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Code allows the use of a wet vent for the bathroom group which in my case is a sink, toilet, and shower drain. About 10-15 feet down stream of this group I have a washing machine drain with its own vent. I also need a emergency drain in my mechanical room. Does this need a separate vent?

diagram

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what's an emergency drain? – Steven Feb 5 at 3:26
floor drain. I thought that is what they are called in a mechanical room. – pdfj Feb 5 at 7:14

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If it's really an "emergency" drain - one that you will never use in normal circumstances - then it shouldn't be connected to the rest of your plumbing at all. It should just have a straight pipe leading into a basement sump pump, outside the house, or even into a crawl space (just put some screen over the open end to keep critters out). The reason for this is that if you connect it to your normal plumbing, you will need a trap. The trap stays partially filled with water to keep the smell of the sewer from entering your house. Over time, the water in the trap will evaporate, letting the sewer smell enter your room. In a drain that gets regular use, this is not a problem, since you refill the trap every time you use the drain. I'm guessing your "emergency" floor drain is only there to get rid of water if the washing machine malfunctions and overflows. In that very rare scenario it's not a big deal to just let the water run into your crawlspace or yard, although directing it to a sump pump (and from the sump pump into your yard) is probably the best option since it keeps the open end of the pipe inside your basement if you have one.

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There are solutions to the dry trap issue - a trap primer hooked up to a commonly used faucet will resolve the issue of lack of water in the trap. There are also trap primers that drip a small amount of water into the trap when there is flow through a valve so it doesn't even have to be connected to a faucet. – Steven Feb 5 at 15:52
@Steven true, but if sewer pipe is blocked, an "emergency drain" not connected to that same sewer pipe is a good idea. – longneck Feb 5 at 20:25
I think the OP just meant a floor drain in your utility room.... – Steven Feb 5 at 20:36
I had not thought about the dry trap issue in this room. Thanks for bringing that up. The drain in that room is for the pressure relief valve on the hot water heater. I will probably have that drain go to the outside under the deck. Still need a vent for it I believe then, – pdfj Feb 6 at 15:50
@pdfj If all you want to do is get the pressure relief plumbed outside, then you don't need a vent. Just plumb the pressure relief valve with PVC to the outside. No traps or vents needed since there are no sewer gasses you need to guard against. – longneck Feb 6 at 21:48
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"The code" allows wet vents but not through horizontal piping. And it can be any nearby fixtures on the same floor of a certain size, the fixture group requirement was removed. Of course, this depends on which code we are talking about, I'm mainly familiar with the IPC formerly UPC. Horizontal wet vents could still be allowed by other codes.

Your bathroom was probably constructed when that practice was acceptable. Doesn't mean you can do it now. In any case, wet venting is allowed for low flow fixtures draining into the sizable vent of high flow fixtures. You proposal for wet venting the washer through the floor drain's vent has the concept reversed, so it fails on principle. There would be a very good chance the washer flow would suction out the floor drain trap. It's hard enough to maintain a proper floor drain trap seal as it is.

The floor drain trap will need it's own vent, though once you are above floor level with the vent, you may make horizontal runs and tie into other vent pipes provided the sizing is appropriate. Some codes allow a mechanical venting device, which obviates the need to run the vent outside somehow.

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Horizontal wet vents are permitted. P3108.1 Horizontal wet vent permitted. Any combination of fixtures within two bathroom groups located on the same floor level shall be permitted to be vented by a horizontal wet vent. – pdfj Feb 6 at 15:47
Thx, I will update my leaky grey file. – bcworkz Feb 6 at 18:28
Note this was for the IRC code so you're grey file might not be so leaky. – pdfj Feb 8 at 4:44

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