What circuit breakers/switchs/fuses do I throw before changing a thermostat?

On Thursday, I attempted to change the thermostats for my first-floor conventational HVAC system and second-floor heat pump HVAC system.

Each time, I threw:

1. A 15A CB labelled Gas Furnace,
2. 2 25A paired CBs labelled Up Heat,
3. 2 25A paired CBs labelled AC,
4. 2 30A paired CBs labelled AC, and
5. 2 60A paired CBs labelled Up Heat.

Thinking I had cut-off all of the power to both systems, I attemped to change the thermostats without success. Both times, after putting the faceplate on, the display did not lightup after waiting 5-7 minutes.

On Friday, the owner of an HVAC repair and installation company that he started 20+ years ago said I burned a fuse in each of the units. He blocked my line of sight so I could not see which fuses he replaced. However, after he left, I was able to determine which fuse in each unit was replaced by the absense of any dirt or dust on each.

I am a fairly successful DIYer and I must have missed something that should have been obvious to me, but wasn't.

Do I throw the same circuit breakers and the replaced fuse until I am done?

Do I pull the two pull-fuses on top of the air handling unit in the attic and the one outside for compressor?

Are there any other switches or fuses that should be thrown?

Should I just through the 200A whole-house circuit breaker?

What am I missing?

Thanks!

gfr92y

 

The thermostats are connected to the air handler or furnace.
That's what you need to shut off.

A furnace should have a service switch in it.
An air handler with electric reheats could have 2 larges breakers or two large and one small.

If you keep the C wire carefully insulated.... you won't have any problems.

 

Yup! I have these two sitting on top of my air handling unit...

I am surprised that throwing all of the circuit breakers listed in my previous post did not deenergize those two boxes on the air handling unit.

Why should I not be surprised by this?

Thanks!

 

Thinking the five circuit breakers "protected" me, I was probably sloppy with crossing the wires.

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These are the boxes you are talking about, right?

gfr92y

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I see two heavy cables.... one into each disconnect.
I see a thermostat cable, a condenser cable and a float wire.
I don't see any other cables which means the box with the orange cable should be the disconnect.

I see the large cable getting pulled out of the disconnect. That should be corrected.

 

Pete,

Thank you very much!

gfr92y