[KISS]

A) In a typical thermostat is the reversing valve controlled directly based on mode of heat or cool regardless of the unit actively cooling or heating

Mode is Heat = reversing valve on (furnace can be actively on or off)
Mode is Cool = reversing valve off (AC can be actively off or on)

Actively cooling = Call for cool
Actively heating = call for heat


-or-

B) Is the reversing valve power changed based on a call for heat or cool.
e.g. heat = call for heat + reveseing valve on
cool = call for cool + reversing valve off

There is energy savings if the latter is used.

More t-stat control options occur if the former is used, otherwise the operation of the HP is identical.

In OFF mode is reversing power removed. I would suppose the answer is yes.

Will all tstats behave like a or b?

Although Carrier surprises me. In "System OFF" mode, the fan can works independently and the daily air filter test still works.

[hvac1000]

If you notice most quality thermostats have a O and B terminal. This is because over the years different manufactures have used different methods to control a heat pump reversing valve.

Most heat pumps today only energize the valve in the cooling mode. The reason is because as a default the heat pump will always be in the heating mode so if a valve or coil goes bad you will always have heat capability. In most areas of the country heat is considered more necessary than cooling on an emergency basis.
Over the years there have been different manufactures that have wired it differently but today the above way is just about universal.

Usually system off mode just covers the heat and cool side of the thermostat. The blower on switch will still activate the blower itself.


Does this cover it?

[KISS]

No it doesn't. Let's try asking it this way:

For example without worrying about logic (e.g. normally closed reversing valve)

Is O and Y energized independently? i.e Where O is controlled just by being in cool mode or just moving the heat/cool switch activates O.

And Y is activated when there is a call for cool. So, in esscence, you set the mode by activating/deactivating the reversing valve and then use call for heat or cool to activate the AC/furnace.

-OR-

Would O&Y be activated at the same time when there is a call for cool. So if the stat was in the cool mode and O was needed to be on for cooling. O would be activated ONLY when there is a call for cool to save energy.

So, is it:

Q1:

A) O&Y controlled independently.
B) O is set by mode switch. Y is activated by call for cool.

Q2: What happens at OFF.

a) O is always turned OFF, independently of the position of
Heat/cool mode.
b) O is only controlled by mode.

[hvac1000]

O is not activated until a call for first stage heat comes to play
Y is activated on a call for cool or as part of the thermostat control for a call for first stage heat.

Off makes the thermostat control for heat and cool a dead issue. Many thermostats will still control the blower fan when the unit is in the off position.

O is controlled by the call for first stage heat and usually never energised without the call for heat.

Rmember there are many different manufactures of equipment and thermostats. Many of them use different ways to control the same thing.

[KISS]

O is controled by the call for first stage heat and usually never energised without the call for heat.

That would make the most energy conscious sense, but separating it under programmable control, I think might open up more options for woodstoves, etc.

Thankx

[hvac1000]

I think might open up more options for woodstoves.

With wood stoves you just need a two stage heat stat. First stage for wood second stage for other heat sources. Been there done that.

Now if you have a heat pump a three stage stat will work or a two stage with a bypass kit (relay)

[acetc]

hvac1000 is incorrect when he said the "O' is energised on a call for first stage heat, just the opposite, it is energised when the stat is placed in the cool mode, the "B" terminal is energised when the stat is placed in the heat mode.Now that being said, the reversing valve can be energised by either the "O" or "B" terminal in the thermostat depending on if the manufacturer had the reversing valve energised in the cooling or heating mode, some do have to energise the valve to put it in the cooling mode such as Carrier's residential heat pumps do.

[hvac1000]

O
Varies,
Orange
Reversing valve
Energize to cool (Changes from heat to

cool on heat pumps)

yep it energizes the coil on the reversing valve to put the reversing valve into cool position.

B is the reverse. Energizes the reversing valve to put it into heat mode usually on older equipment.

That is what happens when you get older. LOL

[KISS]

The history was worth something. The fact that you's like the reversing valve to fail in heat mode. That makes sense.

[hvac1000]

The fact that you's like the reversing valve to fail in heat mode

Yep that is why most modern equipment is set up that way.

Having fun here. Some local idiots have been installing condensing boilers on older systems. The condensing boilers will usually not condense (save energy) unless the water temp is 130 degrees or below. These old systems are designed to use 170 to 190 degree water and the home owners with these new boilers are having a fit because there utility bill did not go down much. LOL The boiler manufactures are actually paying us to visit these homes and deliver the bad news. The factory guarantees the boilers to condense BUT not when they are retro fitted to an existing high temp set up. All the existing systems are high temp (over 130 degrees).

Now I guess it will be time to testify in court as to the installing dealers blunder on some of these jobs.