Calculating BTU Output of REHAU RAUPanel Radiant Floor System

I recently finished installing a REHAU infloor system in my house and ever since I have been on a quest to tweak, improve and learn as much as possible about it.

I chose RAUPanel because the system has been shown in independent tests to outperform any other radiant in floor system. RAUPanel can  deliver up to 32btu’s per square foot on 6″ spacing and 28 btus with 8″ spacing at 110 degree water which in a word is fantastic. (see chart below via Eden Energy)

RAUPanel delivers as much as 50% more BTU’s then the competition which is why I used it in my home. The fact that it is easy to install, involves no concrete and can be done in a day is just gravy.

raupanel-output

As much as a group of ‘researchers’ proved the systems performance I was curious to know just how well my own system was working. So I cracked open my calculator and decided to do some quick math to determine the performance of my RAUPanel heating system.

First off we need to note that my RAUPanel system was installed with 8″ spacing which means that Virgina Tech’s study found the system could deliver in a controlled test 28 BTU’s per square foot at 110 degree water.

Lets start off with the formula for the BTU or ‘heat’ output of a radiant system. It is a pretty simple formula if you know how to do it. It looks like so;

Qo = K(TS-TR)

To break those down.

  • Qo = Heat output from your radiant system
  • K is a constant we need to look up for the material we used which in my case was RAUPanel
  • TS is the fluid temperature in your radiant infloor system
  • TR is the air temperature you want to maintain.
  • To get K we will also need the R value of the floor or the ‘resistance’ value.

Lets get down to the math now shall we ? I am keeping my room at a temperature of 70 degrees. My water temperature on average is 107.5 degrees ( lets call it 107 ) and the Resistance value of my Engineered hardwood floor is 0.63 according to the RPA lookup table.

So now our formula looks like this;

Qo = K(70-107)

How do we finish this formula? We need to figure out what K is and we do that by looking up K on the RAUPanel Table using the temperature of 37 ( 107-70 ) and the R value of .63. You can see how I do this with the table below.

The red lines cross at my R value and the temperature difference between my water and my air.
The red lines cross at my R value and the temperature difference between my water and my air.

Notice on that chart that there are TWO curves on the bottom one for Wall and one for floor. Because I installed my system in the floor and not the walls I drew a straight line from that curve at .63 up to my DELTA T of 37 and I get 22 BTU’s per Square foot.

I have no problem at all heating the rooms with 22 BTU’s per square foot.

Lets talk for a second about the heat source and how RAUPanel impacts that. Boiler efficiency is dictated in part by return water temperature and my return water temperature is 87 degrees. Where does this number come from? My Supply water temperature is 107 and my water temperature delta T is 20 in my system. So that makes my return water temperature 87.

Take that number and now plot it against a condensing boiler to see what that means to overall efficiency. My boiler will be 91 to 97% efficient depending on what firing rate it is on. ( chart via )

Boiler Efficiency on RAUPanelHow to improve the overall efficiency of my RAUPanel system even further.

Virginia Tech’s report showed that a RAUPanel system could deliver 28 BTU’s at 110 degree water and mine fell slightly short so how could I get my BTU output higher?

Right off the bat increasing my water 3 degrees to 110 degree water instead of 107. That in itself would make my K value 40 ( 110 – 70 ) and increase my BTU output to 24 BTU’s per square foot. Small changes can have a huge impact.!

The other thing I could have changed was my choice of floor covering. I used a 5/8ths Engineered Wood because it was the one my wife chose. Had I of used a 1/4″ Engineered floor my R value would drop from .63 to .25. This makes a HUGE difference in the overall BTU output because of the much lower resistance.

Look at the chart below to see how big a difference a much thinner floor makes. I actually outperform the Virginia tech numbers managing over 30 BTU’s per square foot with my original K value of 37 and a floor R value of .25

raupanel btu output resistance changed
Pardon the lines I realize they are not exactly right so use it for general reference only. I am clearly not an Engineer 🙂

A few key take aways when it comes to a floor heating system and RAUPanel in particular;

  • Even if you do not pay attention to floor coverings you can get 22 BTU’s per square foot of heat as I did.
  • If you do pay attention to floor coverings you can get a significantly higher BTU output.
  • RAUPanel will cost you more up front to install but because it delivers more BTU’s it can save you money each and every month on your heating bill.
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