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What You Need to Know about In-Slab Radiant Heating Systems

Thursday, February 23rd, 2012

Radiant Heating

In-Slab Systems

Radiant Heating has come to be known as perhaps the most comfortable available today (next to laying in the sun at least).  In-slab heating is a common option to consider for basements, slab floors, and garages.  It is also commonly found in commercial buildings and other non-residential structures. Keeping in mind that once the slab is poured, any changes to the system are very difficult and very expensive to accomplish, so careful planning and knowing what is required for both a successful installation as well as a comfortable and efficient heating system is a must.  Here is what you and your customer need to know about in-slab radiant heating systems.

What you need to know about in-slab heating

When heating a slab floor, the goal is to efficiently heat the slab and direct as much of that heat as possible into the living/working space above it – practically speaking the 6-8 foot air space above the slab floor.

Slab System in Finished Basement

Slab System in Finished Basement

Items that need to be taken into consideration to maximize the performance of an in-slab radiant heating system:

Insulation:  Insulating beneath the slab is strongly recommended.  Ideally, the use of an insulation tarp that provides both a vapor barrier and a reflective layer over the insulation will ensure that the majority of heat suppied to the slab will make its way to the intended area, i.e, the 6-8 ft area above the slab.

See our separate white paper Radiant Heating:  In-slab Systems – Insulating Under the Slab for a more detailed discussion on this aspect of in-slab radiant heating.

Desired heated space vs. slab size: Does the slab extend beyond the area where heat is required?

A slab represents a thermal mass.  It will absorb heat as well as transmit it.  If only a portion of a slab is heated, it can be expected that some of the supplied heat will be lost to areas of the slab that are not being heated and therefore will not reach it’s intended area, i.e., the 6-8 ft area above the slab.  Sometimes this may be acceptable, but it is best to make sure that expectations of the system performance are clear.  If this is not acceptable, then the placement of thermal breaks into the slab floor should be considered by the appropriate party (homeowner, builder, designer, etc.).

For this reason, it is usually recommended that each segment of slab be treated as a single zone.  If there are thermal breaks in the slab, then multiple heating zones can be considered accordingly.

Thickness of the slab: In an in-slab radiant heating application, you are effectively transferring heat from the hydronic system into the thermal mass.  The greater the amount of thermal mass, the longer it will take to transfer enough heat to allow the mass to pass heat to the desired area (the 6-8 ft area above the floor).  We say longer because it is important to know that concrete has a certain rate of heat absorption.  If there is more of it to heat up, it is going to take a longer time to accomplish this.  The good news is that once the slab is heated, a thicker slab should give off this heat into our desired area for a longer period of time before requiring another injection of heat from our hydronic radiant system.

Size of the tubing:  From time to time, we hear about people wanting to use tubing with a larger diameter than ½” PEX in their in-slab heating application.  This is not recommended.  As mentioned above, concrete has a specific absorption rate.  We design our systems to optimize the performance of the overall system taking into account all the equipment involved with the creation and transfer of the heat.  Changing the size of the tubing will have an effect of the overall system efficiency (not necessarily a positive one) and would need to be accounted for in the overall system design. In addition, larger diameter radiant tubing may likely require a more powerful circulator in order to achieve the turbulent flow required for efficient heat transfer.   Bigger is NOT better when specifying tubing for a radiant heating system.  A larger circulator will increase the size and cost of your radiant system, and may not produce quantifiable benefits.

Manifold placement: In general, manifolds are located in or on a wall in a spot that is (a) hidden or otherwise out of the way yet (b) still accessible for occasional maintenance and (c) where it can be easily reached by the rough-in Pex-Al-Pex tubing from the mechanical room/area.   Care should be taken during the construction phase to make sure that any electrical wires are located safely away from the same area as the manifolds.

Spacing and Position of the tubing within the slab:  Spacing 12” on center using ½” Pex tubing.  Depth of tubing should be in the bottom third of the slab with the top of the tubing at least 2” to 2-1/2” below the top surface of the slab (deep enough to be below any fasteners that might be driven into the slab during or anytime after construction is completed).  It is important that the tubing not be at the very bottom of the slab where it might not be fully encased by the concrete when it is poured.  This would greatly reduce the efficiency of the heat transfer into the slab.

Heat SourceHow much heat and expected performance

Geo vs boiler system:  While boilers are capable of generating much hotter water than geo heat pumps, both types of heat sources should work very well in a radiant slab application.  If all aspects of the slab construction and radiant system installation guidelines are followed, there will be an appropriate amount of heat provided to allow the slab heating to work effectively and efficiently.


An in-slab radiant heating system can be both efficient and comfortable.  Investment in the proper components and attention to detail on all aspects of the design and installation should result in a radiant heating system that will perform optimally on a daily basis.

Condensing Boilers: Efficient Boilers for Radiant Heating

Saturday, September 11th, 2010

Condensing boilers are the most efficient boilers available, and require special attention during installation.

The vast majority of radiant heating systems use a condensing boiler as the main heatsource.  A condensing boiler is any boiler that is over 86% efficient. The truth is that most condensing boilers are over 90% efficient.

Savio Condensing Boiler

Savio Condensing Boiler

In order to understand the condensing boiler, we first have to understand the non-condensing boiler, which is less than 86% efficient. In a non-condensing boiler, the exhaust gasses leave the boiler at about 180 degrees. These exhaust gasses are immediately expelled outside through the use of vent pipe or chimney. The reason that these types of boilers are less than 86% efficient is that there is significant heat loss through exhaust.

Non-condensing boilers are less expensive because they are less efficient.

A condensing boiler, on the other hand, incorporates a secondary heat exchanger to remove excess heat from the exhaust gasses. This improves the efficiency of the boiler due to the fact that the heat lost from the exhaust gasses is then “recycled” by the boiler — helping to raise the water temperature. This process actually reduces the temperature of the exhaust significantly. This significant reduction in temperature, from about 180 degrees down to about 55 degrees, condenses the gasses, most of which is water vapor, forming droplets that collect and need to be drained.

This is where the condensing part of a condensing boiler comes in.