Posted by iain-3 on March 9, 2005, 8:08 pm
It sounds like you agree with Nick, that copper pipe is a waste of
money for the pool HX because without pumped (presumably turbulent)
flow on both sides, at least one water film is going to dominate the
series resistance. I certainly agree that once you go with plastic
tubing, it's worthwhile to err on the side of too much pipe. It would
be very nice if the plastic tube HX got a U anywhere near 125
BUT/hr-ft^2-F, it'll warm the hot tub up in a hurry!
I don't have any sense of how big a deal this oxygen diffusion is.
Supposedly the PEX-Al-PEX tubing is better than just PEX specifically
because the Al acts as an oxygen barrier. I guess the dissolved oxygen
is supposed to eat your metal bits over time. If I use PE tubing for
the pool HX, I assume I'll be pumping oxygen into the cistern water.
The oxygen issue seems slightly different that the open vs closed
system issue. Supposedly open systems that expose the water to air
require more maintenance. My assumption is that open systems
accumulate both oxygen but also organics and bacteria and whatnot,
which then all has to be killed and strained from the system to prevent
accumulation in the pipes and so forth.
For the DHW HX, it seems to me that the dominating issue is safety, and
copper tubing seems safer than plastic tubing: Less likely to leak or
crack, less likely to blow out (stronger), less likely to support some
nasty thing growing. Probably more likely to leach poisonous heavy
metals into the stagnating hot water, but that's why we don't make
coffee from the hot water tap.
Posted by nicksanspam on March 10, 2005, 12:22 pm
Radiant floor people only worry about cast iron pumps.
...which loses 12% of its pressure rating for each 10 F rise above 73 F.
ENDOT has a lifetime guarantee that includes labor for the first 25 years.
Posted by Ecnerwal on March 1, 2005, 2:59 pm
You can pay more for "washed" rock. It's still not pristine. Rather than
dumping a truck into your tank, I'd suggest setting up your own wash
system, and tediously transferring truly clean rock to the hole, by
hand. This should address your concern about the liner falling in, as
well. If that's too much of a pain, reconsider "engineered structure".
Costs money, saves time, saves several of the other complications of
Never is a very long time. Money-sucking engineered structure tends
towards making a system with longer expected life, less chance for
leaks, and more leak repairability. Shooting for "never", I'd stick the
tank in the house basement, or build a new "basement" under the patio
just for a water tank, and skip the rocks (which will get you down to
~17,000 gallons for the same thermal capacity). Traditional approaches
for liner systems are to use protective barriers on both sides, and
multiple (typically 3) independent liners, along with very great care in
placing the liner and anything on either side of it to prevent
installation tears. That way several failures are needed before the
whole system is compromised. A supplementary clay liner (bentonite) is
also a good idea. This all costs money (or labor, and if you are not the
labor, it costs money), and that money could be spent on a more solid
tank, instead. Don't let any trees grow within 50 feet. Make sure there
are no burrowing animals around, too.
You can solve a whole bunch of your potential problems by going to a
heat exchanger system. No longer is dirt in the rocks a problem, and the
system can be pressurized without the thermal store being pressurized.
Does not make the thermal store "never leak", though.
Cats, Coffee, Chocolate...vices to live by
Posted by nicksanspam on March 1, 2005, 4:22 pm
That's close to a $00 24' diameter x 4' deep swimming pool or 2
12'x42'x4' deep tanks made with 2 20'x50' pieces of EPDM rubber.
Posted by iain-3 on March 1, 2005, 8:27 pm
Thanks for all the replies on the tank.
I agree that putting the tank in a basement would be wonderful because
vertical heat leaks would just heat the house. Unfortuneately, there
is no basement for seismic reasons.
If someone could give me a supplier for $.30/ft^2 45 mil EPDM, I'd
Capacity: I think the house will lose about 600,000-700,000 BTUs/day
during the winter. I'm hoping to swing the water in the tank between
90F (minimum I can see being usable by the hydronic system) and 140F
(above which the Heliodyne panels seem to have significant losses).
So a day of storage takes 12,000 pounds of water, or 1500 gallons.
When I was looking at $+/gallon tanks, I was thinking about having
just a couple of days of storage, which put a $000 line item in the
budget, and I wasn't happy about the amount of storage. When I
figured out a cheaper way to build the tank, I just scaled up. When I
think about the amount of money I'm spending on the panels, spending
on a big tank seems like a good idea.
BTW, I'm the Californian that Nick was talking to two years ago about
solar heating a house with the windows open. I've come to realize
that the windows will remain closed in December, January, and
February, during which time the outdoor pool will not be heated
Ppressurized heat exchanger in the tank, between the tank water and
the panel loop: I figure the HX will lose at least 5 degrees, because
I lose stratification, and the HX isn't perfect. 5 degrees is like
losing 12% of the tank capacity and maybe one or two of the panels, so
that's a very nasty hit. The HX can also fail.
Drainback: A solenoid-actuated vent at the top of the panels might
possibly avoid some of the problems of drainback, or at least reduce
the amount of restriction necessary to prevent burbling noises.
Trees: There are valley oaks within 15 feet of the tank.
Washing rock before installation: Ugh. This sounds like a prison
labor joke. But I see that you are serious. The alternative seems to
be a hope that the fines would settle to the bottom, and that the flow
in the tank would be low enough not to disturb them. The more easily
disturbed stuff could be flushed out immediately after dumping the
rock in the hole. Maybe I could run the water through my swimming
pool equipment for a few months to clean it up. This prospect is not
filling me with enthusiasm.
Access: When I was thinking I would build a real tank, I assumed
access ports through the patio. These would be under a bench.
Engineered concrete tank: I estimate 40 cubic yards of concrete for
the structure, which is a lot of money, especially if most of that is
shotcrete for the walls. I talked to a shotcrete contractor who
suggested that it would end up more than $/gallon, so go back to the
buried fiberglass tank idea. The tank lid is not the expensive part,
BTW. It's the tank walls that carry the lid load down to the
underlying rock, since I'm not willing to trust the rock around the
tank to carry the load without slumping.
There is actually another problem that I thought of: If even a very
small amount of the rock dissolves in the hot water, then comes out of
solution when the water temperature drops in the radiant pipes, I can
imagine limestone deposition on the inside of my underfloor PEX
tubing, or maybe even in the solar panels. Maybe some gnarly chemical
could etch this out occasionally. Maybe without eating my expensive
copper bits. Yuk.