The roof is pyramid shaped (about 30 x 30 ft, tan-colored shingles) with
soffits on all sides and several passive vents mounted near the peak, as
well as a powered fan with a 15" diam. opening. A few weeks ago I took
the large steel dome-shaped top off this fan unit and added more screen
and raised this dome by about 10 inches. This allowed the fan to move
the air out of the attic more freely without developing any
back-pressure. The existing height allowed for about 3 inches worth of
clearance for the air to be expelled. This modification alone was
enough to drop the attic air temp by about 5 to 10 degrees.
A few days ago (wed or thurs) I set up one of my sprinklers on the roof
and watched the attic temperature drop after a few minutes of watering.
I collected the water in a couple of 55 gallon plastic barrels and let
the water cool over two days to water various plants. Naturally, the
water was quite warm as it came out of the downspout and into the
barrels. I have a temperature sensor mounted about 1" under the plywood
roof deck in the attic and it went from about 120F to about 100F during
The temperature at this point may be misleading (most affected by the
sun, least affected by venting). Repeat the experiment with the sensor
on or near the ceiling insulation, half way between the peak and an
I don't agree.
Air currents in a soffit-ventilated attic will flow primarily from the
intake point (the soffits) up along the underside of the decking to the
peak where the exhaust fan is located.
By placing the sensor mid-way up this path, on the underside of the deck
(but not touching it) I'll get a good sense as to the heat load that the
shingles are transfering to the attic space - in terms of the
ventilation system's ability to remove this heat.
Because the water was able to carry away the heat from the shingles, the
ventilation system was able to bring the attic temperature very close to
ambient during the time that the water was being applied, and even for a
short time after the water had been turned off and evaporative cooling
If I had the time, and a backhoe, I'd create a water storage reservior
in the ground near my house, probably lined with concrete, maybe
insulated, and circulate water from that reservior to my roof and back
again, storing heat energy to heat my home in the winter. I'd put pipes
in my driveway and melt the winter snow with this heated water, and by
next spring the water would be cold and ready to absorb the next
Anyone know how much water would be necessary to store enough heat to
heat a typical house in the winter? Say, in Chicago, Detroit, Buffalo
or Toronto? Say if you started with a water temperature of maybe 175
degrees and by the end of the winter it was down to 70 degrees? (no
heat pump involved that is).
What you're talking about is a TES (thermal energy storage) via a TER
((thermal energy reservoir).
Collecting heat (or cold) and using it hours later or perhaps a over a
couple of day is very doable
but "moving" energy from one season to another would require very
large volumes of water.
I'd be surprised if it was economically feasible. :(
(I haven't done the calcs so my comments are based on an educated
This sort of thing was done YEARS ago before the invention of
refrigeration cycles...... the "ice house" concept.
Ice was collected and stored in an insulated structure for use later
in the year.
Water is a wonderful substance, useful for all sorts of processes.
For thermal energy "storage"..... raising a pound of water 1 def F
takes one BTU.
Turning water into ice is even better..... taking a pound of water to
(or from) ice takes about 142 btu.
That's why ice works so well to cool things. :)
So if you need a couple 100,000 btu's, you'll need a couple 10,000
pounds of water
(if your operational temp change is 10 degrees).
Of course changing the water temp more means you need less water
but there are limits to the operational temperature change and there
are also desired operating temps.
The University of California at Irvine had an air conditioning problem
back in the 1990's they needed to solve.
The summer afternoon peak cooling load was higher than the capacity
of the steam plant's chiller.
Rather than add more capacity to the plant, the campus chose to build
a TES/TER to address the problem.
A water tank water (~5 million gallons) was built to hold water that
could be chilled during off peak hours.
The cold water was used to supplement the campus chilling capacity
during peak afternoon demand.
About 53,000 ton-hours of cooling was shifted to "off peak" and the
campus was able to take advantage of "night time" electrical rates.
The system cost about $6,000,000 and reduces the yearly electric bill
by about $500,000.
An additional benefit was the avoided the cost to upgrade the chiller
I'm not sure it would be practical for a single family home but its
not totally far fetched.
You better learn about intake. You're pulling air in from your passive
vents near the ridge as well. Air is supposed to flow as a sheet, not some
hacked up system you believe you "designed" by throwing a power vent up
You're probably better off not running the power vent, pulling air _only_
from the soffit. This is of course based on if you have at least the
minimum intake & exhaust venting needed for the square feet involved.
Did you ever calculate how much venting is actually needed?
I've looked at that, and although you'd think so, it's not really a
With the fan running and no cover to impede air flow, I placed strips of
newspaper over the passive vent grill surfaces. I have 3 of these vents
on opposing sides of my almost pyramidal roof (the roof is not quite
square, so I have a 4-foot long peak instead of a point-peak).
The passive vents are pretty much exactly like these:
About 12" on a side.
Anyways, with the fan running, the paper strips would barely stay put
against the passive vents, but yes they were held there by a negative
pressure. This negative pressure was very weak - the strips wouldn't
stay put for any longer than about a minute before a weak gust of wind
would blow in from the soffit area and overcome the negative pressure
and blow the strips from the vents.
The powered vent looks like this (with the cover on):
When I turn on the fan in the mid-morning, when the sun has already
elevated the attic temperature into the 90's, I watch my temperature
readout slowly start to decline by about .2 degrees every 10 to 15
seconds. So don't tell me the fan isin't doing anything - I can see
how it's reducing the temperature. And when I'm on the roof with the
exhaust blowing right into my face, it's just as hot as the exhaust vent
on my gas clothes dryer.
And yes, air is probably flowing like a sheet along the underside of the
roof decking because of where the soffit and vents/fan is located.
There simply isin't another path for the air to take inside the attic.
The roof will easily get 20 degrees hotter without the fan.
I've proved to my satisfaction that this effect of the fan pulling air
into the attic from the passive vents is not happening to any measurable
extent. I would guess that the warm air inside the attic really does
want to rise, and there is more than enough soffit venting to allow
suitable outside airflow to enter the attic as the hot air is evacuated
by the fan. I probably would get a different result if I tried this
newspaper trick in the middle of the night, when the temperature inside
the attic is equal to the outside ambient air temperature.
I have re-worked the eve and soffit area along one side of this roof a
few years ago, cutting out a hole in the existing plywood soffit between
every rafter and then replaced the existing aluminum soffit with one
that is fully vented along it's entire length. The other 3 sides of the
roof have a soffit vent maybe every 4'th or 5'th rafter.
I posted over at rec.metalworking, and got this fantastic response. The
last pdf is incredible in its results, so I believe I have a very
inexpensive simple way to cool this structure.
Posted by Steve Walker in rec.metalworking ...............
No, but I've wanted to.
See below. The last link to a PDF is pretty informative. Watch the wrap.
email@example.com (remove brain when replying)
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