From the block diagram I knew that I can find if the relay is on at the solenoid. I found the solenoids all clustered together. View from side (cover off) showing Gas regulator and solenoids Whether it is making LOW or HIGH heat is determined by the heater, the idea being that if the room is very cold it makes more heat, then to maintain the temperature it runs lower and more efficiently. This unit, which has 2 stages so it can make LOW or HIGH heat. Here is what we are dealing with: A Rinnai Energy Saver-551F Unfortunately, the furnace is an all-enclosed unit with no external thermostat and it has a single temperature control for both stages. This is mostly because it would be expensive, but luckily, this means I don’t need to monitor 6 different thermostats. The electric heat is not expected to be used much if at all. The apartment is a two-floor town-house, so this may be a tall order. The furnace is intended to heat all the rooms and replace the electric heat. Additionally, there is a single natural gas wall vent furnace in the living room. Each room has electric heat baseboards with a thermostat to control that room. This new apartment has 2 sources of heat. By sensing if the thermostat was “ON” I was able to see if the furnace was heating the radiators. This was controlled by one central thermostat located in the dining room (center room in the apartment). Quick re-cap, previously I had a forced hot water system where a furnace heated water and circulated it through radiators in each room. So this is a perfect opportunity to revamp my existing project with my raspberry pi. I moved over the summer and the new apartment does not have the same type of heating that the last apartment did.

Rinnai energysaver 551f