I'm on the Board of Directors of a townhouse association. We have 15 buildings, each with a fire sprinkler room at one end, built into the last unit. Rooms are ~4x3' and house fire alarm equipment. The community was built 2007-2012 by the same developer, so [still surprisingly for me] the only similarity inside these rooms are the components. Their placement and connections are all over, as if every time it was done from scratch with no blueprints by a new crew.

The management company does not give neither straight nor complete answers, at best offering to "hire a handy man to check things up". We see it as just throwing money away, 'cause no explanation will be provided. We'd like to get some deeper understanding of how and why certain things should be done, and what is the proper way - to be able to point out issues without depending on contractors suggested by management company.

The first thing I want to ask about are Potter RTS (Room Temperature Switches), operating at 40°F, commonly used to protect a dry-pipe-valve from freezing (to me that implies some kind of heating elements, which I couldn't identify). They may be hard to see, but the leftmost setup has RTS-O (normally open) while the other two have RTS-C (normally closed) ones:


My thought is that even if the placement is not identical, still the same or very similar, interchangeable components would be used, and therefore electrical connections between them must be same. At least all control panels are of one single model FireLite MS-4, all wireless transceivers are of one single model Keltron RF774F, all water meters and valves are same.

  1. What is the purpose of an RTS here - what does it control (there's no heater near the valves)?
  2. Is there a reasonable explanation why some RTSs are -O while others are -C?
  3. Could this result in equipment failure due to a wrong part being used?

Let me know if more detailed pictures would be helpful. And thank you, guys!

P.S. USA, Illinois, Niles

  • In many jurisdictions, fire control panels, sprinkler systems, etc. in multi-family or commercial buildings require periodic (1/4, 1/2 or yearly inspection/maintenance/testing). If you have a company contracted to do that, ask them. If you don't, check with the management company and/or local authorities (building inspectors and/or fire department, depending on the jurisdiction) to find out what the inspection requirements are. Also, to get any answers here: What jurisdiction? (Country/state/city) May 21, 2021 at 23:08
  • If the switch is tripped, does the panel send a supervisory or a trouble? May 22, 2021 at 0:17
  • All rooms are in normal state (I'd expect Fire Dept to show up if there was trouble), which is weird to me, 'cause the only way I can explain this is that wiring is not the same, right?
    – Astrogator
    May 22, 2021 at 14:10
  • 2
    I’m voting to close this question because it's not about home improvement.
    – JACK
    May 22, 2021 at 18:40
  • How is it not? Please tell me, where should I ask such (and similar) questions then?
    – Astrogator
    May 23, 2021 at 13:51

1 Answer 1


Building plans don't call out every little detail

The differences in layout between the buildings are expected. You may be largely correct: the installations likely were done by different crews and without precise layout diagrams. That's normal. Plans for a building specify a variety of things like which plumbing fixtures, light fixtures, or fire detectors and horns/strobes need to be installed and approximately where they must be located. Most of the time the locations are quite flexible. Plans would not dictate exact placement of the pipes or electrical wiring. It's not practical. Likewise they would not dictate the precise arrangement of the furnace etc in a unit, nor would they dictate the exact placement of the various fire control panels in the riser room.

Fire alarms can work fine with either style of switch

Fire alarm systems deal with alarm events, tamper events, and supervisory events (sometimes there's no distinction between the latter two). Events cause notifications; virtually all events cause a signal to be transmitted to the monitoring service. A fire alarm panel will typically have multiple inputs. Some inputs go to detection devices and trigger an alarm; other inputs go to tamper or supervisory sources and only trigger a report to the monitoring service. Often an input can be configured for either normally open or normally closed operation. Whether NO or NC is chosen by the designer or installer depends on a variety of factors, but most often the detection job can be accomplished with either. Your RTS is offered in both NO and NC styles to accommodate this flexibility. Building plans often will not require a specific style of switch.

Why would a fire alarm system include a room temp switch, anyway?

A room temperature switch is not likely to control anything; its purpose is purely to monitor ambient temperature in a riser room, attic, etc -- places where a freeze could impair the automatic sprinkler system and could go unnoticed because the space isn't occupied. If the switch trips a signal would be sent to the monitoring service, who should then forward the information to the appropriate building management representative who should then investigate and correct the situation.

Inspection verifies it all works

The system should have been inspected and tested for correct operation when it was commissioned. Periodic re-testing may be required by law depending on where you are. It might also be required by a fire insurance carrier. Of course, testing may also be commissioned at any time by a proactive building owner who just wants to be sure that life safety systems like this one are dependable.

Practical difference in zone wiring for NO versus NC

It's true that NC and NO switches won't work exactly the same - the circuit as a whole has to be arranged differently. Suppose I'm a fire alarm installer (and I was, for about a year) and my work order calls for a room temperature switch on a supervisory zone of its own.

If I want to use an NO switch then I might use two-conductor cable to connect a zone input and common to the two terminals of the NO switch. I'd connect the end-of-line resistor across the switch terminals too. Then I'd configure the panel so that if the zone opens that's a wiring fault and if the zone shorts that's a supervisory alarm indicating the room is cold.

If instead I want to use an NC switch then I'd still use two-conductor cable from zone input and common at the panel. One conductor would connect to a switch terminal, the other conductor would connect to the EOL resistor, and the resistor would connect to the other switch terminal. I'd configure the panel so that if the zone opens that's a supervisory alarm indicating the room is cold.

The latter approach doesn't provide any electronic method of distinguishing between a wiring fault and a cold room condition. Maybe that's a little inferior, but if the job specs didn't require differentiating the two, then according to the job requirements the two approaches are equally valid.

As an installer I'd do either one based on which kind of switches I have on hand. Maybe I prefer to use the NO switch but somebody ordered some NC by mistake, or they were left over from another job, or the supplier was out of stock of NO so we bought NC instead..

It's impossible to guess why the systems were built with different style of switches, but yes it's wrong to expect that two similar ("identical" to untrained eyes) buildings built years apart would have their fire systems set up exactly the same way.

  • 1
    "a signal would be sent to the monitoring service, who should then forward the information to the appropriate building management representative who should then investigate and correct the situation." +1. - If that's not you, that's why you don't know. And I don't have time to share my +25y of random experience with you either. What is the error code / what is the problem? Are you willing to pay for parts and labor for my nearly 100% success rate for solving problems? If so, I can be in Niles in about an hour. ;)
    – Mazura
    May 28, 2021 at 22:46
  • Hi neighbor! :) Is there a way to PM? Let me know your rates.
    – Astrogator
    May 29, 2021 at 23:16
  • @GregHill, I understand all that you said. Would you agree though, that NC and NO switches wouldn't work the same in [I want to say same, but can't] similar circuits? Therefore circuits must be different enough. My bewilderment is - why? I get variations in placement of components and such. But at least the logical schema of the circuit should stay at least alike.. Am I wrong expecting that?
    – Astrogator
    May 29, 2021 at 23:26
  • 1
    @Astrogator Yes the circuits would be wired differently (I added more about that). Yes it's wrong to expect the logical schema to be the same between buildings. It's likely an installer had no more instruction than "it has to have an RTS" with the implicit understanding "wire it up however you like."
    – Greg Hill
    May 30, 2021 at 0:29
  • 1
    Note that with the NC switch, what you'll likely get is a trouble that indicates either a cold condition or a wiring fault instead of having a wiring fault deliver a supervisory to the panel, although the configuration you describe is certainly plausible as well May 30, 2021 at 1:17

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