RTU shutdown w/duct smoke

[nhfire77]

I have an RTU that doesn't have a duct smoke shut down input, it's old. The hvac guys have a third party controller added onto it. It won't allow an immediate shutdown upon alarm, it first goes through its purge cycle of a minute if we just break the t-stat.
Obviously not a desirable condition, and the HVAC company has finished this retrofit and will not play nice, their managers answer was 'how often do you really think are they going to have a fire?'.

So I've made the executive decision with the GC's blessing to break the 24 VAC powering the RTU's internal control board. It shuts down immediately, then powers right back up and restores normal operation when the duct smoke is reset.

I see no code compliance issues, might piss of the hvac guys, but they don't really care. Anyone think this noncompliant? (FD doesn't care either long as it shuts down)

 

[ron]

That is the proper way of doing it when it is required. Do not rely on their control logic or a BMS for a life safety issue.

Of course due to size and what it serves, sometimes shutdown isn't required.

 

[nhfire77]

That is the proper way of doing it when it is required. Do not rely on their control logic or a BMS for a life safety issue.

Of course due to size and what it serves, sometimes shutdown isn't required.

Yea, we had it sized over 2000 cfm and it serves multiple spaces, but its the odd one out in on the roof. I would never use the logic, they would just disable the shutdown when it was convenient for them. If fact that is what happened: We had it shutting down on alarm, and then hvac changed the wiring on us the next day. It was a pleasant surprise two days after we tested it, had our final with the FD one day later, it wasn't working.

The other RTU's have an duct smoke input built into the local RTU processor. Its your average lennox RTU, they show the classic 'A4' indicating alarm on the LCD display when you send 24V to TB1 terminal 21. That's the official method based on the Lennox manuals for shut down from an external fire alarm/duct detector source.

The BMS has no control over the RTU, it just controls the t-stat and monitors air temp and pressure in the unit for maintenance issues. So the home office can shut off the local control of the t-stat, but the unit maintains self control over all internal functions. There are IR/heat, temperature and occupancy sensors in the conditioned space Really quite a good cost saving solution without ever having to alter any site equipment.

 

[hysticali]

If fact that is what happened: We had it shutting down on alarm, and then hvac changed the wiring on us the next day. It was a pleasant surprise two days after we tested it, had our final with the FD one day later, it wasn't working.

Yeah. I am going to do a final tomorrow because we failed yesterday due to the Test and Balance guys jumping out the shutdown wire and the units wouldn't shutdown. We had pre-tested them 2 days prior. Would have been simple for them to fix the wiring once they were done.

 

[nhfire77]

Yeah. I am going to do a final tomorrow because we failed yesterday due to the Test and Balance guys jumping out the shutdown wire and the units wouldn't shutdown. We had pre-tested them 2 days prior. Would have been simple for them to fix the wiring once they were done.

I just did a 6 hour emergency service call, had to replace several duct smokes damaged oil (don't know from where exactly, bearing leg go maybe) It lightly coated the duct smokes. They were old, old, old ESL's and were hacked in. After doing a very nice clean up of the hack work, none of the RTU's including one's I didn't touch, shut down. I did find the wiring was in place and had been disconnected and capped in every RTU. From what it seems, they didn't have it hooked up just right so I'm gonna let them tell me how they want it shut down. Those HVAC guys must have disconnected that 'pesky' shut down a long time ago.

Now I get to go back and meet the HVAC doofs! (to be fair the company coming out probably didn't disconnect it).

This time I don't mind it, when its billable, I feel like hugging the HVAC guys.... well almost, maybe a Christmas card! E service=$ every time.

 

[B4T]

I have had problems even with using the 24V. power as a way to shut the blower down.. it doesn't work with the newer units from what I have seen..

You have to go after the load side with one of these powered from the FA system..

 

[nhfire77]

I have had problems even with using the 24V. power as a way to shut the blower down.. it doesn't work with the newer units from what I have seen..

You have to go after the load side with one of these powered from the FA system..

View attachment 6476

All Lennox Package RTU's have an input that will send the RTU in in alarm and shut down when 24 v is applied between term 6 and 21. Some units low volt terminal strips don't go above 16 or 18, but it can be added. However, it can be turned off in programming, but it is default on in most units. Other brands have similar make or break 24 v systems. You gotta get a decent hvac tech (not installer) they'll get you set up.

What units have you seen where killing the control panel 24 v doesnt shut down?

 

[B4T]

All What units have you seen where killing the control panel 24 v doesnt shut down?

Not sure of the manufactures I had trouble with.. most were in residential attics and I was happy to get out of there ASAP..

NOT very happy when I tested the shut down and it took (45) seconds.. :rant:

So now I avoid the 24V. that route and go directly for the load side.. works every time.. :thumbsup:

 

[nhfire77]

Not sure of the manufactures I had trouble with.. most were in residential attics and I was happy to get out of there ASAP..

NOT very happy when I tested the shut down and it took (45) seconds.. :rant:

So now I avoid the 24V. that route and go directly for the load side.. works every time.. :thumbsup:

45 seconds sounds like you are breaking the t-stat and the purge cycle will still run if you just drop the common feigning the t-stat turning off. Also, a resi RTU needing duct smokes sounds unlikely, but I dont know what the application.

 

[B4T]

45 seconds sounds like you are breaking the t-stat and the purge cycle will still run if you just drop the common feigning the t-stat turning off. Also, a resi RTU needing duct smokes sounds unlikely, but I dont know what the application.

I tried using RED.. power and GRN.. blower motor.. same results.. purge cycle kept blower motor energized..

These are residential group homes without duct detectors.. but must shut down on FA activation..

 

[nhfire77]

I tried using RED.. power and GRN.. blower motor.. same results.. purge cycle kept blower motor energized..

These are residential group homes without duct detectors.. but must shut down on FA activation..

Ah good point,

If you can trace from the low voltage transformer to the CPU, breaking that kills it instantly, I realize some don't have a LV powered board, but it's what I did in the OP.