Hydronic Radiant Heating

When designing a home, it is important to consider early in the process what type of climate control system the home will be using. Mechanical systems used for heating and cooling work best when they are properly sized, thoughtfully laid out, and have adequate space for all the components.

There are many options for heating and cooling a home. Many homeowners these days are opting for hydronic radiant heat systems, and we get a lot of questions from our clients about radiant heat systems and how best to configure them.  Radiant heat is a great, energy efficient choice for homes where heating is the primary concern and cooling is secondary.

Unlike forced air heating systems, radiant heating warms the objects in a space, not just the air. Because of this, the space will feel warmer and the ambient temperature can be kept lower than it would be in a space that is heated with warm air.  Other benefits of radiant heat are the lack of moving air that can transport dust and allergens, and the ability of radiant heat to maintain an even temperature without noticeable fluctuations.

Hydronic radiant heating uses a central boiler to heat a fluid that is then circulated through piping concealed in the floor system.  The choice of floor system is a major determinant in the performance of a hydronic radiant system, but the choice of floor system should not be based only on what type of heating the house will utilize.

The best radiant heat systems use a concrete floor slab as a thermal mass (see related concrete slab vs. wood framed floors).  Heavy duty plastic tubing is embedded in a slab that is insulated both on the perimeter and on the underside. The amount of insulation depends on the local climate, the level of efficiency desired, and the budget.  The biggest advantage of this system is the substantial thermal mass of the concrete slab, which will store and radiate heat over an extended period of time.  The slab will also double as a collector and storage medium of any passive solar gain. On the flip side, the slab will take a while to heat up, so this type of system does not lend itself well to turning the heat down during periods of inactivity or absence.

Leaks and damage to tubing that is encased in a concrete slab can be costly and difficult to fix, but thankfully they almost never occur.  Problems due to tubing failure can be mitigated by making sure the tubing is thoroughly leak tested and the slab subgrade is well compacted granular material.

When a concrete slab floor is not practical, radiant heat tubing can be embedded in 1½” or more of lightweight concrete or gypcrete poured on top of a wood framed floor.  This is often done on homes that have hydronic heat on upper floors or where a crawl space and wood framed floor is necessary.  Since a 4” concrete slab is too heavy to be supported by a wood framed floor, a thinner, lighter slab is used.  It has significantly less thermal mass, but does provide some heat storage capacity and also helps dampen floor vibrations common with wood framed floors.  Floor framing has to be more substantial for this type of application than it would be for a floor that doesn’t have to support as much weight.

For radiant heat applications where a wood framed floor is preferred or required and gypcrete overlayment is not used there are a few different options.  Warmboard manufactures a plywood subfloor sheathing that has integral channels milled into it that allow radiant heat tubes to sit below the top of the subfloor.  The channels are clad with sheet aluminum that radiates heat upward into the living space and makes for rapid warming of the floor above.  In this sense, it outperforms the concrete embedment systems, but it lacks the thermal mass and ability to moderate temperature fluctuations.  Warmboard is relatively expensive, but by most accounts it functions well and is a viable alternative when concrete or gypcrete is impractical.  Misplaced nails or dropped tools can easily damage the tubing, so pressure testing is required before covering and after flooring has been installed.

The staple up radiant tubing application is the least expensive and easiest system to repair or retrofit.  As a trade off,  it is also the least efficient and easiest to damage.  Staple up systems involve installing the tubes on the underside of the subfloor between floor joists.  The tubes are held in place by staples and sometimes backed with foil faced rigid insulation or installed with integral metal heat transfer plates.  In order to be reasonably efficient, a staple up system needs to have more than code required insulation in the joist bays, and shouldn’t be used where floors are cantilevered out beyond heated space below because of the potential for condensation.

For more on floor system options to use with hydronic radiant heating, see our article on concrete slab vs. wood framed floors.

Tom Russell, Project Architect, LEED AP

Hendricks Architecture specializes in custom mountain style homes.  Our homes have been featured in Timber Home Living, Mountain Living, Green Building and Design, Cowboys & Indians, Cabin Life and other publications.  We’re located in Sandpoint, Idaho.  Subscribe to Hendricks Architecture’s Blog

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