Can a 4″ diameter radon mitigation piping system be exhausted through using 3″ pipe?
woodschaver
| Posted in Mechanicals on
I’ve installed an under floor radon mitigation system using 4″ PVC perforated drain piping (in gravel and wrapped in filter fabric). The line ties into a sealed sump. Venting to exterior will be accomplished using approximately 12 lineal feet of horizontal and 28 lineal feet of vertical piping. This will be boosted using a 150 CFM (67 watt) in-line fan installed (in the attic) near the end of the stack.
The fan has 3″ pipe size inlet and outlets. Can I use 3″ pipe for the entire exhaust length, or should I modify fan for 4″ inlet/outlet?
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Replies
The sump pump covers that have the pipe fittings for radon have 3" pipe openings too. I can't see why you could not.
Greg,
A smaller duct (in your case, the 3-inch pipe) will have a higher static pressure than a larger duct. As a result, your fan will either (a) use a little more energy, or (b) move a smaller volume of air with a 3-inch duct than it would with a 4-inch duct. It may not matter much, however. Your call.
Thanks for the responses. The 3" size pipe will make routing among the water piping, HVAC duct and equipment, joists and electrical cables significantly easier. Well worth the trade offs.
Where is the fan to be located along the run? Q = VA, so the volume of air moved by the fan should remain the same and the velocity of the air will increase slightly proportionately to the change in area. So no big deal in my opinion, especially just being radon gas.
The fan can be fitted with rubber couplings that are 3*4 or 3*3 and will perform great with either size pipe. Fitting a Manometer with the fan running will tell you how much vacuum you are creating and if the fan is running. You can use charts supplied by the fan manufacturer to estimate how much air you are moving. In the interest of efficiency you will want to use the smallest fan possible that will move about 40 or so CFM on a 1 or 2 thousand foot slab. The system you have described will get plenty of air. You should do pre installation and a post installation Radon test. Sub Slab Depressurization will improve your air quality even if you do not need to mitigate Radon. I am making a number of assumptions about your home and about your experience with this so forgive me if I offend.