Inspecting furnace startup with tstats that turn off with the service switch (eg Nest)

Working on HVAC training. Ben G. says he turns off power to furnace using service switch, removes cabinet panel, tells tstat to call for heat, returns to furnace, turns power back on using service switch to observe startup sequence.

Maybe this is bonehead question but how can you tell the thermostat to call for heat if it has no power to it? Does this procedure only work with old school, manual tstats?

Any tips for how to observe the furnace’s startup sequence?

He just goes up and turns the thermostat up and it kicks on when he turns on the service switch back on at the furnace. This is so the flame and combustion, etc. can be seen at start up.

Thermostat may be battery operated or is it an old anolog with mercury. It won’t need power.

The kill switch only kills power to the furnace, thermostat is battery powered

Jesse, I never turn off the power to the furnace, just use the thermostat. Some thermostats like the Nest will not turn on the Emergence/Aux heat on a heat pump until the outside temperature is below the set point programmed into the thermostat. I do not override any preset programming or reprogram anyone’s thermostat.

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Thank you.

I’ve seen repeated notes in the Nachi trainings to use the service switch when inspecting a furnace. Can you please expand on why you think I shouldn’t follow that advice?

That is proper protocol when opening a furnace. It is also a safety device that should be tested when inspecting a furnace.

I don’t turn off the furnace for the same reason I don’t turn off the AFCI breaker when the house is occupied. The computer that runs the furnace may be damaged just like the homeowner’ computer. Although this is a rare event I just make it a policy not to do it. It’s your company you decide your risk level.

So can I assume you don’t concern yourself with Interlock switches either?

I take off the front panels, if the blower door switch shuts the system down that’s ok with me. I like to see how dirty the air handler is and look for mold. Half the time a service tech has taped or bypassed the switch, again I don’t call that out. I do go beyond the SOP in other areas such as checking the AMPS on each heating element to verify all are working. My father was a self-employed HVAC contractor so I am comfortable doing a little extra on some items.

The thermostat battery only controls the timing / switching portion of the stat. Furnace still turns on by stat (or you) connecting the red (24v) - white (heat) and green (fan). Service disconnect will interrupt the 24v thermostat feed. Stat batteries can not prevent that.