WHOLE HOUSE VENTILATION VS BATHROOM FANS ...
Let me start of this post by saying that I am not a licensed HVAC professional, I'm just a homeowner that wants to build a house that doesn't make me sick and is as easy on the pocketbook as possible long-term. I've found with building this house that you just ask a whole bunch of questions to a whole bunch of people, and eventually you end up with a consensus that you feel good about - no where is this more true than in the World of "not yet conventional" HVAC design. So disclaimer: this information is distilled from a lot of very knowledgeable and talented HVAC professionals, but be sure to use it as a guide and not a bible.
Since our house was built with SIPs (structured insulated panels or really thick walls of rigid foam) instead of traditional 2X4 building, it is extremely air tight. Specifically, a blower door test that was conducted before hanging drywall came back at 1.8 air exchanges at 60 psi, which means that if we could take all the leaks in the house and consolidate them into one opening, it would equate to a 10" X 10" opening - that's really good. In comparison, my blower door technician says if you really focus on air leakage on a new home with stick framing you might be able to hit 5 air exchanges per hour, but typically it is 6-8 - until us he had never seen a structure under 3. The worst house he tested was an old Victorian fixer upper in Bloomington that scored 23. Although we have not retested since drywall went up, we think we're under 1 air exchanges now - which is about a 50 square inch hole - since we were able to identify the existing leaks with an infrared camera (totally cool, eye opening, and total different post) and drywall helps tighten up the house somewhat.
So what does this have to do with ventilation and bathroom fans? Quite a bit. Although you often hear, "You can build a house too tight" and a better statement would be "Build tight, ventilate right." Usually you are building tight, because you are trying to control energy costs - remember, you might have the most insulated walls in the country, but they aren't going to keep you warm if the window is wide open in the middle of winter. Even though an open window would provide most of the ventilation, it defeats the purpose of the original investment if it's too hot or cold outside, and you still haven't solved the higher ventilation needs of bathrooms.
The solution devised by those anally-retentive Germans and adopted in the US for tight homes was to install an HRV, or Heat Recovery Ventilator, that will exchange the heat (but not the pollutants) of the stale interior air with fresh exterior incoming air, while also warming or cooling the air as needed as they pass (but not mix) with each other. Typical systems are 80% efficient, meaning they recover 80% of the heat or coolness of the outgoing air ... the HRV we used is 95% efficient. They are definitely an 'extra' expense in home building, but I'd rather go with cheaper tile and faucets in exchange for a house that won't make me sick (google 'Sick House Syndrome').
So how do I install them? Not too hard, although there are some things you need to make sure your installer does right. Obviously plan this out on paper first, and if you're worried (and have a little extra cash) get a mechanical engineer to make sure your runs will balance out - you want to make sure your diameters and run of ductwork are such that the system will balance out (more on that later).
First, make sure the intake of fresh air and the outtake of stale air is at least 10' apart. Really no point ventilating your house with the crappy air you just vented out. In our case we made sure our intake was pulling in the freshest air (would not be close to a gas grill, the dog bathroom, etc.)
Second, place the supply and exhaust in strategic locations. Just like you don't want the vents outside to be next to each other, you don't want the interior supply and intake of stale air to be next to each other - you want the whole house ventilated, so spread them out. Start with placing the intake for stale air in the bathrooms - they have the worst air in both moisture, microbes, and let's just say 'unwanted smells'. Once you have those set, put the fresh air supply on the other side of the house where it makes sense - just think how the air is going to flow from the supply to the intake across the house and see if it makes sense. Don't try to pull air directly from the kitchen ... too much grease to clog everything up. Again, your ventilating the whole house, so that salmon dinner you just cooked is going to be gone in minutes anyway with the HRV chugging away.
Third, don't use bathroom fans - seriously. Took me a while to realize this one, but basically you are trying to exchange air in a balloon, which means you need to balance the incoming air with the outgoing air - I think installers have the house SLIGHTLY pressurized. Too much coming in the balloon pops, too little the balloon deflates. You add a bathroom fan, then you are randomly increasing the pressure in the exhaust and throwing the whole system out of whack. Remember, you are still meeting the code requirements of venting the stale air out of a bathroom to prevent mold growth etc (you're actually doing a way better job than any small bathroom fan) - you are just letting the HRV do its job the way it's suppose to be done. Some installers will setup a switch in the bathroom to turn on the HRV remotely after the bathroom is used, but that's probably a hold over from the bathroom fan only ventilation design ... HRV's work best with their little touch screen programmable system is left to work its magic - operating at a low constant flow rate to clear the air and maintain the heat recovery process. Yes, the touch screen is not free, but I'd rather pay $300 for that than $300+ in fancy wiring across the house.
OK, that's about it for my diatribe for HRVs ... if it inspires one person to put in an HRV, the purpose is served.
