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In a hydronic (hot water) heating system electronically operated valves control whether hot water runs through a particular zone of the building, or not.

It seems to be a common setup for the valve to 'default' to being CLOSED to flow, and only open when commanded to do so via a thermostat signal. This seems logical (to me), as you only want heat supplied when you actually need it. This is the "normally closed" type.

However, manufacturers also sell a "normally open" type which would be OPEN to flow all the time, unless some control signal says otherwise.

What application(s) requires the normally open variant?


Note -- Ditto for a chilled water system used for cooling, AFAIK.

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Choice of normally open or normally closed for a valve actuator is part of the overall controls design process, part of which is considering safety/protective functions.

As an example, consider a climate where it gets freezing cold (below 0 degrees C). You have a normally closed heating valve that is closed. Something happens such that your electronic valve actuator no longer receives power (board burns out, breaker trips, etc). Now imagine you are away from home for an extended period of time, and you don't know your valve is now stuck closed, and it gets freezing cold. The zone will have no heating, and the consequences of no heating to the zone could be a burst pipe and a flooded home. Had the valve been normally open, you would likely be wasting energy overheating the space, but no burst pipe.

Note that normally open/normally closed is often used to mean fail open/fail closed, but they are not necessarily the same thing. Motor actuators without specific additional features (spring return, capacitor driven return, etc, backup power) do not have a fail safe state on loss of power, they fail in place. Normally open/normally closed can describe the valve state at the control signal input of zero/false/off.

By the way, there are other valve actuators used in hydronic heating systems besides electronically operated actuators. Pneumatic actuators and mechanical (thermostatic) actuators also exist.

  • Great perspective on this, thanks. Would the Pneumatic or mechanical variants you mentioned also have normally closed and normally open versions? I mean that in the sense of what a control signal would cause, not in the failure sense. – DaveInCaz Mar 20 '18 at 18:36
  • @DaveInCaz Yes pneumatic, no mechanical thermostatic. Pneumatic valves are almost always spring return, and so the actuators almost always fail in a defined position. Whether this is normally open or normally closed depends on the construction of the valve to which the actuator is attached. Thermostatic valves usually use thermal properties (like expansion or melting wax pellet) to directly actuate the valve, and if something caused that to break, that's it, it don't work – tmband25 Mar 20 '18 at 20:01
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I have used these N/O valves on a system that had a wood/coal boiler as a back up heating system so if properly piped could be used during periods of "power outages" to yield gravity flow and give some heat to the residence.

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The normally open valve can act as a by-pass so that you do not dead-head the pump in the event none of the zones call for heating. Then when any one or more zones call the normally open valve will close to allow more head to the zones that are calling.

  • Do you mean that you just pick one valve out of many and set it up this way? And in that type of scenario (or maybe in any scenario where a NO valve is used) is a different thermostat required? – DaveInCaz Mar 20 '18 at 9:25
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    No. In some situations, keeping the pumping running all the time is strategically advantageous. Under these conditions when zone valves are used and it is possible for all of them to be satisfied at once you can see where we could dead head the pump; a no-no. They just install a small closed loop, fairly large diameter pipe, which will allow some water to be pumped at all times. This loop doesn't go anywhere. It is just a passage way for water to be circulated through the pump and the boiler. This is an inexpensive way to deal with what could otherwise be an expensive control dilemma. – Paul Logan Mar 21 '18 at 19:43

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