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My smoke detector was awful. Every time I took a shower I had to make sure the door was closed or it would go off from the steam. After realizing that it was a hardwired AC-powered detector I figured to upgrade to a Nest Protect. It's a little expensive, but the idea of phone notifications for alarms was interesting to me. It worked well, but also sparked a discussion from one of my G+ friends about the possibility of getting a Nest thermostat for my AC units. This hadn't occurred to me since the heating/AC units are pretty awful and only have a control panel, but a little investigation discovered a header for an external thermostat control. Finding the manual lacking and the company impossible to deal with I set out to the Internets to try my luck. This page documents what I've found, however this is a project in progress so forgive me if some data is incorrect or misleading.

A Warning

This should go without saying, but I'm not an HVAC expert. I have no affiliation with Nest nor IslandAire other than being a customer. If you kill yourself or your heat because you messed around based on the input of some random guy on the Internet you've got yourself to blame.

I also don't think you should be messing around when it's 3F (-16C) outside...

When not to Nest

The Units

These are parts of my apartment I'd like to break, but I can't justify an outright replacement as of now. They're made by IslandAire, poorly documented, and the company won't deal with me directly even when my control panel failed and it would have been about five minutes for me to replace it instead of a series of expensive service calls. They are AC units as well as supporting hot water-based heating (called a Hydronic Coil) in the winter months, sometimes known as a PTAC unit. They are a rats nest of wires and circuit boards, but at least the components seem to be labeled.

Some things I've learned:

  • Thermostat wire labels are pretty commonly named across vendors. This isn't exclusively so, but for example W1 is almost always primary heating (amusingly I randomly picked a label that is not standard on IslandAire). The wire colors are also somewhat standard, but you should always trust the labels on the units more than the colors.
  • A PTAC unit (common in small living spaces like condos) combines heating and cooling systems into a single element. Most houses would have a heating system and a separate cooling system - even if they run through the same ductwork.
  • If there's a radiator and a fan in this PTAC unit for heating this is called a "hydronic" heating system. It differs from a traditional radiator in that the hydronic coil is usually smaller than a radiator coil since there is an active fan going blowing the hot air through it instead of relying on passive convection.
  • There is usually a valve to control whether hot water from the source flows through this coil, this is often controlled by a mostly passive device called an aquastat. An aquastat can clip onto the plumbing and opens or closes a connection when the temperature is above a preset amount. Mine is open (no connectivity) when below about 100F and closed above 100F but it's feasible this could be reversed.
  • Thermostats are normally ~24VAC, this complicates things like relays or DIY stuff over using DC but it is what it is.

I have three units, from this document all of the models are 110VAC, oddly E is not a defined chassis type but I'm assuming A is appropriate, they're hydronic with an aquastat, and have built-in digital controls:

Location Model BTU
Master Bedroom EZ09E1RKG3S14AA 9,500
Second Bedroom EZ12A1RKG3S14AA 12,000
Living Room EZ12E1RKG3S14AA 12,000

The Thermostat Connector

There is a 8-pin header (which is remarkably difficult to count...) for which I'm trying to source a connector. The header has a "pitch" (pin-to-pin spacing) of 3.96mm (0.156"). The pins are labeled as-follows (information from here and page 37 here, also see this guide):

  1. C (black) The common or neutral wire.
  2. GL (orange) Often green, this is the fan control for the low speed setting.
  3. W2 This is normally a second stage heating control, however it seems to be the primary heating control in my setup for one of my newer units (well, at least newer control boards). When I apply voltage to this pin the hydronic valve starts up and when the aquastat reads an appropriate temperature it will start the fan.
  4. Y/W1 This is a single pin with both labels
    • Y (yellow) This is for cooling/AC control.
    • W1 (white) Normally primary heating.
  5. B (blue) Often used in a heat pump to switch from heating to cooling. I'll have to see in the spring if I need this to switch to AC.
  6. GH (green) The fan control high speed, the Nest only supports one fan speed (please upvote this idea) and I never use high, so this won't be connected.
  7. LS (purple) When you place power on this terminal the compressor is disabled, presumably this would allow fan-only operation? The IslandAire documentation mentions an "energy management system" interface.
  8. R (red) 24VAC power.

While not a perfect fit, I've ordered a 10 way connector to see if I could at least get started (you may also want some extra terminal pins). This meant I got to order a Paladin Tools 2033 die for my crimper. To help prevent needing to recrimp everything if I need to change out a pin I also added a screw terminal which could handle every pin if I needed it.

Note that this connector worked fine in my bedroom and seems to be perfect except for the two extra pins. In my daughter's room I needed to trim the two extra pins off since it was blocked by another item on the board and the connector seems a bit too large - however it does still seem to work OK.

Nest Mapping

The following represents the mapping from the connector above to the Nest. I currently (no pun intended) have a five-conductor AWG18 wire, assuming this continues to work in the summer all I need are four (five in one room). I also installed a switch on the side of my unit to flip between GH and GL, the AC plus the low speed fan meant anemic cooling in my window-heavy apartment.

Unfortunately the second bedroom unit seems to have a newer board than the other units, the thermostat header isn't labeled (but seems to have the same pinout) and the DIP switches are a bit different and, more bizarrely, it has a different behavior.

Nest MBR Unit 2BR Unit LR Unit Color
C C C C Blue
G GL GL GL Green (via switch)
W1 W2 W2 W2 White
Y1 Y/W1 Y/W1 Y/W1 Yellow
N/C B B B --
N/C GH GH GH Green (via switch)
N/C LS LS LS --
RH R R R Red

Some messy insulators on these which I should re-crimp (note also that the white wire is missing since it's an older experimental setup):

IslandAire Connector Nest Connector

DIP Switches

There are some DIP switches on the control board, however their functions aren't overly clear. The only important one seems to be the "Local/Remote" switch which seems to enable or disable the local control pad as well as allowing the Nest to take control. Switch it to "Remote" for the header pins to work or "Local" to use the internal pad. Before playing with these, you'll need to power off the unit (unplug it, or I trip the GFI tester on the plug). They only seem to be read on power on and it prevents touching the wrong thing and blowing something up. I wouldn't recommend touching any setting other than local/remote as I'm not positive what the behavior changes will be.

For the master bedroom and living room the DIP switches look like this (note that I removed some connectors blocking the view). I switched the hydronic from normally closed (which had the valve on all the time) to normally closed (which only opens the valve when W2 has power):

Image Name Original Setting Current Setting
Old DIP Switches Remote/Local Local Remote
Hydronic On On
Electric Heat Off Off
Heat Pump Off Off
Freeze Prot On On
Intellitemp/Base Base Base
Gas Heat Off Off
Hydronic NC NO

For the second bedroom they look a bit different, and that's the thermostat connector just below:

Image Name Original Setting Current Setting
New DIP Switches Hydronic On On
Heat Pump Off Off
Electric Heat Off Off
Gas Heat Off Off
Freeze Prot On On
Restart Off Off
Local/Remote Local Remote
WSHP Off Off
NO/NC (switch on side) NC NC

Nest Configuration

If things are working the Nest should detect things properly. Of course, you'll want to connect it to your WiFi network, but the important bit of the configuration is under the Equipment section. It should show you that Y1, G, W1, C and Rh are connected.

In the master bedroom and living room I set the heat type to "Electric" which will apply power to G and W1 for heat - this will turn on the fan explicitly.

In the second bedroom I set the heat type to "Oil" which will enable just W1 (connected to W2 on the IslandAire) when the heat is on.

Current State

In the master bedroom I've recently made a change. I noticed that the hydronic coil valve was open all the time so it worked well when it was very cold, but now that the cold spell has broken the room is a little warm. As an experiment I've flipped the hydronic control so there's only power applied when the heat is on to see if that helps things out, and I've connected W1 on the nest to W2 on the unit. Now when I test the heat things work as-expected but when I test the fan only it quickly cools as I'd expect. Of course, time will tell...

I think things are working in my daughter's room where the thermostat header is much easier to access. I'm now using all five connectors in the wire I bought. For heating if I apply voltage to the W2 pin (fed via W1 on the Nest) it turns on the hydronic pump (I can read 120VAC at the pump connector) but it only turns on the fan (high speed) if the aquastat reads a high temperature.

Now that it's summer I'm happy to report that both A/C and heat work great! During a recent heatwave I've noticed that the choice of wiring the fan to low speed may work well in the winter, but not in the summer. I've installed an on-on switch inline, the feed from the Nest goes in the input pin and one output goes to green high and the other green low.

Bill of Materials