HVAC Control Tutorial

Residential HVAC (Heating Ventilation & Air Conditioning) is a topic you don't see written about very much. It seems to me to be a pretty brutal business. After all, HVAC systems are rated in "tons" and are pretty massive. They include large ducts that are built-in-place by slicing and bending sheet metal (often by hand). And they draw lots of power and fuel to operate.

While HVAC manufacturers are gradually becoming more sophisticated, our experience with HVAC installers has shown that they are still more comfortable bending tin than pushing electrons. A homeowner that desires anything more elaborate than a single thermostat must design and procure the control portion of the system themselves.

Although this may seem like a daunting task, let me assure you that it really isn't that difficult. As with many "trade" oriented subjects, it just hasn't been written about publicly very much...until now. Let the professionals design and install your HVAC system and ducting, we'll show you how to design and install the control system.

• Types Of HVAC to Thermostat Connections
  • 220V Baseboard
  • Millivolt Heat Only
  • 24VAC Heat/Cool Non-Heat Pump
  • 24VAC Heat/Cool Single-Stage Heat Pump
  • 24VAC Heat/Cool Multi-Stage Heat Pump
• Choosing A Location For A Thermostat
• Thermostat Wire
• Wiring For A Thermostat In New Construction
• Thermostats
  • 220V Baseboard
  • Mechanical Thermostats
  • Digital/Electronic Thermostats
  • Programmable Thermostats
  • Remotely Controllable Thermostats
    • One-Way X-10 Compatible Thermostat
    • Two-Way X-10 Compatible Thermostat
    • RS232 Controllable Thermostat
    • RS485 Controllable Thermostat
  • Other Common Thermostat Features
    • Heating Anticipator
    • Cooling Anticipator
    • Remote Temperature Sensor
• Zoned HVAC Systems
  • Designing A Zoned HVAC System
  • Zoned Hydronic Systems
• Upgrading Existing HVAC Controls
  • Adding a Programmable Thermostat
    • Adding a Timer Programmed Thermostat
    • Adding a Remotely Controllable Setback Thermostat
    • Adding a Remotely Controllable Thermostat

Types Of HVAC to Thermostat Connections

For the purposes of this tutorial, I'll ignore self-contained heaters and air-conditioners. These units, such as window-mount air conditioners, usually have an integral thermostat.

I'll mostly talk about controlling forced air heating and/or air conditioning systems, but the same control techniques are directly related to other types of heating such as hydronic. Every such system I've seen, no matter how complicated internally, has standard thermostat style connections to the outside world. The only difference is that hydronic and other specialty heating systems often require a fan call* along with the call for heat. Our programmable and remotely controlled thermostats can be set up to do this.

* "Call" is industry jargon for "the thermostat requesting..." which in fact simply means that the thermostat shorts the appropriate two wires together!

220V baseboard

Millivolt Heat Only

Millivolt systems have the advantage of being incredibly simple. A very small electrical voltage is created by a bi-metallic device that sits in the pilot light's flame. This is wired in series with a simple thermostat and an electrically operated gas valve. When the thermostat closes and completes the circuit, a small current flows through the very efficient valve which opens and lets the gas through. Completely self-contained and, I think, quite ingenious. Unfortunately, as with most things ingenious, the system doesn't adapt well to other configurations. Most modern thermostats and HVAC control systems operate on a 24VAC standard. Thus, the selection of thermostats for a millivolt system is rather limited.

To control a millivolt heater, you can use a thermostat rated for use will a millivolt system. But you can also use most simple (non-electronic) standard heat-only thermostats. These thermostats are designed to switch 24VAC on and off, but since they do not use the 24VAC for any electronics internally, they work fine for millivolt systems. Just don't turn the "heat anticipator" up -- it won't work and the added resistance could interfere with the proper operation of the electric valve in the heater.

If you are installing a millivolt system, you could install two conductor thermostat wire, but we would recommend installing five conductor thermostat wire to allow future upgrades. Use the white and red wires.

24VAC Heat/Cool Non-Heat Pump

Note that the remaining wire (blue) is often not connected. It could be used for carrying the 24VAC common to the thermostat so that the thermostat can always have 24VAC power. Or it could be used for setback control of a dual-setting thermostat (see below.)

The HVAC system may have a "RH" (24VAC Return -- Heat) and "RC" (24VAC Return -- Cool) terminals rather than a single "R" terminal. Or, if the heating and cooling systems are independent, there may be a separate "R" terminal on each system. If this is the case, there are two independent 24VAC return lines (one for each system). You may need to keep these separate back to the thermostat and use a thermostat or control system that is "dual return" compatible. (All of our thermostats and control systems are.) Check with your HVAC system installer to see if you have to wire for dual returns (this is very rare).

The connections to a standard HVAC system work as follows: The thermostat "requests" heating, cooling, or just the fan from the HVAC system by shorting the appropriate control lines (heat/cool/fan) to the return line.

• Shorting the heat (white) wire to the return (red) wire will cause the HVAC system to begin heating. Most HVAC systems will turn on the forced-air fan automatically, the thermostat does not need to do anything with the fan (green) wire when heating. (If the system does not automatically control the fan in heating mode, you'll need a thermostat that can call for fan when it calls for heat. Our programmable and remotely controllable thermostats can be set up this way.)
• Shorting the cool (yellow) wire to the return (red) wire will cause the HVAC system to begin cooling. Again, the fan will be turned on automatically.
• Shorting the fan (green) wire to the return (red) wire will cause the HVAC system to run the circulation fan. The fan may be left on as long as is desired. Normal heating and cooling cycles can occur, the fan will just never turn off.

Calling for both heat and cool at the same time is undefined and may damage the HVAC system. Most thermostats (including all of ours) have interlocks to keep this from happening.

Note the sequence of colors. The return (red) and heat (white) come first. If you have a heat only system, these might be the only two wires you need. But some heat only systems allow fan control, so the "force fan on" wire comes next (green). Lastly, if the system has air conditioning too, the call for cool (yellow) wire comes next.

This setup is sometimes described as "one stage heat/one stage cool." The vast majority of domestic HVAC control systems fit into this category. Such systems are usually wired with five conductor thermostat wire.

24VAC Heat/Cool Single-Stage Heat Pump

Note: On some systems, the Changeover may be a Brown wire connected to a "B" terminal.

A single-stage heat pump system brings two new facets to the control equation: The changeover connection and auxiliary heating.

Heat pump systems cool the normal way, the compressor (call for cool) wire (yellow) is shorted to the return wire (red), the compressor and fan runs, and the HVAC unit cools the house. But they heat by running the compressor backwards! There is a new connection, called the changeover connection, that reverses the flow of freon in the system. (Also sometimes called the "reversing valve control".) Manufacturers couldn't get together on which way the changeover control worked (shorted=heat or shorted=cool) so all universal heat pump thermostats have switches or different contacts for each type. Consult the documentation on your HVAC system to determine which type you have. Thermostats either have a single output (marked C/O or O) that is switch selectable for either operating mode, or two outputs (marked O and B), where O is shorted=cool and B is shorted=heat.

The other new facet is auxiliary heating. The heat resulting from running the compressor backwards is efficient, but does not put out as many BTU's as may be required during the coldest parts of the year. So there is a supplemental form of heating added to the system, usually electric, and often either baseboard strips or heating elements in the HVAC stack itself. This auxiliary heating comes on when it is obvious that regular heating (running the compressor backwards) is not adequate (temperature is still falling.) Note that first regular heating is requested with the compressor and changeover wires, this leaves the old "call for heat" (white) wire available to use as an auxiliary heat request wire. Heat pump thermostats are designed to call for auxiliary heat when the temperature drops a few degrees below the heating set point.

The connections to a heat pump HVAC system work as follows: The thermostat "requests" regular heating, auxiliary heating, cooling, or just the fan from the HVAC system by shorting the appropriate control lines (auxiliary heat/compressor/fan/changeover) to the return line.

• Shorting the auxiliary heat (white) wire to the return (red) wire will activate the supplemental (usually electric) heating system. The HVAC system will turn on the forced-air fan automatically, if needed for this form of heat.
• Shorting the compressor (yellow) wire to the return (red) wire will cause the HVAC system to run the compressor. Whether the system is heating or cooling is determined by the state of the changeover wire (see below.) In one position, the system will cool, in the other, the system will heat. Again, the fan will be turned on automatically.
• Shorting the fan (green) wire to the return (red) wire will cause the HVAC system to run the circulation fan. The fan may be left on as long as is desired. Normal heating and cooling cycles can occur, the fan will just never turn off.
• Shorting the changeover wire (orange) to the return (red) wire will cause the HVAC system to reverse the compressor. Most systems are "changeover shorted = cool" which means that if both changeover (orange) and compressor (yellow) are shorted to return (red), the system will cool. If only compressor (yellow) is shorted to return (red), the system will heat. A few systems use changeover in the opposite manner.

It is unusual to call for auxiliary heat without calling for regular heat. (I.E.: only white and red shorted.) But some thermostats allow this function in case the compressor is damaged during a cold spell. The occupant can call for "emergency heat" and the electric heating will kick in without trying to run the compressor.

This setup is sometimes described as "two heat/one cool." The compressor only has one speed so it is also known as a "single stage heat pump." The vast majority of domestic heat pump HVAC control systems fit into this category. Note that the additional wire used is orange (or sometimes brown). The orange and brown wires only appear in seven conductor thermostat wire which is used to wire heat pump systems.

24VAC Heat/Cool Multi-Stage Heat Pump

Note: On some systems, the Changeover may be a Brown wire connected to a "B" terminal.

A multi-stage heat pump system brings one more facet to the control equation: The second stage compressor control.

Multi-stage heat pump systems either have a dual-speed compressor or they have two compressors. The thermostat can cause the compressor to run at either a low or high setting. So the HVAC system can cool at two rates, and heat at three rates (remember the auxiliary heat).

Other than an extra "compressor call" wire, the multi-stage heat pump is controlled just like a single-stage heat pump. Thermostats designed for multi-stage heat-pumps are usually pretty sophisticated in how they turn on each successive stage.

This setup is sometimes described as "three heat/two cool." Multi-stage heat pump systems require more than five wires so you must use seven conductor thermostat wire.

Choosing A Location For A Thermostat

To ensure proper operation, the thermostat should be mounted on an inside wall in a frequently occupied area of the building. In addition, its position must be at least 18" (46cm) from any outside wall, and approximately 5' (1.5m) above the floor in a location with freely circulating air of an average temperature. Avoid the following locations:

• Behind doors or in corners.
• Where direct sunlight or radiant heat from appliances might affect control operation.
• On an outside wall.
• Adjacent to, or in line with, conditioned air discharge grills, stairwells, or outside doors.
• Where its operation may be affected by steam or water pipes or warm air stacks in an adjacent partition space, or by an unheated/uncooled area behind the thermostat.
• Where its operation will be affected by the supply of an adjacent unit.
• Near sources of electrical interference such as arcing relay contacts.

Thermostat Wire

1. It is usually difficult to find solid (non-stranded) wire in cables with more than two conductors when the wires are 20 gauge or larger. This is because cables get stiff and difficult to work with due to the solid conductors. Apparently this doesn't bother the HVAC installers. (I guess ducting tin is even more stiff!)
2. The color coding is unique in the cabling industry. In order: White, red, green, blue, yellow, brown, orange.
3. It is not paired, and requires unusual wire counts: 5 and 7 conductors.

While you might get away with running other kinds of cable, you'll have the wrong number of conductors, with either the wrong gauge or stranding, and with a color coding that will not match the directions or the labels on the screw terminals of either the HVAC system or the thermostat.

Look at the descriptions of the various kinds of HVAC systems (above) for thermostat wire color codes.

Wiring For A Thermostat In New Construction

In new construction, just to be safe, you should install seven conductor thermostat wire between the HVAC system and the thermostat location. If you are sure that you will not be using a heat pump system, you could use the five conductor cable.

HVAC End

At the HVAC end, before the walls go up, nail a low-voltage wall-box to a stud near where the HVAC system will go, at about outlet height (12" from floor to wall-box center.) Coil up about 8 feet of cable and tie-wrap it to the back of the mud-ring. (See picture, left.) Choose a location for the thermostat (see above). Drive a large nail into the stud at this location and attach the thermostat wire to the nail with a tie-wrap. Leave some slack inside the wall. (See picture, right.) The sheet rockers will cut a hole for the nail (and wire) in the sheetrock. After the sheet rockers are done, you can finish out the HVAC end by pulling the coil of extra wire through the mug-ring hole and installing a split wall-plate with hole. The cable will be ready to connect to the HVAC system once it is installed.

Thermostat End

At the thermostat end, after the sheetrock is up, cut the tie-wrap, pull the extra wire through the hole, and remove the nail. Then follow the thermostat's installation instructions. Be sure and completely seal off the hole into the wall before final installation of the thermostat! Even a tiny opening can allow enough of a draft to seriously affect the thermostat's reading.

Thermostats

Here's some of the different types of thermostat you might see:

220V Baseboard

Mechanical Thermostats

Mechanical thermostats with a single "set point" control are generally not able to switch from heating to cooling automatically (called auto-changeover). Models with a separate set-point control for heat and cool generally can, if set to "auto", automatically change.

Digital/Electronic Thermostats

Digital thermostats usually operate using a battery or batteries. Some steal a little power from the 24VAC HVAC lines to operate but this can cause problems if the thermostat gets too greedy. (All of our thermostats have tested out clean in this regard.)

Most digital thermostats have auto-changeover.

Digital thermostats usually support heat and cool modes, but could be wired for just heating or cooling.

Certain models of digital thermostats are designed to control single-stage heat pump systems or dual-stage heat pump systems. Be certain that the thermostat you buy is compatible with your type of HVAC system. Note that most dual-stage heat pump capable thermostats can also control single-stage heat pump systems (ours can), but heat pump thermostats won't usually work with non-heat pump HVAC systems.

We have some pretty unique digital thermostats that allow two settings within the thermostat. Opening and closing two contacts select which setting to use. These contacts can be wired to a motion sensor, timer, or computer interface. See "Add a Heat/Cool Setback to an Existing System" below.

Programmable Thermostats

Again, certain models are designed to control single-stage heat pump systems or dual-stage heat pump systems. Get the right model for your HVAC system.

Remotely Controllable Thermostats

One-Way X-10 Compatible Thermostat

Two-Way X-10 Compatible Thermostat

RS232 Controllable Thermostat

RS485 Controllable Thermostat

Other Common Thermostat Features

Heat Anticipator

The net result is that when the thermostat calls for heat, the resistive wire generates a small (and adjustable) amount of heat that will soon artificially lower the temperature at which the thermostat ceases its call for heat. The homeowner can experiment with the heat anticipator: If the heater comes on infrequently, and the temperature in the house fluctuates too much, turn up the heat anticipator. If the heater cycles too frequently, or comes on for too short of a period turn down the heat anticipator.

Heat anticipators compensate for less-than-perfect thermostat sensor locations. Thermostats are usually located in a position that is a compromise between the best location to sense the actual room temperature, and the best location to access the thermostat controls.

Most mechanical thermostats have heat anticipators (ours do.)

Most electronic, programmable, and remotely controllable thermostats have minimum cycle times (all of ours do), and/or some way to remote the sensor for more accurate readings (optional on all our non-mechanical thermostats.) Also, electronic thermostats tend to react to temperature changes more rapidly than mechanical ones, thus reducing the need for anticipators.

Cooling Anticipator

Remote Temperature Sensor

Remote sensors wire using 3 to four conductors. We recommend using the same wire you used to wire the thermostat (5 or 7 conductor thermostat wire) or our quad wire. Run the wire from the thermostat location to the remote sensor location.

Zoned Systems

How do HVAC zone controllers work? They are actually pretty simple. The standard HVAC system has a single thermostat controlling a single HVAC system. A zone controller simply connects multiple thermostats to a single HVAC system. The zone controller itself is a small box that mounts back in the HVAC closet. It has one connection for each zone (which goes to a thermostat located in the zone), it has a connection for the HVAC system, and it has a connection for each damper.

The dampers are the key, these devices open and close the air duct leading to a zone. We use normally open, 2 position, spring return, 24VAC, 2 wire motorized dampers. This is the only way that the zone controller can "force" heating or cooling of particular zone(s). The interesting thing about the way zone controllers work is that they use standard thermostats! The thermostat "thinks" it is controlling the HVAC system directly, when actually it is just "suggesting" to the zone controller what the zone needs. And the HVAC system thinks it is being controlled by a thermostat; it just heats and cools as directed, unaware that the airflow is being diverted to where it is needed.

One of the neat things about this method of zone control is that you use a regular thermostat in each zone. So all the discussion above about thermostats is applicable. In fact, you have to use the proper thermostat for the HVAC system selected: If it is a heat-pump system, you must use a heat-pump thermostat in each zone.

So the concept of what zone controllers do is quite simple. But the actual implementation of the firmware of zone controllers is more complex than you might think. The only thing you really need to know is how to select the proper zone controller: There are three variables: How many zones? Is this a heat-pump HVAC system? And is this an upstairs/downstairs installation? If you are zoning a two-story house you should divide the zones between upstairs and downstairs and use a thermal equalizer version zone controller. This version does some neat things to keep the upstairs and downstairs temperatures balanced.

Designing A Zoned HVAC System

It is very difficult to retrofit zoning into an existing installation, so we'll assume we are dealing with new construction or a major remodel here. Let the HVAC specialists go ahead and design the HVAC system size and register locations. Then you can follow these five steps to turn the design into a zoned system:

1. Determine how the structure will be zoned and how many zones you will have. A multi-story dwelling will normally be zoned by stories. Try to make each zone cover approximately the same size area. Avoid having very small zones as they make it difficult to duct (see below.)
2. Have your HVAC specialist plan ducting that connects each zone to a separate branch of the main duct. This main "branching" often occurs very near the HVAC system but it is also possible that some branching occurs far away. Remember that a damper will go at the branch point to each zone.
3. Now "upsize" the ducting to accommodate the increased volume. Duct sizes will have been calculated based on all zones open. When the zone controller closes one or more dampers, you don't want the back pressure to increase significantly or the HVAC system could be damaged. So the ducting for each zone should be large enough to handle the air volume of the HVAC system alone. The rule-of-thumb is to increase the duct capacity by 25% in a two-zone system. This usually equates to one size larger ducting. In a four-zone system, increase the capacity by 50%, or two sizes larger ducting.
4. Plan the selection and location of the dampers, and make sure you run wire (use two-conductor thermostat wire) to those locations from the HVAC area.
5. Plan the location of each thermostat (one per zone) and make sure you run wire from each thermostat location to the HVAC area. (You can also use remote temperature sensing on each thermostat if you wish. Don't forget to run these wires.)

That's it for the design. Anyone can put the wire in (see "Wiring For A Thermostat In New Construction" above) and your HVAC installer shouldn't have a problem putting the dampers in. The dampers default to "open" so the installer can connect one thermostat directly to the HVAC system and make sure everything is working before you add the zone controller.

As a final check, the materiels you will need to supply are:

• 5 or 7 conductor thermostat wire for connecting the thermostats (and optionally the remote temperature sensors.)
• 2 conductor thermostat wire for connecting the dampers
• One damper for each zone.
• One thermostat for each zone.
• The appropriate zone controller.

We have placed all the manuals for our thermostats and zone controllers on-line. You can consult these manuals for further details. Good luck!

Zoned Hydronic Systems

Upgrading Existing HVAC Controls

Adding a Programmable Thermostat

Adding a Timer Programmed Thermostat

First you have to select the correct programmable thermostat. If you have a standard HVAC system (non heat-pump), buy a standard programmable thermostat. If you have a heat-pump, buy the programmable thermostat for heat-pump systems.

Remove your old thermostat noting the colors of the wires and which terminal each wire goes to on the old thermostat. Put up the new thermostat connecting the wires to the same terminals as before.

That's it! If you have any questions that aren't answered by the installation manual or this document (be sure and consult the wire color-code conventions above), call for assistance.

Adding a Remotely Controllable Setback Thermostat

To implement this setup with the dual-setting thermostat, you will need at least one extra conductor in the cable between your HVAC system and your thermostat. Check the HVAC end of the thermostat connection to see if there is a wire that is not connected. If so, you're in luck. If not, you'll have to either use the next method described or use a fully remotely controllable thermostat described below.

Replace your existing thermostat with a dual-setting thermostat. Use a standard thermostat if you have a non heat-pump HVAC system. Use a heat-pump thermostat if you have a heat-pump HVAC system. Connect as shown in the drawing at right.

Connect either a 6325 Telephone Responder or other contact-closure device at the HVAC area as shown.

Now, when you call up the telephone responder and tell it to open or close its contacts, the thermostat will switch to its day or night setting!

The second way to add a remotely controllable setback thermostat is to add a second thermostat back at the HVAC system to be used as the anti-freeze thermostat and then "switch" in and out the original "comfort setting" thermostat via remote control.

The drawing at right shows a standard thermostat connection above, and the modified connections below. We've added a 6325 Telephone Responder and a simple mechanical thermostat. Both the new components will be located near the HVAC system.

First, we're going to turn the existing thermostat into a "comfort setting" thermostat. You'll leave this thermostat set to a comfortable heating level all the time. We are going to change the wiring so that the telephone responder can disable this thermostats ability to turn on the heater.

At the HVAC system, disconnect the white wire from the W terminal on the HVAC system. Connect the white wire to one terminal of the 6325 telephone responder. Then jumper from the other terminal of the telephone responder to the W terminal of the HVAC system. That's it. Now, when the telephone responder "opens" its contacts, the thermostat won't be able to call for any heat.

The new thermostat should be mounted where it can measure inside temperature, although it doesn't have to be as accessible as the existing thermostat. This new thermostat will be the "anti-freeze" thermostat. The 200403 has an unusually low range, so you can set it at reasonable anti-freeze level or 45 degrees or so.

The new thermostat is directly connected to the R (red) and W (white) terminals of the HVAC system. If you need wire, you can use a our two conductor thermostat wire. Now, no matter what happens with the telephone responder or old thermostat, the new thermostat can "call for heat" and keep the pipes from freezing.

Be sure and test this out before leaving your home or cabin during potentially sub-freezing temperatures: Through the telephone or an X-10 transmitter, turn off the comfort setting thermostat. Now turn up the anti-freeze thermostat and make sure that it can cause the heater to come on.

As a last note. If your HVAC system has air conditioning, you'll need to remember to leave the main thermostat set to heat only whenever you leave. Otherwise the air conditioning might run a lot if the climate is warm. If desired, you can use a universal module (UM506) to interrupt the Y (yellow) wire so that you can turn off the air conditioning remotely.

Adding a Remotely Controllable Thermostat

Finally

Also, I know from all the e-mail I get that you really like tutorials like this, and I truly enjoy writing them. Nothing forces anybody to buy from us, even though they might get all their information from us. I imagine some don't. But I'm also convinced that most do. Generally, our customer seem to be pretty loyal.

All we ask is that, if you find this information useful, that you give us a fair chance at your business when it comes time to buy.

Cheers,

Jeff