Results 1 to 8 of 8
  1. #1
    Join Date
    Mar 2008
    Location
    Portland, OR
    Posts
    9
    Post Likes

    McQuay RTU/MAU furnace package heat fail - HELP!

    We have a 20 year old McQuay RPS115CSA rooftop unit/make-up air unit that serves the kitchen at the facility where I work as a building engineer. It uses a McQuay SuperMod gas furnace package with a Honeywell RM7895A controller that has a R7847A1033 flame rectification module. The furnace package uses a large burner gun assembly that fires on low as a "pilot" and ramps up to full fire (2 million btu) with 6 gas vales piped in parallel. We use Alerton Envision as our BAS. All this stuff is very old and in need of replacement but it's my job to keep it limping along for the next couple years before replacement RTUs and BAS can be budgeted and installed. I have been fighting this unit for some time and I could use some outside help.

    Starting from square one, with the burner control NOT in alarm, the unit will fire and run just fine. When the kitchen reaches set temp the burner shuts off and apparently won't fire again. The unit is regularly faulting out on "flame failure" alarm in our BAS. I've done my best to make sure the sparker and flame rod are clean and adjusted correctly. I even replaced the flame sensor & wire. All connections appear to be tight and when the unit fires initially I get +/-1.8vdc from the flame rod (rectification module needs >1.25vdc for flame proving). I can even turn the furnace control power off and back on and it almost always refires. It just seems to fail every time it tries to refire after satisfying the temp setting for the space (running for 15-20 min), this happens a lot since the unit is bringing in 100% outside air. I'm almost to the point of replacing the burner control module again but I hate just throwing parts at a problem when I don't know the cause.

    As a former HVAC service tech and engineer at another facility I have a lot of experience with gas heating but I have no experience with this kind of McQuay furnace unit. Any and all help appreciated!
    Attached Images Attached Images  

  2. #2
    Join Date
    Aug 2002
    Location
    Southold, NY
    Posts
    25,680
    Post Likes
    What is it shutting off on? If a safety then there's an issue.

    When was the last time it was serviced?

  3. #3
    Join Date
    Mar 2008
    Location
    Portland, OR
    Posts
    9
    Post Likes
    Thread Starter
    I don't know the last time it was serviced. I've serviced what I can but have not removed the entire burner assembly since I've been at this job (March 2019). All of the other maintenance items in the furnace package maintenance manual have been verified. I have observed the unit shut down on flame failure (could be multiple causes). I cleaned & adjusted the flame rod and installed a new flame sensor wire. The spark electrode was also giving me issues so I cleaned and adjusted that. When I'm at the unit (when there's a call for heat and it's in alarm) I reset the alarm, the combustion blower turns on, the combustion air damper actuator goes through it's open-close process, and then the burner fires in pilot mode, flame sensor reads +-1.8vdc at the flame rectification module, and then the main burner fires. I can observe the burner/flame through a small window in the burner box. When it fires the flame looks good, nice and blue and no lifting. The supply air fan runs the whole time. If it's left to run it will do so until the space is satisfied, then it will shut down (main blower continues to run since it's a MAU) and then it will fault out on flame failure when there's another call for heat. The only limit that is manual reset is the ignition controller, all other limits are auto reset.

  4. #4
    Join Date
    Aug 2002
    Location
    Southold, NY
    Posts
    25,680
    Post Likes
    Looks good is not even close! You need someone on site with the proper test equipment!

  5. #5
    Join Date
    May 2004
    Location
    SE Pennsylvania
    Posts
    1,903
    Post Likes
    Those are a pain. Go through every step of the start up instructions as if it never ran.

  6. #6
    Join Date
    Oct 2011
    Location
    Chicagoland Area
    Posts
    8,149
    Post Likes
    Your local supply house that sells the flame safeguards should have a way to bench test the RM7800 chassis and the amplifier card. There is also a display available for the chassis that will give you a more specific cause of failure. If the bench test shows one or the other is bad, great. If not, you need to go back to the unit to narrow it down.
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  7. #7
    Join Date
    Nov 2011
    Location
    East Side
    Posts
    7,059
    Post Likes
    1.8 VDC is kinda low. I would expect to see 2-4.

    Have you cleaned all the grounds? Might try a test and run a temporary ground from the flame sense assembly to the 7800 ground.

  8. #8
    Join Date
    Jul 2019
    Location
    Twin Cites MN
    Posts
    45
    Post Likes
    X2. I have ran grounds directly to a rod in the flame which helps on some. I like it to be above 3 on main flame. Below that they seem to be flaky.

    I made a burner control tester. Transformer, base. 2 solenoids, and a pilot. Found some flaky controls and cards we with it.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •