Range Hood Pressure Switch - Need One that is Normally Closed

[larry4406]

I currently have a Broan 97018853 pressure switch on the range hood exhaust. Remote roof top blower, so switch is connected to the vacuum port.

This switch only has a "NO" normally open set of contacts, meaning upon actuation, the switch closes and allows current flow. This in turn allows a 24VAC transformer to power dampers to activate the makeup air system and it works just fine.

I want a similar switch that is "NC" which is normally closed. I want to dropout (disconnect) the HVAC thermostat blue common wire whenever the range hood is operating. Yes - turn off the the entire HVAC system when the range hood is running.

Broan-Nutone does not offer such a NC pressure switch (I just called their tech support line). Oddly the back of the plastic case looks like its ready to receive a spade terminal for NC.

Can I use a 24VAC contactor powered thru the existing pressure switch to obtain a set of NC contacts? Recommendations for cheap contactor to do this?

 

[fitter30]

https://www.amazon.com/TWTADE-Electromagnetic-Indicator-Socket-YJ2N-LY/dp/B09W25T8Q9/?tag=atomicindus08-20

If you want the blower to run just use the Y in the drawing
Drew it killing both the blower and ac.
Couldn't find just one relay with supplied din rail.

 

[mm08822]

Look into furnace pressure switch replacement parts. These will typically be N.O./N.C. with a single common terminal. (Called form "c".)

Or just throw a dpdt relay in a 4-11/16 sq box and drive it from the existing sensor.

 

[yatg]

I want a similar switch that is "NC" which is normally closed.

That won't work for what you want to do. You've got a 24v system for the dampers, and the HVAC has its own 24V system. Not integrated. It would work if you could find a pressure switch that's DPDT. It
Sorry, that would work if you used a NC pressure switch in addition to your NO switch for the dampers.

I want to dropout (disconnect) the HVAC thermostat blue common wire whenever the range hood is operating. Yes - turn off the the entire HVAC system when the range hood is running. Can I use a 24VAC contactor powered thru the existing pressure switch to obtain a set of NC contacts? Recommendations for cheap contactor to do this?

You actually want to switch the "red" into the tstat rather than the "blue" common. A tstat gets one side of the 24v from the red, and sends it to white/green/yellow depending on the mode. The other "common" side of the 24v is connected to the electronics/relays in the hvac units. Switching the red before it gets to the tstat is just like turning the tstat to off.

The common going to your tstat is a courtesy wire and doesn't do any switching. Switching the common might work if you intercept it right at the hvac transformer, but that might cut off power to stuff in the unit that expects to be on all the time.

Use a 24vac coil SPDT relay.
For the coil, one lead from the xformer, the other from the switched side of the pressure switch.
Connect the tstat "red" feed to the NC terminals.

pressure switch inactive, no power to relay, red is connected (NC), HVAC normal operation
pressure switch active, relay activated, red is disconnected, HVAC disabled

Any regular size relay will work.
You're switching hvac control voltages, usually less than an amp.
example:

Amazon.com: Supco 90370 General Purpose Fan Relay, 12 A Load Current, 24 V Coil Voltage, Single Pole Double Throw Contacts : Industrial & Scientific

Amazon.com: Supco 90370 General Purpose Fan Relay, 12 A Load Current, 24 V Coil Voltage, Single Pole Double Throw Contacts : Industrial & Scientific

www.amazon.com

 

[larry4406]

That won't work for what you want to do. You've got a 24v system for the dampers, and the HVAC has its own 24V system. Not integrated. It would work if you could find a pressure switch that's DPDT. It
Sorry, that would work if you used a NC pressure switch in addition to your NO switch for the dampers.



You actually want to switch the "red" into the tstat rather than the "blue" common. A tstat gets one side of the 24v from the red, and sends it to white/green/yellow depending on the mode. The other "common" side of the 24v is connected to the electronics/relays in the hvac units. Switching the red before it gets to the tstat is just like turning the tstat to off.

The common going to your tstat is a courtesy wire and doesn't do any switching. Switching the common might work if you intercept it right at the hvac transformer, but that might cut off power to stuff in the unit that expects to be on all the time.

Use a 24vac coil SPDT relay.
For the coil, one lead from the xformer, the other from the switched side of the pressure switch.
Connect the tstat "red" feed to the NC terminals.

pressure switch inactive, no power to relay, red is connected (NC), HVAC normal operation
pressure switch active, relay activated, red is disconnected, HVAC disabled

Any regular size relay will work.
You're switching hvac control voltages, usually less than an amp.
example:

Amazon.com: Supco 90370 General Purpose Fan Relay, 12 A Load Current, 24 V Coil Voltage, Single Pole Double Throw Contacts : Industrial & Scientific

Amazon.com: Supco 90370 General Purpose Fan Relay, 12 A Load Current, 24 V Coil Voltage, Single Pole Double Throw Contacts : Industrial & Scientific

www.amazon.com

I don't want an integrated system.

I simply want a set of contacts that is powered off (open) when the pressure switch activates. This then makes/breaks the control of the heat pump.

I have a spare pressure switch (same eff'n NO version) and spare 24VAC transformer which can be dedicated to this and not part of the existing damper system if necessary.

I am thinking a powered contactor inline with the existing damper and pressure switch. This contactor, when having no power has a set of normally closed contacts. These NC contracts have the intercepted thermostat control wire pass thru so the HVAC operates as normal. Then when the hood pressure switch operates, the contactor changes state (meanwhile dampers not affected), and the HVAC system drops out.

The heat pump is a Goodman unit. My HVAC foreman at the day job specifically said to make/brake the blue common wire to ensure that it drops out under all 3 modes of operation - heat, resistance heat, AC.

Existing thermostat is a Honeywell TH5220D1029.

 

[yatg]

 

[larry4406]

Yes - that is what I am thinking.

Red or blue thermostat wire to intercept?

I can't find a wiring diagram for that Supco relay you linked.

Found this one on McMaster Carr. Diagram seems quite clear.

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

 

[yatg]

Whatever relay you want, as long as it has NC contacts.

I vote red.

 

[larry4406]

Update on this project.

It is installed and working!

I installed a dedicated 24VAC transformer, a dedicated NO open pressure switch, switch provides power to a Subco 90340 relay when hood is on (pressure switch closes). Used NC terminals 1/2 on the relay to intercept the RED wire of the thermostat.

On hood shutdown, the large roof top blower wheel takes some time to spin down, thus there is a pressure decay in the hood duct. Appears to be about 45 seconds from hood switch off to when the relay resets and HVAC comes back on.

Project got delayed due to unrelated disconnect breaker failure saga.

Thanks to all that assisted with this project!