Exhaust fan relay question.

[Diesel Dan]

Installing a booster fan in exhaust duct.
Need to control it from 3 separate switches on separate afci/gfi breakers. Looking for a relay, or digital controller, that would have 3 separate inputs/coils for one output, opposite of a 3PST (3-Pole, Single-Throw).
Is there a single, mechanical, relay for this or should I just use 3 cube relays and run the output in parallel?
Quick look at microcontrollers like an Arduino or ESP32 show way more capabilities than I need but still would require a relay to hand the actual fan load.

 

[mm08822]

3 ice cube relays could work as described but all switches (or whatever is driving each relay) would have to be off for the fan to stop. An enclosure would be needed for the relays.

What about 2 3ways and a 4 way arrangement?

 

[wssix99]

If for a standard HVAC system, there is probably an auxiliary relay/output available on your main board or thermostat to control an auxiliary fan.

If a custom system, I would use contactors. Here is an arrangement I have with 4 separate switches, each with a contactor switching on two separate circuits at the same time:

 

[Stuart in MN]

If you want the fan to be on whenever any of the three switches are closed you don't need a relay with multiple coils, just wire the switches in parallel to a standard relay.

 

[dave*99]

Need to control it from 3 separate switches on separate afci/gfi breakers.

If you want the fan to be on whenever any of the three switches are closed you don't need a relay with multiple coils, just wire the switches in parallel to a standard relay.

Probably not the best advice.

 

[Stuart in MN]

OK, I missed the part about separate breakers. It could be done by using a relay with a DC coil. Put a little 12 volt DC power supply in the box with the relay, then run one AC line to power the fan and use low voltage wiring to the switches.

 

[Diesel Dan]

3 ice cube relays could work as described but all switches (or whatever is driving each relay) would have to be off for the fan to stop. An enclosure would be needed for the relays.

What about 2 3ways and a 4 way arrangement?

Correct, would be in an enclosure and only off if all rooms are off.
3&4 way, are you referring to wall switches? If so, it's not practical.

If for a standard HVAC system, there is probably an auxiliary relay/output available on your main board or thermostat to control an auxiliary fan.

If a custom system, I would use contactors. Here is an arrangement I have with 4 separate switches, each with a contactor switching on two separate circuits at the same time:

Not HVAC, just exhaust fan controlled by wall switches.

 

[wyliesdiesels]

Why do they need to be on separate breakers?

 

[Diesel Dan]

Why do they need to be on separate breakers?

That is how it's currently wired.

 

[matt_i]

We should have a logic/wiring diagram, but if the relay NO contacts are all wired in parallel then each circuit has functionality....BUT if you turn it ON from ckt A you won't be able to go OFF from ckt B (etc).

If you desire functionality where you can go ON from any ckt and OFF from any other ckt then you have to use the relays to setup a pathway identical to a 3way-4way-3way switch. For emulating the 3 way function, a SPDT can be used and for emulating a 4 way you will need DPDT. Of course for simplicity all can be DPDT and you just use 1 pole.

 

[Diesel Dan]

We should have a logic/wiring diagram, but if the relay NO contacts are all wired in parallel then each circuit has functionality....BUT if you turn it ON from ckt A you won't be able to go OFF from ckt B (etc).

If you desire functionality where you can go ON from any ckt and OFF from any other ckt then you have to use the relays to setup a pathway identical to a 3way-4way-3way switch. For emulating the 3 way function, a SPDT can be used and for emulating a 4 way you will need DPDT. Of course for simplicity all can be DPDT and you just use 1 pole.

On with any switch, off when all switches are off.

 

[matt_i]

Simplest....

3way/4way functionality.....

Then simplified for wiring....

Sorry they are sideways. B1 and B2 just denote different "poles" which share the same coil.

 

[matt_i]

On with any switch, off when all switches are off.

Think you want the simplest diagram. All 3 NO contacts in parallel.

 

[BillK]

Is there currently three separate fans going into the one duct that will have the booster fan ?

 

[PCustoms]

Is there currently three separate fans going into the one duct that will have the booster fan ?

Not sure if this is where you were going, but if so, is there a pressure differential switch that can be used to turn the booster on when there is airflow?

 

[wyliesdiesels]

how about just putting it on the same circuit? would save a lot of complication

 

[Diesel Dan]

Is there currently three separate fans going into the one duct that will have the booster fan ?

yes but going with one larger inline fan for quieter operation

Not sure if this is where you were going, but if so, is there a pressure differential switch that can be used to turn the booster on when there is airflow?

Had considered powered/sealed dampers for rooms not in use but that adds more complexity with questionable returns.

how about just putting it on the same circuit? would save a lot of complication

If only that simple.

Here is a quick diagram I put together with cube relays, or individual contactors.
This way the AFCI breaker are separate and not causing issues with each other.

 

[PCustoms]

Had considered powered/sealed dampers for rooms not in use but that adds more complexity with questionable returns.

Has nothing to do with what I was suggesting

 

[JohnX14]

Has nothing to do with what I was suggesting

Your suggestion is what I'd have suggested. Airflow sensor that pulls in the booster fan. Regardless of how the upstream fans are wired, airflow will pull on the booster fan. Pretty simple setup. Of course it could be done with relays such that any one fan pulls in the auxiliary fan, but that is far more wiring than your suggestion.

 

[Diesel Dan]

Has nothing to do with what I was suggesting

No use for pressure sensors when there will be no upstream fans.

 

[Diesel Dan]

Your suggestion is what I'd have suggested. Airflow sensor that pulls in the booster fan. Regardless of how the upstream fans are wired, airflow will pull on the booster fan. Pretty simple setup. Of course it could be done with relays such that any one fan pulls in the auxiliary fan, but that is far more wiring than your suggestion.

What air flow sensors are you referring too?
Was looking at a Siemens velocity sensor ($200) but it can't directly control a fan so needs additional controls as well.

 

[BillK]

I didnt think you were supposed to hook multiple fans into one duct ???

Either way if you are doing away with the individual fans then the relays are going to be your only choice. Seems like a wiring nightmare unless all three of the switches are in very close proximity.