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What is a Thermostat and how does it Work?
Honeywell T87 Mercury Switch Thermostat
Home Furnace Thermostat
The furnace thermostat is the nerve center of your home heating system. It's the main control point that determines when and how much heat will be produced by the furnace. But for all its mystery, the thermostat is actually a very simple device. In fact it's simply a temperature sensitive switch.
There are two main categories of thermostats depending on the type of heating system, low voltage and line voltage.
This tutorial covers the low voltage thermostat often made by Honeywell, White-Rodgers, Lux, Robertshaw and others. It runs off of a transformer reducing the incoming line voltage from 120 volts down to between 12 and 24 volts, depending on your furnace.
Within this low voltage thermostat category, there are four major types. They include:
• Mercury Contact (older style)
• Mechanical Contact
• Digital
• Electronic Programmable
The other type of thermostat, the line voltage thermostat, is typically hooked up to a 120 volt or 240 volt electric resistance heating system. This thermostats work similar to a light switch dimmer, varying the amount of electricity to the resistance heater. Line voltage thermostats are not the subject of this tutorial.
Let's take a look at the anatomy of each low voltage thermostat type, starting with the older style mercury contact thermostat often represented by the iconic T87 Honeywell in the photo above.
Mercury Bimetallic Thermostat
Mercury contact thermostat
The mercury bimetallic thermostat is a good example with which to start in describing how an analog thermostat works.
There are three major elements to a non-digital or analog thermostat:
• Bimetallic Strip or Coil
• Contact
• Heat Anticipator
Bimetallic Strip / Coil
In order for a non-digital thermostat to work, something has to move as temperature increases or decreases. That's the job of the bimetallic strip. "Bimetallic" is just a fancy name for two pieces of metal joined together. But the trick in making it work is that the two metals expand at different rates at a given temperature (known as coefficient of expansion). So as one side expands more than the other side, the metal curves one way or the other. You then couple this little bit of physics knowledge with the exact rates of expansion for the two metals and you have a precise instrument that will move a specific amount at a given temperature.
Contact
The contact is another key part of the thermostat necessary for it to work. Think of the contact like a switch for your lights. As the bimetallic strip moves, it either opens or closes the contact. In the case of a mercury thermostat like the Honeywell T87 shown above, the mercury serves as a sealed switch. The mercury is placed inside a sealed glass vial called an "ampoule."
The ampoule is attached to a coiled bimetallic strip and as the coil moves, it tips the vial of mercury until the mercury moves and the switch contact is opened or closed.
Heat Anticipator
The heat anticipator is an electrical resistance wire mounted on a center disc connected to the bimetallic strip and is adjustable to allow fine tuning of when the thermostat turns the furnace burner on and off.
Modes / Programming
The mechanical contact thermostat has modes for heating and cooling and fan positions for on/off and automatic. The thermostat is not programmable.
Mechanical Contact Thermostat
Mechanical Contact Thermostat (White-Rodgers)
The mechanical contact thermostat can also use a simple mechanical contact instead of a mercury contact switch. As shown in the photo using a White-Rodgers mechanical thermostat, the bimetallic coil is used to move and open or close simple mechanical contact points attached to a lever arm. It also uses a heat anticipator.
Another older version of the mechanical contact thermostat uses a straight blade bimetallic strip and not a coiled strip. They are a bit less accurate but work under the same principle.
Modes / Programming
The mechanical contact thermostat has modes for heating and cooling and fan positions for on/off and automatic. The thermostat is not programmable.
Digital Thermostat
Lux Digital Thermostat #DMH100
The Digital Thermostat is an electronic version of the simple mechanical thermostat. It has electronic controls versus a bimetallic coil to sense temperature changes and is much more accurate. These thermostats usually have large, easy to read displays and easily replace the older mechanical thermostat you have in your home.
Modes / Programming
The digital thermostat has modes for heating and cooling and fan positions for on/off and automatic. The simple digital thermostat is not programmable.
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Electronic Programmable Thermostat
Electronic Programmable Thermostat
The Electronic Programmable Thermostat take the digital thermostat and gives it a "brain." It has user controls that allow you to set the days of the week, time, temperature, override, temporary temperature hold and other custom programming. The electronic programmable thermostat can save energy too by allowing you to set back your heat at night or when you are away. These thermostats allow the weekday, Saturday or Sunday settings to be different and even tell you when you need to change your furnace filter!
Modes / Programming
The electronic programmable thermostat has modes for heating and cooling and fan positions for on/off and automatic. It is fully programmable with features for multi day settings, multi period per day settings, hold, override, furnace filter change and other features as may be provided by the manufacturer and model selected.
You Must Match the Thermostat to Your Heating System
Furnace and Thermostat Compatibility
Furnaces and thermostats are not mix and match appliances. There are numerous types of heating systems and thermostat systems and they need to be coordinated for safe and proper operation.
In this tutorial we'll review the three types of thermostat systems used today:
• Millivoltage
• Low Voltage
• Line Voltage
We will then look at what system is used on what type of heating or cooling system in the home, including these systems:
• Gas Furnace System
• Boiler and Oil Fired Systems
• Electric Heat and Cooling Systems
• Heat Pump System
Furnace and Thermostat Compatibility - Introduction
Type of Systems - Millivoltage
Millivolt Thermostat and 750 mV Thermopile Generator
Millivoltage System
These systems are not very common in a home as they are used often in direct /top vent wall furnaces. Thermostats that run on a millivoltage system rely on very low voltage, usually 750 millivolts (mV) or 0.75 volts. They do not require a step down transformer reducing line voltage of 120 volts to 24 volts as do low voltage systems. Instead they are self powered by a special thermocouple device called a powerpile or thermopile generator that produces direct current (DC). The thermopile is a device with thermocouples connected in series to increase the millivoltage output to 750mV.
These types of systems require a special thermostat and will not work with standard low voltage (24 V) thermostats.
Type of Systems - Low Voltage
Digital Low Voltage Thermostat
Low Voltage System
Low voltage thermostats are the most common and versatile type of thermostats found in the home. They are typically 24 Volt AC and powered by using a step-down transformer from the 120 Volt line voltage in the home. Some versions of this thermostat use a voltage as low as 6 volts and as high as 30 volts, but 24 volts is by far the most common. The transformer is usually mounted on or near the furnace.
These thermostats may be digital, electronic programmable, mercury bimetallic or mechanical contact type thermostats
Type of Systems - Line Voltage
Line Voltage Resistance Heating Thermostat
Line Voltage System
Line voltage or high voltage thermostat systems run off your home's 120 Volt or 240 volt AC power and are not particularly common as they are primarily used for electric resistance heating systems. These thermostats are not as sensitive as the low voltage types and may take up to 7°F of temperature change to register a change. As a result expect wider temperature fluctuations with these thermostats.
A way to check if you have a line voltage thermostat is to look at the wires. They will have thicker electrical wires like you would see going to a light switch or outlet, 14Gauge or 12Gauge as seen in the above photo. Low voltage wiring looks like old doorbell or telephone wiring or speaker wire.
Also you can check for voltage listing inside the cover of the thermostat or on the heat pump, furnace (inside the access door), or on the boiler or on the electrical baseboard unit itself.
Line voltage thermostats are typically hooked up to a 120 volt or 240 volt electric resistance heating system.
What to Look For When Identifying Your Furnace and Thermostat
Identifying Your Furnace and Thermostat Types
When you look your existing thermostat and heating or cooling system over, you'll want to look for a few things to help identify what systems you have. The following criteria profiles will be developed for each heating / cooling system type:
• Type of Heating or Cooling System
• Number of Wires to Thermostat
• Voltage
• System Attributes
Let's look at the criteria profile for each of the following heating / cooling systems:
• Gas Furnace System
• Boiler and Oil Fired Systems
• Electric Heat and Cooling Systems
• Heat Pump Systems
Gas Furnace System - Millivoltage
Monterey Direct / Top Vent Wall Furnace Using Millivolt Thermostat
Type: Gas Furnace System - Millivoltage
Number of Thermostat Wires: 2
Voltage: 750mV
System Attributes: Gravity wall (room) furnace; direct or top vent wall furnace for small areas; mobile home direct vent wall furnace
Remove the Old Thermostat
Removing the Old Thermostat
First you must disconnect and remove the old thermostat, being mindful as to the number and colors of the wires.
• Shut off power to the furnace. You will find a circuit breaker or fuse that controls the furnace in the electrical service panel or fuse box. Find it and turn the circuit off by turning the circuit breaker to the "off" position or unscrewing the fuse.
• Remove the old thermostat and expose the old wires.
• It is a good idea to take a digital picture of the wires as they were connected to the original thermostat.
• Disconnect the wires but be careful not to let them fall into the wall!
• If they are not colored, label them as to which terminal they were connected.
• Remove the old thermostat mounting plate from the wall.
Install New Thermostat Mounting Plate
Next, you install the new mounting plate.
• Once the old mounting plate is removed, pull the old wired through the hole in the new mounting plate.
• Place the mounting plate on the wall so that the wires come through the hole in the wall and through a mounting plate hole without binding the wires.
• Using a torpedo level to level the mounting plate, mark two holes on the wall to fasten the mounting plate.
• Install plate mounting screws, check for level then tighten screws.
Connect Thermostat Wires
Connect Thermostat Wires
Once the mounting plate is fastened to the wall, the next step is to connect the wires.
• Match the color coded wires to the appropriately marked terminal connections on the thermostat and tighten to the terminal screws.
• If the old wires had no color, connect them to the appropriate terminal based on the labeling you marked when you removed the old thermostat.
• Refer to the digital picture you took if necessary.
• Once the wires are connected, push excess wires into the wall.
Attach New Thermostat
Attach New Thermostat Cover
Once the wires are connected all that's needed is to connect the new thermostat cover and set your controls.
• Snap the new thermostat onto the mounting plate making sure the electrical connections are correctly seated.
• Turn the electrical circuit back on that powers the furnace.
• Set your thermostat to your desired settings.
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WALL OF SHAME 1
I have a friend – an electrician, who keeps teasing me by saying that even a monkey can do this job.
My answer always is - Well, of course, even a monkey can, but an electrician can't. In retaliation, I asked him several times to do some installations or at least explain the way he would be doing it, but unfortunately for him he never was successful in this regard.
As I already mentioned on some pages of this website: there is no a comprehensive nationwide “Ductwork Installation Guide” except of mine! Every company is doing the residential job by its own unique way and if you are lucky enough to be in a good company, with good traditions you may learn good ways of installation and become a good pro, if not you may end up on my “Wall of Shame” page.
From a homeowner point of view, a bad, sloppy job is going to bring only problems into the house! The problem starts up with a bad aesthetic view of the ductwork system and ends up with insufficient airflow! As one of my apprentices told me – this system of ductwork will blow air anyway! If your heating contractor will support of this kind of attitude then you, as a homeowner, are in a big trouble!
Therefore, here is a deal:
I have visitors on my website from entire country and if you, as a homeowner, are thinking that your heating contractor did a sloppy job take some pictures of the troubled spots and email them to me. Also, give me the company’s name, city and state where you are from. I will publish these photos on this page with my or yours comments to it and the country will know its heroes! In addition, I will explain how to fix it absolutely free!
Below is an example of the very sloppy installation performed by unknown company in Farmington, MI:
 The walls, which you see in the picture was built just recently, but this “jumper” in the middle of the basement was always there !
 They made this terrible “jumper” located in the middle of the basement around the gas pipe. Besides wrong location, this “jumper” has only 90* elbows and it’s not straight!
I could not believe my eyes, but this plenum with the canvas at the base is not suspended! Only drives of the starting transition are keeping it in the air. On my “Remodeling Tips” page, I already mentioned that you could not rely on the home inspector, this one probably also could not believe his eyes and passed it without any problems.
 
The heat run in the pictures above were done by Sun Heating and Cooling company. I don't know how they are doing now, but in the past they were famous for hiring everyone off the street and sending them without proper training right to the frontline of the new housing construction. As a result, there was a huge waist of time, materials and money.
In the pictures above, you can see just one example of that.
I have a question: How many elbows should the installer be using in this case?
The answer is from one to four! How many elbows the installer used - five!
Beside that, look at this installation, doesn't it look like shit?
Do you have the same kinds of BS installation in your basement? Most likely you do, it is why you are here, on this page.
Are you tired to look at it and to be embarrassed to invite your relatives and friends into your basement? Of course you are, it is why you are here, on this page.
Are you planning to fix that and make all heat runs look nice and neat? Maybe not...
However, now eventually you have a great opportunity to turn things around! Now for the very low price of only $1.86, for the price which is so low you probably won’t even bend your back to pick this money off the floor, you can buy Chapter #44 Jumper Installation from my “Ductwork Installation Guide”!
Therefore, now you will have the power to hire anyone to fix your heat runs and if he will try to screw you up again, you can read this chapter, learn how it should be done the right way, and teach him a lesson, or make him to read this chapter!
On the other hand, if you are a DIY-er you can easily do it yourself! Just follow my step-by-step instructions and fix this mess once and for all.
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44. Jumper Installation – the chapter gives a description how to make 45 degree angle from the “elbow”.
 Chapter covers all possible cases of measurement, calculation and installation of the “jumpers”. Also the chapter covers several uncommon cases of the “jumper” installation.
The chapter has 39 pictures; 11 pages. $1.86 (Instant Access).
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OK. Next on my list is a house in Birmingham, MI. “Kotz Heating & Air Conditioning Company” did the ductwork and furnace installation for this house. It is funny what they are writing about themselves on their website:
 “Kotz Heating & Air Conditioning is considered one of the best quality dealers in Oakland County. Unlike many other companies, you can be assured that our company is fully licensed, bonded and insured. In addition, our technicians have been factory trained and tested to prove their qualifications. We use no sub-contractors; our own employees do all our work.”
By the way, several years ago I’ve contacted them by e-mail and offered that I will train an installer for them and they have to only set up an interview with him – that’s it, no obligations, no payments, but of course they, like many others, just ignored my offer!
So let’s take a look how their technicians, which “have been factory trained and tested to prove their qualifications” completely screwed up the job!
 
What you can see in the pictures above are installed blockers. As you can see, it has been done extremely sloppy. Through the gaps, you can easily see inside and there is enough room for a mouse to sneak-in!
 
Picture to the left was taken in Troy, MI. This time around, it was not a ductwork installer’s, but rather stupid electrician’s fault! A homeowner hired this knucklehead to run an additional wire and he destroyed the blocker.
Check out your basement as well, if in one of your second floor bedrooms too hot in the summertime or too cold in the wintertime then you maybe also a victim of such moronic work.
In the picture to the right, you can see another intrusion in integrality of the ductwork system. This time around, it is a plumber's fault! As you can see in the picture, this moron ran a ¼" copper pipe to the fridge and of course partially destroyed the blocker.
However, if you would like to know how to install blockers the right way you can purchase blocker installation chapters from my Ductwork Installation Guide, see them below.
In this picture on the left, you can  see a basement heat. This monster is hanging down way below the joists level and looks like s--t. Of course, I understand this is just a basement, the place where people stock their garbage. However, I would not respect myself if I would be doing work like this!
If you would like to know how to install the basement heat the right way you can purchase Basemen Heat chapter from my Ductwork Installation Guide, see it below.
 
Moreover, here are two jumpers again! It is very difficult to understand how “factory trained and tested to prove their qualifications” technicians could have made this kind of sloppy job!
What you can see in the first picture to the left proves that there was no conducted tests, but maybe only test on the level of stupidity! Everyone in this business of ductwork installation knows that every elbow has its resistance to the airflow. Resistance of one 90* equals 10’ of pipe, so those "factory trained technicians” put in one spot Oval 90*, From Round to Oval 90* and four elbows! As a result, they made resistance equal 60' of pipes!!! (Factory trained my ass). But if there would be Ed’s trained and tested tech he would’ve installed one Oval, two Round 45*-s and one elbow equal 25’ of resistance – as a result more than twice less resistance!
On the second picture, you can see ONLY two mistakes:
1. Instead of the Round 90* they should’ve use Round 45*!
2. "Factory trained technicians" who were doing this job were in a hurry or maybe couldn’t do the job right due to the extensive training, but he used a full piece of 6” pipe, so the pipe passed its goal and he ran a jumper back to the straight boot!
 Therefore, if you live in Oakland county MI you can hire this “infamous” company and your basement will looks like one in the pictures above!
I'm in the midst of resolving issues with my builder over the installation of a new split system that is specs out to be Goodman/16 Seer. I'm not a DIY, just a homeowner trying to get smarter on what is considered to be a "proper installation". Any advice or comments on the installation would be appreciated.
What about plenums for the air handler connections to supply and return?
Thanks!
 
The supply and return "plenums" are non-existent, likely causing high static and low air flow. I'd want the ESP (external static pressure, think resistance to airflow) of the duct system tested.
And it also crushed one of those ducts to about half capacity.
I can not for the life of me understand why contractors don't get air handlers higher off of drain pans so there is plenty of room to put in proper condensate traps and be easier to work on. And they could just cut the trap shorter instead of cutting the deck. Also, not having even a two foot section of supply or return plenum is just asking for unnecessary problems.
In the photo to the right they used band iron to hang the flex and the line set! It’s also wrong! Over time the band iron will cut through the flex. And the same fate, expect the copper touching the steel!
  
In the first picture they centralized cold air return plus two more connected to the box, it may not works because it’s easier for air come through the grill then through small size smashed flexes.
In all the pictures you can see wrong flex connections: no insulation and unnecessary use of sealing mastic which in this case absolutely useless!
If you would like to learn about proper installation of the Air Handler you can purchase Air Handler Installation chapter from my Ductwork Installation Guide, see it below.
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21. Blocker Installation* – the chapter gives general information about blockers and after that gives detail  explanation of all possible cases of their installations.
The chapter has 9 pictures; 4 pages. $1.86 (Instant Access).
*Article:
Thermo-pan Installation Instructions.
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34. Sheeting (Panning)* – the chapter covers almost all possible cases of the joist's panning with a wide variety of  uncommon cases, presents how to deal with penetration of the wires, copper and PVC pipes.
The chapter has 28 pictures; 11 pages. $1.86 (Instant Access).
Summary Report for:
47-2211.00 - Sheet Metal Workers
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46. Basement Heats* – the chapter describes an installation of the heat runs in the unfinished basement.
 The chapter has 3 pictures; 2 pages. $1.86. (Instant Access).
*Tee Quiz.
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56. Air Handler Installation*# – the chapter describes an air handler installation in the attic. The chapter has 10 pictures; 3  pages. $1.86 (Instant Access).
*Register Installation Quiz.
PDF files:
Air handler Installation Manual.
Proper Subcooling Charging Techniques.
#From code:
M1305.1.3 Appliances in attics.
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OK, another day, another basement and of course another shameless installation performed by another shameless company!
This time around it is JMP Custom Heating & Cooling, Inc!
 You know, what is funny about this company it’s that I already tried to deal with this company’s sloppy work in the past. It happened about eight years ago. I was helping a homeowner in his project of finishing his basement off and I encountered multiple mishaps in the ductwork installation done by this company’s employees.
At this time the homeowner had a vacation which he used to finish his basement by himself and every time he came down in the basement I’ve met him with a new revelation. We were laughing all day long, but for him it actually wasn’t a laughing matter because after all it was his basement and it was his heating system!
Of course I don’t remember everything, but, for example I remember that the main duct had a turn for 90* to be run above the I-beam and instead of running it up, at first, they ran it down, way below the I-beam!
Also, among the other mistakes, there was a terrible ticking noise coming from the entire ductwork!
This house was actually part of the large subdivision of about 50 or so houses in Novi, MI and I was naïf enough to think that I had a great opportunity to earn some money! I came to a conclusion that if the same heating company has been involved in the entire subdivision the way they were involved in that house – there should be a lot of problems in the other houses as well!
So, I made 50 or so fliers and in a sunny Saturday’s day put them in every mailbox. There were a lot of people who obviously saw my marked with the company’s name on my truck; they saw me putting fliers in the mailboxes, but nobody even bothered to respond!
I knew there was terrible installation in every house, I knew there were loud and annoying ticking noises and it was zero calls on my phone! Anyone can explain why? If you have any explanations you can type them here.
Ok, let’s come back to the house in Northville, MI. What you can see in this picture is an  80% eff. furnace with an exhaust pipe way off the plumb! Its looks like that an installer was pretty much drunk and didn’t understand what was going on with this installation! As much as he didn’t understand that he should put a piece of pipe with a cap on that tee before the condensate trap!
The reason why it’s necessary to put a trap for this kind of application in the first place is to prevent conditioned air from escaping into the basement through the condensate pipe and the tee without cap pretty much undermine this notion.
 In this picture you can see double sheeting nailed to the studs. Its looks like that before installation and that team of hacks were dancing on it for at least an hour!
 In this picture you can see big gaps between the duct and the other system elements.
 Here I would like to bring a little bit more of your attention.
They ran all the heat runs in insulated flex and you can see how that flex was choked by the hangers!
Someone may say that it’s not a big deal! Down South they are installing all systems in flex, but in my opinion this kind of installation is a crime! I keep seeing on the forum websites people complaining about high electrical bills and I believe they have it because of this kind of installation! Putting heating and cooling duct in the attic space where temperature can reach 120 – 150 degrees is a no-brainer!
Someone can also say that here on this website you have a page were you are showing how you installing the system in the attic! Well it’s true! But it’s a homeowner's decision. He is a millionaire and he obviously doesn't care how much his electrical and gas bill will be! He already has four HVAC systems in his house and I’m in the process of installing of three more! But in his particular case I’m running all the ductwork as low as possible, because I want to bury it under the blow-in insulation. If down south every homeowner is also a millionaire then I’m sorry for my remarks!
Patrick J. Cloninger
InterNACHI Member Home Inspector TN
 
This is something I've been seeing too often in new homes. HVAC installers are putting the operating manual/warranty inside the burner cabinet.
This was from a couple of days ago on a new home.
I found this picture and a post on one of the inspections sites:
 "On a recent inspection, I came across this. A new HE furnace was installed but they kept the hot water heater on the old vent. This vent goes through the roof. I don't mind the vent size but duct tape as a connector. I have searched the threads and since my codes books were "removed from my premises", stolen, I haven't confirmed this is incorrect. The installation was by a very large firm in our area and I certainly don't want to call it unless I have some back up. Can anyone shed some light on this for me?"
So, what we can learn from this post:
First off, those installers from a "very large firm" do not care what they were doing, while completely hacking this job!
And secondly, an inspector is just an uneducated dumbass who doesn't know shit about ductwork installation! Such a disgrace!
If you have something like this in your house send me some pictures and I will publish them on this page.
Ed
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Hacks from the unknown company definitely didn't know how to do a low heat for the finished basement!
First off, they used wrong fitting as a register box, which brought everything far away from the wall.
Secondly, once they realized that they used two elbows to direct it to the wall, stopped on the halfway and ran it aslant to the curved side of the flat 90*!
Therefore, just in one installation they've made five stupid mistakes!
  And here is another ugly installation! The installer used only five elbows, but who knows maybe before he use to work in Serpent Safari...
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