Running Radiant Infloor off Air Temperature Alone

I recently completed work on a radiant smart control that enables just about anyone to run their radiant in floor system from a smart phone but in doing so I have opened up a slew of ‘weekend warrior’ opportunities to test various theories that have been taught to me over the years by industry guru’s like Robert Bean and John Siegenthaler.

My recent foray into experimenting came innocently enough from a chat  I had recently with Robert Bean on Thermal Comfort. No sooner did we talk about how Air Temperature does not dictate thermal comfort my system in my living room captured an example of just that ( read more here ).

On Wednesday I got talking with hydronic’s champion Dean Newberry about using air sensing thermostats alone on radiant in floor and simply could not resist doing a test in my own home.

I am not a fan of air sensing thermostats alone for radiant systems because air temperature alone is not a proper measurement of thermal comfort. In my own home 68 degrees is perfectly comfortable with my floor sensor properly calibrated. Without the floor sensor my stat may still read 68 but the floor is cold and my family starts giving me that sideways look of ‘hey you told us this radiant floor heating is amazing’.

That is not to say that an air sensing thermostat can not run your system my argument is why you would do it when there is a much better option. Below is a few thousands data points from my radiant smart control with my system being controlled by air and floor temperature. The temperature deviates a few points of a degree but is otherwise incredibly stable.

air temperature
Air temperature on my radiant infloor heating system

As an experiment this weekend I disconnected my floor sensor to run the system off the air temperature alone and you can see that the results were noticeable.

Radiant Infloor Running with Air Sensing Thermostat

Normally where my system would only overshoot by a .5 degree I am now over shooting my temperature by as much as 3 degrees and it is happening frequently. Now imagine what if my thermostat was installed on an outside wall or in a drafty location? What would my floor temperature be? Likely it would be overly hot and uncomfortable before my air sensing thermostat noticed.

Lets look at a second example from a week ago. My thermostat was set for 72 it was 72 where the stat is but the floor temperature was cold. It was my wife who gave me that sideways look that brought it to my attention.

The graph below illustrates this point. My air sensing thermostat thought the floor was warm but it was not. The end of the graph is when I finally addressed the issue, my pump had stopped and my air sensing thermostat missed the memo.

raupanel temp

Floor Sensors can not only enhance the comfort in the room but avoid structural issues with hard wood. The Hardwood Flooring folks do not like when we put really hot water under a hardwood floor because they say it will ‘damage the wood’.

In my own home when the flooring sales person saw I was putting in radiant heating he declared ‘woah you can’t do radiant with hardwood’. I am sure he meant no harm but comments like that can hurt our industry. Contributing factor I am sure is the fact he had never even seen my RAUPanel heating sytem before. ( 28-32 BTU’s per square foot at 110 degree water )


It is true you don’t want to run super hot water under some wood floors but its worth noting that it’s not the radiant system itself that can damage the wood its the effect that temperature has on the moisture content of wood. Robert Bean said it best when he said;

100% of all hardwood flooring complaints in buildings heated exclusively with forced air did not have radiant floor heating to blame.

As the water temperature gets hotter beneath the hardwood the wood in turn gets hotter and the moisture content goes down which can cause the wood to shrink. Nobody wants to take responsibility for this which is why a radiant floor sensor is a must have $25.00 addition to hydronic’s projects.

Not only can you prevent flooring issues being blamed on you by setting the max floor temperature to less than 85 but you can also ensure optimal thermal comfort and work to eliminate awkward complaints like ‘this radiant floor system is not working’.

So can you make a system run with an air sensing thermostat? My own weekend warrior experiment shows it can be done but running does not always equate to working properly. Without question a floor sensor can and will make a radiant infloor system more comfortable and can eliminate potential issues with over/under heating the space.

In my own home until I can convince my wife to let me install a globe style mean radiant temperature sensor in the middle of the room I am going to continue using a dual sensing radiant in floor PID thermostat.


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