House is built on a concrete slab. HVAC is in a centrally located
closet. AC drain line is 3/4" PVC going into the slab. After 40
years of use I finally had a drain blockage. HVAC tech said the line
drains into the sewer line and it was probably blocked there and to
use pressure to blow the blockage out. A plumber aquaintance said the
line drains into a dry well under the slab and to use a shop vac
suction to remove the algae blockage. Who's probably right?
I tend to doubt drainage into a sewer line because of the possibility
of sewer gas/fumes getting into the house but at the same time if it
is, I hate to hook a shop vac to a line and suck sewerage into a 3/4"
line causing a bigger blockage.
In your case I would try blowing it first. With an air compressor you can
develop a lot of pressure. The suction will only be about 15 psi at the
Also not knowing where the end of the line is it just seems safer as not
suck something back that you do not want to.
Who knows? I guess you could dig up the slab to find out.
All the drain lines I've seen dump into the sanitary sewer - but they also
have a p-trap.
If you have access to the drain, dump a couple of cups of bleach into the
thing. It will kill the algae without any sucking or blowing.
That's the way they are supposed to be done. And if it
was done as part of new construction it's usually easy
to do. However I've seen plenty of pics of all kinds of
connections. Tapping into a waste vent line in an attic,
for example. That's not allowed, but it;s done. So,
Any chance it runs outside?
If it comes to blowing air or using a vacuum, I don't see
the downside to blowing air. Even if it's a drywell under
the house, which makes no sense, no harm in using
reasonably compressed air pressure.
Worst case if it can't be cleared it could be replumbed
using a condensate pump to an appropriate drain
Kinda what I was thinking too. You don't have to start at 120psi.
Start around 20psi and work up.
Now, just to flavor the discussion a bit --- When I was young (a long
time ago) I happened to be at a cousin's farm home when a local handy-
man was called to clear a sewer plug between the bathroom and the
septic tank. There was a conveniently located clean-out about 20'
from the house but the guy didn't know which side of the clean out the
plug was on. He had a large truck-mounted compressor on his truck.
He hooked one end of the air hose on the compressor. He put a trigger
activated valve on the other and packed rags around it and the clean
out fitting and let 'er rip. Spray painted the bathroom
My a/c is in the garage and the drain line runs across the floor (by
the wall) and outside. I pour Clorox in twice a year to keep it
running clean. The man who checks it when he comes out once a year
blows it out. I think a hose would do the same -- if you are worried
about the pipe breaking and leaking, just try Clorox first. Then if
it still doesn't work you can try a hose and see if the water pressure
pushes it out. I, too doubt it drains into the sewer but you never
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