better than I can explain
Originally Posted by Rambo2000
.3 Disabling Attic Exhaust Fan
The third retrofit involved turning off the attic exhaust fan. Average cooling energy consumption (based on an eight-month cooling season) decreased by an additional 36%. The 14.3 kWh/day saved breaks down to 11.2 kWh/day from the exhaust fan motor and 3.1 kWh/day from a reduction in air conditioning energy. Savings over an eight month cooling season are projected to be 3208 kWh or $289. Assuming a service call cost of $50 for turning off the attic fan, the simple payback would be after two months. Peak electric demand (including attic fan power of 0.47 kW) was reduced by 21% from 3.4 kW to 2.7 kW.
Impacts of Disabling Attic Fan on Ventilation, Humidity, and Building Pressure
Turning off the attic fan produced dramatic impacts upon the office. The large driving force (pressure differential) causing air conditioned air to be sucked out of the occupied space and hot humid air to be drawn into the occupied space is gone when the fan is off. Table 3 is located in the Conclusions section of this paper and summarizes measured ventilation rates, relative humidity, and building pressures during initial, duct repair, and attic fan off monitoring periods. The natural air change rate (all mechanical equipment off) is not shown in Table 3, but was measured on a couple testing days and averaged 0.17 ach.
Building pressure decreased from -0.064 in.WC (-15.9 Pa) to -0.0024 in.WC (-0.6 Pa). This resulted in a large drop in the ventilation rate and the indoor relative humidity. The 24 hour average relative humidity levels plummeted from 77% to 61%. Relative humidity levels are shown in Figure 4 during a composite 24 hour day consisting of one business week of data. Each hour represents the preceding hour of gathered data. For example, hour 2 represents data gathered beginning at hour 1 and ending at hour 2. Non-business hours from 12 AM to 7 AM show the humidity drop from an average of 83% when the fan is on, to an average of 65% when the fan was off. During business hours from 11 AM to 5 PM, the relative humidity drops from an average of 68% when the fan is on to an average of 55% when the fan is off. The building ventilation rate decreased from 0.79 air changes per hour (ach) to 0.33 ach. Due to diminished ventilation, the peak carbon dioxide concentration increased from an average 614 ppm to 1054 ppm during weekday hours of 3 to 5 p.m. Figure 5 shows carbon dioxide concentrations during a composite 24 hour day consisting of one business week of data.
Figure 4 Daily composite of indoor relative humidity before and after attic fan was turned off.
Figure 5 Daily composite of carbon dioxide levels before and after attic fan was turned off.
Measured ventilation rates and carbon dioxide concentrations indicate that this office needs additional ventilation after duct repair and with the attic fan off to be in accordance with ASHRAE 62-1989. Typical occupancy during normal business hours is eight adults. According to ASHRAE 62-1989, the desired amount of ventilation air would be 20 cfm (9.4 l/s) per person which totals 160 cfm (76 l/s) for eight people. Infiltration testing by means of tracer gas decay indicated a total of only about 58 cfm (27 l/s) or just over 7 cfm (3.3 l/s) per person. A suggestion was made to the building manager to contact a qualified air conditioning and ventilation contractor to increase the ventilation rate, but no changes were desired to be made by the business during the time that monitoring was conducted. It was not desirable to turn on the attic fan because it increased the relative humidity significantly. A discussion of how the ventilation could have been provided follows.
text taken from above link
The House Is A System
•Research at the Advanced Energy Corporation showed that 40%of the air exhausted by a PAV is conditioned inside air!
The House Is A System
•Convection loops move only 20% of the heat from the roof deck to the attic floor!
•Radiant heat transfer is responsible for 80%+ of the heat transfer.
•So, you could create Hurricane Katrina in your attic and only affect 20% of the heat transfer.
The House Is A System The House Is A System
3.“Improve”attic ventilation and run the risk of making the house more humid, more likely to grow mold, cause the electric bills to go up and possibly cause a fire or CO poisoning!
Thanks for the research! I did turn of the PAV. If I'm reading everything correctly, the Resnet conference seems to say turn off the PAV.
The building science article seems to indicate that venting an attic is a bad idea unless everything is done perfectly (I don't pretend to assume that my builder did everything perfectly), but I can't "unvent" it now and I'm also stuck with my air handler in the attic (also a bad idea). I don't see where this article said anything about the PAV?
The Florida Solar article seemed to indicate some reduction in heating by the PAV, but at a higher energy cost.
Of course, at this point in time, I'd take anything to make my conditioned space more comfortable. What is the typical cost for a radiant barrier system, and would it make sense in my case?
Thanks again for all of the insights!
Yes. The builder installed the PAV
Price discussion is against the rules here, but I can say it is reasonable. You can get an estimate from a contractor easily enough, it will be a function of the square footage and complexity of your attic. It will probably be a special type of paint and will stop about 75% of the radiant energy. You could look into getting a proper passive attic ventilation at the same time, probably a ridge vent. In Texas there is a company named Efficient Attic Systems that advertises heavily and seems to be pretty good, one can hope they serve Atlanta too.
I put up foil type RB in my own complex attic and boy was it labor intensive! I like the results though they are hard to quantify. FSEC claims energy savings in the range of 8-12% and I believe them. With weather variations a savings of 10% is hard to measure (or even prove it exists). Most people are very pleased the attic is immediately more comfortable, too.
Hope this helps -- Pstu
Yeah, I should've known better than to ask about pricing on a public board (wasn't thinking).
I actually do have a passive system (soffet vents with a lot of daylight coming into the attic space, and a line of about (6 - 8) 12" square vents lining the roof line across the top. Not as good as a ridge vent, but something.
Out of curiosity, did you happen to measure your attic temps before and after the radiant barrier was installed? I'm curious as to what type of temp drop could be expected?
Thanks for your reply.
Adrian F....one of your links strongly suggested that in addition to disabling PAV's that a homeowner should close foundation vents which it said causes mold and mildew when open by bringing in hot, humid air ....is that also your recommendation? All the houses in my (Richmond, VA) neighborhood open foundation vents in the spring, summer and fall....sounds like we may not be doing the right thing ....?????
Originally Posted by adrianf
do a search at the FSEC on radient barrier and read tell you're drunk with knowledge.
Oh yeah if your psuedonym was Rocky2000 we would have alot of jokes at our disposal.
in most all cases the PAV does way more harm than good on multiple fronts.
Measuring attic temps is not as straightforward as I thought. For years I had a temperature probe stuck up through the attic floor, about 12 inches above the insulation and I thought I was "measuring the attic temperature". Well that point is very different from other places to measure. In addition to that I have a remote temperature sensor hanging in the air just below the radiant barrier, and another placed above the RB.
As I write this at 10am in July, the measurement is 88F below the RB and 93F above it. The point above the attic floor is 84F. I did a study on typical temperatures in Sep-Oct 2007, when the reported peak was still low-mid 90's. Generally saw 125F commonly above the RB in late afternoon, with about 110F below the RB and 95F at the attic floor. On such day (10/4/2007 I think) the official high was reported at 92F but my back porch saw only 86F. All that looks pretty nice plotted on a graph but it's hard to figure out what it all means. Whenever someone say "attic with RB should be within 10 degrees of outside temperature" I listen to them but don't know whether I need to do something else to live up to that claim. IMO there is no one number which clearly describes attic temperature.
This is as good a time as any to say I REALLY admire the toughness of those Texas AC guys who have to go up into these attics on summer days!
Hope this helps -- Pstu
I don't have any experience with crawl spaces but what the building sceince folks are saying is you can't reduce the humidity using hot humid air to ventilate.
Originally Posted by VA GENT
Well, more fun. The contractor now believes there is probably a problem with the coil. They measured different temps from the top to the bottom of the coil, so they moved the TXV valve to see if there would be any effect, and they are going to have the local Trane rep come out and check out the system. They think there is an issue with the coil but they indicated that the Trane rep will probably want them to replace the TXV first.
They also aren't 100% sure that my old furnace is moving enough air across the coils. However I told then that since they rebuilt the ducts and plenum, I have better air flow than I've ever had (of course, the old system never worked right either). But, first things first. They want to get the coil operating properly.
Thanks again to all for the informaton and insights. When I'm done with the process I'll know more about residential HVAC than I ever imagined I would!
I actually measured different temps (by 15 degrees) between my East and West facing actic walls this morning. So, I can believe that it's not a straight forward process.
I'm just another novice, but somehow your problem does not sound like the system to me. You are in Hotlanta area (same here), and in the evening have:
(input air temp) 76 --> Furnace --> 61 (supply vent temp)
I seem to remember that 15-20 degree drop was all that you should expect, which by itself should cool the house down. Anyway, my experience in this area has been to reduce the load on the house and duct work, by any means possible. I replaced turbines with continous ridge vents, sealed the ducts in the attic, AND installed 2 Power Vents. The power vents are THE main reason I can keep my systems in the 75-76 range. Recently one died, and the upstairs went to 78, you can believe I replaced that motor quickly!
Two conclusions that I've come to:
1) Attic temps bake the ducts unmercifully, even if they are insulated.
2) South / West facing brick walls store massive amounts of heat from the sun, and keep pumping it into the house till late in the night.
I'll be anxious to see what you come up with, since I can only dream of waking up to 72.