HVAC connections

The "new" thermostat found in an antique mall

In an antique mall I did find a new (well, not really new) WiFi thermostat from Honeywell (Lyric T5), so I decided to change the current one I have at home ( a quite basic and simple one).


I think there is a "right way" to do things, so even for a trivial work like changing a thermostat, I find useful to document and explain what doing and why.

History

Most of the new thermostats, especially the ones that have some extra functionalities, requires now 5 wires.
On the market there are thermostat cables with 4, 5 and 7 wires.
The original furnace we had in the house, was quite old and had only 4 wires.
The original thermostat was the glorious mercury round one like the one below.

The original thermostat we had in the house, years ago

Then it was upgraded to a basic generic one like the one below.

The current thermostat, battery operated

The current furnace we have now (Goodman GMH95) provides 5 connections for the thermostat, thus 5 wires should be used.
Naturally the wired cable in the house has only 4 wires, so I have to replace it with a newer one with 5 or maybe, why not install a 7 or 8 wire cable for future use ?
Something like this one at Lowe's. 
Of course as usual the plan is one thing, doing it another one.
The wall where the thermostat cable is places is not a normal wall, it has double frame wall and the wire is running between logs, easily stuck in very narrow places somewhere.

The option to re-wire are :

• cut big portion of drywall to see where the original wire is stuck
• remove the HVAC from its place in order to reach the back wall to drill a new hole
• find another place for the thermostat and wire from scratch, including from  the crawling area

All these options involve to spend a lot of time not only to re-wire but also to repair the drywall after the job or drill from the crawling space into the furnace area so I opted for a third one - use for the power one of the existing connections (the Lyric allows to do so).

Connections

Every HVAC unit could use different terminology, but as standard de facto we can assume these connections present in every HVAC system towards the thermostat.
Important to remember !
Don't assume the connection to follow some guideline about the wiring colors !
There are colors suggested but as usual who install stuff rarely follows them.

• C
  Common wire - blue wire
• G
  Fan - green wire
• W
  Heat - white wire
• Y
  Air conditioning - yellow wire
• R
  Power (24V AC) from furnace transformer - red wire
• Rc
  Power (24 V AC) dedicated to Heat call - red wire
• Re
  Power (24 V AC) dedicated to Cooling call - red wire

The names of the connections are the initials of the wiring color, but once again, is possible to find everything out there !

My Goodman GMH95 furnace has 5 connections and the suggested color scheme :

Ideal connection to the new thermostat

The connections are quite straightforward and of course it is implied a 5 wire cable is used.
But as stated above, I had to use the installed 4 wire cable.

So I did end up with this connection as suggested by the Lyrica instructions in these cases :

Basically the wire used to manually control the fan (G) is used now to carry the return wire so to bring power to the thermostat and the fan is only powered when the HVAC is running (heating or cooling).

The final effect is to lose the capability to manually turn on the fan, thing that honestly I did use rarely if not when the HVAC had problems.
So in the end soon or later I have to plan to wire the 5 wires cable somehow/somewhere but is not anymore a 20 minutes job since I have to cut drywall and thus repair the drywall after and re-paint everything.

And it become very important the HVAC monitoring project in order to detect if the HVAC unit has problems.

For the moment everything is working just fine.