Run Away Heat in a Roof Top Unit


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.

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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16 Responses to Run Away Heat in a Roof Top Unit

  1. mike says:

    great blog by the way, will be saved in my favourites..thanks

  2. Angel bermudez says:

    Working at school with 20 plus sunline rooftop units from the 90’s any info on this unit will be very helpful.
    Now I have a unit that work fine when I reset it but once it reaches room temp , and call for heat inducer run but it won’t ignite

  3. bobby fitzgerald says:

    Thank u this was perfect helped me alot

  4. Thanks for this. We have a new York rooftop unit but the same thing is happening. When I turn off the power and back on it runs okay for a while but occasionally will lock and either run hot air all the time or will run the blower/fan and blow in cold air. Will test based on your blog post.

  5. Silas G says:

    thanks in advance i have this york roof top DH090N15 the belt was broken i replaced but now soons i open the power it try to run heat even the thermostat its calling for ac (exhaust blower and fan motor runs but not heat), i disconnect all the wires on the board but it still run the fan and exchausr blower by itself. thank you

  6. Luke says:

    Thanks for this blog! This blog is the only one site that I can find some clue about York rooftop units. I finally found the limit switch by removing the whole top cover of the unit. A 3-way or three terminal switch (165F to 65F range) in series with another limit switch? I removed it and put ohm meter across the terminals and it was showing open. shouldn’t this be normally closed so that it should be showing low resistance like 0 to 10 ohms? I ordered the same part from “source 1” and it was showing the same condition, OPEN across the terminal. When I put a jumper across the terminal, furnas starts right up but runs continuously (burner, draft blower, main fan) even the call for heat is off from the thermostat. Any advise will be grateful.

    • the 3 terminal limit has a set of normally open and normally closed contacts. the “W” from the board (call for herat comes in to a terminal and that goes out on the normally closed terminal to the Aux limit and on to the ignition control. Coming in to the 3rd terminal is a “R” from the trannsformer. If the limit trips, it opens “W” to the aux limit terminal and then closes “R” to “W” to create a “backfeed to the control board to keep the fan running. If the limit trips and does not open “W” to the aux limit, you get the run away heat scenario. The aux limit is manual reset so if this were to happen, that would trip due to overheating and shut the unit down. You need to check out which wires come from “W” & “R” of the board and make sure they are on the correct terminals of the 3 terminal limit control. Then you can ohm out the switch, and the normally closed contacts should be between “W” & Aux limit. You need to go one step further and make sure when the limit trips it closes “R” to “W” and not “R” to Aux limit.

    • luke says:

      Thanks for very helpful reply. I did get the three-way limit switch replaced and it worked just fine! until the inducer motor bearing went out. I got the motor replaced and it was working for few weeks and now the runaway heat started again which indicates to me that the main controller wasn’t controlling properly that the heat was probably cycling on the limits. My question is there is another limit switch in series with the 3 way limit switch. Is this the manual reset switch you described above that must be reset manually?

      • the very old units did not have the manual reset limit in series with the 3 wire limit. However, if you ordered parts from a york Distributor, the replacement limit would have included the 3 wire limit, the manual reset limit all mounted on a bracket and pre-wired. If you replaced and properly wired the 3 wire limit, chances of that causing “run away heat” again are remote but could happen. Have you checked your gas pressure, both inlet and manifold? what is your temperature rise? To high could cause the unit to cycle on limit. Is the gas valve functioning properly?

  7. luke says:

    Thank you for your quick response! No I did not check the gas pressure because I did have the ignition and was running properly for few weeks and assumed the gas side was working properly. When it went to the runaway situation, room thermostat was reading 90+F and still running when it was set at 65F.(The heat exchanger compartment was probably a lot higher and the limit switch(the one I replaced – Source 1 setting was stamped @165F). When I replaced it, I took a picture of how the wiring was situated, but since the colour coding was black, white, and red, I thought it was fairly simple, but I may have crossed two black wires. When I go up the roof this weekend, I’ll check them both. (Chicago wind +50 miles/hour and temperature treacherous)! Thanks again for your insights.

  8. Andy Villalva says:

    I have a 7 1/2 ton Predator , 3phase HEAT PUMP, rooftop packaged unit with “run away heat” that stays on even with tstat wires disconnected. No electric heat. Flash Code 9. Why does the heat stay on? I see no limit switches just low & high pressure switches. Maybe defrost board?

    • Temperature Limit
      If the UCB senses zero volts from the high temperature limit,
      the indoor blower motor is immediately energized.
      This limit is monitored regardless of unit operation status, i.e.
      the limit is monitored at all times.
      If the temperature limit opens three times within one hour, it will
      lock-on the indoor blower motor.
      Limit Switch (LS)
      This control is located inside the heater compartment and is set
      to open at the temperature indicated in the Electric Heat Limit
      Setting Table 21. It resets automatically. The limit switch
      operates when a high temperature condition, caused by
      inadequate supply air flow occurs, thus shutting down the
      heater and energizing the blower.
      Electric back up heat energized by W2 tstat call.

  9. luis says:

    What’s the correct o normally for air 20 ton YORK also 15 TON , HEAT FAN ON DELAY AND HEAT FAN OFF DELAY TO WORK GOOD ? Always one circuit of compressor is out and my solution is down the breaker circuit and up . But then I have the same problem in the long time

    • Heat o /off delay is for your heating side of the unit. Has nothing to do with cooling. Heat on/off should be set to help maintain proper temperature rise and to eliminate “cold blow” at the start and end of a cycle. In some cases, if you are experiencing limit trips set the “on” to start sooner and the “off” to the longest time.

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