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Resistor with T8775 thermostat

the old "Green" heads or new "Gold" versus the ESP Valves.

The procedure for Gold and Green is: (Three wire valves)

1. With the thermostat calling you should Have 24 volt power at terminals 1 and 2. If you do not check your transformer or thermostat.

2. With power applied to terminals 1 & 2 the head should start to get warm and "smell" like it is burning (good sign). After about 1 minute to 2 minutes terminals 2 & 3 should make internally (this is the end switch) and the relay should pull in. If that does not occur then place a jumper across 2 & 3 this should pull in the relay so you would have determined that the relay is okay and the end switch did not make. End switch contacts can be adjusted but I would have to show you how to do it. It cannot be explained.

3. One other thing about Taco Zone valves you can run three of them on one 40 VA NEMA rated class II transformer that conforms to standard DC20-92.

4. Set you anticipators on thermostats to 1.2 as this will give you a better cycle with TACO ZV's.


  • carol_3
    carol_3 Member Posts: 397
    using the resistor that comes with a stat

    Here's a question I got about how to use the resistor that comes with a thermostat--and the answer from the thermostat engineer. More on non-merc stats later . . .

    From: John Garis [mailto:[email protected]]
    Sent: Monday, November 21, 2005 5:37 AM
    To: Fey, Carol (CO01)
    Subject: John Garis Petro Oil

    The mercury T87 thermostats are being phased out.

    I picked up a T8775A 1009 Digital round thermostat. I like the idea. As you well know everyone wallpapers and/or paints AROUND the thermostat. If I put a different one on, the white drywall that is now exposed is an issue.

    What is the idea behind the 4074FAB - 202764BAC 1,000 ohm, 3-watt resistor?

    The directions on how to install it were vey good, except on the particular unit is was working on.

    New yorker boiler with a Beckett burner. Hot water base board, 1 zone. Triple aquastat with a domestic coil. T/T wires went to the aquastat. The Honeywell primary did not have screws for the T/T terminals. If I were to install a new 8184G I would have had to put a jumper across T/T.

    I was not able to install the digital round t-stat. I had no place to put the resistor. Or did I? Did I miss how the resistor would work on this system?

    What is the resistor designed to do?

    Thanks, John Garis
    Service Technician
    Petro Oil Co.
    Southampton PA
    [email protected]

    Here's the answer--For oil hot water systems, the line voltage wires on the oil primary are typically connected to the burner output terminals (i.e. - B1, B2) on the aquastat and then the TT terminals on the oil primary are jumped together. The thermostat is then typically connected to the TT terminals on the aquastat.

    The customer did not include the model number of the aquastat so I cannot determine if his particular aquastat supplies continuous power to the thermostat or if the power is interrupted to the thermostat when the temperature of the boiler is below the low limit. If his particular aquastat supplies continuous power to the thermostat at all times, then a resistor is not required (more than likely this is the case). If the power is interrupted to the thermostat when the temperature of the boiler is below the low limit, the customer can do the following:

    1. Install an isolation relay and transformer. This will prevent blank displays during low limit. A resistor cannot be installed because there is no place to connect it on the aquastat.

    2. Explain to the homeowner that the display will go blank on the thermostat during low limit. The only affect this has on the digital round is a blank display. All settings are permanently stored in memory so no information is lost.

  • CO Testing Biz?
    CO Testing Biz? Member Posts: 14
    How to test?

    Hi, I have two questions,

    1) what way is the correct way to test a taco powerhead with before replacement, using a muiltmeter?

    2) How do you test with meter an aquastate for gas boiler.
    what proves an aquastatezone valve head is bad.
This discussion has been closed.