Replies

1. 
   Riversong | Oct 25, 2010 05:59pm | #1

   First, you should have a passive radon vent made of 4" perforated plastic pipe bedded in crushed stone under the tightly-sealed crawlspace air/vapor barrier. This pipe must be vented through the roof with as straight as possible a length of 4" solid pipe. 95% of all household radon problems are relieved with a passive vent such as this. If the radon still reads above 4 pCi/L, then you will have to add an inline fan to the vertical stack, typically in the attic, to create negative pressure at the soil.

   Second, your kitchen hood fan should not move more than 100-150 CFM of air (on high speed). 300 CFM is too much for a tight house.

   Third, it's best to install the clothes dryer in an air-tight room (weatherstripped door with bottom sweep) and provide a dedicated make-up air inlet. I use a 6" duct with an exterior hooded screen and interior louvers for self-actuation. You can also install a hinged or cape-style backdraft damper in the duct to minimize unwanted cold air leakage.

   You don't indicate what type of whole-house ventilation system you've installed, but that's absolutely necessary in a tight house. If you don't have a balanced HRV system, then you can use existing bathroom exhaust fans on 24-hour programmable timers to provide fresh air exchange along with passive make-up air inlets in bedrooms and living spaces (I prefer the American Aldes Airlet 100). Having strategically-located dedicated make-up air will relieve the negative pressure.

2. 
   Armando Cobo | Oct 25, 2010 06:14pm | #2

   Ron,
   Unfortunately, radon mitigation at this point is expensive ($2k-4k per stack) but if your readings are pretty high, I would address it before and separately from the negative pressure issue. Fresh make-up air can be mitigated with an outside air supply to the air handler plenum, a mechanized damper and controlled by an Air Cycler or an IAQ Thermostat; or a mini ERV or HRV depending on your climate.

3. 
   [email protected] | Oct 25, 2010 06:33pm | #3

   Robert,

   Some additional info. The house has an Venmar Eko 1.5 HRV which is tied to the forced air heating system. The range hood is rated at 300cfm, but moves less with the duct configuration. That said, it is depressurizing the home when it is running at low speed which is moving something like 70 cfm. As far as the radon goes, this was a existing remodel with strong limits for both scope and budget. We did not have the opportunity to add crushed stone in the crawlspace. I have received a bid to install a length of pipe in a "T" configuration under the plastic and tied to a radon fan for ~ $700. The first cost for this seems ok, but I am not happy about the 500 - 600 kWh per year to run it. Plus the dueling fan issues seem wrong... As far as the sealed dryer room, this seems like it would introduce lots of cold, unconditioned air into the room. Unless it was both weather sealed and insulated (which would be hard to do in this case) it would have a significant energy penalty. Is there a more passive heat exchanging product on the market?

   Thanks again for helping me think this through,

   - Ron

4. 
   Riversong | Oct 25, 2010 06:47pm | #4

   Ron,

   I was suggesting exactly the passive options you desire.

   Whether or not you can install crushed stone, you can still use a passive radon vent through the roof, even if it has to run outside the house (which is less effective because it will be cold in the winter). Do this before installing any radon mitigation fan.

   If you place a make-up air inlet close to the dryer, which operates only when there's negative pressure in the room, then that cold air will short circuit directly into the dryer and out the dryer vent. It should not cool the room noticeably.

   But you can't eat your cake and have it too. If you want passive systems, then you have to accept a slight energy penalty trade-off in return for no additional electrical consumption. There is no "magic bullet" or free lunch.

5. 
   [email protected] | Oct 25, 2010 07:21pm | #5

   Robert,

   I was actually hoping for a magic bullet that came with a coupon for a free lunch...

   The fresh air inlet for the dryer short circuiting the volume of the room is a good point. Perhaps a manually sealed door is an option (they use the dryer as infrequently as possible).

   The problem I am having with a passive radon pipe is one of location. I could put it outside as you suggested, but I also believe that they are not very effective in the winter when you do this. If I bring it up inside the home, the only place I seem to have access to puts it too close to the fresh air intake of the HRV. Therefore I think I am going to need to install it outside - which will most likely mean a fan.

   What about makeup air for the kitchen hood? I keep seeing makeup air kits that involve electric heaters. What would you suggest for the kitchen?

   - Ron

6. 
   Riversong | Oct 25, 2010 07:52pm | #6

   Ron,

   If you install a 30' radon stack (assuming full basement and two storeys), even at 50° basement soil temperature and 30° outside air temperature, you'll have nearly a 5 Pa pressure difference drawing air up the vent. That should be sufficient unless you continue to create competing negative pressures.

   At the risk of repeating myself, I already recommended installing a smaller kitchen hood. ASHRAE 62.2 requires only 100 CFM intermittent capacity in the kitchen.

7. 
   [email protected] | Oct 25, 2010 10:34pm | #7

   Actually it is Single story, 3 foot crawlspace…

   Lets say we have -5 Pa under the vapor barrier. Because the home is so small, it only takes 730cfm to reach -50 in the conditioned space. Even small fans have an easily measured effect. The two bath fans (50, and 75 cfm) seem to be able to create -4 to -6 Pa in the home. I had hoped that the HRV would have allowed for a bit of a pressure buffer effect, but something that I didn’t think about is the ECM motors ramp down their power as the pressure they are overcoming goes down (which is a good thing) but it turns out that the overall flow ends up being quite constant (you used to be able to get the velocity to go up on one side of the exchanger without effecting the other side by pressurizing or depressurizing the house).

   The result of all this is that the kitchen hood seems to be able to take the home negative with little effort. Even if the fan was half the size, I think I would be in the same boat (although perhaps I would be floating a bit higher :-)

   Regarding sizing the range hood – it is my opinion that ASHRAE 62.2 is very good, but not perfect. I believe it under appreciates the danger of the particulate content of oxidizing foods. The process of "cooking" puts out lots of aerosols that I would like to send out of the house without breathing first (and especially each time I open the oven door) whether you are browning potatoes (or worse burning them). For these cooking “events” even the larger hoods seem small (as in their overhang, not air movement). With lots of actual cooking as stated expectation by the client, a little grease in the filter and an elbow or two later and a 300 cfm rating seems reasonable. In addition, the expectation is that the fan will be used mostly on low with the occasional and short duration ability to go higher.

   It therefore seems that one possible strategy may be to install a fresh air supply next to the dryer, to install one 4” passive radon pipe on the exterior (and prepare for a future fan). I still am left wondering what to do for the kitchen makeup air…

   Regarding the passive radon pipes. Do you know of any reason not to do two 3” pipes instead of one 4”? I will have more placement options with 3” pipes.

   Thanks again for your thoughts,

   - Ron

8. 
   Riversong | Oct 25, 2010 11:58pm | #8

   Two 3" pipes offers more cross-sectional area than one 4" pipe, though a bit more drag and turbulence, so the duo should work fine as long as the runs are as close to vertical as possible with no 90° offsets (use sweeps or 45s).

   There are motorized louvered vents that could be controlled by a relay in parallel with the kitchen fan, and like with the dryer could be short-circuited near the hood, perhaps behind the bottom of the range so incoming air scoops the unwanted aerosols (which most people consider yummy cooking odors).

9. 
   Ron | Oct 26, 2010 01:39pm | #9

   Thanks for your help.

   - Ron