Home Automation Series: Temperature Sensors


The heart and soul of my home automation and HVAC controls are room temperatures. Having a single zone HVAC unit means that the temperature is controlled in one place (usually near the central return). My house was built in 2016, and makes use of the idea that when a bedroom door is closed, the air return can occur under the door. That idea is….well, terrible.

With HVAC units you have registers (the vent that blows air into a room) and returns (it removes air from the room). Our house is unique because it has a second master suite, which has its own air return but has no temperature control and very weak air pressure from the unit. We’ll come back to this in a bit.

If you have a register in a room and it’s blowing air and the door closes you do what’s calling “blowing up” the room. Like a balloon. You are putting more air than you are removing into a room. Now, if you don’t have enough air blowing into a room and you have a return sucking air out you actually get the exact opposite issue. You create a negative pressure issue. When the unit is trying to pull air out of the room but there isn’t enough air from the unit, you start pulling air in from outside.

I mention this because right now in our house we have four bedrooms and two common areas. Two of the bedrooms are controlled by a single vent off the unit. I recently added an air return to my bedroom to help balance the system and attempt to get rid of the build up of air. So, that second master bedroom. The floor vents provide very little air, and I mean, VERY little air. So little its barely detectable by placing your hand over the vent. But, the room has a fully operational air return, which runs straight into the system. In the winter time, that room is very cold and in the the summer very hot.

The issue is, the air return pulls in whatever temperature the air is and sends it to the HVAC for recirculation. The issue is, if we want the house warm, but that room is cold. the house struggles to warm up because we’re sending cold air.

Wow, quite the intro. haha.

So, how will I correct this? By using Shelly H&T Sensors to monitor temperatures in each room, I will be able to tell which room needs more or less air. The two rooms that are controlled by a single vent duct from the unit will need a “jumper duct” to allow better air transfer.

Jump Ducts | Building America Solution Center
A jumper duct lets air from one side of a wall or door easily pass back to the unit.

The Shell H&T sensors have an Open RestAPI, which will make direct automation very easy!

Once per hour my code will query the sensors to get Temperature and Humidity readings. Once the readings are complete, determine which rooms have temperatures that are out of scope for the overall house temperature.

Once we’ve determined which rooms are too cold or hot, we’ll adjust the dampers accordingly and push air.

When the dampers have been adjusted, the room temperature should raise or lower pretty quickly since we’re driving a lot of air to that room. When a room has been marked as being too cold or hot, we’ll check in every 15 minutes after the dampers are adjusted to see how the temperatures is coming. If we’ve checked-in twice and still haven’t seen the temperature come into scope, a push notification will sent to me alerting me.

I haven’t yet added to jumper ducts, and I don’t have the sensors yet. I’ve ordered two of the sensors so that I can begin the tracking of temperatures in my room and one of our other bedrooms.

Thanks for reading!

Dan