Hydronic Radiant Floor Heat 101 (part two)

With the popularity of radiant floor heat and sales rising each year more and more concrete contractors and general contractors are selling radiant floor heating systems with their projects. Radiant Floor heat creates some questions when first starting, in this article we will explain the key points of radiant floor heat and give you the confidence to start including radiant floor heat in your projects.

Can I place the tubing in the sand below the concrete?

Yes and no! There are some advantages to placing the tubing in sand below the concrete. Just be careful that you don’t get the tubing too deep in the sand. We had one job where the tubing was at the bottom of 15’’ of sand. It did not work well, partly because it was off peak electric and 15’’ of sand gets too dry and doesn’t conduct heat well.

Some contractors prefer a couple inches of sand below the concrete to place the tubing in – this allows the troweling of the concrete to be more manageable. The sand also protects the tubing if there is a major crack/shift in the concrete. 

What if the concrete cracks?

The normal cracks that appear in all concrete are not really a threat. I have heard of a couple shifting cracks that have pinched off a loop over the years; however, this is an extremely rare occurrence. This type of shifting crack is from poor compaction of the base. On most jobs, if you were to lose one loop from a crack, the other loops will carry the load.

What type of tubing is used in the concrete?

You can use a Pex-Al-Pex or a Pex tubing. Let’s look at the differences between the two options. (FYI: the word Pex in both types of tubing is an abbreviation for cross linked poly-ethylene.)

Pex-Al-Pex or Pex-Aluminum-Pex features:

(This style of tubing has an aluminum sleeve centered between two layers of Pex.)

• 100% oxygen barrier (the aluminum sleeve).
• The oxygen barrier is protected (centered between the layers of Pex).
  
• Better heat transfer because of the aluminum sleeve.
  
• Ratings: European quality 145 PSI at 203 degrees or Asian quality 125 PSI at 180 degrees.
  
• Ease of lay out. It bends like soft copper and stays where you put it.
  
• Less fasteners are needed.
  
• All advantages of both plastic and metal tubing.
  

• The best coefficient of linear thermal expansion. In other words it does not try to grow very much in length when you start running warm fluid through it.
  
• Kinked tubing – if severe, replace roll or splice.
  
• Price: middle of the road.
  

Pex with an Oxygen barrier features:

• 98% oxygen barrier.
• The barrier is usually located on the outside of the tubing. The oxygen barrier is NOT protected from sun, water and abrasion.
  
• Ratings: 100 PSI at 180 degrees.
  
• Ease of layout – Pex has a memory; it is always trying to recoil itself.
  
• Additional fasteners needed.
  
• Coefficient of linear thermal expansion is poor compared to Pex-Al-Pex.
  
• Kinked tubing – depends on the brand of Pex. Some you need to replace the roll or splice, others you heat up to a certain temperature and it will go back to its original form.
  
• Price: All over the place, many highs and lows. (Be careful – some Pex on the market has zero oxygen barrier!)
  

What is an oxygen barrier?

An oxygen barrier prevents oxygen from penetrating through the wall of the plastic tubing and entering into the boiler solution. And yes, oxygen can penetrate the wall even with a pressurized boiler system. Boilers are designed to be closed and sealed off from oxygen. If they are not, rust and sludge can start to build up in the boiler system.

Should I use hydronic tubing or electric mats?

They both work and are both considered “radiant floor heat”. I feel the hydronic tubing is the better choice over electric mats.

Let’s take a look at why:

• With hydronic tubing you can hook up to multiple heat sources; natural gas, L.P. gas, fuel oil, ground source, electric boilers, bio-mass burners, wood boilers, corn boilers, solar, etc.
• With electric mats you are committed to the electric company.
  
• The track record related to longevity and performance of tubing in concrete is far better than electric mats.
  

What size tubing is used?

The two most common sizes are ½’’ and 5/8’’. Many times this decision comes down to someone’s opinion. Most people assume 5/8’’ does a lot better job compared to ½’’.

Some installers prefer to use ½’’ and others 5/8’’. There are times when the layout of trench drains and manifold locations force the designer into the 5/8’’ tubing. With 5/8’’ you can go up to 450 foot loops, ½’’ is 330 foot loops maximum.

There is very little difference between ½’’ and 5/8’’ tubing. In fact, it is common to install both sizes using the same on center spacing.

What information is needed to get a project started? «read part three of Hydronic Radiant Floor Heat 101

Notice the Pex-Al-Pex staying flat on the Styrofoam. Air test is on tubing and ready for the concrete.