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Radiant Floor Heating/Pex spacing in concrete slab


When considering pex spacing in slabs, I've seen examples where the outer permimeter is more closely spaced than the inner.    I've seen examples where that's not the case. I've also seen online calculates where defining the slab as being "blocked" ( which means it has some flooring such as carpet on it) gives 18" spacing with unblocked (bare concrete flooring) yielding 6" spacing.  Is it fine to have even 11" spacing using 1/2" pex on a bare concrete floor.  Would the considerations be different between a garage/shop space compared with a living room space?

The "consideration" of tube spacing starts with the Manual 'J' 8 heat load for each room that radiant panels will be heating.

The first consideration is to heat output. Closer spacing, higher output.

Second, is to comfort. Generally, closer spacing, more comfort.

Finally, it is efficiency. The closer the spacing the lower the average water temperature.

But all these factors are just a few of many. The most important factor in any hydronic radiant floor design is average water temperature.

We are heating warehouses with tubing placed in the first 4ft of the perimeter. Comfort is not the goal.

We place snow/ice melting PEX 9-12" on center depending on the particular design requirements.

In residential design, floor covering can impact tube spacing, design temperatures and controls.

Variable tube spacing can be quite effective where high loads warrant the extra cost of tube and labor. Only a proper room-by-room heat load coupled with field experience can predict when this increasingly, maybe faddish, design options will make sense. One has to consider that the greatest heat loss is at the perimeter and the warmer the panel, the greater the heat loss.

We have used 18" spacing for commercial applications and could certainly use this wide spacing on many low-load residential applications but prefer to lower the design temperature and use weather sensitive controls, increasing comfort and efficiency at once.  

Radiant Floor Heating

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Morgan M. Audetat


Radiant panel heating design, including floors and ceilings, European wall panels and snow melting for residential and light commercial buildings..............................................................-Master of Steam and Hot Water,City of Minneapolis....................................................--- Master Plumber, State of Minnesota...........


30 years..... Charter Board Member - Radiant Panel Association, former manufacturer of condensing boilers, former national distributor of radiant floor products, son and apprentice of mechanical & plumbing contractor. Current mechanical/plumbing contractor specializing in hydronic based, integrated HVAC systems. Radiant floor heating/cooling. Snow Melting. Condensing Boilers. Indirect water heaters. System design, consulting and technical training world-wide. Licensed contractor, designer, installer and consultant.

B.S. University WI 1981 CONTINUING EDUCATION: Viessmann Condensing boiler and Solar water heating certification 2010, N.D.S.U. Lead Worker RRP certification 2010, Knight condensing boiler certification 2009, Wrightsoft Manual J CAD certification 2009, RPA Designer & Installer Certification 2008, Nate Hydronic/Forced Air Certification 2008, Uponor/Wirsbo advanced design school certification 2007, Buderus Wall-hung certification 2007, Power Limited License (low voltage controls) 40 CE credits 2005, Basic Hydronic Certification IPEX & Northern Alberta Institute of Technology 1998, Charter Board Member - Radiant Panel Association 1994, Residential Off-Electric to Hydronic Conversion Heating School Canadian Hydronics Council 1994, B&G Little Red School House 1993, Rood Utilities (now Auburn Technical Institute) Oil Burner School 1993 ,Tekmar Controls residential and commercial 1993, Division Manager and Advanced Hydronic Seminar Instructor for the first Exclusively Hydronic Radiant Floor Distributor in the USA 1990, Hydronic Radiant Heating Association Workshop participant with Richard c. Bourne, PE spring 1988, Master Plumbing, Hydronic, Solar Course Red Rocks Community College 1987.

Awards and Honors
2009 System Showcase Award - Radiant Panel Association

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