Is it possible to add a C wire to a Beckett Aquasmart 7600?

tim_in_massachusetts

I would like to replace an old mercury thermostat with a smart thermostat, but need a C wire. It's a single zone Buderus home boiler, no fan or cooling. I have a Beckett Aquasmart 7600 on the Buderus. The Aquasmart has two terminals for the thermostat: Tw and Tr and there is 27 VAC across them when its not calling for heat.

As you can see in the photos, there is no C terminal next to the thermostat terminals inside the box. There is a longer terminal strip underneath those wires - feeding the furnace and pump.

Can I add a C wire? I hear the Nest can work without it, but I want to buy a Honeywell RTH9585WF1012 which demands a C wire.

Some people talk about buying an external 24V transformer and running the wires between the C and Rh terminals on the thermostat, but wouldn't that conflict with the 27V from the Beckett?

I would appreciate any help.

Tim

HVACNUT

tim_in_massachusetts said:

Some people talk about buying an external 24V transformer and running the wires between the C and Rh terminals on the thermostat, but wouldn't that conflict with the 27V from the Beckett? 

First, good choice on the thermostat. 
Second, no, you can't pick up a Common in the 7600.
And C, a separate transformer for the thermostat and wiring it to Rh and C is only half the job. The C on the transformer must also go to one side of a 24v coil on a Normally Open relay. W on the thermostat goes to the other side of the coil on the relay. Tr and Tw go to the N.O. contacts on the relay. 
If you're handy you can do it. A 24v, 40va transformer with a 1900 plate. A 1900 box. A R.I.B. relay. Whatever length 14/2 w/ ground BX wire, connectors, straps, some zip ties, whatever length thermostat wire. 
Don't forget the wire nuts.

tim_in_massachusetts

HVACNUT, Thanks for your reply. So you're saying that when the W wire from the thermostat calls for heat, power will flow across the relay coil, connecting Tr and Tw. I can see that.

But this diagram from Emerson makes it look so much simpler. What's wrong with their design?

HVACNUT

There's nothing wrong with that design. They're using Rc to power the thermostat and Rh and W for switching. I prefer it to be hardwired, and fused. 

Steamhead

If you're going to mount a 1900 box, a fan center relay like this:

https://www.supplyhouse.com/Resideo-R8239A1052-40-VA-Fan-Center-w-SPDT-Switching-Action-Includes-R8222B?gclid=CjwKCAiAsNKQBhAPEiwAB-I5zY8_KdRTYMce7qlsRpHr3mzlO3I1xMydraOtfaLm6s9tREokRMJ9ihoCFT4QAvD_BwE

is the way to go. The transformer powers the thermostat, which operates the relay, which closes the thermostat contacts in the AquaSmart.

EBEBRATT-Ed

@tim_in_massachusetts

here's a drawing of my method. A rib relay (RIBU!C) and a 40va transformer from supply house.com will get you going. A few dollars and a little work. Probably less cost than a fan center. You can mount the relay and a transformer on a 4" square box

STEVEusaPA

For me, this way works pretty well, as written by @Robert O'Brien
https://www.indoorcomfortmarketing.com/its-not-so-common/

tim_in_massachusetts

EBEBRATT-Ed: Thank you for the drawing. Totally helpful. Just to be clear, the ORANGE and YELLOW coming off the relay can go to Tw and Tr on my Beckett and it doesn't matter which goes to which?

tim_in_massachusetts

STEVEusaPA, Thanks so much for that link. It looks reasonable.

EBEBRATT-Ed

@tim_in_massachusetts

yes. the orange and yellow is just a normally open contacts. Basically your thermostat runs the relay and when the relay contacts close that starts whatever your stat used to start