How should I wire this Taco501 to my old boiler?

The_DIY_guy · November 29

Hi all,

I'm trying to slowly improve the energy efficiency of my home and I'm on a tight budget so I thought I'd start small and install a smart thermostat. However, I'm one of those unfortunate people who has a thermostat with only a R and W connection and while there is a blue (common) wire available in the wall, it's not hooked up to the zone controller of the boiler, and therefore not supplying a constant 24V for my smart thermostat. I could jerry-rig some kind of relay etc. etc., but I'd rather just replace the zone controller (relay) since that's pretty old too. In short, how can I swap out my current (antique) Honeywell zone controller for a somewhat newer Taco 501?

My setup is as follows:

A Honeywell RA832A zone controller (it's on the older side)
This controller supplies power to a circulator as well as the aquastat control inside the boiler.
My aquastat control is also Honeywell (L4026B, vintage), which basically controls the boiler ignition and pump, and I should not need to do anything to it.

I've taken a few photographs (see below) and made a diagram to illustrate the current situation, but essentially a 120V supply feeds the R832A and aquastat (coming in at the top of the zone controller box), which in turn is controlling the boiler ignition and pump (both traveler/L wires in my diagram); one wire from the aquastat is going to the R832A, connected on terminal 3. The N(eutral) is going to the second terminal on the R832A, the pump and boiler ignition. The thermostat is hooked up to the double T terminals and I believe the double X terminals (auxiliary) are unsuitable to supply a continuous 24V to the thermostat (basically to function as a common) since they are dry contacts. The wires from inside the furnace enter the zone controller through the middle knockout at the bottom; the pump wiring enters the zone controller through the knockout on the left. Tracing all the wires, the whole looks like it has been hooked up as in the following diagram.

Based on the instructions that came with the Taco 501 I imagine the new wiring would be as follows where I assume that the red from the aquastat is a common wire.

the incoming 120V hot will feed the Taco and the aquastat;
the Taco controls the pump based on signaling from the aquastat, wired up to 6 N(ormally)/O(pen) and 5 (common)— the aquastat seems to be controlling the pump currently…
the incoming neutral is hooked up to the N on the Taco 501, pump and boiler ignition.

Unfortunately I have no wiring diagram for the aquastat (it's a hi-lo limiter) so I'm not 100% sure that red is the common (the blue may be as well though that does seem somewhat unlikely because the red terminal is right next to the hot going to the boiler). Given the history of this house I would not be surprised if the current setup is not as it should be. Any advice to help with this project or to improve the situation is much appreciated!