Sorry if this is a stupid question, but any ideas are welcome.

Seems this would be a common use case and probably there is a ready made solution before I dig out my soldering iron out of the attic.

We did a home remodel and added an addition. Code forced me to put in a make up air unit for my stove exhaust vent. ($3K Yikes).

Anyway, we now have 2 furnaces, and when they both get cranking it creates a huge negative air pressure situation.

Simple me, thinks that since I already went through the expense of installing a Make Up Air Unit, that is idle 99% of the time, that I should just tap into that.

I know enough that I should just not splice 2 or 3 24V feeds to damper motor and hope for the best. It seems I would want an OR or XOR logic gate such that when 24V comes in on any one of the inputs (2 furnace fans, 1 vent hood) that it then spits out 24v to the damper motor to make it open and lesson the negative air pressure.

My house is constructed in such a way that I don't have easy access to outside walls (basement is below grade) so my only chance to put in more Make up Air Vents is through the roof.

I've googled until my fingers are bleeding, but all I can find are IC's not a ready made module or board.

Thanks in advance.

 

Welcome to AAC!

It seems I would want an OR or XOR logic gate such that when 24V comes in on any one of the inputs (2 furnace fans, 1 vent hood) that it then spits out 24v to the damper motor to make it open and lesson the negative air pressure.

Sounds like you want a 3 input OR function that could be implemented with 3 diodes.

 

I know enough that I should just not splice 2 or 3 24V feeds to damper motor and hope for the best. It seems I would want an OR or XOR logic gate such that when 24V comes in on any one of the inputs (2 furnace fans, 1 vent hood) that it then spits out 24v to the damper motor to make it open and lesson the negative air pressure.

Do you know the XOR gate function, when any one of input is different with the other two inputs then the output will be high or true, and the OR gate function is when any one of input is high then the output will be high or true?

The OR gate function can be easy done with three 1N4148 and one 24K resistor, but the XOR gate as CD4070 is more complex, maybe you can use 6 resistors to make three Voltage dividers at the inputs of CD4070 and add a voltage shifter to the output.

 

I assume the input voltages are all the same 24V, should match the furnace fans output which most HVAC stuff is 24V. In general if any furnace fan is running or the hood vent sensor is triggered, then the damper motor needs to fire and hold open the flap. Otherwise when no voltage is present the flap is closed by default.

It does sounds like a fund little easy project, I just don't have any spare time to build my own.

I've reached out to local HVAC contractor to see if they have any ideas. The builder that installed it said "NO" and walked away. I assume there is something like this already. The guys that just installed my AC said they were afraid to touch it. I can't be the first person who is too cheap to put in 3 different make up air units. Alternatively, I'll just remove the damper motor and let the pressure differential take care of things.

 

If the 24V is DC then you can just use three diodes of the appropriate current capacity to power the damper motor, as suggested by dl324.
That gives you an OR function (I don't think you want the XOR function which would shut off the damper if more than one input is high).

 

Sounds like you want a 3 input OR function that could be implemented with 3 diodes.

Furnaces are typically 24 VAC, so Oops.

You can give the make up air unit a separate power supply of 24 VAC, then place a 24 VAC coil of a relay at each system (furnaces 1&2 and that's energized when you want the make up air to be on.

Parallel the contacts of all three relays to the new 24 VAC make-up air supply and contactor.

The needed relays may already be present to handle 2 systems.

Other ways exist too. There are AC I/O modules (Isolated) that will detect the presence of AC and provide logic outputs. Here https://www.galco.com/techdoc/crou/iac5_cp.pdf is a quad module that requires a 5V power supply and has open collector outputs.
So, 1 module, a mounting base, a 5 V power supply and a 5 V relay or SSR feeding a contactor of some sort for your make up air.

Other logic supplies e.g (5, 12 and 24 VDC) are usually available and there are single modules.

 

Sounds Like I need to get some specs on the devices first. I assume the damper motor is 24vac, but I'm not 100% certain. Let me start with that. I did not see any noticeable make or model# that was obvious so I probably have to do some disassembly.

 

Modern furnaces might have an EAC outlet. This is usually 120 v that's energized when the unit's fan is running. You can tap off of that in the same way.

Contact closure communication at 24 VAC levels is easier. Let the furnaces and the vent hood just provide the potential free contact closures and integrate those at the make up air controller.

Switching Class II 24 VAC power limited wires doesn't require anything special like conduit.

I had a bunch of strobes to interface to a 24 VAC system and that was challenging. Each strobe was supplied from a power limited circuit.
I used a UL rated supply designed for access control and fire alarms from Alarm-Saf.

I imposed a rule that for any remote panel all lamps must be powered from the local panel. There were really cool thin DIN rail mounted relays that I used a lot of.

You can get some relays with indicator lights and "check buttons" that make troubleshooting really simple.

The "indicator LED" says the relay has power and the "check button" is a safe manual way to momentarily activate the relay.

DIN rail construction (it's like an electrical erector set with reuseable parts) is the way modern control panels are designed.

Look here: http://www.winford.com/products/cat_din.php

and

http://www.winford.com/products/cat_rly.php

for some ideas. Certain common footprints (Octal) have their own modules.

 

Thanks All

 

If you still need an OR function, that could be done with three 24Vac coil relays.

 

Furnaces are typically 24 VAC, so Oops.

GIGO.

The OP said 24V which implies VDC.

 

I'll check the specs tonight as best I can and put a meter on the damper motor to see if I get AC or DC.

 

Dampers are usually 24 VAC as well. They can be 2 or 3 wire. They CAN be a modulating damper.

They are usually a synchonous motor. You should find a capacitor and a common terminal and limit switches. Power is applied to the common and one side of the capacitor depending on open/close.

You can also have spring to close or spring to open.

A moduating damper generally depends on the time to open/close.

Somehow the two furnaces have to actuate the damper. They don;t usually like putting the dampers on the same transformer as the furnace.

You can do independent control of the furnaces with two stats or simultaneous control.