Do I need 24V transformer for Nest even though it’s hooked up to a relay?

ericmhurtado

Hello, I’m having a bit of trouble understanding my heating system and this forum has been great in answering the many questions I have.
I’m trying to see if I need a 24V transformer to power my Nest even though its hooked up to a Honeywell RA89A relay? Thanks

STEVEusaPA

Yup

ericmhurtado

Ok. I thought that the Nest would be powered through the 24V transformer embedded in the relay?

Jamie Hall

Only if that transformer has enough power, and if you have the third common wire to the Nest and they are all wired together correctly.

ericmhurtado

I’m not sure about the power but this Nest only controls heating and thus only has two wires, no common wire. But I have seen it exhibit some power stealing such as a sign showing that it is time delayed for x:xx, and the boiler turning on and off when there’s no call for heat, etc

Zman

The honeywell looks like it has the power needed for your nest.
The lack of common terminal will likely give you trouble.

wcs5050

In my experience Nest will work just fine on 2 wire to a dedicated single relay. I had an issue with an old Argo 6 zone relay once but added a single zone as an intermediary and it worked great. Also had an issue with poor conduction through 20 gauge wire that caused erratic function. Best to have 18 gauge wire.
In general if you have 2 wire and running a 3 wire (for common) is impossible this method works great. Connection to zone valve manifold with 2 wire also appears to work fine.
Nest will not work via 2 wire on a series wired 24 v steam boiler due to voltage drop so relay addition works well here as well.
Typical hvac setups have a multi conductor wire so need for modifications.

Jamie Hall

No common wire? Your Nest may work. Then again, it may not. It may work sometimes and not others. Nests draw a lot of power all the time, and have a rather limited internal backup battery. In a two wire hookup, they are dependent on stealing power from the hot side of the two wires and returning it -- but if those two wires are shorted by the Nest itself (that is, if the Nest is calling for heat) there is no power to recharge the Nest. So... whether it works or not will depend on multiple factors, and the more the system calls for heat, the worse it will work.

Put in a common wire or an outboard power supply (Nest makes one) -- or a thermostat which doesn't have that problem.

wcs5050

This is multiple nest TT on several jobs, no call backs.

ericmhurtado

wcs5050 said:

This is multiple nest TT on several jobs, no call backs.

I’m a bit unclear about your comment can you please elaborate?

ericmhurtado

Could my Nest power relay issue have anything to do with my hydronic loop problem? Problem being that the burner shuts off before high limit set point of 180 is reached?

ericmhurtado

I have created another discussion titled “How to safely wire Nest to this transformer” after I found two extra unconnected thermostat wires. I felt it would be better to discuss separately since it’s a slightly different topic.