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Digital thermostat on an incremetal PTAC (type K) with electromechanical contorl box (4 Posts)
Digital thermostat on an incremental PTAC (type K) with electromechanical control boxHi 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!
JulienThis post was edited by an admin on March 15, 2013 12:52 PM.
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.Jamie
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
Maybe not the best idea indeedHi 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
JulienThis post was edited by an admin on March 22, 2013 12:33 PM.
Your biggest problem isWho/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.