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PTAC On-Off is in the Breaker Box...

12K views 36 replies 5 participants last post by  biggles 
#1 · (Edited)
thank you for your help.

My 1-bdr apartment is in a building constructed in 1968. It has three through-wall type A/C and heating units. Two of these PTACs work fine. The one in the living room apparently has a broken controller, and it can only be turned on and off -- at the circuit breaker box (!). When you flip the breaker the living room unit comes on full blast. Depending on the season it blows hot or cold.

The problem is, in order to shut off the LR A/C (or heat), you have to shut off all power to the living room and one wall of the bedroom. It was like this when I bought the apartment, though I didn't discover it until after the closing. I suspect that maybe the broken electromechanical thermostatic control has been shunted, so that it is "always on". The dial freewheels.

It is probably impossible to find a replacement for the original 1968 controller dial/switch. I have been offered an electronic controller for $750, which seems to me excessive.

What I have in mind is to drill a hole and install a simple on/off rocker switch in or on the PTAC unit. That way I could leave the power on at the circuit breaker, and turn the PTAC on or off as needed. I am an electronic technican -- I can solder, drill and make basic measurements at home.

Is this a realistic idea? I understand PTACs run on 220, but at the level of the controller, I am hoping there is a low voltage/amperage circuit that can be interrupted with a simple rocker switch?

Alternatively, maybe there is a "universal" board that would control the PTAC electronically, e.g., with thermistors and logic instead of an electromechanical thermostat. Or maybe there is someplace I could buy an antique thermostatic control.

Thank you for your insights.

John

An update.
Poking around on the net for an electronic thermostat. Here is one example.

http://www.qkits.com/moreinfo.cfm/VM148

It apparently has a relay closure that will handle 3A. It is powered by a 12V wall wort. Anyone experimented with devices of this type? Could it replace the defunct thermostatic switch?
 
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#25 · (Edited)
Here is the wiring diagram.

The wiring of the real unit has been changed in a few details. It looks like the Klixon wiring has been modified, and there seems to be a jumper from one side of the dual thermostat switch to the other. The relay, which probably ought to be NO or NC, is neither. I isolated the contacts and I read 5000 ohms across them. Neither open nor closed.

I can provide more detail on the problem spots but at the moment I am just trying to figure out how the manufacturer intended for the system to work.

For example, the thermostat. It is a dual line-voltage thermostat with a capillary bulb. In effect it has two switch "channels," and they are labeled on the unit as "1" and "2". One is heating and one is cooling.

The diagram implies that an SPDT Klixon thermal switch "decides" which channel, 1 or 2, should be active.

My basic question is, which channel number is heat, 1 or 2? Put another way which channel is cool, 1 or 2?

Thank you for your insights,

John
 

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#26 · (Edited)
Here is a redrawn version of the schematic with some wiring detail filled in, such as the rotary switch positions and the coil of the relay.

Inside the thermostat "channels" are two thermal switches, one each, one NC and one NO. These have not been drawn.

It appears that even when the fan speed control is set to OFF, the relay can (nevertheless) turn the fan on at its HIGH speed winding, if the thermostat calls for this with a thermal switch closure on Channel 2.

Still not quite clear to me what the 1 and 2 mean, exactly, at the thermostat. It seems to be a standard or convention, though, because the numbers are big and embossed on the unit.

John
 

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#27 · (Edited)
the klixon contols the 115Vvoltage (1) to the thermostats (R)s depending on the pipe water temp thats what swings that (6-11) going to the stat.noww the stat inturn with its dial set and the bulb sensing air return swings the 115V to the fan selector or the relay to control fan LO heat or HI cool speeds...4 and 5 are the closure relay contacts for LO fan ... 1 and 3 are the coil in the relay
 
#28 · (Edited)
Is the relay is for the heating cycle only? Still not clear to me which side of the dual thermostat (1 or 2, upper or lower) is for Cooling and which is for Heating.

Seems to me from the diagram that maybe the relay closure energizes the Red, or HI line of the motor? A puzzle, this thing. Maybe I have misunderstood the rotary switch. The three lines for HI, MED and LO are each assigned to a switch position. The only one left for the relay is the OFF position.
 
#30 ·
There is on my unit a jumper wire that crosses the thermostat from the heating side to the cooling side. I drew it into the schematic with a curved line to draw attention to it.

I first thought the jumper was part of the hack job, but now I think maybe it is part of the manufactured product. It looks manufactured.

It seems to me that for the relay to close, ever, there has to be power on the wiper of the rotary switch. With the jumper, the wiper has power summer and winter. The jumper does not seem to otherwise affect the operation of the heating channel.

John
 

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#32 · (Edited)
At the moment the rotary switch in the control box is bypassed. The blower fan is wired direct from the junction box.

I found a replacement rotary switch, made by the same manufacturer, also intended for A/C, but I noticed there is a difference in the specs.

The old rotary switch was rated for 15A at 120V. The candidate replacement switch is rated at 10A at 125V.

So the amperage rating is lower. On the other hand, the motor in my unit is 1/10 hp, and the new switch is supposed to handle 1/4 hp.

My shaded pole motor draws 1.55 A, according to the plate.

The relay in the system (which draws current though the switch) is rated at 8A.

Two of these Fan Coil Heating/Cooling units (living room and bedroom) plus the wall outlets on one LR wall and one BR wall, are all wired to a single 15A breaker in breaker box.

Is it okay to install the new 10A rotary switch in place of the old 15A rotary switch?

In other words, what does the amperage rating on a switch really mean? It is the typical usage current? Or is it the "Never Exceed" amperage?

Thank you for your insights.

John
 
#34 ·
Thanks Biggles, that's good.

I would like to install the new rotary switch for motor speed control only (Off, High, Normal and Low) and leave everything else out of the circuit.

The wires from the switch are already color coded (Red, Blue, Black for Hi, Normal and Low), so this should be a straightforward wiring job.

The motor return (white) is already wired to normal in the junction box.

At present the "hot" wire is connected directly to the red wire from the motor. I would undo this (hacked) connection. Then I would connect the red line from the motor to the red line from the rotary switch. The hot line would then be connected to the switch wiper.

This should give me control of the fan on/off (and speeds) at the unit, instead of at the breaker box.

Then, over time, I can remove, study and troubleshoot the thermostat, the relay, and the Klixon.

Thank you again for your excellent analysis and help on this problem.

John
 
#35 ·
anytime...your definitly on top of it with your info here...just to verify the hot in the cooling mode goes to the fan speed selector switch and feeds the selected speed and in the heating mode seems it is a dedicated speed with that relay because the hot to the selector is aced thru the stat and klixon swing into heating pipe temp....if you already realized this just ignore my babbling...:wink:
 
#36 ·
Yes, I understand from the wiring diagram that the cooling and heating modes are intended to be different.

But I suspect the Klixon may be fused in one position or the other, and I know from measurement with the VOM that the relay shows an odd value (5000 ohms) across the contacts. Seems to me it should Closed or Open, not something in between. Net of it: I suspect both the Klixon and the relay are bad. I want to cut them both (and the thermostat) out of the circuit.

What I have in mind is a simple fan-speed rotary switch circuit that works in the same way summer and winter.

Basically that's what I have right now, in the hacked system. I flip on the breaker, and the fan runs full blast -- summer or winter.

By installing the new rotary switch I hope to 1) move the on/off to the unit, so that I don't have to keep visiting the breaker box and 2) add the refinement of High, Normal and Low fan speeds.

Would this create a problem, in winter operation for example?

Thank you for your insights.

John
 
#37 ·
the Kloixon swings the 115Vs to the stat and then the stat swings it to the relay/heating fan speed or selector/cooling...the action the klixon does allows the stat to change from summer/winter the klixon is the pipe stat and the unit stat is your control.might consider removing the relay and give yourself all the fan speeds in the heating mode just like cooling....let the unit mounted stat control cycle the fan in heating or cooling at any speed....sounds easy from hear typing away:huh:
 
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