| In
This Article:
An existing thermostat is
removed and replaced with a new energy-saving set-back
thermostat. Please read the mercury warning at the end of this
article. |
Related
Articles:
|
| Skill Level:
2 (Basic) |
Time Taken:
About ½ Hour |
By
Bruce W. Maki,
Editor
Perhaps the easiest way to reduce residential heating and cooling
energy usage is to turn back the thermostat when people are
away from home or sleeping. By "turn back" I mean two
possible things:
- Lower the temperature during heating season;
- Raise the temperature during air conditioning season.
Why Does This Save Energy?
One of the fundamental laws of the science of Heat Transfer
is that the rate of heat transfer between an object and
its surroundings is proportional to the difference in
temperature between the object and its surrounding
environment.
If there is a big difference in temperatures, your house
will lose more heat per hour than if the temperature
difference were small. This is intuitive: your furnace runs
more on cold winter nights than during milder spring and fall
weather.
A couple of years ago I wrote a little article about the science
of heat transfer, and it explains this in more detail. |
 |
The basic Honeywell thermostat.
This device controls both heating and air
conditioning. It also has a fan control switch, which
you can see in the photo, that lets you run the fan all
the time.
|
|
| These Honeywell thermostats have a plastic
ring that just pulls straight off. |
 |
|
 |
Thermostat without the cover.
There are some exposed electrical connections here,
but this is only 24 volts AC.
|
|
| In this house the furnace is in the crawl
space. |
 |
|
 |
I turned off the power to the furnace.
|
|
I also could have simply turned off the breaker for the furnace,
if I hadn't wanted to go into the crawl space. Not all houses have a
separate circuit breaker for the furnace.
All furnaces are supposed to have a disconnect switch (which may
be a simple light switch) within reach of the furnace. On some very
old houses there may not be a disconnect switch.
| There are 3 small screws that hold the
Honeywell thermostat to the base plate. |
 |
|
 |
The thermostat came right off, with no
wires attached. |
|
| The base plate has all the wiring
connections.
The blue wire was connected to the terminal for the yellow
wire, which is marked "Y".
All the other wires were connected to their
respective terminals. |
 |
|
 |
I unscrewed four connectors. |
|
| I found it necessary to use pliers to
carefully unwrap the wires from the screw terminals. |
 |
|
This is why you need to turn off the power to the furnace.
There will be 24 volts at some of these wires, and if you touch the
wires together you will cause the furnace (or air conditioning) to
momentarily turn on. That's bad for equipment.
 |
I removed 2 screws that held the base
plate to the wall. |
|
| I removed the base plate, and pulled the
4-wire cable through the hole. |
 |
|
At this point remember one thing: Don't let that wire fall
back inside the wall cavity, or else you'll have to fish it out,
which can be a real chore.
The Replacement:
 |
This is the Ace Hardware ATX500 Series
programmable thermostat. |
|
| There is a release tab on the bottom of
the thermostat to separate the unit from the base plate. |
 |
|
 |
This thermostat also employs the base
plate as a connection point to the control wiring.
There are 5 metal pins that provide electrical
connections to the actual thermostat.
|
|
This thermostat has 5 connections, but two connectors are joined
with a jumper wire, which you can see in the photographs.
| A small problem: If I simply mounted the
new base plate, there would be some unpainted drywall
exposed. |
 |
|
 |
So I enlarged the hole with a knife.
This will allow the wire to move over.
|
|
| I held the base plate against the wall and
marked the screw holes. I was able to reuse one of the
old holes. |
 |
|
 |
I drove in the two mounting screws. I was
surprised to find wood behind the drywall, so I didn't
need any drywall anchors.
I used my cordless impact driver here, but using a
simple screwdriver might be a better idea because it's
easy to over-tighten small screws and ruin something.
|
|
| The instructions showed which wires to
connect.
There are several different wiring schemes out there,
and I'm not familiar with all of them. This 4-wire
scheme (for heating and air conditioning) is quite
common. |
 |
|
 |
I connected the 4 wires to their proper
terminals. I didn't need to curve the wire into a hook,
just slide the wire under a metal piece and tighten the
screw. |
|
| I installed the batteries in the new
thermostat. |
 |
|
 |
I snapped the new thermostat onto the base
plate.
Then I turned on the power to the furnace.
|
|
| I cranked up the temperature to see if
everything worked. |
 |
|
 |
The finished installation.
The programming for this thermostat is explained on
the fold-down cover.
|
|
This thermostat has 4 set points per day, which is meant for all
you workin' fools that spend all your time away from home. The
temperature can rise in the morning, then fall back during the day
when your gone. When you get home late in the day the temperature
has risen again, and then falls back at night while you sleep.
This product has default time and temperature settings, but of
course you can re-program the thermostat to your own preferred
settings.
Warning: Old Thermostats May
Contain Mercury!
Many old thermostats, such as the Honeywell unit that I
removed in this article, use a mercury switch to turn
the furnace on and off. Mercury switches are very effective
and are part of an excellent design, but with one dark side:
if that mercury ever gets out of its little glass bulb it
becomes a serious health risk and an evil environmental
pollutant. A recent article in Discover magazine illustrates
how truly wicked mercury can be. The liquid metallic form,
which is inside the mercury switch in a thermostat, isn't so
bad, but the vapors given off by the liquid can be
lethal in amounts measured in micrograms... that's millionths
of a gram! When mercury gets into the environment, whether
it be the soil, air, or water, it just wreaks havoc. Forever.
DO NOT throw away an old thermostat in your household
trash.
Call a local heating and cooling supply shop (the kind that
mostly cater to mechanical contractors) and ask if they will
dispose of the old thermostat for you. In my area there are
several heating supply companies that will gladly recycle old
thermostats with mercury switches. Also, many cities and
states have a hazardous waste disposal place for consumers
with small amounts of toxic stuff like mercury. |
|
Tools
Used:
- Cordless Drill/Driver
- Screwdrivers
- Pliers
|
Materials Used:
- Setback Thermostat, Ace
Hardware ATX500 Series
- AA Batteries (2)
|
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