This is my HVAC home automation program.

Background on my house:

My house is a rural farmhouse in the midwest of the US.  As a farm house it was built
and added on to many times over the past 120 years.  So insulation varies, building
techniques vary, etc.

The heating system is a natural gas fired boiler, with three zones of base board
radiators (think 1980's aluminum fin style).

Being in the midwest, while rare, our winters can get as cold as -20F (or colder).
The house and heating system struggle with temperatures that cold and I ususually
need to use a supplimental portable plug-in electric heater.

Since it's a radiant heat system, there is no central air -- and thus no central
air conditioning.  This means I need to rely on a series of window air conditioners
in the summer time for cooling.  Again in the midwest we can get to temperatures that
exceed 90F with humidity at 90%+.


Thermostat control:

The Nest thermostats do a wonderful job controlling the ups/downs of radiant heating.
However, on the coldest days they can't cope with the overall heat loss of the house
so everything runs continuously and the house can get colder then I'd like.

On these days I use a plug in space heater, however if I don't regulate it
perfectly, I end up using more electricity then necessary raising my electrical
bill.  (Natural gas heating is far cheaper for me.)

I want a way for the Nest to call for supplemental heat, which can be as simple
as turning on an IFTTT enabled WiFi plug-in switch.  This will allow the Nest to
make a more efficient choice concerning both comfort and cost.


In the summer with the window air conditioners, I can cool my house with one
air conditioner per zone, roughly equivalent to where the thermostats are placed.
However, they are noisy and either the fan has to run all the time, or I have to
deal with the 'eco' mode where the fan is off -- but the temps swing wildly in
the house.

I was finally able to find some WiFi / IFTTT enabled window air conditioners
(more on that below).  So I need a way to sync up the Nest to control the
air conditioners in a way that is both comfortable, and keeps some of the fan
noise and power usage to a minimum.


Hardware involved:

* Boiler (180F set) & pump - 24 VAC driven
* 3-Zone Control Valves    - 24 VAC driven
* 3-Nest thermostats (2 Gen3, 1 Gen1) - 24 VAC driven
* Numato 8-port GPIO USB board (GND to GPIO driven)
* 8-port 24VAC Relay board
* IFTTT enabled plugs (Meross from Amazon)
  * Baseboard style plug in heater
  * Ceramic element, fan heater
* GE (Haier) Window Air Conditioners, IFTTT enabled


With this integration, the Nest thermostats can drive the 24 VAC relays.
The relays can then drive the GPIO lines, and the GPIO can be used to
determine what the nest is calling for.  Fan, Heating, Air Conditioning,
Humidity, Dehumidification, etc....

Along with that, Nest makes a 'Works with Nest' developer API available.
This API appears to be available for both hobbyists, as well as commercial
developers and provides the opportunity to read more then just the on/off
that would be available from the GPIO board alone.


Air Conditioning theory:

The GE air conditioners are actually pretty 'smart'.  In cooling mode they
will simply cool until they reach the set temp.  However, in eco mode they
will cool to the set temp, and keep the fan running just long enough to
use the cooling capacity left in the condensor.  Using this principal with
the Nest thermostats you can end up doing something like:

1) Wait for the GPIO to signal cooling.

2) Read the desired temp from the Nest.

3) Set the mode to 'cool', set the temp UNDER the desired temp.  (I think -2F
makes sense.)

4) Once the Nest says cooling is complete, switch the mode to Eco, and set
the temp OVER the desired temp.  (I think +2F makes sense here.)


The idea with this is if in case of a power outtage, IFTTT outtage or other
things going wrong.. a reasonable base temp will still be available in case
it is needed.


Heating theory:

Using a simple on/off plug heater, with the portable space heater set to a
reasonable max temp should be fine.

It may be necessary to occasionally check the temp in the house for a sign
that it's 'too warm' and send the power off signal in case it was missed
previously.


Program theory:

The GPIO, Nest API, and IFTTT work will each be in their own worker threads.
They are each responsible for getting or sending the data, and retrying in
case of failures.

A thread for each 'zone' in the house will also be present.  This thread
will be contacted via events when the GPIO or Nest API changes.
(Presumably if I want integration with IFTTT, I could add an event for
that as well.)

So all of the control algorithms will be local to the zones, and all of the
data I/O will be local to the workers.


Limitations with current design:

The 24VAC to the boiler is the primary heat source (Stage 1).  Stage 2 heating
is accomplished via the GPIO to the zone.  While the Nest Gen3 supports a
stage 3 heating, it has not yet been added to the general zone framework.  It
should be possible to enable a stage 3 though with Zone specific programming.

Cooling is very similar, except there is no central air conditioning.  So all
cooling control happens via the GPIO configurations.  Similarly, there is only
a single cooling stage implemented.  In one case, I had a need for a two
staging cooling and introduced a second 'virtual' Zone for this cooling phase.

The only issue with this design is that cooling and heating for that virtual
zone come from two different thermostats.  Currently there is no method for
adjusting the Nest in use for different system modes.

Updated 2018-10-12
--Mark