I am considering installing the following thermostat in my system, but
before purchasing and installing it, I want to understand the wiring.
I believe that I understand the theory of operation of the controls system.
My question is in regard to the particular design of one of the components.
Here is the question:
Is a backfeed from the COOL-H to the HEAT-H fan terminals on the furnace
control board a concern?
Here are the gory details if you need them to answer the question:
Here is the (relevant) equipment currently installed. Note that the furnace
control board is the one that comes with the Trane unit.
Furnace Controls (White-Rodgers):
Thermostat under consideration is a White-Rodgers 1f95-391
I have downloaded and reviewed the detailed thermostat installation
instructions, and note the following:
"If you have a single stage compressor system see the diagram below. A relay
(customer provided) should be installed as shown in Fig. 5 to switch the
fan speed to the next lower speed on a call for dehumidification from the
thermostat. The reduction in air flow allows the coil to remove more
humidity from the air."
The fan in this system is a 4-speed fan, currently wired to the control
board as follows:
HIGH - Black (Connected to COOL-H terminal on the board)
MED.-HIGH - Blue (Connected to HEAT-H terminal on the board)
MED.-LOW - Yellow (Connected to "Park" (i.e., not connected))
LOW - Red (Connected to "Park" (i.e., not connected))
Whether dehumidifying or cooling, the thermostat tells the furnace board
(and compressor) that the AC should be running. There is no difference in
operation from the point of view of the furnace board and compressor.
However, during a call for dehumidification, another output from the
thermostat "DHM" is energized. The purpose of this output is to energize
the coil of a relay. This relay acts as a fan speed selector, downstream
of the furnace control board. The furnace control board does not know
about this relay.
The recommended wiring diagram shows the contacts of the relay wired as
Common: COOL-H output from furnace control board
N.C.: HIGH speed blower wire (what was connected to COOL-H before)
N.O.: MED-HIGH blower wire (blower speed 1 step down from normal COOL speed)
At this point, I note that the MED-HIGH blower wire is already used (it's
connected to HEAT-H output). Presumably, this is the correct speed
selection, determined by the installer. So, I plan to continue to use this
fan speed when heating.
If I blindly follow the instructions supplied by the thermostat
manufacturer, I would:
Wire the MED-HIGH (Blue) lead to both the N.O. contact of the relay while
leaving it connected to the HEAT-H output of the control board.
This means that when the system is operating the compressor and fan on a
dehumidification call, the furnace control board will energize the COOL-H
terminal. This terminal will be connected to the common of an SPDT relay.
The relay coil will be energized and the N.O. contact of the relay will be
connected to the MED-HIGH (BLUE) blower motor lead. As stated earlier,
this BLUE lead will also be connected to the HEAT-H terminal of the control
Therefore, the COOL-H terminal of the furnace control board will be
energized and connected to the HEAT-H terminal.
If both the COOL-H and HEAT-H relay contacts and not wired to anything else,
backfeed is a non-issue. If I knew this was the case, I would use a SPDT
relay and not overcomplicate the system.
But I do not know this is the case, and don't want to let the smoke out of
the control board!
So, again, the question in 4 parts:
Has anybody done such an install?
Did you run into this problem (Backfeeding from COOL-H to HEAT-H)?
Is this an issue, or is it safe to ignore the condition?
How did you solve the issue?