This post is a supplement to my other post on “How To” install a Home Style WiFi thermostat (t-stat) in your RV. This one is a ‘standalone post’ meant to provide insight into the working of these analog devices which can help troubleshoot or replace your RV’s Original Equipment t-stat. It is meant for those who desire to take on the challenge of DIY as well better understand their RV’s systems. For this write up, I used a Coleman analog style 7330F3858 that is found in many RVs and used it to take pictures, provide examples, etc. Again, this is about the typical analog / manual RV thermostat that have been around for decades, not the newer serial bus / digital t-stats
Enjoy!
Note: LONG POST
First off, a discussion about relays is warranted as the t-stat and the rest of the systems work using them. In simplest terms a relay is a switch that is turned on or off with a signal. The idea is that the signal can be carried via small wires (low current) and the ‘switch’ part of the relay can handle a heaver current. Relays allow low voltage signals to turn on and off large loads. Think of your car. It has a large power-hungry starter motor that requires thick wires to power it (the battery cables). Yet these same thick wires do not come all the way up your steering column to the ignition key. Instead, when you turn the key to "start", there is a relay getting its ‘signal’ and it, in turn, closes allowing a large current to flow to the starter. The relay has a ‘switch side’ can handle the large current and is closed by a low voltage signal. Relays ‘turn on’ when its internal electromatic coil is powered by causing the ‘switch’ to close. The magnetic coil literally pulls on the switch to close it.
Picture of the inside of an automotive style relay:
Here is what it looks like electrically both ‘off’ (at rest) and ‘on’ (energized):
To power the coil inside the relay you need a closed (complete) circuit, so a Positive (Pos) and a Ground (Gnd aka Negative). For A/C, there is not a true positive and ground, instead there is “Hot” and “Neutral”, but idea is the same > need a complete (closed) circuit to power the coil. To complete the coil’s circuit, you can switch either the Pos or the Neg, depending on how its wired. Convention is to switch the Pos, as shown in the diagrams, and that has been my experience in RVs. But the opposite is possible.
You will also encounter the terms “Normally Open” and “Normally Closed” abbreviated “NO” and “NC”. These indicate what the switch is ‘at rest’ (not energized). In the example above, it’s a NO type. A NC type will, once energized, pull the switch open.
Side note about convention I use. For a 12VDC source, like a RV battery (or your RV system) I use +12VDC to mean the positive terminal and Ground (Gnd) to mean the negative terminal. Some think in terms of +12VDC for positive and -12VDC (negative or minus) for the Gnd. However, if it truly was +12VDC AND -12VDC then the electric potential between these two would be 24VDC, which it’s not. For the Multimeter, when I use minus 12VDC or -12VDC this is referring to the
display of the Multimeter.
Now onto thermostats.
In its most basic concept, a t-stat is simply a switch to turn on the A/C or Heat. Not unlike a light switch. When your t-stat calls for Heat, it closes an internal switch that turns on the heat. But unlike a light switch, the power flowing thru the t-stat switch is not what powers the heater. Instead, the t-stat switch causes another switch (a relay) within the heater to close and thus turn on the heater. Here is a mockup of that:
That coil of wire in the Heater is the electromagnet coil of the relay. When the t-stat switch closes it powers this coil and in turn it pulls on its switch to close it as discussed above. In fact, the t-stat switch is itself a relay > it has a tiny coil that is controlled by the switches and electronics of the t-stat. So, it’s a relay causing
another relay to close. When the switches are set to Heat the electronics sense the temperature and when too cold, the electronics power the tiny coil for the t-stat relay and, in turn, it powers the coil for the relay in the Heater, turning on the Heater. Here is a picture of the relay in the Coleman t-stat:
With this understanding we can visualize the t-stat as simply a bunch of switches that open and close depending on the setting. Like this:
Thus, when you select the heat function AND the temperature is such that heat is needed, the t-stat closes the switch for Heat. Like so:
Here is a picture of the Coleman with Heat on and the temp up high enough to call for heat and a multimeter connected to the Gnd (blue wire in this case) and the Heat (white wire in this case):
Notice that the White Wire has 12.76VDC going to it. This white wire leads to the relay in the heater and causes it to be energized (closed) and thus will turn on the heater.
The rest of the settings follows the same principle. When set to A/C with the Fan on Auto / Low and there is a need for cooling, the electronics will power the little relay inside the t-stat which will cause the A/C Wire (yellow wire in this case) and the Fan Low (gray wire in this case) to both have 12VDC to them. And these wires lead to the relays inside the A/C and Fan causing them to turn on.
This one little internal relay does it all. It can since it is not doing the actual switching of the high current devices. Its simply closing allowing a 12VDC ‘signal’ to be sent to those high current devices. And these devices in turn have their own (and larger) relays that get turned on from the 12VDC signal from the t-stat.
Here is another mockup showing the t-stat relay powering up the A/C compressor relay:
But only one relay in the t-stat? Yep. Its all in the selector switches on the t-stat. When you select Heat the 12VDC output ‘signal’ of the relay is routed via the switches to the Heat wire. When switch to A/C with Fan on Auto / Low the output of the relay is now routed to the A/C and Low Fan wires. And so on. Here is a more refined diagram of that concept that includes the t-stat relay.
So that’s how they work.
Tying it back to my post about ‘How To’ Home Style WiFi t-stat at
https://www.forestriverforums.com/fo...-a-253684.html the reason you do not need the Gnd wire (aka minus 12VDC) in the Home Style t-stat is because the relay in the Home Style WiFi t-stat uses 24VAC to work. You do need the +12VDC wire as that is the ‘signal’ that powers the Heater, A/C and Fan relays. The Home Style WiFi t-stat works just the same principles as described above. Thus, when its internal relay closes calling for Heat, it will pass the connected +12VDC ‘signal’ to the Heater relay, causing it to close and the heat to come on.
Is it also passing the 24VAC signal to the Heater Relay? You bet. But since there is not a complete circuit for the 24VAC, it does nothing. It just sits there. And because its low voltage its fine. Instead, the 12VDC signal does make a complete circuit casing the Heat or AC to come on. I address the concept of 24VAC riding on the 12VDC RV system in my post on WiFi Smart Thermostat Install
https://www.forestriverforums.com/fo...ll-207971.html under the subheading of “The Theory Part”.
Here is a mockup showing how at Home Style t-stat in your RV is passing both +12VDC and 24VAC:
Hope this helps tying it all together or at least providing you a better understanding of your RV systems