I have been trying to connect a new honeywell rth8500 to a Trane unit (believe it is a heat pump). The Trane unit is currently hooked to a Trane thermostat with only 4 wires. They are set as follows

Black - Zone
Red - Common
Blue - Cool setpoint
Green - Fan sw

The old Trane thermostat has only numbers displayed. I cannot figure out how to properly connect these wires to my honeywell thermostat. Can anyone assist in the wiringÉ

Go to the other end ,in the furnace/airhandler. Write down what each tstat wire color is attached to,, whatever its attached to should be labeled

Get back to us with info

The information I posted in the question is the info from the unit itself. There was no labels on the terminals in the existing thermostat.


Black - Zone
Red - Com
Blue - Csp
Green - Fan

Let me also clarify that I live in an apartment building where the unit is mounted in the ceiling. I was trying to find some useful product identification but there was only a ridiculously long model number that did not resolve to anything in Google. The existing thermostat that I am trying to replace has a temperature dial and a switch with the options off/auto/low/medium/high.

It's not going to happen. This information is why: http://www.trane.com/CPS/Uploads/UserFiles/Controls/Sensors/BAS-PRC029-EN.pdf

There is a variable resistor which Csp is the cool set point in the stat. The Common is a signal common. Zone is the zone temperature which also uses a thermister and Fan is the Fan speed selected by different resistors.

It looks like some systems have an occupancy override.

Although, I could probably do it, it would not be easy. The steps might be something like:

Existing stat must stay in place. Off must never be used. It would measure the temperature and select the fan speed, so Common and Zone remain connected to the old stat.

The tstat control would be modified to include a 24 vac relay and 2 DPDT relays. Each of the two DPDT relays would be activated in the Fan and Cool positions.

In the FAN position, it would select say Roff , the resistor necessary for off and allow the fan terminal to continue though in the Fan position.

and Rcool and Rcoo-off, a resistor necessary for cool. These resitors would have to be found. Could use something like the resistor for 60 F for Rcool and the Resistor corresponding to 85 F for Rcool-off.

So, dooable, but not without a fair bit of work like mount a big plastic enclosure (something like 2" x 6" x 8"L on the wall with two stats side by side and somehow get power to it.

The box would contain a transformer, 2 relaysand a couple of resistors.

Looks like the handler is only used for cooling unless heating and cooling modes are set by the apartment. Tis would complicate the design. I didn't think about that one.

I thought about it a bit more and:

Assumptions:
Cool only
You can rrun an extra cable
24 VAC is available in the air handler

Another suggestion

Move the relays and resistors to the air handler.
Mount all parts on a DIN rail.

Make a nice stained or painted plate to mount two thermostats side by side. It would be nice if the old stat was mounted in low voltage wall plate

Have two cables back to the air handler. One for the old stat and one for the new one.

Put all the parts on a DIN rail. A few resistors and relays and DIN terminals. Have terminals for both stats and air handler.

A DIN rail is a T shaped rail that's used as an electrical erector set. Relays, resistors and terminals are used to make a circuit.
It's a re-useable relatively expensive method that's easy to wire and troubleshoot.

It could work. Maybe $100 in parts.

KISS has me so confused,, I guess I`m reading the original question wrong.sounds awful complicated for a 4 wire thermostat

If your trying to replace a four wire zone sensor its not going to work with a normal t-stat.

wmproop:

Read the link in post #5. The thermostat in the link has the same pin labels and it's a Trane, so I suppose it's similar, if not identical.

If you look at the internal schematic, the thermostat consists of a bunch of resistors, a few switches and a temperature dependent resistor.

There are NO RELAYS in this thermostat, hence it's not a conventional or heat pump thermostat.

1st clue that something is different. Multiple fan speeds selected at the thermostat. This you will find on FAN COIL units. This is not something we see here. I've seen it once on this forum.

FAN COILS typically are just that. A fan and 1 or two coils and a few valves. One heating coil filled with hot water and one cooling coil filed with cold water. So, it's a chilled and hot water system where boilers and chillers are involved and would be typical for a hotel situation.

In one system, there is only one coil and the heating and cooling mode is selected by the loop temperature at the central plant.

The FAN COIL units usually operate in one of two ways.

#1: The building selects the mode.
#2: The mode is selected at the thermostat

This may be a fan coil or it may not be. There was no selection of heat or cool, so I'm assuming cool only. It could well be a FAN COIL cool only system.

It's NOT a 4 wire residential thermostat.

The variable resistor in the stat is likely part of a resistance bridge and thus the UNIT can determine how many degrees it's away from setpoint. The stat could be used for heating and cooling if the mode is set PLANT wide. e.g. A high temperature loop or a low temperature loop.

Usually there are a couple more sensors in a FAN COIL stat or the loop temperatures, so it may not be a FAN COIL either.

So, my guess is it's a FAN COIL thermostat with a single water loop and the heating and cooling mode is set for the entire complex based on season.

Doing both heating and cooling would complicate the design a bit to use a conventional thermostat. Having B and O available at the same time might make it easier. I'm not thinking about it for now.

For fun, look here: http://www.iaqsource.com/product.php?p=honeywell_tb8575a1000&product=172896

KISS is correct. The breaker for the unit is marked fan coil. I guess if I still want to use a programmable thermostat I need to determine how many pipes the unit has and who has control over heat and cool. I am not even sure yet since I just moved in.

Programmable FAN COIL stats seem to be non-existant.

How do you pay for your HVAC? Setback is usually used for energy savings, but can be used for comfort sleeping.

That seems to be a TRANE zone control which means it's specific to TRANE. The one I found only had COOL. You seemed to indicate that the stat had no HEAT/COOL/ positions.

The stat I found which is not necessarily yours, looks like it can be used for both heating and cooling provided the mode is set by the central plant which can also be determined by the temperature of the pipe(s).

Low temp - cool. High temp heat.

A hotel stat tends to work as follows. There is a minimum or maximum temperature set and occupancy sensors allow the user setpoints.

A condo stat, may allow constant temperature all of the time.

The FAN COIL stat's have some built in smarts, so it doesn't blow hot air if set to heat and the coil is in cold mode.

But, it looks as if it's part of a zoning panel.

Check the thermostat for a HEAT/COOL/OFF mode.

Try to get heat in the summer by setting the mode or setting the setpoint.

Based on what I see, I think it's possible to use the original and a programmable stat.

Another option is to use two of the zone stats and connect to a 7 day timer, so you have a choice of 2 setpoints.

So, again:
Figure out if you can get heat in the summer.

How do you pay for heating and cooling?

What purpose are you trying to achieve with a programmable thermostat?



Probably 30 years ago, I did a setback system in an apartment. It wasn't smart, but it required no modifications. I took a timer and attached it to a bakeilite box with a dimmer switch mounted in it. On the short edge I put a screen and a resistor inside controlled by the lamp dimmer. It faked the stat out during the day from the extra heat from the resistor. It worked well enough.

I found this article useful as well:

Troubleshooting Trane air conditioning controls without Trane's manual? (http://www.air-conditioning-and-refrigeration-guide.com/troubleshooting-trane-air-conditioning-controls.html)

There is an absence of Hsp which suggests it's non-heating.

I'm still curious how heating and cooling are done and if there are heat/cool labels on the control.