using bathroom fan as a house ventilator
I’ve seen the following idea suggested for cheap whole-house ventilation (spring and fall, when windows are closed but boiler not running): Have a 24-hour timer on the bathroom fan that turns it on periodically to cycle the house air. Based on our occupancy (3 humans, 2 cats) and 110CFM Panasonic, I figure that’s 20 minutes of every hour.
Every electrician I’ve discussed this with is dubious (it comes up with electricians when I explain how I want the fan switch wired). First the amount of air circulated is small (I have to explain that the attic fan is too much). But the other point is that I’m just going to prematurely burn out the fan motor. Given that the beast will be buried under cellulose in the attic, so replacement will be a major pain, that is a concern.
What do people think of this approach? I originally got it from Bruce Harley’s “Insulate and Weatherize”, I presume it showed up at some point in an issue of Fine Homebuilding.
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If you are looking to add an air exchange each hour, a 110 cfm fan would, if run continuously, exchange air in a 825 square foot house (pretty small!)
I don't worry about burning out the fan because they are usually pretty cheap and easy to replace - the motor and fan that is. The whole housing is a pain, but the motors are designed to be servicible from below.
You speak of doing it when the boiler isn't running, but even in the summer, there's the (gas?) HWH and the range hood.
Seems either your house is tight and then you DO need ventilation but probably more. And unbalanced ventilation is bad for backdrafting. So an HRV is a better solution, albeit more expensive.
Or you house is not very tight, then while out of balance and backdrafting is not a concern, you don't need the fan to run because the normal leaks are already ventilation.
If only for high temps in the summer, go ahead - it will help a bit, not a lot. Opening windows on opposite corners of the house (NW up, SE down) will do a lot more.
I have considered a similar plan. The situation I'm dealing with is that I'm in Florida. By 1PM the exterior heat has heated up the attic spaces and it is beginning to radiate downward into the living space. By the time I get home the interior is quite hot.
Interior heat sources are part of this problem. I have a few pilot lights and a refrigerator adding heat. The refrigerator is an issue because it is dependant on the interior air for its cooling. The hotter the ambient temperature surrounding it the greater the loss through its insulation making it run more often and the harder the unit has to work to pump the heat into its surroundings.
My thought was that if I could place an exhaust fan, thermostatically controlled, very near the ceiling at the highest spot within the building and exhaust the hottest air I could reduce the heat load on the refrigeration units inside and the AC when I get home. There would also be some additional advantage to having the exhaust fan blowing this air, hot as it is, into the much hotter attic space as this causes the hottest air in the attic up and out of the existing ridge vent.
This would not be the same as the usual whole house fan. A whole house fan is designed to keep the house cool enough for comfort of the people. Primarily this is done by moving air to increase the evaporation of sweat. This system would not move enough air to help cool people directly. Its goal is to keep the air at the level of the ceiling below as set point. Something like 100F sounds about right.
The idea is to take the ragged edge off the temperature at ceiling level so that interior surfaces, interior refrigeration units and, when I get home, the AC unit can operate more efficiently. In theory, IMHO, there could be substantial advantages to such simple system.
Your plan seems a little more challenging in that you are attempting to keep people cool. While removing hot air at the ceiling in such small amounts could help to keep interior temperatures below some, relatively high, maximum in all but the most moderate outside temperatures the maximum temperature will be so high that none but the damned themselves could call it comfortable. There just is not enough air movement to cool bodies even if the surrounding air could be kept within a dozen degrees of comfortable.
In summary a small exhaust fan might, possibly, serve to save energy, by lowering heat loads felt by the AC when it is turned on and refrigerators before the AC is turned on. A positive and favorable outcome but the amount of air moved is so small that it would not significantly decrease AC usage. The fan would not significantly, IMHO, change the apparent or sensible temperature felt by individuals within the house.
4Lorn1: Your approach would be much more succesful than the bathroom fan. Because you propose to move more air, from the hottest place (ceiling), displace even hotter air in the attic, and use a thermostat.
This is done by some venecular architecture in the SE. A tall center structure to create a chimney affect. Often combined with an extensive porch to shade the ground floor.
If you do the t-stat fan thing, leaving a north window open would help the most.
Some reverse variant of "hot-rocks" i.e. "cold-rocks" that are cooled each night but the replacement air flows through each day would help more, but I wouldn't recommend it. The payback is much much less than for the simple fan and it would be a better match to a very tight house that wouldn't need a lot of cool air to effect it.David Thomas Overlooking Cook Inlet in Kenai, Alaska
The purpose of whole-house ventilation is not to cool things down, but to bring in fresh air from "outside" and remove moisture from the internal air resulting from exhalations and such. It wouldn't be necessary in summer, the AC would be running, and it wouldn't be necessary in winter, the boiler would be drawing in fresh air. (This does require that air is drawn in through desirable spaces and not e.g. through the crawlspace or the garage.) It is very much a background kind of thing, so it's important to have a quiet fan. Recommended 10-15CFM per person, say 15CFM per bedroom plus 15CFM for master bedroom (taking this from Harley's excellent book). So our 3-bedroom house needs 60CFM, so run our 110CFM bathroom fan for 30 mins every hour (at least while people are in the house).
In a moist environment like Florida, you want to run it less because you're pulling in moisture from outside. And once you're cranking the AC, it isn't necessary at all. For reducing AC load, I think what you really need are (high-efficiency) ceiling fans in conjunction with the AC, to make people feel cooler than it is.
An alternative to the bathroom fan, and to a central ventilation system, is a supply-only air system ducted through a CAC air handler. Remember this is for the time of year when you don't need AC itself. You have to put the air handler on a timer for this, and put a damper on the air input in winter. This alternative will provide better ventilation than the bath fan (you control the input, and the fresh air is distributed throughout the house), but it'll be less efficient to operate than a bath fan. Seems to me replacing an air handler suspended in the attic will be easier than replacing a bath fan buried in cellulose, but a lot more expensive.
Edit: I didn't notice Dave Thomas' comment about replacing motor from below, that would certainly help. I'll have to check the fan for this.
Edited 4/30/2004 5:40 am ET by Taylor
Lovely, I've now had an electrician ask for a wiring diagram for this (using EFI 24-hour timer and switch). Claims one must be a master and the other won't work unless the master is turned on, which defeats the whole purpose. If anyone has a wiring diagram that they could share... No one has ever tried this? I assume it originally appeared in FHB at some point.
This would be an unbalanced system--stale air out, but no fresh air in. You would theoretically produce a negative pressure in the house, which could lead to dangerous backdrafting problems. The new Panasonic fans mention are designed for 24 hour use in just the situation you describe. Get a smaller CFM unit and run it all the time with a pressure sensitive source for fresh air in.
You might try this http://www.jlconline.com/cgi-bin/jlconline.storefront/409260050015178c27177f0000010532/Product/View/0306cool
If I remember correctly the author was with a some "group" (university, solar institue, energy conservation) and I think that they have a web site. Try googling on his name along with things like solar, roof insulation, etc.
I don't think that blowing hot air out of the attic in the evening will do much good. One of the things that his article showed was that a lot of the things that they tried just pushed back the peak time that the heat energy would "hit" the inside. By this time the structure and insulation is alread heat soaked.
Also I would be concerned about the humidity in the air that is drawn into the house.
You may want to look at installing a radiant barrier to channel cool air from soffit to ridge vents. Your attic is probably approaching 140 deg, and you are right about the saturation.(it hurts you all night) A good barrier/vent system can keep the attic close to ambient air temp. You also may have your duct work in the attic, which is another source of hot air into the house. The ducts quickly fill with hot air and expand into the house. After a rad barrier, I would bury the duct work in blown cellulose. Is your air handler in the attic as well?
I lived in Clearwater for 5 years. After some modifications, cut A/C bill in half, and kept house cooler than design delta of 15 deg to ambient.
There are a lot of improvements that can be made to "typical Florida construction". PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home