Tubing depth matters!

Anyone who has installed hydronic floor heating has likely watched as his or her neatly placed tubing circuits get buried in concrete. Sometimes the tubing and reinforcing mesh it is attached to 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 is not even there.

DOES TUBING DEPTH MATTER?

Unlike relocating a sensor or unsweating a pipe, there is no chance of changing tubing depth once that screed slides over the concrete. The slab’s performance over decades of future service life is now fixed. The irreversibility of the situation should give us pause to consider if we are installing the tubing in the best manner possible. If the tubing depth does not have much of an effect on performance, why worry about it? However, if tubing depth does significantly affect performance why be ignorant of it? Why sacrifice performance to a detail that adds very little, if anything, to the installation cost?

There are several ways that tubing depth should affect the performance of a heated slab:

• The deeper the tubing, the greater the thermal resistance between it and the floor surface. The higher the thermal resistance in the path of heat flow, the higher the water temperature must be to achieve and 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.

• When the tubing ends up near the bottom of the slab more of the slab’s thermal mass is above the horizontal plane at which heat is being added. This increases the time required to warm the floor surface to normal operating 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.

Considering these facts, it seems intuitive that placing the tubing higher in the slab will improve its performance. The harder questions to answer are:

1. How much is performance affected by tubing depth?

2. Is the change in performance worth the necessary jobsite oversight to ensure it happens?

NUMBER CRUNCHING

The answers to these questions require credible numbers. One way to get them is through specialized software known as finite element analysis (FEA). This software allows a physical situation to be mathematically modeled and simulated. The calculations FEA software can do in a couple of seconds are far beyond what any person could attempt to solve through manual methods.

One of the FEA models I constructed is shown in Figure 1. It consists of a four-inch concrete slab sitting on one-inch thick extruded polystyrene insulation (R-5 ºF•hr•ft²/Btu), and covered by ³/₈ in. oak flooring. The latter is assumed to be perfectly bonded to the top of the slab. The tubing is assumed to be spaced 12 in. apart.

Several versions of this model were used to simulate tubing at different depths in the slab. Each time the model was run it determined the temperature at hundreds of points within a small region of the slab including points spaced
1/2 in. apart along the floor surface.

Figure 2 shows the isotherms (e.g., line of constant temperature within the slab and surrounding materials) that are generated by the FEA software.

When the FEA model was run for several tubing depths the following trends were observed 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 half way between adjacent tubes decreases. This is a desirable effect because it makes the floor surface temperature more “homogenous.”

3. The area under the surface temperature profile curve changes with tube depth. This implies that the upward heat output from the floor changes as tubing depth changes.

Using the temperature data from several simulations, I estimated the heat output from the system for water temperatures of 100F and 130F. In each case, heat output increases as the tubing is lowered through the upper portion of the slab and decreases as the tubing gets deeper. This implies that there is an optimal tube depth where the slab delivers maximum heat output. The simulations I ran suggest it is about ¼ of the slab thickness down from the slab surface. However, 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/ft². The results are shown in Figure 3.

These results imply that the average water temperature in the circuit has to increase about 7F to yield an output of 15 Btu/hr/ft² if the tubing is located at the bottom of the slab. The average water temperature in the circuit must be about 14F higher to yield an output of 30 Btu/hr/ft² with the tubing at the bottom of the slab.

Can the system’s heat source provide the higher water temperatures required by the deeper tubing? If that heat source is a conventional boiler, this change in water temperature would likely have a very small (but none-the-less undesirable) effect on boiler efficiency. However, if the heat source was a condensing boiler, solar thermal collector array or heat pump, this change in required water temperature would have a more pronounced negative effect on efficiency, as well as the heat gathering ability of the solar collectors or heat pump. Higher water temperatures in the tubing also mean reduced capacity through mixing devices, higher piping heat loss and higher underslab losses, all of which are undesirable.

BARE SLAB SIMULATIONS

I also wanted to see how tubing depth effects heat output for uncovered concrete slabs. The FEA model was easily modified to turn the ³/₈ in. oak flooring into ³/₈ in. thick concrete and the simulations were rerun. The results for upward heat output at a water temperature of 100F are shown in Figure 4.

The results again show that heat output decreases as the tubing is placed lower in the slab. The highest output for the simulations I ran occurs when the tube is centered about ¾ in. below the slab surface (about 25.1 Btu/hr/ft² at 100F water temperature). Lowering the tube so that its centre is two inches below the slab surface (e.g., tubing centred on four-inch slab thickness) reduces output to 23.8 Btu/hr/ft². These changes are relatively small. However, look at what the simulation predicts when the tube is located at the bottom of the slab. Here the output is only 17.8 Btu/hr/ft². That is a 25 per cent decrease in upward heat output compared to when the tubing is centred in the slab’s thickness. The only way to compensate for this would be to increase water temperature several degrees Fahrenheit.

I also looked at downward heat loss as a function of tubing depth. When water temperatures are adjusted (as shown in Figure 3) to allow tubing placed at the bottom of the slab to produce the same upward heat output as tubing centred in the slab, downward heat loss increases by about 10 per cent.

OTHER CONSIDERATIONS

There are factors other than thermal performance that have a bearing on tubing depth within a slab. One of them is protecting the tubing near sawn control joints. The depth of such saw cuts is typically 20 per cent 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. A typical detail is shown in Figure 5.

Another consideration is penetrations by fasteners used to secure equipment to the slab. In most cases it does not 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 several inches 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 this plan with the building owner.

WHAT DOES THIS MEAN?

Is finite element analysis guaranteed to predict reality with 100 per cent accuracy? No. There are hundreds of possible variations on factors such as soil temperature, flooring resistance, tube spacing and so on, that make it hard to draw generalized conclusions based on a few simulations.

Still, for the limited simulations I ran, the predicted upward heat outputs agreed fairly well with other sizing tools used for system design. The predicted increase in water temperature required for tubing at the bottom (rather than the centre) of the slab is both believable and significant. The 10 per cent increase in downward heat loss caused by higher water temperatures in bottomed-out tubing also seems reasonable.

Keep in mind that these results are also based on steady-state conditions. They do not predict the consequences of the longer response times associated with deeper tubing. In buildings with significant and often unpredictable internal heat gains, this longer reaction time will surely lead to wider temperature swings and compromised comfort.

Considering all these tradeoffs perhaps it is time we all find better ways of ensuring that tubing and reinforcing mesh end up near the mid-height of the slab, (except under any sawn control joints).

For products such as “knobby” foam panels, or plastic staples that clamp PEX directly to the underslab insulation, manufacturers should provide accurate thermal performance data that accounts for this tubing placement.

Be sure to make your requirements clear within plans and specifications. It is also worth having a discussion of these requirements with the “accountable” person overseeing the concrete crew. Make sure they know that tubing depth does affect system performance. Do this several days before the pour, not while the first concrete truck is backing down the driveway, so there is no excuse for being unprepared.

John Siegenthaler, P.E., is a mechanical engineering graduate of Rensselaer Polytechnic Institute and a licensed professional engineer. He has over 34 years experience in designing modern hydronic heating systems. He is also an associate professor emeritus of engineering technology at Mohawk Valley Community College in Utica, NY.