TACO SR501-4 / Honeywell Digital Thermostat RTH6500

TonyNY

I just installed a Honeywell Digital Thermostat RTH6500 to a TACO SR501-4. The issue I have is when the boiler kicks on to heat up to temp 170 degrees, the green light on the SR501-4 goes out and then comes back on once the boiler reaches the temp of 170 degrees. Is this normal?
When the thermostat request heat the red lights come on and goes off once the temp is reach which I know is normal.

Alan (California Radiant) Forbes

No, it's not normal. The green light should always be on.

How does the SR501 get its line voltage? From the boiler or a junction box? How many pumps are wired to the SR501?

TonyNY

One pump is wired to the SR501-4. I believe It’s powered from a junction box.

HVACNUT

Is the relay new as well?
Is seems as if terminal #1 on the SR501 is receiving power from the Lo side of the dual aquastat. The limit is 170°, so assuming the Lo is set to 150°, if the boiler temp is at 150° or below there wont be any power at the load side of the Lo. This is to help insure that during a heat demand, the circulator will shut off at the Lo setting to prevent condensing of the flue gasses AND keep temp in the boiler so you dont run out of hot water using the tankless coil.
For constant power to the SR501, terminal #1 must be wired for constant 120v. So power the relay direct from the load side of the service switch. Remove the jumper between terminals 1 and 3. Remove the existing black wire from terminal 1 and move it to 3.

STEVEusaPA

Do you have the C wire connected properly on the relay and the tstat? Your thermostat requires one.
Looks like you might, but I can’t see it in the pic that well.

TonyNY

Yes the Com off the relay is connected to “C” on the Tstat.

HVACNUT

How is the line volt wired?
What powers terminal 1 on the relay?

Joe Mattiello

Hi Tony
as I said in our email correspondence, green light going out was an important observation.
When green light is illuminated, that means you have power to the control, apparently, voltage is interrupted causing it to be inoperable.
Need to determine where voltage source is, and why it’s intermittently cutting out.
HVACNUT provided a good explanation of what may be happening because of type of wiring. It appears to be wired per alternate wiring diagram for tankless boiler application. the 120v is interrupted when there is a DHW demand so the boiler can focus energy to make hot water. Did you contact Taco Tech support, as I suggested?

TonyNY

@HVACNUT see attached.
@Joe Mattiello I have not had a moment to call as of yet.

TonyNY

All, after troubleshooting the issue I found that the wiring from the switch goes to a White Roger aquastat 11C15-11. As HVACNUT mentioned that this maybe the issue. @Joe Mattiello

TonyNY

I want to thank everyone for there help. The Fix was as @HVACNUT suggested with the confirmation from Mario and Joe Mattiello at TACO. All good now.

ratio

Thanks for the follow-up, @TonyNY.