NEST Thermostat Wiring Help!

120volts

Joined Sep 26, 2014
62
Hello, Folks. I am installing two NEST E's for a customer. I have installed these before but this order is a little unusual. First he has two existing thermostats for each floor with one heater/AC unit. And I am pretty sure I can just replace the two old thermostats with the new ones. But what's been bothering me is this: When I opened up the old one, I not only saw the usually jumper from the Rh to the Rc, but two extra jumpers with what looked liked (red) capacitors. I've never seen that. So now I'm wondering if I should jump the same into the new NESTs or not. I'll be contacting NEST support on this soon but wanted to know from anywhere here what those two capacitor looking like components are and their functionality. Help!

wayneh

Joined Sep 9, 2010
17,152
Hello, Folks. I am installing two NEST E's for a customer. I have installed these before but this order is a little unusual. First he has two existing thermostats for each floor with one heater/AC unit. And I am pretty sure I can just replace the two old thermostats with the new ones. But what's been bothering me is this: When I opened up the old one, I not only saw the usually jumper from the Rh to the Rc, but two extra jumpers with what looked liked (red) capacitors. I've never seen that. So now I'm wondering if I should jump the same into the new NESTs or not. I'll be contacting NEST support on this soon but wanted to know from anywhere here what those two capacitor looking like components are and their functionality. Help!

View attachment 158011
I believe those are varistors for surge protection. They're not normal components and I have to think they're somewhat optional. It'd be nice to know why they were put there!

LesJones

Joined Jan 8, 2017
3,143
Are you sure they are capacitors not MOVs. I suspect they are fitted to reduce switching spikes if the contacts feed an inductive load. Look at the back face of them for a part number.

Les.

120volts

Joined Sep 26, 2014
62
I don't hit the worksite again until the week of the 26th of August and will post an update and the backside of the components. I don't know a thing about varistors but that may explain.

I just got off the phone with NEST and they, too, were stumped. They thought they were just jumpers used to power and told me they weren't necessary in the new NESTs, but I'm not confident in their answer about them just being "jumpers". They just didn't know. But I promised to inform them for their edification once we all figure this out!

Thanks, all!

wayneh

Joined Sep 9, 2010
17,152

120volts

Joined Sep 26, 2014
62
So sorry about the long delay. I didn't have occasion to return to the customer for weeks...

...I removed the "varistors" from the old (two) thermostats and installed two new NESTS without them and both 'stats work fine.

I took a pic of the backside of the suspected varistor but couldn't find anything on google, not that I'd know what the specs meant anyway.

I'll take this knowledge of the varistors as controlling voltage surges and leave it at that???

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LesJones

Joined Jan 8, 2017
3,143
The first hit in a search for "P100Z4" (Using duckduckgo.com) takes you to this web page. If you go to the third page and the
to the eighteenth entry in the table you have the data for your P100Z4 varistor.

Les.

KeepItSimpleStupid

Joined Mar 4, 2014
4,686
I put a bidirectional TVS diode across R & C at the furnace when my thermostat's backlight stayed on all the time even though it was programmed to be off. I turned the furnace off for two weeks to fix the underlying problem. Meaning, that shut it off.

The TVS fixes the issue due to power glitches. it should also protect against nearly >$400 furnace boards and >$400 thermostats. >12 years and no issues.

AC contact protection is usually a ZNR while DC protection can be a capacitor. Both were recommended by a Hydride Toxic gas alarm and Hydrogen alarms I hooked up for work.

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