Taco makes two variants of zone valves - one type for general hydronic use, and another type marketed especially for geothermal use.

What is the difference in valves that particularly suits them for one application or the other?

All the manufacturer's literature I can find (so far) basically reads identically for both products. But the geothermal variants are more expensive. Having looked at samples myself, I don't see any superficially obvious differences. The actual valve portions appear to be potentially identical; but there could be some differences in the actuators ether mechanically or electronically which would not be obvious.

(Perhaps other manufacturers have a similar offering, I just happen to be a little familiar with Taco's).

Assuming there are substantive differences, in a pinch could one be used in the "wrong" application successfully? (e.g., using a regular hydronic in a geothermal system, or vice-versa).

Note - both samples I've seen are normally closed, so apparently the answer is not that one is normally open.


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There may be other differences as well but the geothermal valve is designed for a higher head pressure. The standard hydronic system is built around reduced pressure. The geothermal valve is expected to deal with standard water pumping and or city pressure.

  • Thanks Paul. Is the difference in pressure capability going to be like a PSI rating in the literature? And is this a difference in the valve body or in the actuator (or both)? – DaveInCaz Mar 20 '18 at 9:15
  • Regarding pressure either body can handle all kinds of pressure. The actuator of the geo valve is going to be loaded all together different. – Paul Logan Mar 21 '18 at 19:35

This comment pertains specifically to Taco valves, it may or may not be true of other valves intended for geothermal applications. Not all geo systems are open loop.

[The geothermal valve] is made from materials that can resist oxygen corrosion more effectively than those in a Zone Sentry valve. Zone Sentry valves are designed for use in closed-loop systems, but Geo-Sentry valves are designed for use in open-loop geothermal systems. Open-loop systems contain fresh oxygen.

(From Q&A section at this page.)

This was also confirmed by information I received directly from a manufacturer in reply to asking this same question:

A standard hydronic valve is made of free machining brass. When exposed to oxygen, it will dezincify. (The oxygen pulls the zinc out of the brass making it brittle and porous.) A geothermal specific valve uses brass that can handle open systems (O2) and will not dezincify. The bottom line is that the normal hydronic valves are not made for open systems.

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