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Hot water tank thermostat problem?

Hi, I have a Vaughn S500TTP hot water tank with a TPI thermostat (model ETC100). The tank and stat are 10 years old.
I recently heard a loud clicking noise coming from the system. On investigating I found that the recirc pump to the tank was turning on and off along with relays in the Taco SR503 Circe pump switch relay box and in the Honeywell Aquastat Relay model L8148A.
The TPI thermostat on the tank had its green led flashing as it does when calling for heat. However, the flashing was different to how I have seen it operate previously.This time it is flashing rapidly for say 1/2 sec, turns off, then flashes again. The frequency of flashing/no flashing ties in with the frequency that the relays and circumstances pump are operating. This cannot be good for the system. Eventually, the system fires up correctly and the hot water tank is supplied with heat until the led is continuously on.
Is the TPI unit the problem?


  • BPH
    BPH Member Posts: 39
    I'm assuming your tank aquastat is getting power via the SR503 TT and the boiler aquastat is receiving the on off via the SR503 end switch as weIl the pump is energized through the 503? I'd be looking at the tank aquastat.
  • speedyg56
    speedyg56 Member Posts: 3
    I took some voltage checks at the TPI on the hot water tank. There is a 24vac continuous supply to the unit on 1 pair of contacts. When the green light on the TPI is flashing to call for heat, there is another pair of contacts pump/tt that outputs a 24vac signal to a pair of contacts in the 503. The signal pulses from 0 to 24vac at the same frequency as the light. The 503 output zone is putting out 110vac to the circulation pump but pulsing from 0 to 110vac at the same frequency. At the same time a relay in the aquastat is clicking at the same frequency. Eventually the the aquastat relay stops clicking, but the circulation pump is still getting pulsed and not a constant 110vac. The TPI then has the green light on continuously with 24vac continuous on the pump to contacts and the 503 has 0 vac at the circ pump contacts, i.e., tank is at required temp.
    Is this pulsing normal?
    Note that there are two other zones that are used for the house forced air system, and these get 110vac supplied to their respective recirc pumps continuously when heat is required.
    I am not sure what the aquastat unit does, so please pardon my ignorance.
  • speedyg56
    speedyg56 Member Posts: 3
    Problem solved. It was the water tank boiler. Replaced with Honeywell unit.
  • dav
    dav Member Posts: 29
    What water tank boiler are you talking about?
  • dav
    dav Member Posts: 29
    I have a Vaughn indirect water heater with the toilet connected to a Honeywell 845a relay (tt terminals). I also hear relay chatter when it first calls for hot water. The 845a is wired to control the zr zc on the aquastat by the 5 and 6 dry contacts. The 3 and 4 dry contacts energerize a standalone relay that I use to disable the other zone valves for domestic hot water priority. What I have done is directly shorted the tt terminals on the 845a and forced a call for domestic hot water by shorting the tt terminals. There is no chatter while directly shorting the tt terminals. Now if I leave the hot water to control call for heat by turning the temperature adjust up and down I get chatter. I can only assume that there is some resistance in the dry contacts on the tpi control. Any other suggestions for this?

  • dav
    dav Member Posts: 29
    Not toilet... Tpi control sorry spell checker
  • Jamie Hall
    Jamie Hall Member Posts: 21,093
    Your problem is just as likely to be a poor connection somewhere in the wiring as in bad contacts. Try cleaning and retightening all the connections first.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • dav
    dav Member Posts: 29
    Did that already but thanks.
    JUGHNE Member Posts: 10,562
    I have seen any number of relays chatter because of voltage drop. Or the coil is weak and needs more sustained voltage applied to hold in.

    The cases I speak of are that when the coil is energized the contacts pull in to feed the load. The load drops the voltage enough to let the coil drop out. It immediately pulls back in again as the voltage recovers. This will continue until by chance the coil holds the relay in.
    Could be marginal supply voltage or marginally weak coil winding.....or both together.
  • the_donut
    the_donut Member Posts: 374
    Adding to Jughne, excessively long, thin gauge wire can also cause voltage drop. Don’t know how long of a run you have, but we are talking 100’+.