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Life is full of choices. In the contracting business, many decisions are financially based. The buyers of our products and services are many times concerned with the cost of the job. Or at least they say they are.
In truth, many shoppers appreciate better quality and efficiency, if you are skilled at explaining the reasons and benefits of the more-efficient products and installations. I know of projects where the homeowner or customer trusts the contractor to select the best option.
Time is money; I think we all agree on that. Contractor labor rates are, or should, represent the cost of providing services; that applies to all trades. If labor rates are in the $200-plus range, there are good reasons to take labor cost out to be competitive.
The cost decision also may come down to product selection; balance it with the convenience factor. Would you consider a faster or easier-to-install product, if there is a trade-out for performance?
Here is a story that illustrates convenience vs. performance. Nowadays, we have pretty solid information about radiant slab performance based on common installation methods. Radiant slabs, for example, have been around long enough to offer some documented working data.
We can measure and, in some cases, see the performance with heat-seeking cameras. And we can use finite element analysis (FEA) to predict or confirm a radiant heating system’s performance.
We all know the performance and output of a radiant slab are dependent on a number of factors. One consideration is where the tube is placed in the slab. The numbers — and common sense — indicate that the deeper tubing is placed in the slab, the lower the output. This is based on the resistance of the material over the tube; for example, we know the specifics of concrete resistance.
I recently decided to build up a couple of sample slabs to try and put some color to the question of tube placement and performance. Over the years, we have read articles showing how tube placement affects the output. Most of the PEX manufacturers analyzed and provided this information. John Siegenthaler has run some FEA models and come up with some predictions going back to 2005.
I built three different common slab samples.
Slab 1 has the tube installed over 2-inch EPS and the suspended about 2 3/8 inches below the surface; 1/2-inch PEX, 5/8-inch PD was supported on 1-inch tall chairs.
Slab 2 was a 2-inch thick pour on one of the knob-type insulation products; this method is sometimes referred to as a topping slab. It would be placed over the top of an existing structural slab, for example.
Slab 3 was a 2-inch knob board with a 4-inch pour. The decision on this type of installation is where to measure the slab thickness. I chose to pour a full 4 inches over the top of the knob, so the slab ended up 5 inches between the knobs. This puts the top of the tube at 4 3/8 inches below the surface of the slab.
We took the 4-foot by 2-foot formed slabs to the ready-mix plant and poured them in their lot with a typical six-bag, 7-slump, 3/4 aggregate mix.
I chose to use a Flir E40 thermal imaging camera on all three slabs and record the warmup every 15 minutes over 90 minutes. Water was supplied at 105 degrees, with the ambient temperature around 65 degrees.
As expected, the thicker slab, or the slab with the tube at the bottom of the 5-inch pour, lagged behind the other two. I planned on running the three slabs until the surface temperature reached a commonly accepted 82 degrees.
My recorded information matched fairly close to what Siggy’s FEA predicted. Comparing the slab temperature against the ambient, you can closely predict the output in BTU/square foot. So, the slab with the PEX tube at the bottom had about 30 percent lower output.
Sure, this could be accommodated by raising the supply temperature, and I found about a 15-degree warmer supply would be needed on the slab with tube at the bottom to equal the performance of the slabs with PEX tube closer to the top.
Deeper tubing, lower performance
So, what is the takeaway? Clearly, a designer or installer should know this information. The system may not meet the heat load on design days if the output is this much lower than the requirement. The additional supply temperature required will be reflected in the operating cost. One boiler manufacturer suggests that for every 3 degrees you lower the supply water temperature, you gain 1 percent. So the additional 15 degrees cost you 5 percent.
Bottom line: The tube deeper under the surface carries some penalties. Same issue with tube stapled to the foam insulation. In addition, the knob foam products add some additional insulation against heat transfer at all the contact points to the tube.
I have used the knob foam products and also stapled to foamboard. I understand the benefits regarding installation time, and this is where the decision between performance and convenience comes into play. The knob panels are super-easy to install PEX tubing. Often you do not even need to bend over; you just walk the tube into place.
But be sure the system can perform as you expect. If someday a heat pump is used, that low water temperature operation will make a big difference in its efficiency.
At the least, it is good to know this data when you do a design. Understand that the more contact the tube has with the concrete, and the closer to the surface it is, the quicker the ramp-up and lower SWT required.
A design or below day will be when you notice the lack of performance. The common fix is to increase supply temperature. This can lead to uncomfortable surface temperature and a slab very slow to ramp down.
Remember to take all the factors — labor, ease of installation, material costs, efficiency and performance — into the total price of your jobs. Develop the sales and communication skills required to help your customers understand enough about the mechanics to make the best decision. Until the value of your recommendations has been established, you’re stuck talking about price.