Defrost Thermostat Wiring Diagram

[Gaetan Boren]

Onething to note; the cutoff thermostat may be open circuit until the evaporator tubing is frosted fully, so the timer may not run until the fridge is down to temperature. Some of them are in the 30 degree turn-on and 55 degree turn off range. So if the fridge has not been running, the defrost timer may not run for an hour or more until it cools the evaporator down.

Jake; with all due respect, the timer motor has a complete circuit through the components highlighted below. The power path will change when going from Cooling and Defrost mode as shown by the highlighted path.

With an appliance such as this, there can't be an intentional connection from power "Live", to PE (equipment ground) through a component. Whine it's true that the timer motor does draw only a tiny amount of current, it's just not something which would be done with a 120V plug-connected appliance.

For instance, when in cooling mode, the current for the timer comes from the "Live" side of the power cord, the cold control, through the timer motor, and then back to Neutral through the defrost thermostat and heater. The heater has a very low resistance (maybe a few hundred ohms) and the timer motor has a very high resistance (thousands of ohms). Therefore, the voltage will be divided between the two resistances proportional to the resistances. The timer motor will get almost full voltage, while the heater gets a tiny voltage. For this to work, the heater and defrost thermostat must be good, and closed.

Also, while in Defrost mode; the current for the timer comes from the "Live" side of the power cord, through the cold control, through the timer motor, and then back to Neutral through the compressor motor. The compressor motor winding has a very low resistance (less than 10 ohms) and the timer motor has a very high resistance (thousands of ohms). Therefore, the voltage will be divided between the two resistances proportional to the resistances. The timer motor will get almost full voltage, while the compressor gets a tiny voltage (probably less than one volt).

Can you send a model or part number of the defrost timers used? It may need a nonstandard wiring configuration to function in this circuit. If the timer came with some wiring diagrams, that would be helpful.

My system is composed of the following equipment installed in 2015 (the wiring diagram in the Dropbox folder link below is from the Trane system documentation):

Trane 4TWR4 Heat Pump

Trane TEM4 Air Handler

Honeywell FocusPro 5000 Series Thermostat (replaced with Wyze)

Documentation for these is easily found online.

Wyze Thermostat has a Differential Temperature setting available which allows you to customize the threshold for when the heating or AC will turn on. This allows you to set the threshold for when your heating or AC will turn on, with a range between 0.5-2.0 degrees in half-degree increments. This allows you to fine-tune your system to work how you want.

I had an issue with how to handle the white wire wired to Aux/E which is my heater coils. Wyze told me to wire to * and then when I set up the app it asks what is wired to * and I tell it emergency heat.

I had the same question, how does it know when to kick on the emergency heat? Also it will get cold enough at my latitude where the heat pump is ineffective. Does it also kick on emergency heat based on outside temp too. Most units do I thought and that would be the time I will really need it to be smart enough to automatically do it.

Is there an official bug request or report, some support ticket already open that I can also tag onto? I have used at least 50% more electricity since I switched and this is obviously unacceptable for numerous reasons.

Depending on which model thermostat you have, it is actually correct that the auxiliary heat is typically wired to the W2/AUX terminal. The real question is, did the technician make changes at the air handler or the outdoor unit at the same time? I asked this question because in a heat pump system, it is the outdoor defrost control board that initiates the auxiliary heat. Between the outdoor heat pump unit, and the indoor air handler, the defrost control board has a W2 terminal that is connected to the W2 terminal in the air handler. A dual fuel system would typically be a heat pump with a gas or oil fired furnace. You employed a workaround when you had it wired as a dual fuel system and the temperature cut over at 40F.

The tech did do something in the air handler. But just to clarify, I have a Nest Learning thermostat (3rd gen), a heat pump, and heat strips in the air handler. The Nest wiring diagram currently shows a white wire into W1. Should it instead be in W2?

GL4, the way heat pumps work between the defrost control board and the indoor air handler is that the defrost control board uses a W2 signaling wire to activate the electric heat in the air handler. Typically a thermostat is wired to the very same auxiliary heat unit using an E wire to enable the homeowner to activate the emergency heat in the case of a failure of the outdoor unit. As for auxiliary heat, when activated by the outdoor unit, the signal wire sends a signal to the thermostat on W2 to allow the thermostat to know that auxiliary heat has been turned on. In some cases, smart thermostats can be programmed in such a way that if it detects that the heat pump is running, and the outdoor ambient temperature is so low that the heat pump cannot keep up with the demand for heat, the thermostat can also activate the auxiliary heat to boost the heating capacity of the system. The nest is one of these types of advanced thermostats.

The goal here is for you to, that is, if you are comfortable, to ensure that your thermostat has the white wire on W2/AUX and in the air handler, the same color wire is also on the W2 of the control board. Do you have access to the indoor Air Handler?

If he indeed connected all three stages together then it should work. The problem is if the control board has indeed 3-stages of heat, and if the air handler does not have 3-stages of heat, it can act erratically. You should not tie W1, W2 and W3 together in the air handler. For me to look up the control system, I need the model number of the air handler. Some systems have multiple banks of heating strips and get activated is a sequence or as Stage 1 and Stage 2.

Sometimes the installer uses differ colored wires instead following industry standards. What ever color is in W2 should be in W2 at the other end. Your Trane air handler can indeed have up to 3 stages of electric heating coils. In order for that to be the case, separate electrical contactors are used for each stage. Additionally, you air handler has an Elecyriclly Commuted Motor (ECM) and can greatly vary the speed of the blower. Can you get me the model number of the outdoor unit?

Your outdoor unit is a single stage Heat Pump. Not sure how many stages of electric heat you have in the indoor air handler unit. The auxiliary heat kits are installed by the contractor and not by the factory. Since the outdoor unit does contain an outdoor temperature sensor, the connection between W1 and W2 and W3 is not a valid configuration. The proper configuration between the Nest thermostat, the air handler and the outdoor unit typically W1 from the thermostat to W1 in the air handler, W1 from air handler to W1 on the outdoor heat pump, W2 from the outdoor heat pump temperature sensor W2 going to the air handler W2 to the W2 of the thermostat.

The AC Cooling Wizard

The Nest App just uses different colors to simply differentiate the wires. What matter is what color wire is in the terminal backplate and the other end. If you think about it, the thermostat is not smart enough to see the color of the wire. So, the software uses the industry standard wire colors for each terminal.

I have a Kitchenaid (model ktrp20khbt00) and whenever it goes into defrost mode, it turns the lights (in freezer and refrigerator) off and it doesn't even perform the defrost. I've changed the defrost timer and it is still doing the exact same thing. Now I have ice building up every 2-4 days and no lights. If it's not the timer, what else could it be?

@jayeff I did hire a service tech and he's been here 5x in the last 2 months and still cannot fix it. The last time he was here, he took out the defrost timer and put in some sort of loop to make the light and icemaker work, which obviously causes the ice to build up. I have since put back the defrost timer and we're back to square one. What I can't figure out is where do the two wires in the front go (the brown and pink wires coming from the defrost timer connection)?

Disconnect the power to the refrigerator and then unplug the defrost heater- part #WP2315530 (supplier example only) which is located under the evaporator unit, from the wiring harness and use an Ohmmeter to test it for continuity. I don't know what its resistance value is but it shouldn't test open circuit.

If the heater is OK, here's a video that shows how to test a bi-metal defrost thermostat -part #WP4387503 (supplier example only). The defrost thermostat is also located near to the evaporator unit inside the freezer compartment.

Here's the tech sheet for the refrigerator that has the wiring diagram. Both the full detailed schematic and the simplified version that may also help, especially if it is a wiring problem with the lights.

That end of the brown and pink wires go nowhere. They are test points as shown in the wiring diagram. The other end of the wires go to the defrost timer - bi-metal defrost thermostat connection and the bi-metal defrost thermostat - heater connection - see image below.

The temperature thermostat opens when the correct temperature is reached and this stops the defrost timer motor, compressor motor, evaporator fan motor and condenser fan motor from operating. When the temperature in the compartments warms up, the thermostat closes and reconnects power to the defrost timer motor, compressor motor, evaporator fan motor and condenser fan motor and they start to operate again.

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