Fresh-Air 1.0

Fresh Air Intake - Residential HVAC Controller

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A typical residential air system only circulates air in the home and relies on infiltration, opening doors and windows, to provide fresh air. Adding some duct work and this controller allows us to provide fresh air into the home without having to leave windows and doors open. The controller allows us to decide when to add the air for the most economical impact.

You can buy one of these things from several sources and probably have one installed by your local A/C repair guy. What's the fun in that? I've owned two of these from various companies and have been highly dissatisfied, hence this project.

Indoor Air Quality/Air Side Economizer Theory: If you can use fresh air of the appropriate temperature and humidity instead of conditioning (heating or cooling) return air you should be healthier and wealthier.

There are two schools of though on how to accomplish this. The simplest is the sample the outside air temperature and if it's within range let some in. A more complicated scheme samples the outside air temperature and the return (inside) air temperature and if the difference is within range let some air in.

We can complicate things even further by introducing humidity into the equation. Here along the Gulf Coast this is a big deal in the summer.

This is project basically a customized 'shield' that sits on top of an Arduio UNO. The biggest issue I have with the commercially available controllers is the lack of flexibility with the set points and logic. By using the UNO we can do whatever we want with regards to the logic.

The basic system has two operating modes, just like those mentioned early. In the 'absolute' mode we just sample the outside air temperature and go from there. In the 'differential' mode we sample outside and inside, take the difference and go from there.

The mode is controlled by the switch #1 on the 4 position dip switch.

The system controls the damper and the furnace blower independently. If the temperature, or differential, is within a larger range we open the damper and run the outside air (OA) fan. This provides fresh air into the system especially when the furnace runs. Within a narrower temperature range we also force the furnace blower to run for a prescribed amount of time. Switches 1-3 define the run time, between 0 and 30 minutes per hour.

***Disclaimer - This project involves messing with the control wiring on your furnace. i suspect this will void the warranty should anything go wrong. You have been warned ***

There are 3 main pieces of hardware -

1) The controller board is basically a marriage of a 5-VDC power supply and a relay control board commonly used with micro-controllers. The terminal blocks make it easier to connect to the house thermostat and temperature boards. The on-board power supply lets us run everything from the existing 24-VAC control voltage.

2) The temperature boards are general purpose single or dual channel boards that support both digital, DHT11/22 type temperature/humidity sensors, or simple ADC TMP36 analog temperature sensors. We can use a DHT11 or TMP36 on either channel and both channels can be different by changing jumpers on the controller board. Since the DHT11 needs a pull-up resistor we can't have both DHT11 and TMP36 on the same channel.

This board is configured for 2 DHT11/22 devices. The temperature sensor needs to be mounted out of direct sunlight in a dry-ish place. I put mine under the eaves on the north side of the house. I put a small, screened opening in the box so the temperature sensor is exposed directly to outside air. In a harsher climate you could seal the box and it would work just fine. It would take a bit longer for changes in temperature to register but that's all.

3) The fan control relay box. ****WARNING - HAZARDOUS VOLTAGES PRESENT**** This box simply houses a 24V relay controlling a standard 120V 15A outlet which runs the OA fan.

Ok, so actually there's a 4th piece of hardware that's not electronic but critical. That's the air intake system. My second major beef with commercially available systems is they use flexible duct and take no precautions with routing and distance. This in essence guarantees almost zero outside air will actually make it into your house. Since I live in the south, my installation is skewed in that direction. If you actually live where there's snow in the winter you'll need to adjust. Or move.

Step 1 - locate a source of outside air as close to your return plenum as possible; I put a vent in my roof on the north side of my house. You could consider just using attic air but keep in mind it will take longer for attic air to...

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  • Captain's Log, Stardate....oh come on, it's funny.

    Jeff Thomas10/31/2014 at 12:22 0 comments

    Day 1 - Got it all installed and it's working. Sort of. I had to run a USB cable from the attic so i could see what was going on. For some reason the dip switches aren't pulling down the inputs like they were on the bench. I have no idea. I'm going to build up a second unit and see if I can replicate it on the bench.

    In the meantime I just modified the software to ignore the dip switches and it's working as expected.

    Day 2 - Temperature dropped into range about 1800 hours yesterday and the unit operated as expected. I have it set to run for 15 minutes every hour.

    Day 3+ - System is working as expected. The problem with the dip switches was resolved. The USB connector is too tall for the header pins that extend into the UNO. Basically there was no connection, or a very tenuous one at best. The quick fix is to extend the pins with stackable headers.

    Second problem is the accuracy of the analog temperature sensors. Using the coefficients provided by the vendors they still read as much as 10 degrees lower than the DHT11 based units. I'm going to switch back to a DHT11 outside. This also gives me the flexibility to control based on humidity.

    Day 14 - Decided to add a display so I can see what's going on. The 50' USB cable into the attic was problematic at best. The unit is running as expected I just wanted to see what it was thinking:

    Ok, so what is this telling us? RT = Run Time, WT=Wait Time and these items will flash and count down during their intervals. DO/DC = Damper Open/Closed, FO/FF = Fan On/Off, the temperature is 71.4 and the relative humidity is 35.7. Mx = Mode, Absolute or Differential. In the lower right hand corner, not shown here, is a little blinking thing that will flash every couple of seconds if the unit is waiting for a new temperature sample.

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    I have a few of each type of PC board available and I have the Express PCB files. If you're interested let me know and we'll work something out.

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