Nest Wiring to Old Boiler (R8184G and L8124A)

Ben_jamin321

Hello. First time using this forum. Hopefully someone can help me! Recently brought an old house in upstate NY and I am trying to wire a new Nest thermostat to the existing boiler. I have a Smith 8 Series S/W5 oil fired boiler from the late 90’s . It has a Honeywell R8184G controller and Honeywell L8124A aquastat. The aquastat has 120V (B&W) wired to a very old thermostat (only thermostat in the house). The aquastat is also wired to two circulating pumps (not sure why there are two since it is a single zone).

My plan was to remove the old thermostat and abandon the 120V wires behind the wall (with wire caps on ends) and then run new 24V thermostat wire from the two ‘T’ terminals on the aquastat to the Nest. To power the nest, I was going to add a 24V transformer at the single pole switch and run 24V to the Nest separately.

Hoping someone can tell me this will work or if I’m obviously screwing it up! I have a sketch of how the system is currently wired as well as photos that I’ll post as soon I figure out how to do that via an iPad!

Thanks!

Jamie Hall

That will -- probably -- work, with one modification and it is much the simplest way to do it. However. If that thermostat and aquastat are really 120 volt, and directly controlling the circulating pumps, you will need a relay and power source for that, so that the 24 volt circuit from the Nest can switch the 120 volts which the pumps and aquastat are set up for. Now... I'm not at all sure that you actually have 120 volts at your thermostat -- or indeed, any voltage at all. Older thermostats are simple switches -- on, off. Aquastats are simple switches -- on, off. And the Honeywell 8184G has T-T terminals -- which are looking only for a closed circuit or open circuit -- on, off -- and no outside power input at all.

So... I have a suspicion that there is something incomplete about your circuit description. Something you are missing in there. It would be a very good idea to come up with a complete circuit diagram... before you fry the Nest, the controller, the pumps, or all three.

Ben_jamin321

Jamie - thanks for your response. I have uploaded a number of photos as well as my poorly made sketch of the current wiring scheme and what I'm proposing to do. Appreciate your feedback.

STEVEusaPA

Wrong, wrong wrong. Don't do it that way. Don't know why line voltage wires are on T-T. With an aquastat, no wires should be on T-T. You're in over your head.
You should update your aquastat, it's obviously not reading temperature correctly if you low limit needs to be set to 180° and your high limit is on 210°-or that's the only way you can get decent domestic hot water. In which case you have a problem with your tankless coil.
And where's the ground wire?
Put your money into some decent modern controls.

Edit: To avoid confusion, usually with an aquastat no wires should be on T-T on the primary control.

Ben_jamin321

Alright Steve, guess I’m going to have to hire a professional! Thanks for the response.

Jamie Hall

@STEVEusaPA beat me to it.

EmptyNest

Get the Nest Power Connector "Pro Installation Guide" here: https://support.google.com/googlenest/answer/10523126#InstallAdvanced&zippy=,nest-power-connector-tech-specs
The "Pro Installation Guide" is different (but only by adding a few extra diagrams and use cases) from the ""Installation Guide" also with a link on the same support page.

The Pro Installation Guide has instructions for how to install the Nest Power Connector if your furnace (HVAC) has no C terminal. In short, "You’ll need an external 24 VA/20 VA transformer".
Since the R8184G Honeywell and other furnaces like yours have no C terminal, this use case would seem to apply.

There are suggestions on some forums suggesting that you can use the outer "F" terminal as a C terminal. From what I gather, doing this might draw too much power and blow your transformer. Given that there are alternatives to the Nest (as well as the option to use an external transformer), that risk doesn't seem worth it. 

It may also be possible to avoid using the Nest Power Connector, and just use a 24V transformer plugged into a wall socket near your thermostat. You can find these products, e.g., on Amazon, with searches like "Thermostats 24 Volt Transformer C Wire Power Adapter for Nest". In this case, I am not sure if there is an advantge to using the 24V transformer AND the Nest Power Connector at the location of the thermostat (assuming you have only one thermostat) or if the Nest Power Connector would be redundant.

STEVEusaPA

F wire…that’s sneaky, only known by a few. And only on an older primary, never on a newer one.