contacts between “W” and “B”and close the normally open contacts between “R” and “W”creating a “back feed” through the “W” wire to keep the blower and inducer relays energized to keep the blower and inducer running to cool down the heat exchanger. If for some reason, the normally open contacts close but the normally closed contacts do not open, the “R” from the transformer not only keeps the blower and inducer running, but it also send continuous 24 volts to the ignition control through the centrifugal and roll out switches keeping the heat on constantly.
To fix this, the limit should be replaced. If you do replace the limit switch, of course air flow, dirty filters, temperature rise, manifold pressure, etc should all be checked to see why the unit has been cycling on the limit switch so much to cause it to fail in the first place. One other item to check is how long does it take to turn on the blower when a call for heat is present? The fan is controlled by a time delay relay and if this relay becomes weak or “slow”, the blower may be taking too long to turn on the fan and, as a result, the limit switch trips. Normally, the fan on these units should turn on at a maximum of 60 seconds. If it is taking longer, this could account for the limit tripping frequently and causing it to eventually fail and possibly causing a run away heat situation.
Another thing to check would be on D7 and newer small Sunline units is the IR or RW1 (ice-cube) relay. This is an isolating relay just for the heating. When the “W” call comes into the board, it send out 24 volts to the coil of the relay. When the relay energizes, it closes it’s normally open contacts and sends 24 volts to the heating section of the unit. If this relay sticks closed , it is sending a constant 24 volts to the ignition control and, again you have run away heat.
There are, of course, other things that could cause run away heat, like a stuck open gas valve or shorted wires. If it is not a mechanical/electic short failure, then either the 3-wire limit switch or the IR or RW1 relay may be the problem. Remember to do a complete diagnostic to make sure you not only corrected the active problem, but have found the cause for it occurring in the first place.
Hopefully, this information will help get you off the roof quicker and help you solve the problem of run away heat in a Sunline roof top unit.