Nest thermostats have a blue wire that supplies power to the t stat.
Since the Nest thermostats are new- the system worked fine in the past.
Sounds like the Nest thermostats get their power from that 20 amp breaker that is being tripped = air handler.
Who wired the Nest t-stats?

A breaker is $10. Replace it.

The units are 22 years old. Start planning on replacing.

Okie dokie.

Finally had a cool enough evening that climbing up into the attic wasn't going to be immediately regrettable.

Can confirm that both forced air furnace units are on the same circuit. You can see one furnace plugged into a single gang outlet box and follow the Romex out about 15 feet to another outlet where the second furnace is plugged in.

Killed the breaker, and can confirm that the power is out at both outlets.

Edit to add: I also had the HVAC tech out to look at it and give the whole thing a "tune-up". Can confirm that everything is working within spec, amp draw for both furnaces up top and compressors outside is normal, coolant a little low, but acceptable.

So the problem just went away, or were there repairs made?

Repairs as Skins said?
Did you separate the units?

Units operating within specs, but the problem still persists intermittently. HVAC guy recommends contacting Electrician to further resolve.

I'll swap out the breaker switch to rule that out.

Going forward, I may need to separate the two furnaces onto separate circuits. I may wait until it's time to replace the whole system as they are aging, but that's okay because we rarely need to run both upstairs and downstairs at the same time.

I'll move my other project of mapping out circuits higher up in priority. Perhaps if I find a nearby 20amp circuit that is getting low utilization, I may hire an Electrician to add an outlet and move the furnace over.

Breakers are cheap, it's worth a shot. Easy way to tell if you have a electrical problem and save a trip to home Depot would be to run an extension cord to the attic and plug both units into a circuit with no other load on it. Kitchen outlet or bathroom (if it's 20a). If it trips that circuit, then you need an HVAC tech. If it works without issue then you have an electrical problem.

The 20 circuit in question, is that a single or two pole breaker?

Also it's against code to run any non HVAC related loads on a furnace circuit. So if it is a single pole breaker and you want to split the furnaces up, you'd need a new line to electrical panel.

The model number on the breaker is Cutler Hammer BD2020.

Markings are the same as this Eaton breaker from Home Depot.



https://www.homedepot.com/p/Ea...ker-BD2020/100124683


When the breaker is triggered, it's the bottom switch only that goes.

quote:

Originally posted by Aeteocles:
The model number on the breaker is Cutler Hammer BD2020.

Markings are the same as this Eaton breaker from Home Depot.



https://www.homedepot.com/p/Ea...ker-BD2020/100124683


When the breaker is triggered, it's the bottom switch only that goes.

20/20s are two single pole breakers. I can't believe they used BR breakers in your house? That's the cheapest breaker on the market. The older ones tend to fail a decent amount (still fractions of 1%). So there is a chance it could be a bad breaker.

Try the extension cord idea I suggested. It will tell you for sure if you have electrical or HVAC problem.

quote:

Originally posted by Skins2881:

quote:

Originally posted by Aeteocles:
The model number on the breaker is Cutler Hammer BD2020.

Markings are the same as this Eaton breaker from Home Depot.



https://www.homedepot.com/p/Ea...ker-BD2020/100124683


When the breaker is triggered, it's the bottom switch only that goes.

20/20s are two single pole breakers. I can't believe they used BR breakers in your house? That's the cheapest breaker on the market. The older ones tend to fail a decent amount (still fractions of 1%). So there is a chance it could be a bad breaker.

Try the extension cord idea I suggested. It will tell you for sure if you have electrical or HVAC problem.

Doesn't surprise me. Cookie cutter homes in a subdivision. It's an upscale neighborhood, but these aren't custom homes.

I'll try the extension cord this weekend.

Also, thank you for your help so far. Very much appreciate your expertise in the matter.

Also, thanks to everyone else also in providing their thoughts and expertise as well.

Am I reading this correctly that your air handlers are plugged into outlets in your attic? If that isn’t the case, how would extension cord be used to test anything? If that is the case does that meet code? I would have thought air handlers would have to be hardwired in a covered box in the attic.

quote:

Originally posted by trapper189:
Am I reading this correctly that your air handlers are plugged into outlets in your attic? If that isn’t the case, how would extension cord be used to test anything? If that is the case does that meet code? I would have thought air handlers would have to be hardwired in a covered box in the attic.

My furnaces (and the forced air blowers) are 110/120 volt and attached to outlets. Heat itself is generated using natural gas that's plumbed up there. The outlet just powers the blowers. That's my understanding of it anyway.

This is all original from 1999, so code may likely have changed since then.

Edit: Quick reading on some home inspection forums indicates that molded receptacle plugs are not uncommon for furnace/blower in attics. The plug serves as the local disconnect when servicing. Again, not sayin it's still up to code, but it appears that it was not unusual at some point in the past.

quote:

Originally posted by Aeteocles:

quote:

Originally posted by trapper189:
Am I reading this correctly that your air handlers are plugged into outlets in your attic? If that isn’t the case, how would extension cord be used to test anything? If that is the case does that meet code? I would have thought air handlers would have to be hardwired in a covered box in the attic.

My furnaces (and the forced air blowers) are 110/120 volt and attached to outlets. Heat itself is generated using natural gas that's plumbed up there. The outlet just powers the blowers. That's my understanding of it anyway.

This is all original from 1999, so code may likely have changed since then.

Edit: Quick reading on some home inspection forums indicates that molded receptacle plugs are not uncommon for furnace/blower in attics. The plug serves as the local disconnect when servicing. Again, not sayin it's still up to code, but it appears that it was not unusual at some point in the past.

An outlet can definitely serve as an disconnect. As I stared earlier you may not share an HVAC circuit with any other loads, I'd have to dig deeper than my memory to determine if two furnaces can share a load, it's not anything I've ever seen before, especially since running a three wire (two circuits) up there and splitting the plugs (top furnace 1, bottom furnace 2) or making quad outlet would cost less than $30 in material and no additional labor.

As for being common, it's pretty rare to use an outlet for a disconnect for a furnace, but there is nothing wrong with doing so. I've converted from disco switch to plug and receptacle a number of furnaces for people with small portable generators that only desire heat, fridge outlet, and a single outlet for a lamp/phone charger.

quote:

Originally posted by trapper189:
Am I reading this correctly that your air handlers are plugged into outlets in your attic? If that isn’t the case, how would extension cord be used to test anything? If that is the case does that meet code?

No, it doesn't meet code.
Nor is it the only code violation which is present with this installation.

OP, please call a reputable electrician to correct the problems with your installation.

And since your current HVAC company *thinks* that everything is ok, please locate a reputable HVAC company for your future needs.

Skins, you obviously didn't think about what you suggested to the OP, on the use of an extension cord for diagnostic testing.
(I know you know better, as I know where you stand on the use of electric heaters and overloaded circuits)

quote:

Skins, you obviously didn't think about what you suggested to the OP, on the use of an extension cord for diagnostic testing.
(I know you know better, as I know where you stand on the use of electric heaters and overloaded circuits)

What? I do it all the time. Especially for telling if a fridge is broken and tripping a GFI or if a GFI is broken. There is no danger to plugging a furnace into a extension cord for 20-30 minutes. Most furnaces only draw 5-7A.

quote:

Originally posted by Skins2881:

quote:

Skins, you obviously didn't think about what you suggested to the OP, on the use of an extension cord for diagnostic testing.
(I know you know better, as I know where you stand on the use of electric heaters and overloaded circuits)

What? I do it all the time. Especially for telling if a fridge is broken and tripping a GFI or if a GFI is broken. There is no danger to plugging a furnace into a extension cord for 20-30 minutes. Most furnaces only draw 5-7A.

Recommending anyone to plug an appliance (which contains a motor) into an (unknown) extension cord is recklous.
You know most people are going to have standard 50-100' cords in the 16ga range.
16ga at 6-50' 3-wire is 10 amps, loaded to 80% is 8 amps.
16ga at 51-100' 3-wire is 7 amps, loaded to 80% is 5.6 amps.

There's reasons for selling short 3-6' appliance specific, heavy duty 14-12ga extension cords.

You also didn't state to just plug ONE furnace into the extension cord (of unknown specs/length/gauge), but TWO furnaces, each containing a blower motor.

quote:

would be to run an extension cord to the attic and plug both units into a circuit

Now, you can't believe the OP's 20+ yr old, 111,000 btu furnace, has a blower only pulling 5-7 amps?
In fact, the old beast happens to have a 3/4 hp PSC blower motor, pulling 11.1 (RLA) running load amps.
Furnace is rated at 14.4 maximum load amps and requires 12ga wiring.
This is all for ONE furnace.

So,if that's not bad enough, multiply the specs x 2.
22.2 running load amps??? on an extension cord???

Nothing states testing for only 20-30 mins.
Run it that way to see if it trips, could mean days depending on frequency of the tripping.

Now, if you come across this installation;
Are you going to recommend changes or are you on the same page as his previous HVAC tech, seeing nothing wrong with the wiring/installation?

No worries guys, I'll take it real easy.

First, I'm going to head back up there with a clamp meter and a line splitter and see what the power draw of each unit looks like, one at a time.

If the power draw is excessive, then I'll run the units one at a time until next year when prices are back to normal, then I'll shop for new appliances and an electrical contractor to bring it all up to standard. We've lived here for a year and a half only ever running one unit at a time, we'll be okay for another year. Running one HVAC unit at a time has not ever tripped the breaker.

If the power draw looks to be OK, then I'll swap a new breaker in there. If it continues to trip, then I'll test the equipment on an extension cord (I've got 12 gauge USA made cords in 25ft and 50ft runs for my little portable generator) to another circuit with all other loads removed. If the "test rig" trips, then there's something with the HVAC causing the breaker to trip. If the "test rig" holds, then there's something wrong with the first circuit beyond too much load.

I'm all for hiring someone to do the work. I, however, never trust anyone who is in a position to sell me anything. I want to know what's going on, or at least narrow down my possibilities, before I give someone else an opportunity to lie to me.

In the HVAC guy's defense, he's the third guy to have serviced this system since I've moved in--and nobody has said anything about the two furnaces very obviously sharing the same circuit.

My home inspector also didn't say anything about it.

quote:

Originally posted by Aeteocles:
If the power draw looks to be OK, then I'll swap a new breaker in there. If it continues to trip, then I'll test the equipment on an extension cord (I've got 12 gauge USA made cords in 25ft and 50ft runs for my little portable generator) to another circuit with all other loads removed. If the "test rig" trips, then there's something with the HVAC causing the breaker to trip. If the "test rig" holds, then there's something wrong with the first circuit beyond too much load.

I'm all for hiring someone to do the work. I, however, never trust anyone who is in a position to sell me anything. I want to know what's going on, or at least narrow down my possibilities, before I give someone else an opportunity to lie to me.

In the HVAC guy's defense, he's the third guy to have serviced this system since I've moved in--and nobody has said anything about the two furnaces very obviously sharing the same circuit.

My home inspector also didn't say anything about it.

If you want to head up there and check the amp draw to see if they're operating in spec, cool, go for it.

If you want to change the breaker, test them on an extension cord, etc... waste of fucking time!

Tripping another breaker while on a test cord... absolutely going to happen.
You think you'll have a equipment problem because a new breaker tripped(?), you'd be wrong.

I have nothing to sell you, nothing to gain here.... no reason to lie.
I'm here to help inform people, so hopefully they don't get taken to the cleaners by the hacks of the industry.

No need to defend the HVAC guy. You called them with a problem and he was too stupid to know what the hell is wrong.
As to the home inspector, their normally nothing but a talking head spewing all kinds of mis-information.

You have an electrical issue. The issue is TWO furnaces on the same circuit (overloading said circuit).
It's not to code, it's not to manufactures specifications, it's flat out wrong from the word go!

Why is it that I have NEVER laid eyes on the system, never saw a photo of this system, yet I know where the problem lies?
But your so called HVAC guy and inspector (which has laid eyes on it), been there to test and check things, doesn't?

Because I know what the fuck I'm doing and talking about.

You have a 20amp circuit, loading to 80% capacity is 16amps.
Yet you have TWO motors with 11.1 RLA (each) x 2, thats 22.2 amp load on the said circuit. That's not counting any start up loads, transformer loads, etc... the freaking circuit is OVERLOADED. The breaker is doing its job (tripping) and protecting the wiring.

As I previously posted;

quote:

OP, please call a reputable electrician to correct the problems with your installation.

You can either believe me or go chase your tail looking for a ghost (the problem).

Excam,

I appreciate you.

My confusion is that I have got to assume that the wiring and HVAC work must have worked properly at some point in this house's 23 year history. It's a tract house, all the houses would have been wired the same way if I had to guess.

quote:

Originally posted by Aeteocles:

My confusion is that I have got to assume that the wiring and HVAC work must have worked properly at some point in this house's 23 year history.

Worked properly, no.
Worked, yes.

Why, because everything is new, clean, tight, strong, and getting by, yet working outside of normal parameters.
Then over the years everything changes, ducting/coils/blowers/etc get dirty. Dirty equals more amp draw.
Amp draw is heat, adding stress to the components. Breaking them down over time.
Things can work lose or become lose due to heat and/or vibrations. Connection points can become corroded, adding in amp draw (heat).
Over the years the breaker/wiring/connections are stressed, break down and can no longer work past normal capacities.

Now adding in all those little increases over the years (to an already overloaded circuit) and you have the situation you're in now.

Most everything will work outside of the manufactures specifications, for how long is the magical question.

Had it simply been done correctly, you wouldn't be here asking for help.
Because even with all the added amp draw/wear/tear/time it would have still been working within parameters.