Now that we have the height settled, can anyone tell me how much John is going to save in heating cost by raising the pex 1 1/8"??

Somewhere between not much (4%?) and nothing, depending on the heat source and various other conditions. Negative savings if you are trying to use a slab for off peak energy storage.

I knew there was something about jonr too. A smart guy indeed.

The actual amount of additional energy or fuel required to provide the same upward heat gain with the tube at the bottom of the slab as compared to having the tube at middle of slab is dependent on the heat source (e.g., heat pump, solar collector, condensing boiler, etc) used to heat the hydronic fluid and the magnitude of upward heat gain required (which depends on the building heat loss). To gain some insight into how significant or not tube placement may be, we can quantify how much higher the average circuit temp would have to be to to provide the same upward heat gain with the tube at the bottom of the slab as compared to having the tube at middle of slab.

It turns out that Sieg did exactly this and published the results. Sieg's floor assembly was 4 inch thick slab-on-grade with 0.5 inch PEX tube spaced 12 inches with 2 inches of insulation below the slab. To provide an upward heat gain of 15 BTU/Hour/Square Foot, the required average circuit temp with the tube in the middle of the slab was measured to be 95 deg F. To provide this same upward heat gain, the required average circuit temp with the tube in the bottom of the slab was measured to be 102 deg F. So for this case the heat source would have to heat the hydronic fluid 7 deg higher with the tube placed in the bottom of the slab.

To provide an upward heat gain of 30 BTU/Hour/Square Foot, the required average circuit temp with the tube in the middle of the slab was measured to be 120 deg F. To provide this same upward heat gain, the required average circuit temp with the tube in the bottom of the slab was measured to be 134 deg F. So for this case the heat source would have to heat the hydronic fluid 14 deg higher with the tube placed in the bottom of the slab.

Sieg didn't publish data for 7.5 BTU/Hour/Square Foot which might be a more typical upward heat gain requirement for the energy efficient buildings that we prefer on this forum. However, I believe we can successfully extrapolate the above data and conclude that for this case that the heat source would only have to heat the hydronic fluid 3.5 deg higher with the tube placed in the bottom of the slab.

So the efficiency you give up by placing the tube in the bottom of the slab tends becomes increasingly less significant as your upward heat gain requirement decreases. So Jonr's one sentence reply pretty well captures the essence of this significance for well insulated, energy efficient buildings.

the significance of this also decreases with reductions in resistance above the pipe (siggys test had a wood floor). Also reduced with tighter tubing spacing.

Posted By sailawayrb on 28 Apr 2013 10:01 PM
The actual amount of additional energy or fuel required to provide the same upward heat gain with the tube at the bottom of the slab as compared to having the tube at middle of slab is dependent on the heat source (e.g., heat pump, solar collector, condensing boiler, etc) used to heat the hydronic fluid and the magnitude of upward heat gain required (which depends on the building heat loss). To gain some insight into how significant or not tube placement may be, we can quantify how much higher the average circuit temp would have to be to to provide the same upward heat gain with the tube at the bottom of the slab as compared to having the tube at middle of slab.

It turns out that Sieg did exactly this and published the results. Sieg's floor assembly was 4 inch thick slab-on-grade with 0.5 inch PEX tube spaced 12 inches with 2 inches of insulation below the slab. To provide an upward heat gain of 15 BTU/Hour/Square Foot, the required average circuit temp with the tube in the middle of the slab was measured to be 95 deg F. To provide this same upward heat gain, the required average circuit temp with the tube in the bottom of the slab was measured to be 102 deg F. So for this case the heat source would have to heat the hydronic fluid 7 deg higher with the tube placed in the bottom of the slab.

To provide an upward heat gain of 30 BTU/Hour/Square Foot, the required average circuit temp with the tube in the middle of the slab was measured to be 120 deg F. To provide this same upward heat gain, the required average circuit temp with the tube in the bottom of the slab was measured to be 134 deg F. So for this case the heat source would have to heat the hydronic fluid 14 deg higher with the tube placed in the bottom of the slab.

Sieg didn't publish data for 7.5 BTU/Hour/Square Foot which might be a more typical upward heat gain requirement for the energy efficient buildings that we prefer on this forum. However, I believe we can successfully extrapolate the above data and conclude that for this case that the heat source would only have to heat the hydronic fluid 3.5 deg higher with the tube placed in the bottom of the slab.

So the efficiency you give up by placing the tube in the bottom of the slab tends becomes increasingly less significant as your upward heat gain requirement decreases. So Jonr's one sentence reply pretty well captures the essence of this significance for well insulated, energy efficient buildings.

I could be wrong, but I thought Sieg modelled the stack up. I was not aware he actually measured it. And with his own model platform that would have had his own bias built into it. Furthermore, it did not appear that he calculated the increased thermal conductivity of the rebar laid between the pex and the surface. I'm sure that the increased conductivity of the bar would sieg - nifiantly increase the efficiency of the pipe on bottom model to where it out performs the pipe in middle model by at least .00123%. I take the last sentence to mean you really agree with Badger's position of value and waste, despite spending more time/dollars defending the middle of slab position then OP can save in one to ahh ten years??

Sieg both constructed/measured and did finite element modeling of several floor assemblies. However, it is not entirely clear to me whether there was or was not mesh/rebar in the measured slab-on-grade floor assembly Sieg data that I provided. Several years ago we built some slabs to validate our finite element thermal mass models. We intentionally did not include mesh/rebar in our slabs to reduce the effort required to decommission and remove these experimental test slabs. We have not observed any significant difference between our design models and actual building slabs that contain mesh/rebar. In any event, I don’t see any way that placing the tube in the bottom of the slab would ever equal the heat transfer performance of placing it in the middle of the slab, much less ever outperform it. I do wish that there was some magic that would make this so as then optimal heat transfer efficiency and installation efficiency would go hand-in-hand and everyone would agree on how best to place tube.

I don’t believe Sieg has any bias here. Sieg just did a scientific investigation to better understand the heat transfer characteristics and to properly validate his math modeling. What would be Sieg’s motivation for recommending tube placement in the middle of the slab beyond from this simply being better from a heat transfer perspective? How does Sieg profit over others by making this recommendation? I guess I am not in know regarding the conspiracy theory here.

Yes, as I originally indicated, I would agree that there is certainly a tradeoff between chasing this improved heat transfer performance versus the additional time/hassle factor/cost when making this tube placement design decision. Which path one should take can really only be determined after considering both the increased average circuit temp needed to achieve the required upward heat gain and the actual heat source that will be used. So what might be rightly considered an insignificant efficiency loss for one design could also be rightly considered to be a very significant efficiency loss for another design that would best be avoided even given the increased time/hassle factor/cost.

Posted By sailawayrb on 29 Apr 2013 06:13 PM

I don’t believe Sieg has any bias here. Sieg just did a scientific investigation to better understand the heat transfer characteristics and to properly validate his math modeling. What would be Sieg’s motivation for recommending tube placement in the middle of the slab beyond from this simply being better from a heat transfer perspective? How does Sieg profit over others by making this recommendation? I guess I am not in know regarding the conspiracy theory here.

Sailaway - I was not in any way trying to say that Sieg was intentionally misleading. I have no reason at all to suspect that. When you build you own models, you naturally carry your own thoughts into it. Like if you believe an ICF wall only has R20 than the outcome of a model that you build that shows the cost of heating ICF houses at ten different locations will reflect that bias.