Comfort is Not About Air Temperature, Thermostat #FAIL

I have two thermostats in my home and both are set to the same air temperature of 71. Both are great thermostats and both I love immensely so understand this post is not about bashing a product it is about better understanding the application of the products.

One of my thermostats is an ecobee 3 the other is a REHAU radiant thermostat.

The ecobee 3 is the ‘Jedi Knight’ of thermostats and my REHAU radiant thermostat…. well not so much.

The ecobee 3 is a learning thermostat with wireless sensors that can turn itself on and off whenever I come home or leave. The ecobee 3 provides temperature averaging across my entire home and has HomeIQ which learns how my home heats up and records indoor temperature, outdoor temperature, wind speed, weather and oh so much more to heat my home and save me money every month. The ecobee 3 is in a word cool and thanks to its HomeIQ saves me money on my home heating bill.

The REHAU radiant thermostat? It has a button on the front I push to turn the heat up or down. You definitely wont find any tech bloggers raving about it as the hot new trend in thermostats.

The Dual Sensing REHAU Smart Thermostat wont be winning any design awards anytime soon.
The Dual Sensing REHAU Smart Thermostat wont be winning any design awards anytime soon.

Which stat do you think keeps me the most comfortable? If you guessed my ecobee 3 you guessed wrong.

It is my super simple low-cost dual sensing radiant thermostat from REHAU.

As much as I love my ecobee that does not change the fact that a device that only measures air temperature does not measure the actual thermal comfort of a room.

More significantly and key to this post the ecobee 3 being incredibly smart and sophisticated does not change the fact that its hooked to my gas furnace.

The reason that my REHAU thermostat actually works better is because it is hooked to a radiant infloor heating system. Thermal comfort is determined by 6 factors as noted by HSE, ASHRAE and just about every respected PhD;

  • Air temperature
  • Radiant temperature
  • Air velocity
  • Humidity
  • Your Clothing
  • Metabolism ( how many BTU’s your body is putting off )

My REHAU thermostat is not pretty like my ecobee 3, it does not have an app, it does not log any information that I can remotely access ( which is all a bummer ) but what it does do is cover 2 of the 6 items on what makes us ‘comfortable’.

It allows me to set not only the air temperature but also the floor temperature and by doing so it makes sure that the floor temperature is similar to my body so that when I walk on the floor instead of loosing heat due to a cold surface it is the same temperature or slightly warmer keeping me super comfy.

The floor doesn’t ‘feel warm’ but what it does do is take the cold off the floor and prevent my body from radiating btus into the floor and gives me 2 out of the 6 factors in thermal comfort.

Radiant temperature has a greater influence than air temperature on how we lose or gain heat to the environment. Our skin absorbs almost as much radiant energy as a matt black object…. – HSE

The point of this post is to just point out that yes air temperature is important in keeping us comfortable but make sure you understand the application and more importantly your heating source.

If you are in fact heating with radiant in floor and using an air sensing thermostat you are completely missing the point of radiant heating. My REHAU dual sensing radiant thermostat and floor sensor combo are set typically to 68 degrees air and 80 degree floor and the room is FAR more comfortable than the rest of my drafty house run by my high-tech and awesome ecobee 3.

Dont have radiant? Only have a forced air heating system? Then do what I did and install an ecobee 3. I openly ENCOURAGE it as I have one in my own home but understand your heating system and more importantly what actually controls comfort and if you have a  radiant system use the right thermostat.

For those wondering I hacked my REHAU smart thermostat to give me some cool remote accessibility features because I simply could not help myself. 🙂

My REHAU smart control monitors supply and return water temperature, BTU output, run time and oh so much more fun stuff!
My REHAU smart control monitors supply and return water temperature, BTU output, run time and oh so much more fun stuff!
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4 Comments

  1. Stupid comparison. The normal thermostats are not meant for radiant heating system. It would be like complaining that my expensive thermostat is not turning down my wood stove properly. Duh!

    1. FYI air temperature is most certainly part of a radiant control and is one of the 10 contributing factors to thermal comfort in the space.

      Mean Radiant temperature contributes more to thermal comfort because the human body looses 62% of its BTUS via radiation losses.

      Next time rather than calling people stupid why not share your views maturely.

  2. I’m a building services engineer, and I came across your blog looking into radiant/air temperature control. I love all the sensors you have!

    “Only have a forced air heating system? Then do what I did and install an ecobee 3. I openly ENCOURAGE it as I have one in my own home but understand your heating system and more importantly what actually controls comfort and if you have a radiant system use the right thermostat.”

    With forced air heating, a radiant temperature sensor is less useful as a control due to the increased convective heat to radiative heat transfer. However, if you can assess the mean air velocity in the space you can use the equation below to take into account radiative and convective heating/cooling to give a combined figure more suitable for rating thermal comfort against.. Depends how geeky you are!

    Another way of looking at control/sensing is to use the ‘operative temperature’, which takes an average of the radiant and air temperature (so if it’s very sunny, the heating will be off, and the cooling likely on).
    This doesn’t apply the same for forced air heating, where the mean air velocity is over 0.1 m/s (0.33 fpm).

    At air speeds over 0.1 m/s, the equation becomes more complex due to the change in heat transfer ratios.
    For air velocities > than 0.1 m/s 0.33 fpm, Operative temperature =
    ([air temperature] * sqrt(10 * [air velocity, m/s]) + [mean radiant temperature]) / (1 + sqrt(10 * [air velocity, m/s]) )

    Source CIBSE Guide A 2015

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