Maybe someone knows for sure what this drain is. It's in the furnace room in a concrete floor. It's 6" across, 2.5" deep, with a removable grill cover. There is a 2.5" plug in the center, possibly never removed, and a 2" plug in the sidewall in regular use.
There is one water line upstream of this drain. It drains the clothes washer on the floor above. The line carries that water under the house and thence to the city sewer running the periphery of the property.
That was all this drain ever did until 1995 when we added central air. Needing someplace to drain the condensation, and with this floor drain only 4' from the air handler, a 5/8" hole was drilled through the 1" nut of the 2.5" plug in the bottom of the drain and a condensation drain line stuck in. Not a tight fit, just snug. Water could seep around the pipe.
After a decade or so, the condensation started to occasionally back up. Something blocked the line (a trap?) under the drain. To fix, I'd remove the 2" plug on the sidewall of the drain and the backed up water (a quart or two on the floor) flowed out.
I installed an alarm to announce when the water backed up.
If the clothes washer pumped out while the 2" side plug was removed, a small quantity of wash water, a pint or so, would puddle around the drain and then flow out.
For maintenance the system got a summer monthly dose of chlorine at the uphill air handler end of the condensation line, and a once-a-year dose of root eradicator through the 2" plug. So far, so good.
Stepping back for a minute, I have lived in this house for 28 years and always believed the house had two lines to the city sewer, one terminating at each of the manholes on the property. The line I imagined for this drain ran under a heavily wooded section, so roots entering a 1952 sewer line seemed reasonable.
Two years ago on June 24 we had a 60-second wind microburst in town that leveled that wooded section. Leveled means trees 40" in diameter were snapped off and decimated everything downwind. After 12 grand spent for cleanup, we had lawn where once there was woods.
Forward to last month. There was the usual hotspell condensation back up. I applied the chlorine, and an extra large dose of root eradicator through both drain openings, the 2" plug and 5/8 drilled drain hole. Time to wipe out those dying roots once and for all.
The drain plugged up. Zero flow.
I called a rooter service who ran the line through the 2" plug. Cleared that line in 15 minutes. No problem. But the smaller rooter going through the 5/8 hole got nowhere. That pipe stayed blocked. He said it was a trap. To learn more, the 2.5" plug needs to come out.
It can be speculated that root eradicator pellets clogged the trap under the floor. Since they dissolve, the blockage will eventually pass without need for removal of the rusted plug. But I'm too far into removal to quit merely because it's pointless.
The $229 paid to the rooter didn't all go to sewage transit. I also had the house sewer line checked where it entered the manhole, to make sure it was clear at the lower end.
Then came a shock. There was no customer sewer line in that manhole.
So we drained the clothes washer to see where the flow led, and found that water left the washer, flowed to the line under the floor drain, and crossed under the house to connect with the main line leading to manhole #2. And there was free flow, no blockage whatever.
So this week, the condensate is temporarily going out the side hole while we contemplate further action with the trap. A powerful shop vacuum sucking out the root eradicator through the 5/8 hole is one possibility. That worked when my granddaughter blocked the bathtub.
If it were up to me, it would be illegal to sell a house without passing a full set of plans to the new owner including plumbing, heating, cooling, and electrical.
