In progress, pictures coming.
One of my hesitations surrounding this project has been with burning out my HVAC systems blower motor because I do something lame like closing all of the vents and running the blower at full. Or perhaps opening and closing the vents in rapid succession causing static pressure surges. Fun, but not likely to gain me any brownie points with the family when the HVAC is down and it’s 110 outside. To that end, here are some of my safety concerns and how I addressed them.
Overloading the motor by having too many vents closed at once.
The HVAC blower motor has to move air at a sufficient velocity to overcome the static pressure present in the system and force air out of the registers and into the house. Pressure is built up in the plenum as that air moves through and increases more as it has to negotiate vertical surfaces e.g. upstairs. Each register that is closed increases the pressure inside the system and makes the motor have to work that much harder in order to do its job. While I’m not sure that closing all of the vents would actually burn out the blower motor, I’m pretty sure it would do some bad things to the heat exchanger/manifold simply due to lack of airflow. So to protect against that, I’ve attached a Freescale MPX4250 gauge type pressure sensor to the main plenum in the basement. To calibrate the sensor, I opened up all of the registers in the house, replaced the furnace filter and let the system run for 30 minutes taking pressure measurements every second. I then averaged those measurements and came up with my wide open number (230). From there I closed off all of the vents and registers and ran the system for a few minutes to get my danger-danger-Will-Robinson number (300). I arbitrarily decided, based on those numbers, that my safe zone was 230 to 260. If a reading between 261 and 279 is encountered, no additional registers are allowed to close and I get a twitter message. If a reading of 280 or greater is encountered, all registers are forced open and I get a twitter and an SMS.
The controller fails and all of the vents close anyway
That’s all fine and dandy assuming that the controller is working correctly. Should it flip out, the servo motors don’t return to 0 on their own and I’m not controlling the thermostat so a condition exists where all of the registers could still be closed while the blower is running and the software would not prevent it. I went with a very low tech solution to fix that little problem and used return springs attached to the servo motors. It does make the servo work much harder and I’m not sure how I’m impacting their life expectancy, but I can burn out a lot of $10 servos before I can break even with a $1200 HVAC service call.
In progress, pictures and more info coming.
June 9th, 2009 at 4:42 pm
What about putting a NO relay on the HVAC power switch? The arduino would always have to power the relay in order for the system to run. If the arduino looses power, the system shuts off, regardless of vent status. I could, however, see this solution becoming very unpopular with the family if it occurred too often.
Just a thought.
June 9th, 2009 at 9:41 pm
not a bad idea. I want to keep this all low voltage, so I wonder if I could achieve the same end by connecting the relay to the thermostat. I haven’t had the arduino freak out yet, but like so many things, my enjoyment in developing for and with the arduino is disproportionate to my ability, so it’s just a matter of time I think.
June 29th, 2009 at 9:11 am
If you monitor (vacuum) pressure across the return to the plenum you can also gauge when you need to replace your filter.
You can use a device like this as a guide to get you started.
http://www.airfiltergauge.com/
June 29th, 2009 at 9:37 am
Not a bad idea. I went a slightly different direction. The pressure sensors I’m using are dual ported and differential, so I just plugged one port into the low side and one into the high. I haven’t got a good number on when the filter is too dirty yet, but it’s only been a few weeks on a new filter.
June 29th, 2009 at 9:53 am
Almost forgot…
Overloading the motor isn’t the only issue you can run into. When heating if your not passing enough air over the heat exchanger it could possibly overheat. Causing it to crack, then CO gasses can get into the house. Although the furnace should shut itself down if you go above the allowable temps.
As well it’s possible to ice up the evaporator.
I’d add a temp sensor to both your supply and return vents. Watch the temperature differential…
Another spec on your furnace is the “heat rise” you don’t want to go over this number. This is the differential from the return to supply (measured immediately out of the furnace) while heating.
June 29th, 2009 at 9:58 am
Dude, you have spent far too much time reading the manual
Good call, I have a few extra sensors I’ll wire in this weekend and we’ll see what goodies she’ll give up! If I remember correctly, 15 degrees is about the max temp diff that you want to see during cooling, correct? Too much and it’s a sign that their may be a blockage somewhere, too little and it could be low on freon, etc?