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re: pex radiant heat tubing in collector
25 mar 2003
robert scott  wrote:
>i am looking around for ways to reduce the cost of a fluid-based solar
>collector.  specifically, i am wondering about using pex (cross-linked
>plyethelene) radiant heat tubing.  i know that it was not designed for
>direct exposure to sunlight or temperatures above 200f, but i think
>ways can be found to avoid those problems.  what if i took sheet
>aluminum and formed a half-round indentation...

how would you do that?

...into which the 1/2" pex could be pressed on the side away from the sun?

warmboard (, 877-338-5493) for radiant floor heating
is similar, with 1/2" pex tubing on 12" centers pressed into u-shaped
troughs in 0.025" aluminum glued to 1" plywood with similar u-shaped
troughs. they say put a bead of caulk in the u before pressing the pex
tubing in, and claim a square foot of warmboard under r0.5 flooring will
transfer 20 btu/h of heat from 90 f water (average of supply and return
temps) to a 70 f room.

>that would shield it from the sun and still allow thermal contact with
>the collecting surface.  i know the efficiency will not be that great
>if the tubes are spaced 6" apart, but i am willing to live with that
>if the price is low enough.

what's the thermal conductance of pex tubing walls? can you estimate
the cost and solar collection efficiency vs aluminum thickness and
tube spacing?

>overall temperature can be limited with a fail-safe vent system (in case
>of a power failure).

how would you do that?

>i am thinking single-glazed, which further limits stagnation temperature.

some temperature-sensitive louvers might open to let air flow underneath.

>has anyone tried to make radiant heat tubing work in this way?

>-robert scott
>ypsilanti, michigan

i started making some homemade "big fins" attached to copper pipes
like this, viewed in a fixed font:
                                                  .  .
                                               .  .  .  .
                   aluminum flashing   ssbss . .  1/2"  . .   
                .........................o... .  copper  . .  
                .........................l... .   pipe   . . 
                                       sstss . .        . . 
    sssss might be a 1/2" x 1/16"              .  .  .  .
    thick aluminum strip or angle --- .           .  .
    or drywall l-bead                 .  
                                   |  . <--foil-faced foamboard
                                      .    on the back side?
...then open it up like this:         .  
                                      .           .  .
                                      .        .  .  .  .
     home depot sells dark-           .ssbss . .        . .   
     brown-painted aluminum       ~5" ...o... .  copper  . .  
     roof flashing for $46.50         ...l... .   pipe   . . 
     for a 50'x2'x0.027" roll.        .sstss . .        . . 
                                      .        .  .  .  .
                                      .           .  .
         [use courier font.]          .  
                                   |  .    this solar collector might be
                                      .    60% efficient heating 120 f
                                  --- .    water inside an 80 f sunspace.

the aluminum didn't want to cling tightly to the copper pipe by itself...


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