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Suspended Tube Radiant Heating System

The ability of Pex-AL-Pex to handle relatively high water temperatures makes it possible to install a suspended tube system. The tubing is placed within the air cavity between the floor joists. The tubing gives off direct radiant energy to the surfaces within the joist cavity. The outside of the tubing also gives off heat to the surrounding air, establishing a gentle convective circulation within the joist cavities. The warm air flows across the underside of the subfloor transferring more heat to it.

Suspended tube systems have some unique benefits. They don’t require heat dispersion plates and thus reduce installation cost. They operate at high water temperatures under design load conditions and thus can often be piped directly to a boiler without needing a mixing valve. When the tubing is suspended below the subfloor, it is not subject to puncture from the nail points associated with installation of hardwood flooring.

Pex-AL-Pex piping is ideal for suspended tube systems. Its aluminum core provides the structure that prevents the tubing from sagging between supports when operated with high water temperatures.

As with all floor heating systems, it’s imperative to install underside insulation.

This must be a reflective insulation system meaning that there is a shining reflective metal surface facing the pipe. There has to be an air gap between the pipe and the reflective layer minimum 2” or more.

Foil faced batting insulation or the aforementioned “bubble” insulation can be used. The “bubble” insulation is different from the one used with concrete. The aluminum layer is exposed on one side minimum and is always facing the piping. The insulating layer can be one or two layers of plastic “bubble” depending on the amount of insulation required.

When the space below the heated floor is also heated, use a minimum of R-11 underside insulation. If the space below the floor is partially heated, install a minimum of R-19 insulation. If the space below the heated floor is an unheated crawl space, install a minimum of R-30 underside insulation. Although these suggested underside R-values are conservative, the installer should verify they meet or exceed local energy code requirements.

Threading the pipe into the joist space is identical to the method explained under the section discussing joist space heating with heat transfer plates.

The fastening of the pipe is different in this case. There are three main ways to secure the pipe; stapling to the underside of the subfloor; using a pipe hanger to suspend the pipe in the joist cavity; or use a nail clip to nail the pipe directly to the side of the joist.

Stapling to the floor is very simple, however the pipe is close to the surface and can be punctured easily from above. The other two overcome this problem, but an extra item pipe hanger or nail clip is used. In high heat load installations, the direct stapling to the underside can result in high and low temperature “lines” on the floor.

The fastest and easiest to install is the nail clip method. They all have their advantages and disadvantages.

It’s possible to staple Pex-AL-Pex directly against the underside of the subflooring without using heat dispersion plates. As discussed above this approach is only suggested for low heating load situations such as rooms that have minimal if any exterior exposure. Without either a slab or aluminum heat dispersion plates, the floor’s ability to spread the heat laterally away form the tubing is more limited. Still, when the design heat load of the space doesn’t exceed 15 Btu/hr/sqft, this installation method can deliver adequate heat output at reasonable water temperatures.

As a summary, it is probably fair to say that piping can be fitted into any floor surface and there are numerous variations to fit the project circumstances. It should also be clear that there are important differences between these methods and some are better suited than the other for effective heat transfer.

The following image illustrates the heat transfer process during joist space installation using heat transfer plates or direct staple up. The image speaks for itself and gives very good reasons to consider using the heat transfer plates wherever it is possible.

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