Exploring all the aspects of tubing depth
What difference does it make?
No, this column isn’t about the comments offered to a congressional committee by a certain presidential candidate. It’s about the effect tubing depth has on the thermal performance of a heated floor slab.
On some installations 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 and the tubing inevitably ends up at the bottom of the slab.
Does it matter?
Unlike relocating a sensor or changing some piping, there is no way to alter tubing depth once that screen slides over the concrete. The slab’s performance over decades of future service life is now fixed. The irreversibility of the situation should give heating professionals pause to consider if they are installing the tubing in the best manner possible.
If the tubing depth doesn’t 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 any to installation cost?
There are several ways 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. While higher water temperatures are often achievable, they usually make the heat source operate at lower thermal efficiency compared to what that efficiency might have been at lower water temperatures.
- The closer the tubing is to the bottom of the slab the greater the underside heat losses should be. Downward heat flow is global warming in the literal sense. Although the main deterrent to excessive downward heat flow is good underside insulation, tubing depth does, in part, determine what the rate of downward heat loss will 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 cooldown 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 the above, it seems intuitive that placing the tubing higher in the slab would improve that slab’s thermal performance. The harder questions to answer are:
1)How much is performance affected by tubing depth?; and 2) Is the change in performance worth the necessary jobsite oversight to ensure it happens?
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 simulated. The calculations FEA software can do in a couple 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 4-in. concrete slab sitting on 1-in. 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 2shows the isotherms (e.g., line of constant temperature within the slab and surrounding materials) that were 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:
- The floor surface temperature directly above the tube decreases due to the greater R-value between the tube and the surface;
- The difference between the floor surface temperature directly over the tube and that halfway between adjacent tubes decreases. This is a desirable effect because it makes variations in the floor surface temperature less noticeable; and
- The area under the surface temperature profile curve changes with tube depth. This implies 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 floor construction shown in Figure 1 for water temperatures of 100º F and 130º. In each case, heat output increases as the tubing is lowered through the upper portion of the slab and then decreases as the tubing gets deeper. This implies there is an optimal tube depth where the slab delivers maximum heat output. The simulations I ran suggest it’s about 1/4 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 the average water temperature in the circuit has to increase about 7º 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 14º 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 nonetheless undesirable) effect on boiler efficiency. 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 significant effect on efficiency.
Higher water temperatures also mean higher piping heat loss and higher underslab heat losses, all of which are undesirable.
Bare slab simulations
I also wanted to see how tubing depth affects heat output for uncovered concrete slabs. The FEA model was 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 100º are shown in Figure 4.
The results again show upward heat output decreases as the tubing is located deeper within the slab. The highest output for the simulations I ran occurs when the tube is centered about 3/4-in. below to slab surface (about 25.1 Btu/hr./ft2 at 100º water temperature). Lowering the tube so its center is 2 in. below the slab surface (e.g., tubing centered on 4-in. slab thickness) reduces output to 23.8 Btu/hr./ft2. These changes are relatively small.
However, look at what the FEA simulation predicts when the tube is located at the bottom of the slab. Here the output only is 17.8 Btu/hr./ft2. That is a 25% decrease in upward heat output compared to when the tubing is centered in the slab’s thickness. The only way to compensate for this would be to increase water temperature several degrees.
I also looked at downward heat loss as a function of tubing depth. When water temperatures are adjusted (as shown in Figure 3) to compensate for tubing placed at the bottom of the slab, and allowing the deeper tubing to produce the same upward heat output as tubing centered in the slab, downward heat loss increases by about 10%.
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 saw control joints. The depth of such saw cuts is typically 20% 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.
The penetration depth of fasteners used to secure partitions to slabs also needs to 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 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. It’s also a good idea to document these areas by taking several photos of the tubing placement along with some reference measurements before the concrete is placed.
What is the ‘take away?’
FEA analysis is not guaranteed to predict reality with 100% accuracy. There are hundreds of possible variations on factors such as soil temperature, flooring resistance, tube spacing, etc., that must be considered when making any 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 radiant slab 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% increase in downward heat loss caused by higher water temperatures in bottomed-out tubing also seems reasonable.
One also can take stock in the FEA results on a comparison basis. The relative changes that occur when only one parameter – in this case tubing depth – is changed tends to “cancel out” some of the error that may be present due to other assumptions, such as the conductivity of the concrete or the bond resistance between the finish floor material and the slab.
Keep in mind these results also are based on steady-state conditions. They don’t 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 surely will lead to wider temperature swings and compromised comfort.
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 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 the tubing being at the bottom of the slab. Engineers should specifically ask for such information rather than assume it is of no consequence.
It also is important to clearly and repeatedly communicate your requirements on tubing depth on all plans and specifications. Take the time to discuss 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’s no excuse for being unprepared.
So to answer this article’s title question: Tubing depth within a slab does make a significant difference in that slab’s thermal performance. Furthermore, that difference will be there for the life of the building.
Don’t treat it trivially.
This article was originally titled “What difference does it make” in the April 2016 p