Need advice on how to operate make up air fan using room pressure monitor

Thread Starter


Joined Oct 24, 2013
I bought a GE room pressure monitor (RPM-1), and I'd like to install it in my house to operate a make-up air fan for when the house becomes depressurized (e.g., my wife turns on both gas clothes dryers, or a piece of wood is burning in the fireplace, sending a ton of air up the chimney). The sensor will be on the outside of the house, effectively turning the house into the 'room'.

The room pressure monitor literature indicates:

--The room pressure monitor comes standard with a SPDT relay (for supplemental controls such as audible alarm or remote status indication), and 3
analog output signals, 0 to 5 VDC, 0 to 10 VDC and 4 to 20 mA. These analog outputs are proportional to the room pressure.

From this description, I believe it means that as the pressure difference between the outside of the house and the inside increases, that the analog output signal will increase as well.

I believe I can connect the analog output signal from the RPM to an electronic fan speed controller (EFSC):

Here is the description of an EFSC by sentera-controls:

The EVS automatically controls the speed of single phase (230 VAC – 50 Hz) voltage controllable electric motors with a 0-10 VDC or 0-20 mA control signal selectable by switch. It is possible to invert the control signal to 10..0 VDC/20..0 mA. To power on, an external switch on the power supply is provided. A supplementary terminal block is foreseen to branch off 230 VAC not controlled for 3-wire motor connection, a telltale or to control a valve or damper.

There are two working modes, internally selectable by switch: kick start or soft start.

  • Supply: 230 VAC 50 Hz
  • According to Low voltage directive: 2006/95/EC, EMC directive: 2004/108/EC
Does this seem feasible?

Any advice would be appreciated--I am experienced with 120 volt residential wiring, and have a multi-meter, but this low power circuit stuff is a little foreign to me.




Joined Feb 11, 2008
Umm, maybe this is an oversimplifaction, but why do you need a fan or electronic system at all?

If the clothes dryer or fireplace are making your house "depressurised" ie lower pressure than the outside air, why not just have a vent? Surely the air will automatically flow from the higher pressure area (outside?) to the lower pressure area?


Joined Sep 9, 2010
... why do you need a fan or electronic system at all?
I have a dryer and a couple fireplaces (which I don't think can substantially reduce pressure) and so on, and I certainly don't need anything other than the cracks.

To be a problem, your low pressure would have to fall 3 psi or so below the ambient, equivalent to gaining 5000ft of elevation. I think this would be essentially impossible in any normal house. All your windows would implode.


Joined Mar 14, 2008
I think you have a solution is search of a problem. I don't see how your clothes dryer or fireplace can cause any significant (or even measurable) change in the internal pressure of a house with normal cracks and vents to the outside.


Joined Nov 30, 2010
This seems very strange. Some appliances are required by law to have a make-up air supply, but this can usually be done with a piece of PVC pipe and a filter. Some gas fired water heaters use coaxial pipe so the make-up air enters in the space surrounding the hot flue gas. I have heard of houses that are 60,000 square feet, but the owner of that house wouldn't be designing the HVAC system. So...what led you to believe you need a forced air make-up system? How did you arrive at the necessary CFM?

Thread Starter


Joined Oct 24, 2013
A vent will work, but it requires a large opening. A powered fan allows the use of a much smaller opening/ duct. Plus, a heater can then be used to temper the incoming cold air.

The house will be sealed very tight via the use of spray foam--no cracks or vents for air to naturally infiltrate.

A common problem with tightly sealed houses is backdrafting of chimneys and standard water heaters/furnaces (the homeowner runs the dryer and range hood, and suddenly you get ashes shooting out of the fireplace due to backdrafting).


Joined Mar 14, 2008
Back to your original question. Yes it would appear that you can use the output of the pressure monitor to drive the fan controller but you may need a way to control the gain of the fan speed versus pressure signal, if the controller doesn't already have that.


Joined Jun 26, 2012
As a first broad cut, pick off the 0-10V proportional output of the room pressure monitor and use it as the signal to the motor controller. Set up the motor controller for inverse sensing (SW1 up) so that a low room pressure means more fan drive. Use SW2/PT3 to enable and set an ON threshold.

This winds up being a proportional (only) loop control so expect a lot of tinkering and I would expect that the make up fan will pretty much run at some fixed maximum when its called for to avoid oscillations but since you have the stuff, give it a try.
Yes it would appear that you can use the output of the pressure monitor to drive the fan controller but you may need a way to control the gain of the fan speed versus pressure signal, if the controller doesn't already have that.

BTW Its not a bad idea. Running at a slight positive pressure with filtered make up air sure helps to keep the dust and other contaminants out, gas heaters notwithstanding. I did controls work for a biological company way back when and our labs and fume hoods all had various degrees of pressure management (usually determined by pulley sizes and dampers/diverters). #12 would be the guru here for that.

Datasheets for the two units:


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Joined Nov 30, 2010
Trying to get a sense of proportionality. Normal range of vacuum on a 1000 cfm air handler is about .1" of water column. I have measured the outside half of a 2 ton air conditioner at .15 inches of water column. The point is, .1 inches of water column can move an awful lot of air.

Meanwhile, air is very compressible and has more inertia than you would expect. The sensor can NOT be close to the fan. It would respond in a fraction of a second and it is not strange for a pressure change to take 3 seconds to move through a building. The weakest pressure fan, a propeller type, is sufficient for this job, but a squirrel cage fan has a smaller inlet and can be fitted with a filter more easily.

The air flow in a chimney is directly proportional to the altitude of the chimney and the temperature difference. Measuring the air flow of a cold fireplace is a fools errand.

A lot of air conditioners are fitted with outside air intake ports. They use the existing equipment, fan, and ductwork to filter, dehumidify, cool, heat, and deliver the make-up air throughout the building. Problem is, the central air system will often not be running when the fireplace or clothes dryer is running and it is difficult to control the speed of a fan that is dedicated to a different job than what you want. You can use the ductwork to deliver the make-up air, but you will need to provide ways to control the amount. I'm thinking about a separate, 1 speed fan motor and a proportional electric damper. Your electronics would control the damper, not the motor. You could add a relay in "or" configuration to activate the central fan motor.

OK. That's thoughts for the moment, considering that this moment has zero information about how much air you will need.


Joined Mar 31, 2012
One concern I'd have about measuring the pressure difference between inside and outside, given the very low pressure differentials you are looking for (the unit maxes out at 0.1in of water, which is roughly 0.004 psi) is being extremely sensitive to airflow around the house due to winds and breezes. My understanding is that even moderately strong winds can result in pressure differences between windward and leeward sides of a bulding that on this order. I would imagine that even fairly gentle breezes could produce differences that are in the range that you will be using as your threshold.


Joined Nov 30, 2010
That is a legitimate concern. A steady breeze (which can blow for days) will reveal any defect in the house. Just keeping up with the air that goes out the chimney puzzles me because the apparent cure is a steady flow of outside air, and the heat and humidity that comes with it. I think a glass door for the fireplace is in order. When you add a glass front, my first instinct is to look for an outside wall nearby to use for a pipe to bring combustion air.

Short term movements can be handled with an RC time constant or a pneumatic time constant made with a length of small tubing and a rigid container.

This could turn into a nightmare very quickly. The only possible route now is to quantify the air flow and find out if this is even practical. Many electric companies provide a whole building leak measurement for than $100. They believe they are checking the duct work, but they are actually checking the whole building. Good place to start.