Re: [GBlist] Solar Hot Water Heating - Slab on Grade

[Speireag Alden <Joshua.M.Alden.91@alum.dartmouth.org>]

Sgrìobh Troy Glasner:

>I have two thermodynamic panels at 4' x 8' and wondering if there is 
>any use on attaching to these to my slab to help heat or pre-heat 
>the hot water for my radiant floor  4" slab on grade.  Some people 
>feel that there is no appreciable energy savings and that it is a 
>waste of time and money.  My heater is an propane fueled on demand 
>hot water system that heats both potable and slab water.  Two loops.
>
>My location is Ottawa, ON Canada - similar latitude as that of 
>Minneapolis MN - and same weather pattern as Burlington VT - at 
>least it seems that way anyway.
>
>We are thinking of doing the solar hot water heater as a drain back system.

     I don't know how much good it will do you in the cold season, 
when you'll be wanting heat in the floor.  I have a radiant slab 
which I run through my demand heater, and it works fine.  (I also 
have a solar pre-heater, but I disconnect it for half the year 
because it's just pressurized black pipe in the sun and would freeze 
in the cold.)  Water goes into the floor at about 49°C and comes back 
out between 32°C and 41°C, depending on how hot the floor has gotten. 
The trouble is that you only want to run the water which comes back 
out of the floor through the solar panels if it's cooler than those 
panels.  Sooner or later, the panel temperature will drop, the floor 
temperature will rise, and then you're losing heat to the outside. 
So, I suggest a bypass for the solar panels, activated when they 
become radiators instead of collectors.

     If this water ever comes out of a tap anywhere, then you'll need 
to set things up so that all piping is flushed regularly, so that 
nasty bacteria like Legionella can't grow in warm piping.  If it's a 
closed system, then this isn't an issue.

-Speireag.

-- 
Understanding a person does not mean condoning; it only means that 
one does not accuse him as if one were God or a judge placed above 
him. -Erich Fromm, psychoanalyst and author (1900-1980)

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