View Full Version : Attic fan control question
11-07-2006, 10:16 AM
I was lucky last year and didn't have any ice dams given the winter we had in Boston. But my attic stays too warm. I've had a more free air flow ridge vent installed and when the HVAC system was modified in the attic I had them use R-8 insulation on the ducts (which carry forced air heat in winter). But the attic stays too warm still - those ducts are 45-50 degrees on the surface so that just warms the whole place up. I have plenty of insulation so an attic fan is my next step.
I'd like to have a smart control set up so it doesn't operate unless it has too. For instance, if attic temp over 120 degrees which is easy to do, but more importantly for ice dams; I want an indoor and outdoor temp sensor that monitors the difference. If indoor attic >32 and outdoor <32 then I want the fan to kick on so it reduces risk of ice dams when snow is on the roof. I know you could manually turn in on or off but that doesn't work if you are on vacation or you forget and you constantly are monitoring temperature, etc. I was thinking X-10 might be the way to go but no outdoor sensors are available. Any ideas how I can make this work guys?
11-07-2006, 11:37 AM
I think that would drive your heating costs up. You might be further ahead to install the heaters on the lower shingles.
Ridge vents are ok, but unless you have vents in the sofit's or eves they dont do much.
11-07-2006, 08:40 PM
add more insulation
better would be to have the ducts in the conditioned space! = within a "drop ceiling" in the central hallway.
I would bet that your problem is with leaks in & around the ductwork --
apply mastic to all seams & joints.
caulk around all elec boxes & HVAC boots in all walls & ceilings
caulk around all pipe & wire holes into the attic.
be sure any "can" lighting fixtures have gaskets -- most can NOT be covered with insulation -- that requires a big can in the attic & special design.
with next roofing install have mastic applied at bottom edges -- even better if put on whole roof
11-07-2006, 08:50 PM
actually the attic has foam insulation on floor - almost complete air barrier...all ductwork sealed as good as possible...won't roof heaters cost more and won't help remove heat load in summer when it hits 130? I do have soffit vents too.
11-07-2006, 11:38 PM
get a couple of thermostat/humidistat fans - set the humidistat lower to get the fans running when a large temperature difference exists between outside temp and attic temp (condensation & high humidity in attic). Around $100 ea at the homestores.
11-08-2006, 03:43 AM
1st differential is the wrong word. Differential would mean dT = outside - attic or dT = attic - outside.
Dewpoint is really what you need to monitor. You would need outdoor RH, outdoor temp and surface temp and you would know what temperature water vapor in the air would condense on a surface, but trying to answer your question:
You could do it the industial way with RTD's or thermocouples and possibly even thermisters, but that way is way to expensive.
An industrial way is using two to three of these which use RTD's. RTD's not included, but you can get the RTD's at http://www.omega.com. Thermister or thermocouple based systems might be available too. Type K is a widely used thermocouple.
#1 would be set to close when outside < 32
#2 would be set to close when attic > 32
#1 and #2 contacts would be in series with a power supply and contactor which would be wired across the "normal" line operated T-stat.
For giggles, someone could design something using these parts from analog devices. http://www.analog.com/UploadedFiles/Data_Sheets/AD22105.pdf
They are IC's with built in temperature sensors. Note
the operating range is -40 to 150 CENTEGRADE not deg F.
Even Thermo-O-disc's from http://www.tod.com/ might work, but good luck getting them. They are used inside of microwaves to protect against the magnetron from overheating.
It's always difficult measuring outdoor air temperature. A really cool place to use is the hooded space under the NID where your telephone cable enters your house. It's protected from the sun and thermisters can be mounted in free air.
There are lots of issues and these are a few technologies which can be combined or even used independently. A regulated power supply, if needed, that operates in an attic reliably is sometimes no easy feat. I have a few antennae amps in the attic that have gone south.
Then there is the distributed approach. Put some or electronics in the basement with some or all the power supplies, but you need a disconnect at the fan.
So I'm thinking...2 solid state sensors in the attic with some glue logic that operates an SSR (Solid state relay) which drives a contacter with a 120 VAC coil. Surge supression and a small amount of regulated power.
Unregulated or regulated power in the basement and an RTD temp sensor to outside wired to the alarm module. A minimum of a isolated contact and unregulated power to attic.
LED's to monitor switchpoints in the attic and possibly the ability for force the fan on from the basement and LED's to know when the fan is running.
These are ideas, nothing more. What I call "Thinking out loud."
11-08-2006, 01:05 PM
From what you have said, I think you are overly concerned about a problem that won't happen. Ice dams are caused by heat transfer from the rooms below to the attic roof. This normally happens along the eve where the rooms are close to the roof sheathing. Air sealing, proper R value insulation, and normal ventilation should keep ice damming from happening. Adding a power vent will only pull more hot air from rooms below to cause problems, not solve them. Keep a close eye on eve area. If a problem developes, check insulation and more importantly check for air leaks!
11-08-2006, 09:00 PM
I actually found a controller some guy has made - fancontroller.com - seems like it would do the trick. And if I have a well sealed attic and hatch it shouldn't suck air from the house, just pull it faster from gables/soffits. Let me know what you think if you check out the controller. Thanks.
11-09-2006, 06:37 PM
newhomeboston---wrong! a fan will place a negative pressure in the attic. It will pull different amounts of air through any hole depending on the size of the hole. One cannot make the attic floor system air tight--it will pull air from inside increaseing utility bills.
11-09-2006, 08:14 PM
Attic fans are a disaster down here.
11-10-2006, 12:13 PM
Attic fan on one gable, opening by design of fan smaller tha opening on other gable. Air will come from the path of least resistance which is the other gable vent and/or ridge vent and not through all of the little cracks that the builder forgot to seal unless you have one mighty powerfull fan.
11-11-2006, 08:59 PM
Keepitsimplestupid--Wrong! Need to go back to science class. If pressure is greater on one side of an opening air will go from high pressure to low no matter what size the hole is. Test after test on homes with attic fans show increased infiltration rates, and higher utility bills.
11-13-2006, 10:12 PM
what if the fan is just up there and turned on when ice dam conditions are present, what is the harm in using it then?
11-14-2006, 12:56 AM
I agree with your statement 100% and as I said it depends on the size of the fan and the size of the openings. A fan sitting in the middle of a large room with essentially infinite openings on input and output will not change the pressure in the room. It will just move the air around a little. Neither would a window fan with all the windows open in the house. Let's put a model airplane propeller for a fan in the attic. Your telling me that it would draw the air from the rest of the house. Doubt it.
11-14-2006, 01:46 AM
look at the amount of soffit venting you have - if you have a bunch - get a bigger fan, if you have just a little get a small fan. jeez - this is not rocket science. almost any small fan just running longer and therefore minimizing air leaking from the conditioned space will equalize the temperature enough to prevent the ice-dam issue. like said before attic fan at homestore $100 or less. depending on size.
11-14-2006, 02:06 AM
Comments on the fan controller. Some just don't look professional especially the electrical box with the plugs at all angles, but he may be onto something. Here is a dewpoint table:
so you can get an idea when to ventilate. Thinking out loud again:
There is creteria A:
Bringing in outside air with T(o) with RH(o) into an attic with Temperature T(a) and you don't want it to condense inside.
There is Creiteria B:
Temperature of the house T(h) (on the side of the attic floor) and the RH(a) and T(a) such that no condensation occurs at the ceiling.
Then there is creteria C:
When the outdoor temperature temperature is < than attic temperature then ventilate unless A or B is true.
When the temperature at the eve is below freezing and the temperature at the middle of the roof isn't then ventillate unless A or B is true.
11-14-2006, 07:23 AM
You should just run your fan for one season turn it on as you see fit and then gat back to us with your update.
You have two issues cooling season and heating season.
11-14-2006, 01:58 PM
Keepitsimplestupid--Your analogy of a fan in the middle of the room does not pertain to a fan on the roof decking plane or a gable vent fan. Your analogy of a window frame fan not pulling air from the other open windows inside the house is dead wrong. We are suppose to give advice that is based on science, not B. S. Many tests and much information shows that pv's should not be placed in an attic. In winter ice dams are caused by lack of insulation or air leaks near the eve of the home. Lets cure the problem by putting adequate insulation and/or seal the air leaks, not only to save energy, increase comfort, but to solve the problem. Pv's create more problems than they solve.
11-14-2006, 02:19 PM
Efficiency loss can cost a lot with today’s prices.
11-14-2006, 02:45 PM
Agreed, the insulation/infiltration issue would fix the ice dams, the ventillation issue MIGHT save on enery bills to run the ventillator in the summer. Good post.
11-14-2006, 07:25 PM
Not trying to be a "harper" but pv's have never shown to save energy and usually increase energy use. Pv's are holes in the roof that use electricity, and cause problems. Its amazing how this myth is so hard to eliminate.
[Edited by uktra on 11-14-2006 at 07:28 PM]
11-14-2006, 07:43 PM
the argument of energy savings vs energy consumption of attic fans is not the argument here as far as I have understood it. The question was about avoiding ice dams on the roof - not about energy consumption.
convenience often comes at a price - is this case one would think at a pretty darn low price too. If we all only cared about energy consumption we would just skip the HVAC all together for homes as it costs a lot of money to operate.
11-14-2006, 10:57 PM
Tundraotto--You are missing the point. The person who started this thread was worried about ice dams, not that he had ice dam problems. Why stick a hole in your roof that uses electricity and can cause problems when there is no ice dam problem? If there is an ice dam problem, why not cure it by eliminating the cause (insufficient insulation/air leaks), rather than putting a hole in your roof that uses electricity and can cause other problems? This isn't about saving energy, but putting in pv's will create the problem of increased utility bills and the way I suggested of dealing with ice dams will save on utility bills. Another point that that no one has mentioned is what happens when the pv you put in to keep the attic cold freezes up and quits in the middle of winter? Pv's are a joke.
[Edited by uktra on 11-14-2006 at 11:05 PM]
11-16-2006, 12:14 AM
So keepitsimple: do you think that fancontroller isn't safe or won't work?
And to those discussing the insulation need, I have R6 insulated ductwork up there that radiates at 40 some degrees when I gun it - methinks all the ductwork, despite being well sealed just keeps the attic warmer than it should be and I can't do much about that now...I had it increased from R4 earlier this year but not much difference.
11-16-2006, 11:05 AM
During winter, the natural forces are moving warm, humid inside air to the attic(stack effect). If you must ventilate, consider blowing in outside air. This would partially offset the stack effect. Keep in mind that cold attics cause condensation inside attic ducts that contain more humid inside air. This is tricky bussiness. Might consider foam insulating, ridged or spary, the parts of the roof deck that melt snow. TB
11-16-2006, 11:18 PM
So the forced air ductwork warming up the attic is or is not the problem? I don't want to spend the money or make a mess up there by insulating the floor of the attic if I'll still have the problem from ductowrk radiating heat. I believe I would have had an ice dam last winter as I was getting a massive area of icycles but the next day the snow all melted away in 60 degree weather. I think we got lucky. Its a low slope roof so natural soffit to gable ventilation may not be the most effective which is why I was thinking about a fan.
11-17-2006, 01:28 AM
the temperature difference between the ambient air and your attic air is the problem as far as ice dams. as described you have several approaches you can take to prevent ice dams from forming - some expensive - some cheap.
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