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 watering.
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 outer wall.
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 was happening.
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 summer's heat.
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).
formatting link
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 guess)
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 capacity.
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 there.
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 factor.
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:
formatting link
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):
formatting link
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.
Steve
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.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.