Nest thermostats on a 2-wire system?

J

JoeNorm

Senior Member
Location
WA
I have a customer who wants Nest thermostats or some other "smart" controls. She has 2-wire thermostat wire currently installed that runs back to her boiler controls. Can this work? I know very little about Nest or smart retrofits.
 
It does depend on the model of the stat.
My sister has a Nest that works on 2 wires because it has a battery. My son has a Nest he was never able to use on his 2 wire system even he tried the manufacturer recommend solutions.
 
I fail to understand why people like Nest. You can't even set the thing on a vacation schedule. You have to trust it can read you mind.

Personally, I like the Honeywell IAQ thermostats that use the Redlink wireless. They only need two wires for power and all the communication to the HVAC unit is done wireless with a box mounted on/near the air handler. You can also get a Redlink gateway and use Honeywell's app, which does not require any subscription fees. It also can take various sensors like outdoor temp and duct before and after air handler temps. This can alert you to a problem.

Here is one example:
 
It will trickle charge the battery with only two wires, much like motion sensors that don’t require a neutral, but don’t work great. You might luck out, and it’s more than a two wire conductor, but being a boiler, not likely.
 
If you like callbacks and customer's calling on weekends with no heat 'emergencies' always be sure use a battery powered thermostat.
I'd take a close look at the t-stat cable, the last one I thought was 2 wire had 2 more wires cut off (18/4). I got lucky and was able to pull some slack to repurpose them. On many old hydronic boiler systems the thermostat is closing a tiny relay on a 'taco box' or zone controller not controlling the zone valve directly, so a battery wont trickle charge thru the load. Same trick like old wall timers and occupancy sensors did, same issues.
When someone calls with a Nest, my plan is to run 18/4 (or 18/3 if you stock that) and connect the blue wires to 'common' bringing 24V to the stat. Then the stat is normally powered from the 24V.
 
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JoeNorm said:
I have a customer who wants Nest thermostats or some other "smart" controls.
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As @tthh says above, there are plenty of other programmable thermostats besides Nest and Ecobee. Seems that Nest and Ecobee make wild claims about saving energy in their ads so that's what people ask for. Honeywell (who you'll probably never see an ad for) has any thermostat you could ever want, and they work with all heating and cooling systems because that's what Honeywell was founded on. Not some techi toy company.

I just happened to watch this video last night. Shows what you get into with those things.


-Hal
 
hbiss said:
As @tthh says above, there are plenty of other programmable thermostats besides Nest and Ecobee. Seems that Nest and Ecobee make wild claims about saving energy in their ads so that's what people ask for. Honeywell (who you'll probably never see an ad for) has any thermostat you could ever want, and they work with all heating and cooling systems because that's what Honeywell was founded on. Not some techi toy company.

I just happened to watch this video last night. Shows what you get into with those things.


-Hal
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Honeywell spun off their residential thermostat business several years ago, creating a techi toy company called “Residio.” My daughter had a Honeywell that would no longer do scheduling properly once Residio took over the software. Replaced it with a Nest that works perfectly.
 
JoeNorm said:
Thank you. I do not think she is attached to Nest. So The recommendations for the others are helpful.
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When she says "smart", what does she want it to be smart about?

For example, that Honeywell I posted about alerted me to a problem with the air conditioning because the temp sensor in the air handler said the air after the air conditioning coil was too cold and that might indicate a problem. Sure enough, the expansion valve had an issue. Had this not altered me, the coil would have probably frozen up and I would not have found out so quickly. It is also smart when you have the outdoor temperature sensor so it can anticipate when to start the heat or cooling to get the house where you want it when you want it. It has the ability to have an occupancy sensor so it can know you aren't home and do something. If she has two staage heat or cooling, it will choose what to do based on how far it has to move.
 
Honeywell bought Novar, an energy management company that is used at a lot of chain stores such as Home Depot and Walmart, along with others. They wanted us to do office buildings and such, but wanted us to keep (buy) a large inventory of parts. Novar is pretty simple, but can get fairly complicated on the refrigeration end at Walmarts.
 
retirede said:
Honeywell spun off their residential thermostat business several years ago, creating a techi toy company called “Residio.” My daughter had a Honeywell that would no longer do scheduling properly once Residio took over the software.
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I was going to mention that Honeywell Home is now Residio. I wouldn't call them a techi-toy company. Alot of the Honeywell commercial line was rebranded Honeywell Home before Residio. ("Home" made no sense.)

If you have something as simple as a thermostat that gets software updates via the internet, you're in trouble already. I've seen it before in the telecom business. A company sells out to another company; they let the software engineers go and outsource to India. The new software people don't understand how the original software was written or how the device works and weird things happen when they make changes.

The smartest thermostat I had was the Honeywell Round. Set it to 70 degrees and every time you look at it, the house is 70 degrees. Amazing.

-Hal
 
Here is what I do. My nephew installed a Nest to his oil-fired boiler. The nest from what I know is supposed to charge on the boiler off cycle if hooked up 2 wires. His worked for a year and the battery went dead. The primary control on the burner has a dinky transformer so it did not charge the Nest. Operating them on 2 wires doesn't work reliably. The fix is in the drawing I attached. Add a transformer and a relay
 

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  • RIB RELAY & TRANSFORMER FOR NEST.pdf
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Eddie702 said:
The nest from what I know is supposed to charge on the boiler off cycle if hooked up 2 wires. His worked for a year and the battery went dead. The primary control on the burner has a dinky transformer so it did not charge the Nest. Operating them on 2 wires doesn't work reliably.
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It's not a dinky transformer, it's that the Nest's battery went dead. A sales decision no doubt, operate it from a rechargeable battery that won't last long because rewiring and additional equipment for the "C" lead would kill sales. Customer thinks it works until it doesn't. That's why most other thermostats use a couple of replaceable AA batteries. But that's so low tech.

-Hal
 
farmantenna said:
you use the Nest power connector if don't the have a common wire. have to check for compatibility . I used it for my neighbors house. About $24.
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Yeah, OK. Seems to be a top secret what's inside. W, R, B in and W and R out. Of course they don't say what systems it is needed with. Zone valve controllers and power pile come to mind because there is little or no current in the loop to speak of. But other systems certainly should be able to provide the "tiny amount of power" required.

Like I say, these things are not designed by HVAC professionals.

-Hal
 
farmantenna said:
you use the Nest power connector if don't the have a common wire. have to check for compatibility . I used it for my neighbors house. About $24
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It didn't work with my son's boiler system. It's not a guarantee.
Like other I use HVAC company products. But I am amazed how hard it is to get a DIY stat that will also control my older heat recovery ventilator. They require you to have a dealers secret password in order to enable dry contacts for a relay or go with a commercial stat.
 
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