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Nest 2nd Gen Installation

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Joined: Dec 29 2013

I'm installing a Nest Gen 2 Thermostat in my Eichler and am using a Honeywell Relay RA89A to convert from high voltage to low voltage.
I'm wondering if I can place the relay right behind the thermostat and if this would cause any issues.

I'll post the DIY photos as soon as I complete this project.

Joined: Apr 20 2006


The other ends of the 120V wires behind your original thermostat are in the small electrical box on the garage  wall that connects power to your recirculating pump.   This location is always the simplest for the relay.   Nice that you posted the inquiry before taking your thermostat wall apart.  When I installed the relay, more than 20 years ago, I attached it to the metal lid of the box.  Your Honeywell assembly has a box that you could install right over the existing box....making for a very neat install. 

Randy from Dura-Foam Roofing and Solar Center

Joined: Apr 28 2007

Hi SuperSonic Buddha-

It sounds like Randy answered your question.  But I just wanted to say that I'm very interested in your Nest project and how it works with radiant heat.  I'm specifically interested in if the Nest can manage the huge lag in time to heat the space to the desired temp--that's always interested/confounded me.  I can see how the features like determining if someone is home and adjusting accordingly would be nice with forced air where you can increase room temperature in 10 minutes, but I'm curious how it would work with radiant, or if features like that would just be disabled.


Joined: Apr 5 2003

Yep, looks like the Nest does really understand the radiant heat lag.

We installed a Nest at Christmas, and I managed to grab the raw data for house temperature, thermostat setting, and boiler status for the last week.  (Ask for details; it involves writing a computer program to poll the Nest servers every 10-20 minutes to get the latest data.)

I'd adjusted the Nest so it would set the temperature at 68 at 6 am; for the last week, the Nest has been turning on the boiler at 4am so the temperature will be up by 6am, and turning off the boiler while the temperature is still below the thermostat setting.  It'll be interesting to see how it does when the temperature dips again, but so far I'm optimistic.

Our thermostat has auto-off set, but the house hasn't been empty so I haven't seen what the thermostat will do.


Joined: Dec 29 2013

Thank you everyone for the responses.
I have all the equipment I need before I begin -just one last question.
I opened up the old thermostat and there are two wires there - a black one and a white one.
I'm not sure which wire connects to which spot on the nest.

Joined: Nov 13 2012

(bump)  I have the same question as Supersonic Buddha.  Once you make the line- to low voltage conversion at the pump/boiler side of the wiring, you have not altered the actual fat black and white line voltage wires that were connected to the line voltage thermostat the Nest is now replacing.  Do you use wire screws to get to a thinner gauge or merely use the line voltage black and white?  For someone who has done this, a picture of the back of the connected nest with the black and white line voltage wires installed\spliced, etc. would be the last piece in finishing this project before next winter.

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