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Old 05-20-11, 01:19 PM   #1
Weed Dog
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Default Underfloor PEX Efficiency

Originally Posted by AC_Hacker
AC_Hacker here...

The GSHP and Hydronic floor is a match made in heaven... but the key word is "match".

With this combo, the boiler can output 140 to 160 degree water easily. To match this, floors are usually designed with loop spacings every 9 to 12 inches (usually 12 inches. Or the floor uses tubing underneath a suspended floor. This will all work out if you have a high feed-temperature system.

GSHP run well at lower temps, so some GSHP can hit 120F, but at reduced efficiency. They are much happier (and cheaper to run) at temps between 90F and 110F. So to get the heat into the room, you'll need to space your tubing closer... like maybe 6 inches. This is less than the bending radius of 1/2" PEX will allow, but if you run staggered rows, it is possible..'s a photo of what I mean...(The photo did not survive the move to this new post.) This is an underfloor setup. It should be possible to do something like this inside a slab or on the surface of the floor.

Under floor hydronic heating is not so good for GSHP...


Above you find AC Hacker's commentary. I'm reading up on GSHP, doing some preliminary measurements and calculations, and may attempt a small-scale, proof-of-concept installation and measure the performance of same. My home has finned-tube baseboard radiators, most installed on perimeter walls, a set-up that would not lend itself to connection to a GSHP, given that the water from my oil-fired boiler is heated to 160 degrees and a GSHP would heat water to about 90 to 110 degrees, optimally.

The home is a two-story, balloon-framed farmhouse, built in 1903, though I've done extensive remodeling to reduce air infiltration and between-the-studs air migration, sealed the exterior walls and foundation, and thickened the insulation. I also moved the stairs, moved the first-floor bathroom and kitchen, resized and reshaped the first floor rooms, moved, resized, added and replaced windows, finished off the second floor (it had been a huge, cathedral-ceilinged, knee-walled and uninsulated attic of sorts since 1903) and have lived in this on-going experiment for some 23 years.

The unfinished basement ceiling gives me access to the underside of the first floor, where I could install loops of PEX, as illustrated in AC Hacker's photo. While my finned-tube baseboards heat primarily by natural convection, and I suspect to a lesser extent by thermal radiation, the underfloor-PEX installation heats by conduction. I note the PEX is secured by metal (possibly aluminum) heat transfer plates which aid in transferring heat to the floor. I suspect a bit of thermal radiation may be in play here, too.

I've seen and read about underfloor PEX installations which were rendered less effective due to the casual and untutored manner of the installation. Some installers simply stapled the PEX directly to the subfloor. Only a small arc of the exterior face of the tube actually contacts the subfloor surface, reducing the efficiency of heat conduction. And, you can imagine, some sections of the PEX may not actually touch the underside of the subfloor. Heat transfer in these locations occurs primarily by convection, which will not be very efficient if the water in the PEX is only 90 to 110 degrees. The use of heat transfer plates is vital because they have a “U-shaped” cross section. The PEX is pushed into this groove, allowing much more of the exterior face of the tube to contact the plate, increasing the efficiency of heat conduction. I've also seen projects in which the space between the floor joists, after PEX installation, was left open, i.e., completely uninsulated. Insulate that space, effectively enclosing the PEX in a sealed cavity, and less of the heat is lost downward to convection and radiation.

Were I to build a home from scratch, I'd install a thin-slab radiant floor. But until that ship comes in (and I just received an iceberg advisory), I'm going to tinker with an underfloor PEX-style install, optimized to any reasonable degree possible.

AC Hacker's photo shows two PEX runs between joists. But you could squeeze in three runs between joists. Many of the commercially-available heat transfer plates are five inches wide, the space between 16” O.C. joists is 14.5”, so some trimming of plates, or a 90 degree bend on the outside edge of each outside plate, and voila, three PEX runs between joists. Or you could make your own plates...

Still to be considered are the responsiveness of such a system, (GSHP, remember) to a call for heat (I suspect that in such systems “overshooting” the called-for temperature is not common), and...?

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Old 05-22-11, 10:59 AM   #2
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Originally Posted by Weed Dog View Post
...I'm reading up on GSHP, doing some preliminary measurements and calculations, and may attempt a small-scale, proof-of-concept installation and measure the performance of same...
Weed Dog,

I think that doing a proof of concept is a great way to proceed. I have tried to do this along the way in the Homemade Heat Pump Manifesto. In fact, it has been the results of the various 'proof of concept' stages that has kept me going.

Regarding the hydronic radiators AND an under floor PEX installation, using the two together should work to your advantage. You'll know how much to your advantage, when you try it out.

But, you might enjoy this post from an EcoRenovator in Canada, wherein he used both PEX-in-slab and PEX-underfloor. His description is pretty vivid.



I'm not an HVAC technician. In fact, I'm barely even a hacker...
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