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Dumping heat or saving heat

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Posted by northstar on March 3, 2006, 1:17 pm
I just installed a solar heating system.  Hurray!

Now I need to figure what to do with the heat during the summer.

Any ideas on seasonal heating strategies?

Also,  if I were to buy a hot tub, what brand/model is the easiest for
introduces a heating loop.

Posted by SJC on March 3, 2006, 2:25 pm

  I have no recommendations for hot tubs, but one thought I had was
geothermal storage of heat for the fall months. We might need a =
of your system (flat plate, number of collectors, etc) But it seems =
possible to
take the heat from the summer and store it in the ground for later use.
  By using a fluid source heat pump you can recover the heat and turn it =
into high
grade heat for hot water, radiant floor hydronic and air hydronic. I am =
no geologist,
but i would think that a mass as cheap as dirt might be a good solution =
as a large
thermal store. I read that you can recover 80% of the heat you put into =
one of these
stores, but it did not state over what length of time, nor the =

Posted by Ecnerwal on March 3, 2006, 2:38 pm

It could be a good idea, but it depends a great deal on whether you own
a backhoe, or are paying someone else to dig up the yard and bury a few
thousand feet of pipe. I own the backhoe and have still not opted to do
this...the idea is fine before money gets involved, but the economics of
doing it are often not so good...and you might do as well or spending
the same money burying a large insulated water tank for direct storage
of high-quality heat - however, in general the economics of long-term
storage do not stack up well .vs. the economics of enough collection to
only need a few days of storage in the cold season.

As for other things to do with excess heat, run that hot tub, put a
sauna in too, and then build a thermal chimney (think a cupola full of
radiators or fin-tube) on the top of the house so that your excess heat
can help ventilate your attic and/or house (depending on climate).

Depending on climate, one of several thermally driven air conditioning
schemes might also be worth investigating, if you are up to that level
of chemical tinkering (not a job for the casual DIY).

Cats, coffee, chocolate...vices to live by

Posted by SJC on March 3, 2006, 4:15 pm

  You may not need a backhoe. Some of the folks that live in cold =
have basements. You could use part of the basement for the thermal =
Whether you use water or something else is a matter of density, cost and
preference I would imagine. Just a thought for consideration. I like the =
of storing a few hundred therms during the summer months to replay in
October and November.

Posted by Ecnerwal on March 3, 2006, 5:39 pm

That's a lovely idea. Now do the math. Problem 1 - your basement is
almost undoubtedly NOT built to handle the weight of a substantial water
store. Assume you get past that. Let's give you a pretty substantial
store - 7 feet high, 10 feet square - 700 cubic feet, roughly 44,000 lbs.
Surface area is 100+100+70+70+70+70H0 square feet. Let's say you crank
that sucker up to 200F, and your basement is sitting at 65F.

Posit 6 inches of foam, about R24-30 depending on the foam. Say R30. U
value is 0.03334. Conductance (area * U-value) is 16 BTU/Hr-F. 2160
BTU/hr is leaked into your basement at 200F (135F differential), heating
your house up slightly in the hot season. 59 therms are stored, assuming
that you can use it all the way down to 65F. Perhaps a bit more if you
use a heat pump on it (but that will cost you buying and running the
heat pump), or a lot less (25-30 therms) if you need it to be 120-130F
for typical direct use in heating and domestic hotwater.

Building this rather substantial water storge facility in your basement
will be rather costly.

Next comes one tedious calculation (iterating the heat loss as the
temperature differential drops over time, as the temperature of the heat
store drops over time), and another rather less tedious one (what is the
heat loss rate of your house, if you are actually using the heat stored
to heat things. For completeness, you should also model the winter
output of your solar panels as an input to the heat store, perhaps using
the NREL 30 year solar radiation data.

Looking at constraints, rather than bothering with the full treatment,
if the store lost heat at the same rate as when at 200F (a case
considerably worse than reality, as heat loss will slow down as delta T
drops) this particular store would take about 4 months to cool from
200F-65F, so in reality it would take longer - with no use of the stored
heat - so it will carry heat from summer to winter. The case when you
actually start using the heat is somewhat different.

Cats, coffee, chocolate...vices to live by

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