How to make sense of the choices? Thread split off.

[beenthere]

I broke out the thread so they can keep going on and the OP doesn't get a bunch of needless notifications.

[54regcab]

The power is not cut to the thermostat when a high limit switch opens. I've never seen a furnace that cuts the power to the thermostat when a high limit circuit has opened. Follow the power from the transformer in order to understand what that circuit is doing.

Please help me understand on the path to the 24V power to the thermostat. I've highlighted what I'm seeing on the power path, what am I missing? If LS1 were to open, how does it tell the CPU to go into "high limit" mode? The 24V powered thermostats definitely go blank and 24V to the thermostat back plate is gone when the burners go off on high limit and the "33 code" shows up. The blower continues to run while the 33 code is being displayed. Once the furnace cools back off the power is restored to the thermostat. I'd like to get to the bottom of this since I see a lot of them.

[54regcab]

This is all indicative of an improperly sized duct system, air blockage such as dirty filter or coil, loss of blower capacity etc. None of this has anything to do with the furnace being oversized.

Fair enough, the furnace isn't necessarily oversized for the house, just the duct system. That of course is if it doesn't have any of the other possible issues you mentioned.

[carmon]

I see a yellow line...... lol

[54regcab]

You know it's one thing to want to learn the trade, and just humble yourself and sit back and watch and listen, but when your argument is not fact, and you don't just stop for a moment to research something before you post, you not going to appear to be the brightest bulb in the house. It's like me going on a TV repair forum and and telling all the pro's there how a TV works or is wired.

Fair Enough

[carmon]

oh my.....

[RoBoTeq]

Please help me understand on the path to the 24V power to the thermostat. I've highlighted what I'm seeing on the power path, what am I missing? If LS1 were to open, how does it tell the CPU to go into "high limit" mode? The 24V powered thermostats definitely go blank and 24V to the thermostat back plate is gone when the burners go off on high limit and the "33 code" shows up. The blower continues to run while the 33 code is being displayed. Once the furnace cools back off the power is restored to the thermostat. I'd like to get to the bottom of this since I see a lot of them.

That would be DIY training which is not allowed.

[54regcab]

That would be DIY training which is not allowed.

Do you really think I'm doing these thermostats as some sort of DIY project?

[RoBoTeq]

Do you really think I'm doing these thermostats as some sort of DIY project?

I never said anything about your installation of thermostats, just that you don't know what you are talking about when it comes to furnace operations. What is the model number of one of the furnaces that schematic you posted is from?

[syndicated]

The power is not cut to the thermostat when a high limit switch opens. I've never seen a furnace that cuts the power to the thermostat when a high limit circuit has opened. Follow the power from the transformer in order to understand what that circuit is doing.

On a Lennox G8, which uses the honeywell/trade line fan&limit combo.
The primary side of the transformer is wire to the "limit" side of the fan switch, which opens of temp rise, but keeps blower running.
Therefore, when the limit setting is reached, the whole 24v control circuit is dead.... Gas valve and t-stat.
Many old furnaces use this configuration.

[syndicated]

I never said anything about your installation of thermostats, just that you don't know what you are talking about when it comes to furnace operations. What is the model number of one of the furnaces that schematic you posted is from?

It's from a carrier.

[coopbro]

The York D1NA048N09006C gas pack breaks power to the R terminal when the limit, aux. limit, or rollout opens.

[RoBoTeq]

On a Lennox G8, which uses the honeywell/trade line fan&limit combo.
The primary side of the transformer is wire to the "limit" side of the fan switch, which opens of temp rise, but keeps blower running.
Therefore, when the limit setting is reached, the whole 24v control circuit is dead.... Gas valve and t-stat.
Many old furnaces use this configuration.

I know what old furnaces use, but there aren't that many furnaces out there that are that old any longer. We are discussing current systems and why oversizing new furnaces is a problem.