W1, W2 & E

I got this question via email (edited slightly for length)

Some things I’ve done because I’ve been taught to do them yet I don’t know why I do them. One of those things is putting a jumper between w1/e and w2. Sometimes, in the case of a Goodman for example, I’ve been taught to combine the brown wire along all the whites at the air handler. Do you mind just clarifying the whole situation with w1/e jumped to w2? And also maybe x2 on some stats? Thanks for your help.
— J

Back in the early 2000’s when I was the lead trainer for another company some of the most common miswiring issues has to do with electric heat. So much so that I created a bunch of different wiring diagrams with a fancy program called “Microsoft Paint” to illustrate how to wire different combinations of equipment. Here is one of them.

In older thermostats (older than the diagram shown here) there were no installer setup programs in the thermostat where you could designate the type of system the thermostat was connected to. Each terminal performed a particular universal function and you would configure the operation based on how you wired it up. Which terminals you connected where, which you left open and sometimes, which ones you jumpered out.

So first, let’s give a quick look at the meaning of each terminal

W – When you see a W terminal it just means heat. Usually you will only see W when the control only has one stage of heat

W1 – Means first stage heat. In a heat pump first stage heat is the same as first stage cool. It just means the contactor / compressor is turning on. Whether that is heat or cool is actually dictated by whether or not the O/B terminal is energized. This is why on many old thermostats you would jumper Y1 and W1 in a heat pump application.

W2 – Means second stage heat. This could be the first stage of heat strips in a common southern heat pump, the gas furnace backing up the heat pump in a modern “hybrid heat” application or just a second heat strip bank in the case of a straight electric system. W2 is generally called on based on temperature differential between setpoint and space, outdoor temperature and / or run time.

W3 – Is just the next stage of heat after W2

E – Is emergency heat, usually just a way to manually drive on what would normally be the secondary form of heat without stage 1 heating.

Emergency heat only makes sense when there is some sort of secondary heat source and really even then, it only helps if the secondary heat source is sufficient to heat the space as in the case when the secondary is a furnace, hydronics or a large heat kit. In Florida most of our units have 5KW auxiliary heat which will never be sufficient to heat a home in an “emergency”.

Many of these other terminal designations are a hold over from a time when all the controls in the thermostat and defrost board were electromechanical, and much of it was for indication / trouble lights and some of it was for the thermostat to be able to perform staging based on outdoor temperature due to the fact that run time logic was not available. So for your X2 question, have a look at the thermostat and diagram below.

In the modern thermostat they have usually relegated these staging configurations and terminal designations into the installer setup and every thermostat is a little different. In general in the south we jumper w2 to E because they truly are the same, in some cases this does nothing, in others it just ensures that the aux heat comes on quicker if the user chooses emergency heat.

Are there some cases where emergency heat could be totally different than aux heat? I’m sure… I have just never seen one personally. Like usually, it all comes down to knowing your particular piece of equipment and your controls, reading the installation instruction is a good first step.

— Bryan



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