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Supreme Member

Posts: 5,288

Location: Metro NYC


Sunday, May 5th 2013, 2:14pm

I believe you are overstating the current requirement of those valves. One thing that is helpful, is that the valve 'operator' is a simple resistance heater, as opposed to a solenoid, which draws more current than its wattage indicates (which is why both solenoids and supply transformers are given 'VA' ratings, rather than simple watts and amps)

Just for grins, I run the 800-foot maximum of run for number 14 zone wire through a calculator, and it returns as 2 ohms, which calculates to just about 2 amps maximum of valve draw. That raises the possibility that a controller capable of powering at least 3 modern solenoids per zone might be capable of operating those Thermal Hydraulic valves. I would be looking for any controller with a large (50 VA minimum) power supply transformer, and that rules out even the good entry-level models.

What was the original controller? What did you install to replace it?


Active Member

Posts: 46

Location: Kansas City


Sunday, May 12th 2013, 9:21am

The old timer was a Rain Bird Rain Clox RC-8A. I replaced it with a digital timer, Rain Bird ESP-RZX6 Outdoor. After talking with the engineers at Rain Bird about this particular issue, they advised me that no current timer manufactured by Rain Bird will power the valves due to the internal logic of the new timers. They stated that the amp out put is limited to 0.5 to 0.75 amps and the Thermo-Hydrolic valves require 1.5 amps. Their solution was mentioned by Wet earlier, put a second power supply, isolated from the timer and a bunch of relays.


I had the power supply from the old manual timer that was in the house. The power supply was still functioning, so it became the second power supply. It got wired with to the 120V AC feed with the new timer. I wired the field common to one of the power supply wires. The power supply wire went to all 6 relays on the input point of the relay. The always hot side of the relay got covered to prevent shorts. The always off of the relay got wired to a single zone per relay. The timer got wired to the isolated trigger on the relay. Wired the common from the timer to each relay and each zone to a single relay.

How it worked:

When the timer is run the zones activate the relays which allows the old power supply to power the system up. Everything work with no errors.

How to identify the valves:

Thermo-Hydrolic valves are very old and should only be found in old systems (30+ years old) on the residential side. They can only be powered by the old style manual timers. There is no digital timer on the market (that anyone I talked with knows of). The single biggest clue is the valves take almost a minute to open and close. You will start the zone and about a minute into the run the zone will finally start. The nice part about the valves is they are basically bullet proof. The valves I encountered had been in he ground for 37 years with no replacements.

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