Relay controlled fans from furnace

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Mustwin351 said:
Excuse my ignorance but having never used a relay in a box I'm guessing there is not a need to install a barrier between the high and low voltages? Does that only apply to switches?
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If the control circuit is class 2 or 3, the nipple into the RIB is listed but whatever you supply it with needs to have separation between class 2/3 conductors and power conductors. It doesn't necessarily need to be a barrier that separates them though.
Class 1 control circuit would not require any separation at all.
 
kwired said:
If the control circuit is class 2 or 3, the nipple into the RIB is listed but whatever you supply it with needs to have separation between class 2/3 conductors and power conductors. It doesn't necessarily need to be a barrier that separates them though.
Class 1 control circuit would not require any separation at all.
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So is it allowable by code to mix a class 1 and class 2 circuit in the same box?
 
Mustwin351 said:
Ahh...thank you.

I am guessing a device like this relay:
https://www.functionaldevices.com/downloads/datasheets/RIBHX24BA.pdf

You could by code only use it for replacing the same relay in a listed piece of equipment being that there is not separation between the high and low voltage. Is that correct?
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That is an interesting question. Equipment is not covered under the NEC, except that in some parts it talks about "listed for the intended use" or similar such language. If you are replacing like for like you probably don't have an issue. If you are replacing an internal component with an external component, now I'm not so sure.
 
gadfly56 said:
That is an interesting question. Equipment is not covered under the NEC, except that in some parts it talks about "listed for the intended use" or similar such language. If you are replacing like for like you probably don't have an issue. If you are replacing an internal component with an external component, now I'm not so sure.
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I was thinking the same. So with that particular RIB where there is no separation between high and low voltages.....what situation or place could you actually use it legally by code?
 
Mustwin351 said:
I was thinking the same. So with that particular RIB where there is no separation between high and low voltages.....what situation or place could you actually use it legally by code?
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The high and low voltages are wired through the same hole, so no separation there. But the wires are properly insulated to allow the low voltage circuit to be classified as Class 1 and wired in the same raceway with functionally related power wiring. And the coil of a relay is very much functionally related to the relay output circuit!

Just make sure that the circuit driving the relay does not have to be Class 2 and be sure to use Chapter 3 wiring throughout. (On further consideration, that may take away the attractiveness of LV control in the first place.) :(
 
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Mustwin351 said:
Ahh...thank you.

I am guessing a device like this relay:
https://www.functionaldevices.com/downloads/datasheets/RIBHX24BA.pdf

You could by code only use it for replacing the same relay in a listed piece of equipment being that there is not separation between the high and low voltage. Is that correct?
Click to expand...

so are you saying the UL "part" can only be used as a replacement part, and not allowed to be used for making your own relay system? that doesnt make much sense to me because the risk you describe is the same whether or not you add it to a 4x4 box to make a relay, or if is attached inside a hvac air handler. or am i missing something?

how does one obtain isolation? isnt that just a transformer? so is the goal to use a 24vac isolation transformer between furnace and rib ?? maybe use two or 3 isolation xfrmers in series, waste a little power, but heck, it will be super-duper isolated in the event the rib coil shorts into one of the contacts ;)

its almost like xfrmers used for landscape lighting. if worried about using the UL rib that is by itself not in UL equipment, then use a gfi to protect both a short in the rib and the fans? now thats very very very very safe.
 
The thread mounted RIB is not just a UL Recognized Component (backwards RU) but a Listed device. It can be used in the field as long as the restrictions in my earlier post are met.
 
purduephotog said:
I have a number of booster fans I need to turn on and off from the furnace. My original thought was to use the aux 24v relay in the furnace to switch a second circuit (load would be about 400w).

This would require running 14/2 everywhere.
Jason
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Just out of interest, what decides when to turn the fan on or off? Is it a manual operation or is there some sensor?
 
Besoeker said:
Just out of interest, what decides when to turn the fan on or off? Is it a manual operation or is there some sensor?
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For my money I'd put in room thermostats that call for the fans to come on if the temp falls outside a certain range.

Lets say you want the house to always be at between 68 and 74. As long as it is, the thermostat doesn't call for fan on. Otherwise, whether it's too warm or too cold, the thermostat says "Fan on!" The neat thing about that is it doesn't matter if the furnace is set to Heat, Cool, or Auto. If the room is too cold, presumably the house generally is trying to warm, so you won't be demanding cold air when it's cold. Likewise if the room is too warm. The only thing is that the thermostats will need an independent power supply. Probably a 24V transformer off the same circuit powering the fan it controls.
 
Besoeker said:
Just out of interest, what decides when to turn the fan on or off? Is it a manual operation or is there some sensor?
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sounded to me like he would use the 24v from furnace and extend that out to low-v switches in each location. that assumption based on his comment that 1rib per fan was he choice for independent control of each fan. but to do that some other switch/device would need to be installed to switch the rib but only works when furnace is running.

if that is the case, i would simply rib the 120v right there at the furnace and then run 14/2 like any other ckt, out to switch box(es) then up to fan(s). 1rib & indpendent control.
 
gadfly56 said:
For my money I'd put in room thermostats that call for the fans to come on if the temp falls outside a certain range.

Lets say you want the house to always be at between 68 and 74. As long as it is, the thermostat doesn't call for fan on. Otherwise, whether it's too warm or too cold, the thermostat says "Fan on!" The neat thing about that is it doesn't matter if the furnace is set to Heat, Cool, or Auto. If the room is too cold, presumably the house generally is trying to warm, so you won't be demanding cold air when it's cold. Likewise if the room is too warm. The only thing is that the thermostats will need an independent power supply. Probably a 24V transformer off the same circuit powering the fan it controls.
Click to expand...

Admittedly I haven't work with HVAC for years, but that doesn't sound like a standard TSTAT from the local Big Box. Maybe with a bit of tweaking.
 
Besoeker said:
Just out of interest, what decides when to turn the fan on or off? Is it a manual operation or is there some sensor?
Click to expand...

What he calls the "aux 24 volt relay" is only energized (contact closed) when the furnace is running. These are commonly called the 'X / Y' terminals on the furnace control.
It is common to add a local thermostat in series so that the booster fan only runs when the furnace is running AND the thermostat is calling for heat.
 
ptonsparky said:
IMO, Boosters are the result of poor design to begin with.
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They're often needed when air conditioning is added to an old house. The duct design that was perfectly adequate for distributing warm air often needs a little boost to get cool air to the second floor, now that gravity & buoyancy are opposing airflow instead of aiding it.
 
gadfly56 said:
For my money I'd put in room thermostats that call for the fans to come on if the temp falls outside a certain range.

Lets say you want the house to always be at between 68 and 74. As long as it is, the thermostat doesn't call for fan on. Otherwise, whether it's too warm or too cold, the thermostat says "Fan on!" The neat thing about that is it doesn't matter if the furnace is set to Heat, Cool, or Auto. If the room is too cold, presumably the house generally is trying to warm, so you won't be demanding cold air when it's cold. Likewise if the room is too warm. The only thing is that the thermostats will need an independent power supply. Probably a 24V transformer off the same circuit powering the fan it controls.
Click to expand...

OK - furnace as in domestic use...........I should have had my American hat on...............:D
 
Mustwin351 said:
So is it allowable by code to mix a class 1 and class 2 circuit in the same box?
Click to expand...
Yes, but generally still need to provide some separation between them. If not allowed in same box we could never place a relay/contactor with power/lighting and class 2 control coil within the box.

Mustwin351 said:
I was thinking the same. So with that particular RIB where there is no separation between high and low voltages.....what situation or place could you actually use it legally by code?
Click to expand...
24 volts can still be class 1 control circuit in some cases, but in such cases the entire control circuit is class 1. You can not have class 1 and class 2 or 3 "sections" on the same control system - the entire system is the same class.

gadfly56 said:
For my money I'd put in room thermostats that call for the fans to come on if the temp falls outside a certain range.

Lets say you want the house to always be at between 68 and 74. As long as it is, the thermostat doesn't call for fan on. Otherwise, whether it's too warm or too cold, the thermostat says "Fan on!" The neat thing about that is it doesn't matter if the furnace is set to Heat, Cool, or Auto. If the room is too cold, presumably the house generally is trying to warm, so you won't be demanding cold air when it's cold. Likewise if the room is too warm. The only thing is that the thermostats will need an independent power supply. Probably a 24V transformer off the same circuit powering the fan it controls.
Click to expand...
If I were to do it that way I would probably look into line voltage thermostat and directly control the fan with it.
 
kwired said:
Yes, but generally still need to provide some separation between them. If not allowed in same box we could never place a relay/contactor with power/lighting and class 2 control coil within the box.

24 volts can still be class 1 control circuit in some cases, but in such cases the entire control circuit is class 1. You can not have class 1 and class 2 or 3 "sections" on the same control system - the entire system is the same class.

If I were to do it that way I would probably look into line voltage thermostat and directly control the fan with it.
Click to expand...

Yep, that's a better idea.
 
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