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2017-10-01

How to Choose Thermostat?

How to Choose a Residential Thermostat?

This post is about choosing and installing a residential universal thermostat for most homes in Canada and USA with central force air HVAC.  All the houses that build in 1980 and furnace and A/C sold since 1990 are using natural gas furnace and split system (force air) A/C with a big outdoor unit.  Heat pump, hot water heating, electric heat board are not cover in this post, as home builds in last 20 yr are not common to use those, especially I am talking about residential homeowner.

There have been so many "how to choose thermostat" articles in the Internet, and why do I need to write one again?  Following are the reasons:
  1. Most of them are not telling that modern HVAC is applicable to units that is 20 yr old.  Reader will think it only applicable to HVAC sell in last 5 yr
  2. None of them tell you how to identify the thermostat support multi-stage HVAC
  3. None of the thermostats installation/user guide explains the installation of multi-stage furnace/AC, as well as the wiring.  They all telling you to plug in what is existing to their
  4. Labels on furnace, and A/C do not tell how to connect to thermostat

Introduction

Furnace and A/C that sold ~ 20 yr ago are starting to become 2-stage (simply put as low and high speed blower), or 3-stage (low, med, high speed blower).  They equip with ECM blower motor that can run at slower speed when the room/house temperature is stables.  These are called multi-stage HVAC (furnace and/or A/C).  They are widely see in all HVAC marketing brochures, but sadly not properly connected, especially for a home owner who bought a used or new home.

I have lived in several houses, which built in 1980's (37 yr old), and some replaced their HVAC once, which probably has a 20 yr old HVAC or more commonly 10 yr old.  Some have 1-stage furnace, some have 3-stage furnace, and current one has 3-stage furnace + 3-stage A/C.  However, the thermostat is not connected to use those mult-stage feature.  Some doesn't even connecting the thermostat cable to the furnace controller board at all, except the base minimum 4 wires (G-fan, R-power, W-heat, Y-cold).

Problems

In reality, not much home owner know whether their house come with a multi-stage (2 or 3) furnace or multi-stage A/C.  Most homeowner bought an existing home, instead of new home, so it becomes norm that the all these houses might have replaced with a HVAC (furnace/AC) 20 yr ago, which are multi-stage.  It is sad that most of these multi-stage HVAC system is not properly connected to the thermostat, and resulting not utilizing these multi-stage features.  These HVAC are running as 1-stage system, which always run at high speed, except the first 5 - 15 min where it will try to run at low/med speed thinking that it will save energy.

Homeowner Frustration

Following is summary of my discovery in HVAC for 10 yr of homeowner experience:
  1. All furnaces have no clear label on its casing, and often have to go to basement and open the front cover to find its model #.  Sadly, it is impossible to determine whether it is multi-stage furnace from its label
  2. All A/C has no clear label on its casing to indicate it is multi-stage A/C, although it is easy to find its model # on the casing
  3. All thermostats are often have 4-wire connected to the thermostat, but it needs more, and have to hire electrical contractor to re-wire (fish the wire) and spends few hundred dollars, or few thousand dollars if stuck in the dry wall and need to remove/install/sand/paint
  4. All thermostat can only control 2-stage for furnace, and 2-stage for A/C, i.e. low, and high.  For high, the control board in the blower unit will spin at mid for 15 min, and high after that.  Latest smart/WiFi/bluetooth enabled brands, e.g. Nest, ecobee, Honeywell, which cost CAD$300+
  5. For thermostat to fully control all the 3-stage or multi-stage HVAC, you have to buy the manufacturer's specific thermostat, which has V wire connection in the thermostat.  It comes impossible when the manufacturer for furnace is different than A/C.  The V wire uses PCM (pulse code modulation), which is manufacturer specific
  6. 2-stage or more furnace requires additional 1 cable called W2, in addition of W.  This will certainly missed out in 4-wire thermostat cable.  This requires 6-wire thermostat wire
  7. 2-stage or more A/C requires  additional 1 cable called Y2, in addition of Y.  This will certainly missed out in 4-wire thermostat cable.  This requires 6-wire thermostat wire
  8. To control both multi-stage furnace and A/C, it requires 7-wire thermostat wire
  9. Battery-less (wall powered) thermostat requires additional 1 cable called C.  This will certainly missed out in 4-wire thermostat cable.  All smart thermostat requires C wire, as battery won't be able to operate WiFi for months, and could probably last only 1 day, if one exists
  10. Controller board in the furnace blower casing has mult-stage disabled by manufacturer as default.  Technician always leave it as default, and running as 1-stage instead of multi-stage.  So the W2 wire is always ignore for furnace, and Y2 wire is always ignore for A/C even if they are connected
  11. When multi-stage HVAC runs as 1-stage, they will run in low speed at first 5 min, mid speed at 5-15 min, then high speed after 15 min.  Thermostat can't control their speed at all
  12. Modern thermostats requires wall power, which is the C wire (min 5 wires).  So a re-wire is required (and spend money), or have to live with battery operated thermostat, AND can't control HVAC's multi-stage feature
  13. All the thermostat which mention about multi-stage, they often refer to heat pump, electric heater, unclear "second level heating," 

Research

If you have a 15 yr old or newer furnace & A/C, then you will most likely have 2 or 3 stage HVAC, with 2-3 speed ECM blower.  Following are the steps to do the research in order to choose the right thermostat:

Multi-stage Furnace Research

  1. Find out the model of the furnace - Go to basement and open up the top and bottom cover of the furnace unit.  There should be a white sticker and shows the model #.  Take a picture with your phone as you will need to refer to it often
  2. Find out the thermostat cable connections at blower motor - Before close the cover of the furnace blower cover, look at the edge of the PCB controller board for wire connections.  There should be a series of wires with label (on the PCB board): R - power, C - ground, G - fan, Y - cold, Y2 - cold (low speed), W - hot, W2 - hot (low speed), V - variable speed (or called modular in my model)
  3. Trace the cable to exterior of furnace - Above cable will connect all the way to the wall thermostat upstair.  Take a good look at the cable
  4. Count how many wires/conductors for thermostat wire at furnace blower motor - don't close the door and continue to examine the thermostat wire in #2.  Spread the wire out, and cut the sleeve if needed to find out how many wire (technically we called conductor) contains in it.  If you have 4 wire, then you have to re-wire to use battery-less thermostat or multi-stage HVAC
  5. Find furnace installation manual - most manufacturers do not have PDF copy of their manual, if you don't have yours.  Go to HVAC forum and asks people to give you the PDF copy, or call/email the manufacturer.  For 10 yr old model, manufacturer might not keep it.  They change their model after 10 yr, even internally has almost no change.  It is more often that their supplier, e.g. ECM motor supplier, replaces/upgrades some part rather than building a more energy efficient unit
  6. Enable multi-stage feature - read the furnace installation manual how to enable multi-stage feature.  For my model, it is my flipping 2 DIP switches to ON.  Manufacturer often leave it off, so that if only W wire is connected, then it operates as 1-stage, and if both W and V wire are connected, then it operates as multi-stage (support multiple stages instead of 2)
  7. For multi-stage furnace, there must be a W2, or V cable.  If it is not connected, then connect the existing wire from thermostat wire into W2.  Leave V alone, as we are installing universal thermostat.  Only manufacturer specific thermostat can make use of V, and not universal thermostat.  If you have both multi-stage furnace and A/C, then connects both W2 and Y2 to the thermostat wire
Figure 1: Left edge of the controller board are the thermostat wire connections labeled W, W2, Y1, Y2, G, C, R

Multi-stage Thermostat Research

  1. Go up stair to the thermostat and remove it, so that you can see the thermostat wires
  2. Take a picture for all the connected wires with labels.  Move them around so that the pictures clearly show the label for every wire.  It is helpful to refer to it whenever you swap another thermostat, including replacing a broken thermostat
  3. Count how many wires/conductors for the wire at the thermostat - it must match the number of wires in #4
  4. If the thermostat wire at the thermostat has 4 wires/conductors, then you have to re-wire to use battery-less thermostat or multi-stage HVAC
  5. Check your thermostat whether it has a V connection.  If it has, then it is a thermostat that capable of controlling multi-stage HVAC.  This is the thermostat that is specific for your HVAC.  If you replaced it universal thermostat, e.g. Nest, Ecobee, Honeywell Lyric (app name is Total Connect Comfort), then you will only able to control 1 low speeds, and mid-high (the blower will slowly increase from mid-high, then remain at high)
Figure 2: Universal Honeywell RTH8500D, which has W2 (2 stage furnace) and Y2 (2 stage A/C), as well as AUX (heat pump to be used as additional heater/A/C) and E (typically heat pump used as backup).  No Y cable so it can't fully control all the multi-stage/speed of the blower

Multi-stage A/C Research

  1. Find out the model of the A/C - Go outdoor and look at the bottom at each corner for a label which indicating the model of the A/C unit.  Take a picture with your phone
  2. Find the thin thermostat wire - trace the thin thermostat wire that comes out from the house to the A/C.  We need to open the cover to find out its wiring
  3. Open the cover where the thermostat cable is connecting to.  You should see a series of wires connecting to a board with label, C, Y, Y2, R.  Take a picture as you will need to refer to it often
  4. Count how many wires/conductors for the wire at the A/C - If it has only 2 wire, then you have to re-wire to use multi-stage AC feature
  5. Examine the controller board for Y2 or V wire.  If it is a 2-stage A/C, then there should be Y (regular 1-stage A/C, and treated as high speed), and Y2 (low speed).  If it is 3-stage or more, then there should be a V on the board
  6. Find A/C installation manual - most manufacturers do not have PDF copy of their manual, if you don't have yours.  Go to HVAC forum and asks people to give you the PDF copy, or call/email the manufacturer.  The reason for lack for PDF is basically the same like furnace
  7. Enable multi-stage feature - read the A/C installation manual how to enable multi-stage feature.  Manufacturer often leave it off, so that if only Y wire is connected, then it operates as 1-stage, and if both Y and V wire are connected, then it operates as multi-stage (support multiple stages instead of 2)
  8. For multi-stage A/C, there must be a Y2, or V cable.  If it is not connected, then connect the existing wire from thermostat wire into Y2.  Leave V alone, as we are installing universal thermostat.  Only manufacturer specific thermostat can make use of V, and not universal thermostat.  If you have both multi-stage furnace and A/C, then connects both W2 and Y2 to the thermostat wire

Architecture


Above architecture diagram shown how a universal thermostat will be connected to multi-stage furnace, and multi-stage A/C.  The controller board in blower motor unit (air sender unit) has wire termination for outdoor A/C unit, so all the thermostat wire connections are connected to thermostat.  There is no direct connection between wall's thermostat with outdoor A/C unit.

Please note about following label:
  • R can be labeled as Rc (24V for cold), or Rh (24V for hot)
  • There could be just 1 R instead of Rc + Rh
  • Both Rc and Rh often has a jumper cable connecting both together in the thermostat, because they are connecting to the same furnace air blower controller board
  • Y is Y1.  For single-stage A/C, you only connect to Y (or Y1), and Y2 is not connected
  • W is W1  For single-stage furnace, you only connect to W (or W1), and W2 is not connected

Choosing Thermostat

As shown in above architecture diagram, the you must know:
  1. Whether your furnace is multi-stage, and wires C, W, W2 are available at the thermostat wall unit
  2. Whether your A/C is multi-stage, and wires C, Y, Y2 are available at the thermostat wall unit
  3. If wires are not connected, is there a spare wires that you can connect C, W2, and Y2 to the air blower controller board (air sender unit)?
  4. If you need to re-wire, find out the quote for that.  1' of 8-wire thermostat wire cost CAD$1, and labor is extra.  If dry wall needs to be cut, then find out the quote to patch the drywall, including painting.  If you have basement's ceiling covered, then the cost will be more to remove and patch them as it is impossible to re-wire
  5. If you have both units are 1-stage system, then you need to find out whether you have 4-wire (no C wire), or 5 wire (with C wire) to power the thermostat.  C stands for common ground, which used together with R (24 Vac) wire to power the thermostat, so that it doesn't need battery
  6. Do you want WiFi thermostat?  It will need wall power, and won't run on battery
  7. If you have C wire available to connect to your thermostat as wall power, you can still consider buying a thermostat that support battery, so that during power failure, the thermostat setting still available.  Most high end thermostat has a coin battery (might not be replaceable) to retain its setting, while low end model don't
  8. If you want to control humidifier, then the thermostat wire will need to have 1 - 2 extra wire
  9. If you want to have outdoor temperature, then the thermostat will need to have 1-3 extra wire, as well as you need to buy the accessory for that thermostat.  Smart thermostats are using Internet to obtain the outdoor temperature, so no longer relies on thermometer sensor

Choices #1: No C wire (only has 4-wire), No multi-stage HVAC

This thermostat can't utilize multi-stage HVAC.  It is rare to find it as this become less common.  Smart thermostat is not going to work
  1. Buy battery operated thermostat without C wire connection
  2. Buy battery operated thermostat with C wire connection.  C wire is future proof, in case you decided to re-wire it with 8-wire thermostat cable

Choice #2: With C wire (only has 5-wire), No multi-stage HVAC

This thermostat can't utilize multi-stage HVAC.  Most low end thermostat will work, which is cheap and commonly available.  Smart thermostat will work
  1. Buy battery operated thermostat without C wire connection
  2. Buy battery or wall power thermostat with C wire connection.  C wire is future proof, in case you decided to re-wire it with 8-wire thermostat cable
  3. Buy smart thermostat

Choice #3: No C wire (only has 5-wire), 1 multi-stage HVAC

Either you have 1 multi-stage furnace, or A/C, but not both.  Most mid model thermostat can be battery operate without C wire, and they have C wire connection for future use.  Smart thermostat is not going to work
  1. Buy battery or wall power operated thermostat with 2H1C (2H means it has W and W2 wire connection to the furnace) if you have multi-stage furnace, or 1H2C (2C menas you have Y and Y2 connection to the A/C) if you have multi-stage A/C
  2. Buy battery or wall power thermostat with 2H2C (this means it has W, W2, Y, Y2) so it is future proof for multi-stage furnace + A/C
  3. Skips 3-stage thermostat models as those are used to control split A/C system with heat pump, which is not common in Canada, USA

Choice #4: With C wire (has 6-wire), 1 multi-stage HVAC

The thermostat choice is same as #3, but buy a unit without battery.  Smart thermostat will work
  1. Buy smart thermostat

Choice #5: No C wire (only has 6-wire), 2 multi-stage HVAC

Smart thermostat is not going to work
  1. Buy battery operated thermostat with 2H2C (this means it has W, W2, Y, Y2).  Recommend to buy a unit with C wire for future proof

Choice #6: C wire (only has 7-wire), 2 multi-stage HVAC

Smart thermostat will work
  1. Buy thermostat with 2H2C (this means it has W, W2, Y, Y2)
  2. Buy smart thermostat

Multi-stage HVAC Wiring Preparation

  1. Smart thermostat needs C wire.  Ensure C (common ground) wire is available and can be connected to thermostat.  Measure the Volt (AC) between R and C at the thermostat, and it must show 24V (same voltage as the small transformer in the furnace blower case)
  2. Follows furnace's installation guide to enable multi-stage furnace, else the thermostat can't control the low speed furnace.  This means activating both W (aka W1) and W2 wiring
  3. Follows A/C's installation guide to enable multi-stage A/C, else the thermostat can't control the low speed A/C.  This means activating both Y (aka Y1) and Y2 wiring
  4. Press the special key in the thermostat (for smart thermostat, it is configuration menu), and configures the thermostat to operate with 2-stage furnace (conventional 90% high efficiency) and 2-stage A/C (conventional).  If you only have one of those, then choose 2 for one, and 1 for another one

Wire Connections

You need 7 wires to connects a multi-stage furnace + A/C, e.g. R, C, G, W1, W2, Y1, Y2
  • If you have a multi-stage furnace, but single-stage A/C, then connects W1, W2, Y1, but leave Y2 empty (not connected)
  • If you have a multi-stage A/C, but single-stage furnace, then connects W1, Y1, Y2, but leave W2 empty (not connected)
  • If the furnace has 3-stage (3 speed) or more (as of today, they have only 3 speed), universal thermostat still going to connect to W1 (also called as W) and W2
  • If the A/C has 3-stage (3 speed) or more (as of today, they have only 3 speed), the same thing apply, universal thermostat still going to connect to Y1 (also called as Y) and Y2

Wire Connections Limitation

If you reluctant to spend money to re-wire the thermostat wire, and you don't have spare, then you are not going to make use of multi-stage HVAC.  There is currently no wireless solution available in the market, although this is possible to have one.

Wire Connections Workaround

If you have some spare wires, pull an extra thermostat wire (with 4 wires or more), but not sufficient to connect to all, then you have some workaround
  1. Only use multi-stage furnace, wire W (aka W1) and W2 - this only need 6 wires, i.e. R, C, G, W1, W2, Y1
  2. Only use multi-stage A/C, wire Y (aka Y1) and Y2 - this only need 6 wires, i.e. R, C, G, W1, Y1, Y2

3 comments:

Anonymous said...

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michaelbelow said...

These are some of the finest tips given above about furnace repair. Such a useful article.
Heating and Cooling Mississauga

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