Debugging/Troubleshooting apparent zone fault

JimM66

My system:
Buderus boiler controlled with a Logamatic 2107, Taco SR503 switching relay. 2 heating zones and DHW, each with it's own Taco 007 circulator pump.

I noticed that one of the heating zones was colder the other morning when outside temps dropped to the high 20s. The thermostat's batteries are good, and the zone LED on the SR503 is lit, but the circulator pump isn't running. Let's call this Zone 1.

Since my other heating zone is working as expected (let's call this Zone 2), I swapped each of the following (bad zone<->good zone):

1. Thermostats - no difference in operation (Zone 1 still doesn't enable its circulator pump, Zone 2 still normal).
   
2. 24V relays - no difference in operation (Zone 1 still doesn't enable its circulator pump, Zone 2 still normal).
   

I then swapped the wiring inside the SR503 so that the thermostats now controlled different zones (Zone 1 thermostat-> Zone 2 circulator pump, Zone 2 thermostat-> Zone 1 circulator pump).

When the Zone 2 thermostat called for heat, the Zone 1 circulator pump started operating.
When the Zone 1 thermostat called for heat, the Zone 2 circulator pump did not start operating.

It's clear that the circulator pumps aren't the issue.
It's clear that the thermostats aren't the issue.
It's clear that the relays are good.
The wiring is good.

I'm placing a call to my home heating professional in the morning - just wanted to ask here what your opinion on the problem might be so as to prepare myself for the professional's diagnosis.

Jamie Hall

You say that you are sure the thermostats and their wiring aren't the problem, but as I read your post I see this: Zone 1 thermostat connected to zone 1 circulator, no joy. Zone 2 thermostat to zone 1 circulator, good. Zone 1 thermostat connected to zone 2 circulator, no joy. Zone 2 thermostat connected to zone 2 circulator, good.

Am I missing something? Or confused? From that I would look at the zone 1 thermostat or its wiring?

JimM66

Maybe I didn't do the best job describing what I did.

Both thermostats called for heat on Zone 2, and the circulator pump on Zone 2 started up.

Both thermostats called for heat on Zone 1, the Zone 1 LED on the SR503 lit, but the circulator pump on Zone 1 did not start.

Setting the thermostats back to their original positions, I then swapped the wiring inside the SR503.

Both thermostats, when calling for heat on Zone 1, saw the Zone 1 circulator pump start.

Both thermostats, when calling for heat on Zone 2, did NOT star the Zone 2 circulator pump.

Jamie Hall

Controller

Zman

Do you have a voltage meter? You need to check if the transformer is putting out 24 volts. You also need to be sure you have 120 volt power to the pump relay as well as a reliable neutral. From there, do you have power to the load side of the relay?
A quick check for these is to jump the t-stat terminals.

JimM66

Heating professional came today - diagnosed the problem as a faulty SR503.

There is some intermittent fault with the controller, as he was able to get the faulty zone operating (evidenced by the circulator pump activating) by re-seating the relay head. But if he touched the head, the pump stopped. There is something amiss with the contacts for the relay head within the SR503 (he swapped in a known good relay head, so we know it's not the relay head).

New SR503 to be installed.

Thanks for the input!