Hello: I have a two story addition on my house built by the previous owner. There is one thermostat upstairs and one thermostat downstairs. The system directs air to the downstairs through a damper. The upstairs thermostat controls the temperature and the downstairs one appears only to turn on a fan.

The system stopped working and in my infinite wisdom I started trouble shooting, disconnecting some wires. I'm almost done but have the following situation.

The downstairs thermostat has a Red (24v) White (Common) and Blue (Fan) wires connected to it. I disconnected the blue wire both at the thermostat and at the 24 volt transformer and now don't know how to reconnect it.

My question is: Where should the Blue wire be connected at the 24 volt transformer? At the hot (24 volt side) or at the Common side?thanks in advance.

Richard

The blue wire will not go to your transformer at all it will go to a relay or fan center if you take that blue wire and touch it to the red one a relay will close and the fan should come on. On most residential houses there is no common to the thermostat r is hot g is fan y is cooling , if you have a damper r would nomallly be power [hot] the other two would either power the damper closed are open

Hi:
Thanks for your reply>
I'm afraid I don't understand your answer.
There is a "C" terminal on Thermostats which is the comon side of the 24 volt transformer.

The Blue wire is so located that there is no place for it to be connected except to one or the other sides of the 24 volt transformer.

Richard

It is hard to say because you are in a realm of unstandardized wiring, the installer knew what he was doing and only with a blue print would I advise you on what to do.

Actually I would go as far as to say if this is a zone system, there should be a zone control board to control the dampers and HVAC system. I hate those hacks that try to skirt the additional cost of doing it right when they damper different zones.

You do have a zone system from your description. Some of the older zone boards told the blower when to come on(it controls the blower). The master T-stat was the only stat that had the G circuit hooked up to control the blower. You may be in luck if you have a three zone system and only using two zones. If zone two was bad, move the T-stat wires and zone damper wires to zone three. It's a good chance it will work.

Thanks fellows for you insights.

T-Top your comment is a good one but unfortunately it is a two zone system.

Checking the control board the master T-stat has one G wire and from what I know does control the blower and damper.

At the control board terminal strip there are two cables connected. One cable has 5 wires going from the terminal strip to the master T-stat. The other cable has only two wires, Red and White - the Red wire is inserted into the Y slot and the White wire into the C slot.

I thought this cable went to the downstairs T-stat but now I'm not sure because at the downstairs T-stat the Red wire is in the R slot and a White wire in the C slot. And I think the Blue wire is to be inserted into the G slot.
Does it make sense that the Red wire at Y on the control board terminal strip would go to the R slot on the downstairs T-stat terminal strip?
Also, at the 24 volt xformer there is the cable with Red, White and Blue wires that I thought went to the downstairs T-stat. The Red wire is connected to the 24 volt side of the xformer and the White wire to the common side of the xformer. Thses wires are the ones which raided the question about where should the Blue wire be connected.

However, since the cable at the control board terminal strip has a Red wire conneced to the Y slot I am in total confusion. Is it possible these are not the same cable? I will try to determine if they are the same cable.

Please let me also ask a few more questions:

1. At the downstairs T-stat there is a jumper between Y and W. Anyone have any idea what this is for?

2. If the downstairs T-stat doesn't turn the fan on and off what is its purpose?
Is it only a thermometer?

3. If the upstairs T-stat is Master and controls the fan and damper how does it know how much to open the damper to cool the downstairs if the downstairs T-stat is only a thermometer.

3. When I check the upstairs T-stat I get 24 volts from Red to Green and 0 volts common to Green. Does this imply that the Blue wire to the downstairs T-stat should be connected to 24 volts at the xformer and to the Fan connection at the downstairs T-stat?

I need to do more checking and if you bear with me I'll be back with more information.

Thanks for all your help.

Richard

If you smell smoke you know you wired it wrong. You have a much more complex system than the standard residential heat system. Without being there it is near impossible to know the layout of the original system. Most were laid out on a if come on basis. That is if the thermostat called it would come on and by pass the setting of the other thermostat. The bypass would also activate a relay and run the blower motor weather or not the main stat is calling for heat or cool.

I would draw a complete map of the entire system before you start to pull wires off here and there other wise you could get into some trouble.

Richard, You are a brave soul to try to diagnose the problem, however, your system is a "field constructed" control circuit. Typically, systems have their own transformers and thermostats. You seem to have a modulating control that positions dampers by changes to a temperature sensor. May I politely suggest that you call a licensed air conditioning contractor and explain the circumstances. They all seem to have at least one technician able to solve your problems. PS... blue wires are generally used to tie in all of the controls to the secondary side of the transformer. Red is the input side that leads to the various control relays or contactors through switching devices, i.e. auto/on, cooling, heating, emergency heat, and LED indicators. If you have lights on the thermostat, the blue is usually the secondary side (L2) that power the lights... If there are no lights or indicators, then everything will work from the red through green (fan) yellow (first stage heat or cooling) orange (reversing valve if a hat pump).

Hello:
Well, I was able to talk to the contractor that installed the air conditioner and he cleared up all my questions.

FYI: The upstairs T-stat is the master controller. The downstairs T-stat controls only the opening and closing of the Damper depending upon the temperature in the room and the T-stat setting.

The jumper on the downstairs T-stat is also connected to the white wire that goes to the Damper and allows this T-stat to open/close the Damper regardless whether it is calling for heating or cooling. I had wondered if this T-stat could be replaced with an On/Off switch and the contactor said it could. A switch would greatly simply the understanding of what this T-stat does.

The second cable at the control board terminal strip (Y and C connection) controls the compressor.

I can now relax a little as this has been a very, very frustrating experience particularly because I did not understand the operation of the downstairs T-stat.

Thank you for all your help.

Regards,
Richard