Messages and Online Forum Q&A

Re: Underslab Radiant Hot Water Heating

From: info@builderswebsource.com
Category: HVAC
Remote Name: 207.82.209.107
Date: 26 Jan 2001
Time: 03:35 PM

Comments

Design of radiant heating systems requires proper up front engineering. Many factors influence the tube spacing, such as the thermal mass of the slab, the ambient temperature rise, the water temperature, the length and diameter of tubing, as well as the recirculating velocity.

Since heat rises, placing the tubing near the bottom of the slab is a good idea, as this will heat the slab above, taking full advantage of the concrete's thermal mass. It's also important to install a vapor barrier over the ground. Some manufacturers offer barriers with heat reflective coatings. Also, keeping the tubing low in the slab will help to protect it from nails or other penetrations in the slab.

However, if you are using rebar, you may risk crushing the tubing under the weight of the rebar as the concrete is poured in place, impacting water flow. For this reason, it may be better to place the tubing on top of the rebar or wire mesh. Of course, if the wire mesh or rebar is too low in the concrete (such as right at the bottom), it won't provide much tensile strength...so, it's a balancing act to keep the rebar at the right depth while providing enough protective cover and thermal mass on top of the tubing. Insulating the slab perimeter is also important to help retain heat.

In the case of a garage, you might be able to space the tubing further apart, such as on 12-24" centers, but inside, you'll generally want to keep things on 6"-12"  centers or so. Again, this should be designed with proper attention to the engineering calculations. Fine Homebuilding magazine outlines the following rules of thumb for spacing and length of tubing in an excellent article: Hydronic Radiant-Floor Heating by John Siegenthaler:

Tubing Length

"As a rule of thumb, circuits using 1/2-in. ID tubing should not be longer than 300 ft. Circuits using 5/8-in. or 3/8-in. ID tubing shouldn't be longer than 450 ft. These values help to limit the pressure drop the system's circulator must operate against as well as the temperature drop that occurs over the course of a circuit."

Spacing

"Tube spacing depends on several factors, including the rate that heat must be released from the floor, average water temperature of the heating system and the type of floor covering installed over the slab. Typical spacings for slab- on- grade jobs are 6 in., 12 in. and 18 in. o. c., which correspond nicely with the 6-in. grid pattern of welded-wire reinforcing mesh. Closer 6-in. spacing is common near the edges of the slab where heat losses are greater, or in areas with higher- thermal- resistance floor coverings, such as carpeting. Twelve-in. spacing is common for many residential applications in buildings with average heating loads and average areas of glass. However, 12-in. spacing should be limited to rooms where the upward-heat-flow rate does not need to exceed 20 Btu per sq. ft. per hr. Interior rooms with relatively low heating loads can sometimes get by with 18-in. spacing.

Other spacings, though possible, can complicate fastening the tubing to standard reinforcing wire. Combinations of spacing can sometimes be used within the same room. For example, the beginning of the circuit may use 6 in. o. c. spacing near an exterior wall with a large area of windows, and then make a transition to 12 in. o. c. spacing after it has progressed 3 ft. to 4 ft. in from the wall."

The full text of this article is available at:

http://www.taunton.com/

Another handy source of information, particularly as it relates to slab radiant heating, can be found at the Radiant Design Institute's website:

http://www.radiantdesigninstitute.com/

Since a radiant floor is something you only want to do once, take the time to study best practices and pay particular attention to heat loads and protecting the system against kinking and damage during installation.

Good luck!

Builders Websource

Article Tools

Builders Websource® is a corporate member of ASTM International—a non-profit engineering body dedicated in part to the standardization of building materials specifications, products, systems, and test methods for improved health, safety, and reliability of residential and commercial structures.

Copyright © 1999 - 2018 Builders Websource®, a BUILDERA™ company. All rights reserved.