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John Harris 06-06-2011 11:06 AM

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


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

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?

Marty S. 06-06-2011 04:50 PM

Honestly as cheap as the things are I wouldn't mess with a 43 year old one at all . A brand new one is under $500 if you DIY install.

biggles 06-07-2011 07:02 AM

PTACS and window units are mostly 115V breaks on the fans and compressors so that amperage is what breaks thru the stat.check this site out for a replacement stat that will mount in the PTAC sensing capillary bulb across the return/filter grill and your good to go.there should be a volt/amp rating on the old switch keep it around or above that number.....even if you see somebody throwing out an AC unit clip the stat from it note: manufacturers don't control with 24v unless it is a solid state board mounted transformer especially back in 68'.

John Harris 06-07-2011 11:41 AM

candidate controls
Thank you for your ideas on this. The building is 43 years old, but I think the PTAC is probably more recent. It works fine. In summer it blows very cold, and in winter it blows hot. It is the controller, on one of 3 PTAC units, that seems to be out.

I looked for PTAC thermostatic controls with bulb sensors on ebay and found a couple. Is this the general type of unit that could work?

Many thanks for you counsel on this. I now think maybe I might be able to get at the real problem, which looks like a permanently open circuit in the thermostatic control. A replacement thermostatic control would sure be a better solution than a workaround.


biggles 06-07-2011 11:54 AM

if its not that old might be just the board because you said it cool and heats in the related seasons so its not the stat if it is swinging the modes just doesn't turn off so i take it it over cools and over heats or does..if you have solid state within the PTAC it might be a relay on the board that is welded shut the loads for the relays on any ss board are the supply fan and the compressor...and with PTACs the condenser fan and the evap fan are the same off a double shafted the unit water source or air cooled with a condenser coil need to know....:wink: so i take it the fan/compressor run all the time if you don't touch the breaker:huh:i've been doing HVAC 30yrs and hate PTACs they are so limited in applications with hi end residential installs

John Harris 06-07-2011 02:34 PM

Yes it overheats or overcools. It just runs headlong in either mode unless it is turned off by throwing the breaker.

I will take some photos, study the components, and also see if I can sketch out a schematic wiring diagram or at least a block diagram. I think it is pre-solid-state but will check.

It is possible the system has been jimmied or hacked in some way -- I thought maybe someone had deliberately bypassed the controller to produce the "Always ON" condition. I will also look for a relay. It makes excellent sense that relay contacts might have fused together.

Thank you again for your help.


biggles 06-07-2011 04:57 PM

sure it could be rigged but the different cycles heating and cooling are you switching them or does it just do it a heat pump if it is the 3-way valve is the cycle difference in the seasons

John Harris 06-07-2011 07:40 PM

The changeover from cold to hot is either automatic or something accomplished remotely by the building supervisor.

From my point of view it just happens, seasonally. All 3 PTAC units make the change from cold to hot and back without my flipping switches.

I have tried to discuss the problem unit with the building supervisor but he won't talk about it. Thinks it's funny. I don't think he knows much about HVAC.


biggles 06-08-2011 11:36 AM

it sounds like you being sent chilled water in the summer and hot water in the winter and the fan is the cycle for the setpoint which says this is going to be easy...just break the fan with a genetric stat with that cap tube bulb across the filter rack just slid it into the filter medium or thrre is a plastic slip bracked or it might mount on the fan squirrel housing....can't see a building controlling it electroniclly is over when the seasons chage you might notice wow:huh: theres no heat this morningFALL or man its hot this morning spring :mad: thats the building changing over pumps and systems from hot to cold or visa versa.... just tell me you don't hear compressors running on those PTACS if you do then thats a different animal and i will elaborate on those types..will check back :thumbup: do you see a control valve of some type on the water lines coming into each unit?

John Harris 06-08-2011 05:01 PM

Many thanks, this is encouraging.

It makes excellent sense. In the spring and fall the building managment distributes a memo announcing the change-over date.

If it is just a fan, and not a fan+compressor rig, then it is probably not that much of a current draw. The circuit breaker is not big and impressive. And part of the problem is that this single ordinary breaker has been assigned the A/C plus the bedroom and living room wall outlets. When you flip off the A/C at the breaker, the lights go out too.

I think it must be a fan coil only, running on 110 VAC.

I am traveling but when I get back I will take off the housing and document the unit with my camera, and note the wiring, valves, sensors and so on.

Thank you again for your patient and systematic analysis of this problem.


biggles 06-08-2011 05:58 PM

that amperage for a PTAC unit might in the 10ths .9A even if 1.2A is still breakable with a window unit stat if somebbody dumps one this summer...those usually pull the compressor and fan to cycle...we'll go with that thought and keep watch for the fix here:wink: that 3 units on one breaker seal the deal plus some lights compressor'd ptacs would never be wired stacked up...100w light draws .4 amps so they are definitly a low draw style...just a fan cycle is almost minimal with a temp control and 3-way valve on the inlet piping is a little finer controlling temp'll get it??? having dedicated CB panel in each apt/condo the building doesn't care how long you run the fan $$$ there chiller is what they pay for keeping 45F water supplied to the PTACs and 160F hot water off the boilers...hi-rise commercial office buildings run this way

John Harris 06-11-2011 08:54 AM

Troubleshooting question
1 Attachment(s)
The problem is that the unit cannot be turned off. When the motor speed switch at the unit is set to OFF, the motor continues to run full blast. The blower keeps running at full speed unless power is cut at the circuit breaker box in the next room.

My question is this: If the rotary speed control switch on the unit is set to OFF, can it be over-ridden by the thermostat or the relay? Or is it more likely the switch itself is shorted?

The unit:
This is a York/Borg Warner FC4V. It is a fan coil unit. There is no compressor. Hot or chilled water for the coil is supplied by the building, which is a high-rise condo built in 1968.

According to the plate the system runs on 115V and draws 1.55 Amperes. The fan is driven by a shaded pole induction motor.

The system has 4 basic components:
1) the motor
2) an electromechanical thermostat that can be dialed Warmer or Colder. No numbers.
3) a rotary switch with 4 positions: OFF, LO, NORMAL, HIGH
4) a relay

In addition there may be an electrically driven water valve or sensor.

Problems and troubleshooting clues.
1) Rotary Switch: The rotary switch shaft has two flats, but the plastic dial has been stripped out and turns freely without engaging the shaft. Someone tried to repair this by epoxying the dial to the shaft, but it didn’t help.

However, when you turn the rotary switch shaft with a pair of pliers, it clicks solidly through each of its four detent positions: OFF, HIGH, NORMAL, LO. This has no effect whatsoever on the unit’s operation. It is always ON and it always runs on HIGH.

When you open the unit and study the control box, the Rotary Switch looks intact. By this I mean I could not see any evidence that the switch had been by-passed or shunted out of the circuit or hacked – or even touched.

2) Relay: The relay has been messed with. Someone disconnected a wire – and left it disconnected. I did not re-connect it because I could not immediately guess the wire’s purpose. I have since figured out the wire is one end of the relay coil. The coil connector could not “vibrate loose”. It must have been disconnected by a troubleshooter working in a methodical way – probably a long time ago.

3) Thermostat. No modifications, has probably never been replaced.

How does it work?
According to the internet, the speed control rotary switch selectively taps into points along a series string of inductors. I think the OFF position should open the circuit. It does not. So maybe the switch has shorted?

The relay was suspect and it makes sense that the contacts might be welded together. Not clear to me how the relay is wired into the circuit, however – or what its purpose is. If welded contacts mean “Always ON” then the relay circuit would have to bypass the rotary OFF switch. Seems kind of unlikely to me, but I don’t have a schematic.

So that’s where I am. I can replace the relay for about $5, and the rotary switch for as little as $9. The thermostat I have not researched. However, I would like to fully understand this problem before I start changing out parts.

Best guess at the moment.
If the system is turned OFF at the rotary switch, I don’t see how the thermostat or the relay could over-ride the rotary switch and turn the unit ON. This seems to suggest the rotary switch is shorted.

Does this make sense?

Thank you for your ideas and insights.

John Harris

biggles 06-11-2011 10:27 AM

the speed selector should change the motor speeds seems its wired for hi as you guess that relay might be for a low speed more common on the heating side during the winter hi always with cooling....but the noise at night people alway tweek it down...whats up with the other 2 are they intacted and doing speed changes with the dials.unless you see a unit mounted water valve within the cabinet then were back to that stat controlling the fan cuycle at setpoint.the stat works with 12o'clock position being the "universal:yes:world" and plant 68F setting...with no numbers CCW for colder longer fan...CW for hotter longer fan depending on the building changeover.NOTE to you once they change over there is no changing back in the mechanical rooms(on that water hot/cold) until the season comes around...the only place you'll feel hot water in the summer is your faucets...:wink: eye ball the other 2 units and lets hear back on valve or not on the water line into the unit.that long copper bulb on the fan shroud is the stat sensing point...the selector isn't wired or the motor was changed a while bach and is one speed your close....if you don't have a water control valve take the exsisting stat and break the motor 115V hot thru there and you have the cycle forget the speed dial...

John Harris 06-11-2011 11:07 AM

3 Attachment(s)
Re the other 2 units, the rotary switches do work -- or anyway, they have less pesky problems. In both units, the OFF position works, and at least two other positions (HI and LO) also work. In one of the units, when you switch to NORMAL, the fan stops.

Attached are a three pictures:

The first shot is the relay. I dismounted it to take the photo. The shiny brass terminal on the relay is one of the coil terminals -- somebody, long before me, unhooked the connector and wire.

Second is a shot of thermostat, which is mounted at the bottom of the box.

Third is the waterworks end of the unit. It has two hand valves and what looks to me like one electrically driven valve. Although it might be a heat sensor instead.

Best, John

biggles 06-11-2011 11:22 AM

OK now for the final synopsiiiiiiis<<<<tech spelling it:thumbsup: you have no control water valve just shut offs then the stat control the fan either HI in the summer or low in the winter that sensor mounted on the pipe is the inlet water cool/heat supply is for the heating side when you turn the unit fan on in the inter the fan willln't come on till the pipe heats up to the stamped temp on that see it with a mag glass140/160/170..depends but totlly for heating fan control so you don't blow cold air right out untill that heats up.if the building runs there heating pumps off the boiler 24/7 it is always closed so fan is constant until that return capillary feels the setting temp over the filters....that relay looks like an add on but if you mirror the wiring on the best of the 2 working even into that selector fan switch you should have 3 working units....for now if you want disconnect the stat wires and break the fan wire with it...that will control the cycle...but eyeball the other 2 before touching the the other 2 have that add on relay..if not the selector might be shot in the problem unit and that is why the relay is there to split the LO HI.NOTE lo is always for heating takes air longer to gain heat so it is slower over the coil...cooling the want as many air changes over a coil is the summer to pull that heat of of the room air...makes sense...right:wink: will check back

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