BY THOMAS R. FOX
This MCU-controlled Digital
Thermostat not only replaces mercury
thermostats but also monitors your
furnace/AC air filter and will let you
know if there is a problem with your
■ PHOTO 1. Completed
and installed SmartStat-Plus
mounted on a wall.
Did you ever wonder what would happen if your furnace’s blower motor failed, belt broke, or
something completely blocked your furnace’s air filter?
Well, most furnaces would get hotter and hotter, and the
thermostat won’t know it until it’s too late. If it wasn’t for
the furnace’s built-in high temperature limit control, the
furnace would keep going and going and not only destroy
itself, but perhaps a fire. If this adjustable safety control
happens to be set too high (or is faulty), damage can
occur to your furnace before the safety control shuts it off.
A similar thing can happen to a central air system.
■ PHOTO 2. SmartStat-Plus’ completed main
printed circuit board.
Here, the worst scenario is perhaps a fried compressor
(Note: A few systems also use a mechanical “Sail
Switch” which is supposed to help solve this and other
low air flow problems. However, they are used mainly
with electric furnace systems and aren’t suited for use in
established heating/cooling systems.)
The SmartStat-Plus presented here doesn’t just control
the temperature, it continually monitors the air flow. This
air flow information can be used to determine the relative
dirtiness of the filter and is used by SmartStat-Plus to
determine if there is a problem with the blower or filter.
An option switch on the SmartStat-Plus will give it
permission to shut off the furnace and/or A/C if the air
flow gets too low. Other safety features include a system
failure alert, a dangerously high room temperature alert,
and a freeze alert/control.
Other than the filter monitor/alert system and the
abundant safety features, the SmartStat-Plus differs from
commercially available digital thermostats because it is
What does the wind chill factor
have to do with SmartStat-Plus’
air flow sensor circuit?
Simply stated, they both depend upon the same heat
transfer phenomenon. The explanation goes something
like this: If an object is internally heated, the air is warmed
next to it. If air movement displaces this air warmed by the
object, the object will cool to near the air temperature.
Once this warmed air is blown away, stronger air
movements have no further chilling effect. This is why
winds over 70 mph do not feel colder than 50 mph winds.
SmartStat-Plus’ air flow sensor board uses two LM34