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Digital thermostat on an incremetal PTAC (type K) with electromechanical contorl box

Jmar Member Posts: 2
Hi everyone,

I have an "incremental PTAC unit", a McQuay type K (design A), with DX cooling and a steam heat coil.

In cooling mode, the compressor and the fan are controlled by the Tstat.


heating mode, only the blower fan is controlled by the Tstat. I do not

have a shut-off valve so the steam coil is energized even if the Tstat

doesn't call for heat.

The control box is currently unit-mounted.


like to install a programmable thermostat on it (a NEST or something else internet-enabled). A McQuay service rep

told me the current control box is electromechanical and that in order to

wire in a digital thermostat I would need a need new control box (~$500) with digital inputs. I am definitely not interested in paying $750 per PTAC for that.

I feel like there might be another way to do it myself but I am not very familiar with exactly how these things work.

I have attached the wiring diagram for my PTAC unit, as well as a couple of pictures of the PTAC and the control box.

Has anyone done something similar before? Can you give me some pointers?

Any insight or information you can provide would be very helpful.

Thanks a lot!



  • Jamie Hall
    Jamie Hall Member Posts: 16,532
    How are you with electronics?

    In principle, one could create a controller using the closed/open input from, for example, a Nest, to drive a relay which would substitute for the control thermostat in the existing control box.  You would, of course, need an outboard power supply for the Nest or what have you and the relays which you used.

    In practice, it might be a nice hobbyist project -- which would void any warranties on the equipment, of course, and make it quite difficult for anyone but the hobbyist to work on or troubleshoot.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Jmar
    Jmar Member Posts: 2
    edited March 2013
    Maybe not the best idea indeed

    Hi Jamie,

    Thanks for the answer.

    From the drawing, I was thrown off by the legend at the bottom that has a bunch of indications that the control box could be 24V:

    - "TB2, Terminal Board (24V), Control Box"

    - R1 to R7 are relays...

    But I couldn't find any of those guys on the drawing itself... Until I just realized all drawings for that size unit had the same legend whether they were electric heat, hot water heat or steam heat...

    It does seem like it would be possible but indeed a tad complicated and not necessarily aesthetically pleasing since the bunch of relays, the external power supply etc are likely to not fit in the existing cabinet.

    Plus, you are right saying that troubleshooting by a third party would be tough.

    I think I am going to drop this idea. Thanks a lot for your insight anyway.

    Out of curiosity, do you have any idea how long that would take to figure it out and how expensive it would be (thermostat excluded)?

    Edit: Looks like I would need 3 24VAC coil  to line voltage relays, a 24VAC

    wire-in transformer and some time to make it work. so about $80 + the Tstat +

    some time to make it work... Not worth it I think.

    Thanks a lot

  • unclejohn
    unclejohn Member Posts: 1,736
    Your biggest problem is

    Who/s gonna push the buttons and where are you gonna cram all the relays needed? You have to order the PTAC's for wall mount tstats if thats what you want to use. Sounds cheep to me for the whole new controller board. Any other way is a can of worms.
This discussion has been closed.