Nest output current capacity?


  #1  
Old 03-19-15, 10:16 AM
B
Member
Thread Starter
Join Date: Dec 2011
Location: Usa
Posts: 67
Received 0 Votes on 0 Posts
Nest output current capacity?

I hope that a Nest can operate 3x Taco Sentry zone valves (.48A each) all in parallel as 1 big zone (1.44A x 24 = 34.6VA) ?

The Nest website is all about flowery advertizing for sales & lacks technical specifications.

2 weeks ago I asked their tech support via email about a second question that I had for sizing the 24VAC Xfmr:
"What is the Nest power requirements, in amps" their answer was 24vac. That says alot.

BBP
 
  #2  
Old 03-19-15, 11:29 AM
Houston204's Avatar
Forum Topic Moderator
Join Date: Oct 2009
Location: Texas
Posts: 7,425
Received 17 Votes on 14 Posts
I would use a relay to isolate the load from the stat.
The Nest would only need to make the coil of a relay.
 
  #3  
Old 03-19-15, 11:47 AM
B
Member
Thread Starter
Join Date: Dec 2011
Location: Usa
Posts: 67
Received 0 Votes on 0 Posts
Houston204: Good idea on the relay. That is my backup plan if I can't find an actual amp output capicity, as I don't want to zap some nice new Nest$$.

I am designing a new system install at my Son's new house (zone 7 Crested Butte Colorado). He wants to only install 3 Nest to control 3 large areas (aprox 1200 ft2 each) of house. I think that I will need 6-7 zone valves to provide enough heat at 10.3Cv each.
 
  #4  
Old 03-19-15, 12:01 PM
B
Member
Thread Starter
Join Date: Dec 2011
Location: Usa
Posts: 67
Received 0 Votes on 0 Posts
Actual heat loss at design -19*F is unknown at this point. Architect is not done with 1st pass yet.
Finding Solid State Relays that operate on 24vac input (coil) is VERY hard so far.
 
  #5  
Old 03-19-15, 01:12 PM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
Received 36 Votes on 28 Posts
Why would you want to use a solid state relay? A simple 3-pole mechanical relay such as a Potter & Brumfield KRPA14AG-24 would do just fine. If you MUST use an SS relay then just put a cheap bridge rectifier between the AC and the SS relay input.
 
  #6  
Old 03-19-15, 05:07 PM
B
Member
Thread Starter
Join Date: Dec 2011
Location: Usa
Posts: 67
Received 0 Votes on 0 Posts
Thanks Furd.
For a relay that may operate many, many times, especially if powering an inductive pump, I like SSR's
They only turn on at zero crossing time of the AC cycle, thus no arcking & easier on the load.
 
  #7  
Old 03-28-15, 02:20 PM
B
Member
Thread Starter
Join Date: Dec 2011
Location: Usa
Posts: 67
Received 0 Votes on 0 Posts
Just got off the phone with Nest Senior Tech support, Joey

Nest average power use is under .03KWH / month, so average power draw is under 25 milliamps.
Maximum power draw for sizing transformers is 100 milliamps @ 24vac = 2.4VA

Nest output contacts can handle a maximum of 3A @ 24vac = 72VA
It has an internal fuse of unknown value, if you pop that the Nest has to be returned.
 
  #8  
Old 03-28-15, 02:57 PM
Houston204's Avatar
Forum Topic Moderator
Join Date: Oct 2009
Location: Texas
Posts: 7,425
Received 17 Votes on 14 Posts
Are you going to use an isolation relay?

If you choose not to use a relay, post back if you end up replacing the thermostat in a few years.

I would put all 3 valves on a standard dpdt relay before connecting them directly to the stat.
 
  #9  
Old 03-28-15, 05:10 PM
B
Member
Thread Starter
Join Date: Dec 2011
Location: Usa
Posts: 67
Received 0 Votes on 0 Posts
It looks like in my Sons house, I will need 2x of the Taco Sentry zone valves on a Nest (.48A each) = about 1/3 of Nest output 3A capicity. S/B fine.

I have done a mechanical 24vac relay on my nest in my house. House is wired with only 2 wires to each thermostat (R&W) NO C wire. Nest worked fine power stealing through the coil of relay. It hummed but didn't energize until Nest did a heat call.
 
  #10  
Old 03-28-15, 07:39 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,889
Received 1,480 Votes on 1,368 Posts
I would be extremely cautious about the max load I'd put on a solid state thermostat.
I personally would never connect more than a 1A load.
 
  #11  
Old 03-29-15, 09:14 AM
B
Member
Thread Starter
Join Date: Dec 2011
Location: Usa
Posts: 67
Received 0 Votes on 0 Posts
PJmax, I agree 100% (.48A + .48A = .96A)

I could even use a short time delay relay to stage the 2nd zone valve operation.

I used the past tense in my post #9 above about my Nest power stealing through the relay coil, because after a year of fighting with my very remote poor internet service, I gave the Nest to my Daughter, who has great internet & A/C in her house & could take better advantage of the Nest capibilities.
 

Last edited by Buffalobillpatr; 03-29-15 at 10:04 AM.
  #12  
Old 03-29-15, 05:45 PM
S
Member
Join Date: Mar 2015
Posts: 11
Received 0 Votes on 0 Posts
I'd definitely do the relay. I just blew a brand new programmable stat on the first call for heat due to excessive draw on the W lead. All indications were an intermittent issue in the furnace ignition module but I never did get to the bottom of it. Furnace was a 32 year old 70%er, so I ran an old mercury stat until I could replace the whole furnace (finished it up last night) Ran a new stat wire as well just in case it was a short on that line. In any case, the more crap you have hanging off of the stat, the more chance something going awry could take it out, and Nest stats aint cheap...
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: