Tumbling Dice

Taking a break from our 1974 retrospective to pay tribute to this talented lady’s retirement…

Linda Ronstadt in ’77, covering a Stones classic.

Kinda makes Miley Cyrus look even more pathetic in comparison, doesn’t it?

So, do you feel like rolling the dice?  Here’s a quiz for you…

What’s the easiest way to raise the floor surface temperature with a radiant floor heating system?

Do you:

  • A:  Boost the supply water temperature in the system?
  • B:  Install more tubing?
  • C:  Adjust the balancing valve to allow for more flow?
  • D: Turn up the thermostat?

If you answered D, go to the head of the radiant class!

Turning up the thermostat isn’t only the easiest is way to raise the floor surface temperature, it’s also the most effective way.

And given the options, it’s also the only way.

Radiant floor heating is a beautiful thing – it gives off BTU’s based solely on its surface temperature.  The warmer the floor surface, the more BTU’s per square foot the radiant panel emits.  That’s simple physics.

But what determines the required floor surface temperature?

Simple math, my friends…

PST = (BTUH/SF load ÷ 2) + SPT


  • PS = Panel surface temperature required to heat the room
  • BTUH/SF = the room heating load expressed in BTUH per square foot
  • 2 = the heat transfer coefficient of radiant floor heating
  • SPT = the Set Point temperature, or the temperature at which you set the thermostat

Let’s say the heating load for a 200 square foot room is 4,000 BTU’s per hour. The outdoor design temperature is 0 degrees and the indoor design temperature is 70.  To determine the BTUH per square foot load,  divide 4,000 BTUH by 200 square feet, which is the output requirement for the “radiator” at design conditions:

4,000 ÷ 200 = 20 BTUH per square foot.

Next, divide the BTUH/SF heating load by the heat transfer coefficient of 2:

20 ÷ 2 = 10

Finally, add the indoor design temperature (70*) used in the heat loss:

10 + 70 = 80

So to heat this room at design conditions, the floor surface temperature – the surface temperature of the radiator itself – needs to be 80*. Doesn’t matter whether the finished floor is carpet, hardwood, tile or goatskin, or whether the installation method is slab or aluminum plates – under design conditions that floor surface needs to be 80 degrees to heat the room up to an air temp of 70 degrees.

Here’s a curveball: let’s  say it’s only 35* out.  That would cut the BTUH/SF heating load in half, from 20 down to 10 BTU/SF…

10 ÷ 2 = 5

5 + 70 = 75 required floor surface temperature.

So at 35* outside (or 50% load), the floor only needs to be 75 degrees to heat the room up to an air temp of 70.

How about that!

But what if the customer wants the floor warmer?  One of the most common complaints with a properly designed and installed radiant floor heating system is that the floor doesn’t always feel warm. The house may be perfectly comfortable, but the customer wants warm floors.

They want warmer floors? Turn the thermostat up!  Look at what happens when we turn the thermostat up to 72* when it’s 35* degrees outside, and the BTU/SF heating load is 10:

10 ÷ 2 = 5

5 + 72 = 77 floor surface temperature.

By turning the t-stat up, we are, in fact, increasing the BTUH/SF heating load.  To satisfy that new heating load, the surface temperature of the radiator (the floor) will have to increase.

Make sense?

What about the other choices?

Will raising the Supply Water Temperature increase the floor surface temperature?  Not in the room with the thermostat.  When the thermostat’s happy, the floor is delivering the required BTUH per square foot, which means the floor is the temperature it’s supposed to be.  Raising the Supply Water Temperature only helps the zone satisfy quicker.

Installing more tubing isn’t easy, especially after the job has been installed, and will only result in lowering the required supply water temperature for the system, depending on the installation method.  That’s not a bad thing, but it won’t do anything to increase the floor surface temperature.

And don’t get me started on adjusting the balancing valves on the manifold to increase the flow. The next guy to pull that off successfully will be the first.

The thing to remember with a radiant system is that there is no substitute for a thermostat.  The t-stat is king! And when the king is happy, everyone’s happy.

She’s nominated this year, so it’s Rock ‘N Roll Hall of Fame time, don’t you think?

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