heat-pumphvacthermostatwiring
I cannot figure out how to wire my 30+ year old AC to my lux dmh110a thermostat, I do not have the old thermostat to reference.
It seems to be 4 wires coming out of the air handler and to the thermostat Blue, Red, Orange, and White.
I have been looking at the schematic but doesn't seem to correlate to anything that works.
Here are some pictures of how it is wired at the air handler.
Thanks for any help.
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This is a Heat Pump wiring diagram, if you only have an AC unit then the AC unit outdoors will only require the Yellow and the Common wire as far as the 24 volt control circuits go. Common is the side of the 24 volt control circuit that every 24 volt circuit returns to, to complete the circuit, this is why it is called Common, cause every circuit terminates there. Every 24 volt device has a 24 volt hot leg and a Common, the AC unit will have the Yellow 24 volt hot leg of power and the Common side of the 24 volt control power.
The furnace or air handler has low voltage terminals R/ red, W/ white, Y/ yellow, G/ green and C/ common. The circuit board will have them if no low voltage terminal strip is seen. Follow the 24 volt wiring to see where the terminals are, the pics you gave show nothing of any use, its hiding the components, google 24 volt hvac transformer, or 40 VA HVAC transformer to see what one looks like. Red is the 24 volt hot leg of power White is the Heat circuit Yellow is the Cool circuit Green is the Fan on circuit Common is the side of power every 24 volt circuit terminates upon. 1 of the 24 volt wires going to the AC unit will be Common.
Simply nut the blue wire in the four-wire cable to the junction of the white wire from the two-wire cable and the brown? wire going into the air handler. We know it's your C wire because that two-wire cable has only one place it can head: to your outdoor unit and its contactor. Since the red wire in that cable is the Y wire that switches the contactor, the white wire, and by extension the wire connected to it, must be C wires otherwise the contactor would never work.
Best Answer
So, reverse engineering this...
From the schematic, we can at least tell that your air handler has no heat strips, just a fan and a direct-expansion (DX, aka evaporator or indoor) coil in it. As a result of this, and the fact that your heat pump appears to be a fairly simple unit with only a reversing valve and a compressor contactor, this is actually not a terribly complicated system to reverse-engineer.
From the wiring photo you showed, we can tell that the current thermostat is wired using the black, red, orange, and white wires. If at all possible, you'll want to strip back a bit more jacket, extract enough blue wire from it to strip and wirenut a connection onto, and then use a jumper wire (a piece of 18AWG TFFN will do) to attach that stripped blue end to the existing junction of the thick yellow wire from the transformer (the whatzit in the middle with wires coming out both sides), the thick red wire going to the fan relay (black lump at the top right), and the thin red wire going off to the heat pump outdoor unit.
With that done, we can tackle the rest of the wires. We know the red wire to the thermostat is R because it's connected to the other yellow wire to the transformer, first off. From there, we can look at the fan relay and trace its white wire over to the black wire from your thermostat, which makes that black wire your G wire. This leaves your white and orange wires, which both go off to the heat pump outdoor unit. Conventionally, orange is used for the reversing valve which determines whether your heat pump is in heating or cooling mode, so that's your O (or O/B) wire, and that leaves the white wire as the Y wire that turns the heat pump compressor on.
So, in short, your wire designations go as follows:
Your W/W1/E/AUX terminal will remain unconnected, unless it's hooked up to a different system altogether. Also, you'll want to make sure that the O/B setting on your thermostat is set for O when you first program it, so that your thermostat energizes the orange wire when cooling and leaves it de-energized for heating. (Unless you have a Rheem or Ruud outdoor unit that is, where you'll need to set the O/B setting to B to begin with instead.)
If things don't quite work right
If your system heats when calling for cooling and cools when calling for heating, then you'll need to flip the O/B setting on your thermostat. (You'll only likely need to do this if your outdoor unit is a Rheem or Ruud, though -- they're the only units that commonly do things the other way around.) If your system will not change over (i.e. heats when calling for both heating and cooling, or cools when calling for both heating and cooling), then your orange wire will be your Y wire and your white wire will be your O/B wire in that case.