Tubing depth DOES affect the performance of heated slabs.

Anyone who has installed hydronic floor heating has surely had their neatly placed tubing circuits get buried in concrete. Sometimes the tubing and reinforcing mesh gets lifted into the thickness of the slab as the concrete is placed. Other times, the masons trample over the tubing and mesh as if it’s not even there. Any instructions given them by the radiant installer seemingly vanish when the concrete begins flowing down the chute.

Figure 1

Deep Questions

Unlike relocating a sensor or unthreading a pipe fitting, there’s no way to change tubing depth once the concrete is placed. The thermal performance of the slab with its embedded tubing is now fixed for the life of the building. The irreversibility of the situation should make us question if we’re installing the tubing in the best manner possible.

If the depth of the tubing doesn’t have much of an effect on performance, why worry about it? On the other hand, if depth does have a substantial effect on performance, why be ignorant of it? Why sacrifice performance to a detail that adds very little, if any, to the cost of the system?

There are several ways tubing depth would theoretically affect the performance of a heated slab.

  • The deeper the tubing, the greater the thermal resistance between it and the slab surface. The higher the thermal resistance, the higher the water temperature required to maintain a given rate of heat transfer.

  • The closer the tubing is to the bottom of the slab, the greater the underside heat losses should be. This is true with or without underslab insulation. Thermal losses are obviously greater in the latter case.

  • When the tubing ends up near the bottom of the slab, more of the slab’s thermal mass is above the level where heat is being added. This lengthens the time it takes to warm the floor surface to normal temperatures following a call for heat. Its also lengthens the cool-down time after heat input is interrupted by system controls. A fully charged slab can hold several hours worth of heat that will continue to flow into the space as long as the air temperature and/or interior surface temperatures are cooler than the floor surface. This can be a real problem in buildings with significant internal heat gains from sunlight or other sources, especially when those gains follow a cold night in which significant heat was added to the slab.

    In light of these issues, it seems obvious that placing tubing higher in the slab will improve its performance. The harder questions are: 1. How much is performance affected by tubing depth? 2. Is the change in performance worth a few well-chosen words with a disinterested mason?

  • Figure 2

    Slab Simulations

    Like most engineers, my comfort zone with a design issue is usually bordered by numbers. To get a handle on the tube-depth issue, I turned to a specialized technique called finite element analysis (or FEA for short). FEA software allows one to build a model of the situation you want to test, run it and then see what the temperature would be at any point you’re interested in. The calculations this software does in a couple of seconds would take weeks to complete by hand.

    One of the models I constructed is shown inFigure 1. It consists of a 4" concrete slab sitting on 1-inch (R-5) polystyrene insulation and covered by 3/8" oak flooring. Several versions of the model were used to simulate tubing at different depths in the slab. Each time the model is run, it determines the temperature at hundreds of points within a small region of the slab, including points spaced 1/2-inch apart along the floor surface.

    The curves inFigure 2show the predicted surface temperature profiles for the model ofFigure 1. They indicate the following things happening as the tubing is placed deeper in the slab:

    1. The floor surface temperature directly above the tube decreases due to the greater R-value between the tube and the surface.

    2. The difference between the floor surface temperature directly over the tube and that halfway between adjacent tubes decreases. This is actually a desirable effect that makes it harder to feel where the tubing is located under the floor.

    3. The area under each surface temperature profile is different. This area is proportional to upward heat output.

    Using the temperature data from several runs of the FEA software, I estimated the heat output from the system for water temperatures of 100°F and 130°F. For both water temperatures, heat output increases as the tubing is lowered through the upper portion of the slab, then drops off as the tubing gets deeper. This means there’s an optimal tube depth where the slab delivers maximum heat output. The simulations I ran suggest it’s about one-fourth of the slab thickness down from the slab surface. This depth could vary depending on flooring resistance and other factors.

    Figure 3

    I also used the FEA results to determine the average water temperatures required to deliver heat outputs of 15 and 30 Btu/hr/ft2. The results are shown inFigure 3.

    Notice that the average circuit water temperature required to deliver 30 Btu/hr/ft2 is 14°F higher when the tubing is at the bottom of the slab vs. centered in the slab thickness.

    Can the system’s boiler provide the higher water temperatures required by the deeper tubing? Sure it can. If it happens to be a conventional boiler, the effect on efficiency for operating the circuit 14°F higher is probably quite small.

    But what if the heat source is a condensing boiler or geothermal heat pump? The increased water temperature required to deliver the same rate of heat output lowers the condensing potential of the first, and deceases the COP (efficiency) of the latter. Higher water temperatures also mean reduced capacity through mixing devices, higher heat loss from distribution piping and higher downward heat loss from the slab. These are all undesirable.

    So much for a typical residential floor slab scenario. Next month, we’ll continue the discussion with a look at bare slab performance as a function of tubing depth. We’ll also look at downward heat loss and discuss locations where tubing should not be lifted in the slab. In the meantime, be sure you’re specifying that tubing should be lifted to mid-slab thickness. It DOES make a difference.