The depth of hydronic tubing can affect the performance of a heated concrete slab.
Anyone who's installed hydronic floor heating has surely had opportunity to watch the neatly placed tubing circuits get buried in concrete. Sometimes the tubing and reinforcing mesh gets lifted into the slab as the concrete is placed. Other times the masons trample over the tubing as if it's not even there. Any instructions given them by the radiant installer become less imperative as each additional yard of concrete flows down the chute.
More than once I've found myself outnumbered on the issue of lifting the tubing during the pour. Each new crew of masons sets the stage for another "discussion" over this issue. Come to think of it, I'm probably lucky not to have the imprint of a concrete rake across my forehead.
Unlike relocating a sensor or unsweating a pipe, there's no chance of changing tubing depth once that screed slides over the concrete. The slab's performance over its long service life is now fixed. The irreversibility of the situation should give us pause to consider 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, there is no need to worry about it. On the other hand, if depth does have a substantial effect on performance, it would be foolish to ignore 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 should theoretically affect the performance of a heated slab:
- The deeper the tubing, the greater the R-value between it and the floor surface. The higher the R-value, the higher the water temperature has to be 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. Obviously the losses are 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. It 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.
In light of the above it seems obvious that placing the tubing higher in the slab will improve its performance. There are harder questions to answer: 1) How much performance is affected by tubing depth, and 2) Is the change in performance worth a few well-chosen words with a disinclined mason?
Like most engineers, my comfort zone with a design issue is often bordered by numbers. To get a handle on the issue of tube depth, I turned to a specialized method of predicting how heat flows through an assembly of different materials. It's called finite element analysis (FEA). It's computer software that gives you the ability 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 built is shown in Figure 1. It consists of a 4-inch concrete slab sitting on 1-inch (R-5) polystyrene insulation, and covered by 3/8-inch 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 in Figure 2 show the predicted surface temperature profiles for the model of Figure 1. They indicate the following things happen 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 for less noticeable variations in floor temperature.
3. The graphical area underneath the surface temperature curves varies with tube depth. This area is proportional to upward heat output from the floor. The greater the graphical area under a given curve, the greater the heat output.
Using the temperature data from several runs of the program, I estimated the heat output from the system for water temperatures of 100 degrees F and 130 degrees F. For both water temperatures, heat output increases as the tubing is lowered through the upper portion of the slab, and 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 would be about one-fourth of the slab thickness down from the slab surface. This depth could vary depending on flooring resistance and other factors.
I also used the FEA results to determine the average water temperatures required to deliver heat outputs of 15 and 30 Btu/hr./sq. ft. The results are shown in the table above.
Can the system's boiler provide the higher water temperatures required by the deeper tubing? Probably - if it's a conventional cast-iron or steel boiler. But what if a condensing boiler or hydronic heat pump were used as the heat source? The increased water temperature required to deliver the same rate of heat output lowers the condensing potential of the first, and decreases the COP (efficiency) of the latter. Higher water temperatures also mean reduced capacity through mixing devices, higher piping heat loss and higher underslab losses.
I also wanted to see how tubing depth affects heat output for slabs without any finish flooring. A few mouse clicks changed what had been a 3/8-inch oak floor covering into 3/8-inch concrete. The results for a water temperature of 100 degrees F. are shown in Figure 3.
The results again show that heat output drops off as tubing depth increases. The highest output for the cases I ran occurs when the tube is centered about 3/4 of an inch below the slab surface (about 25 Btu/hr./sq. ft at 100 degrees F water temperature). Lowering the tube another inch into the slab reduces output to 24 Btu/hr./sq. ft. Taking the tubing down yet another inch lowers output to 22.3 Btu/hr./sq. ft.
These changes are relatively small. However, look what the model predicts when the tube is located at the bottom of the slab. Here the output is only 16.6 Btu/hr./sq. ft., about 31 percent lower than when the tube is centered 1.7 inches below the surface. The slab with the bottomed-out tubing needs 115 degrees F water to yield an output of 25 Btu/hr./sq. ft, compared to only 101 degrees F if the tubing were centered 1.7 inches below the surface.
The graph in Figure 4 shows what the simulations predict for downward heat loss to soil at a constant temperature of 65 degrees F.
Notice that (for a given water temperature) downward losses are higher when the tube is centered in the slab. You might think this means leaving the tubing at the bottom of the slab reduces downward heat loss. But the problem with this reasoning is that it doesn't consider upward heat output. When water temperatures are adjusted (as shown in Table 1) to allow tubing at the bottom of the slab to produce the same upward heat output as tubing centered in the slab, downward heat loss increases about 10 percent.
Of course there are factors other than thermal performance that have a bearing on tubing depth. One of them is protecting the tubing near sawn control joints. The depth of such saw cuts is typically 20 percent of the slab thickness. I prefer to keep the tubing near the bottom of the slab at such locations to give the blade a wide berth as it passes over.
Any penetrations by fasteners used to secure equipment to the slab must also be considered. In most cases it doesn't make sense to leave all the tubing at the bottom of the slab just to accommodate what might be a future bench or lift post. Find out where such equipment will be placed and keep the tubing a couple of feet away from where the fasteners are likely to go.
Block out and note these areas on your tubing layout drawing. Be sure to leave a copy of the plan with the owner since nobody will remember where the tubing is a few years after it's installed.
What's It All Mean?
Is finite element analysis guaranteed to predict reality with 100 percent accuracy? No. There are so many possible variations regarding factors like soil temperature, flooring resistance, tube spacing, etc., that it's hard to draw generalized conclusions based on a few simulations.
Still, for the limited scenarios I ran, the results agree fairly well with many performance models used for system design. The predicted increase in water temperature required for tubing at the bottom (rather than the center) of the slab is both believable and significant. The 10 percent increase in downward heat loss caused by higher water temperatures in bottomed-out tubing also seems reasonable. Remember, too, that these results are based on steady-state conditions. They don't predict the consequences of the longer response times associated with deeper tubing. Mix strong internal heat gains into such a situation and you've got a tiger by the tail. This is a situation even sophisticated controls have yet to tame.
Considering all these tradeoffs, perhaps it's time we all find better ways of ensuring that tubing and reinforcing mesh end up near the mid-height of the slab - except under sawn control joints.
Be sure to make your tube depth requirements clear to the "accountable" person overseeing the concrete crew. In case you're not sure, he's the one who pays them, and thus holds considerably more sway over their actions than you do. Be professional and persuasive. Tell him - and any of the crew who'll listen - that tubing depth does make a difference. Do this several days before the pour, not while the first concrete truck is backing down the driveway. This lets the crew come prepared with lifting hooks rather than excuses.