If your basement has ever taken on water, you already know that one sump pump is only as reliable as its float switch, its circuit breaker, and the last time it was tested. I run two pumps down there, and they're not set up as overkill. They're set up as insurance. One is on a dedicated circuit and handles normal duty. The other is on a separate circuit so a tripped breaker can't take both of them out at once. At some point I'll put the second one on a battery backup so even a power outage doesn't leave me with a wet floor.
The other reason to run two pumps is volume. During a really heavy storm, a single pump can fall behind if water is coming in faster than it can discharge. When that happens, the second pump kicks on and both of them are pushing water out together through one shared discharge line. That's the setup this guide covers. It's a pretty straightforward plumbing job and you can put it together in an afternoon with materials from Home Depot.
I used to run the discharge through a corrugated 1-1/4" discharge hose and it cracked on me during a flooding event, which is about the worst time for that to happen. PVC is what I'd recommend for the main run, and I'll explain why at each step. But if you already have hose fittings on your pumps or you're renting and don't want a permanent install, I'll cover the hose option at the end too.
Why run two sump pumps to one discharge line
Most people think of a second sump pump as a backup that only runs if the first one breaks. That's part of it, but the real value is a little more nuanced than that.
First, the redundancy angle. If you wire the two pumps to different circuits, a single tripped breaker can't kill your whole system. A battery backup on the second pump takes that a step further and covers power outages too.
Second, the capacity angle. A sump pump rated at say 1/3 HP has a finite flow rate, and during a heavy storm you can hit that ceiling. When the water level rises above the first pump's float threshold and the second pump kicks on, you've now got twice the discharge capacity. Both pumps are pushing water out through the shared line at the same time, and the tee fitting and check valves make sure they don't fight each other doing it.
Running both pumps through one discharge line instead of two separate lines also means one hole through the rim joist instead of two, one run to your discharge point outside, and a much cleaner install overall.
What You'll Need
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Materials
Tools
Step 1: Connect the check valves to each pump
Check valves are the reason this whole setup works. Without them, the stronger pump will push water backward through the weaker pump's discharge port when both are running. Worse, when a pump shuts off, all the water sitting in the discharge line will flow back down and flood into the pit, sometimes triggering the float and running the pump in short cycles all night.
Each pump gets its own check valve. I use the Superior Pump 99555 universal check valve because both ends accept 1-1/4" or 1-1/2" MPT or FPT, which covers almost every residential sump and utility pump you'll find. You don't need to glue or cement anything at this stage. These are threaded connections.
Screw the check valve onto the pump's discharge port. Hand tight plus about a half turn with pliers is enough. Go slow and make sure the threads are not cross-threading before you apply any real force. The flow arrow on the side of the valve body needs to point up and away from the pump, toward your discharge line. If you install it backward it'll block flow completely.
Step 2: Attach the slip couplers
The check valve outlet is threaded, and PVC pipe is a slip fit, so you need an FPT-to-slip adapter to bridge the two. These are the C fittings in the diagram.
Screw one adapter onto each check valve. Again, hand tight is fine here since you'll be gluing the slip end. At this point you've got a threaded stack coming off each pump: pump, then check valve, then slip coupler. The PVC starts from here.
Step 3: Cut and glue the PVC
Before you cut anything, dry fit the whole assembly on the floor next to your pit. Measure the distance between the two pumps, figure out the pipe lengths you need for the short risers that run from each coupler up to the tee fitting, and make sure your discharge run exits where you want it to.
The sanitary tee has two inlets and one run outlet. One pump goes into each inlet through its riser pipe. The run side of the tee connects to your discharge line going out. The 90-degree street elbow lets you change direction to get the line out through the rim joist or up through a floor penetration depending on your basement layout.
Once your dry fit checks out and everything is aligned, glue it up in sections. For each joint:
- Clean the pipe end and fitting socket. Wipe off any dust or moisture. Both surfaces need to be dry.
- Apply primer. Brush it onto the outside of the pipe end, then the inside of the fitting socket, then the pipe again.
- Apply cement within 5 minutes. Brush it on the pipe, then a thinner coat inside the fitting, then one more application on the pipe. Work quickly.
- Push and twist. Push the pipe fully into the fitting and give it a quarter turn as you go. Hold it together for about 30 seconds and make sure the fitting is oriented the right direction before the cement sets.
- Wait before testing. The joint is set enough to handle in about 15 minutes, but give it at least 2 hours before running the pumps. In a cold basement, give it longer.
Step 4: Set your float switches
Some sump pumps have built-in float switches, but a lot of utility pumps don't, and even the ones that do can benefit from an external controller if you want to stagger the trigger heights between the two pumps.
The way I set mine up, the primary pump triggers first at a lower water level. If water keeps rising, the secondary pump kicks on at a higher float threshold. That way for normal rain events only one pump runs, and the second only adds capacity when it's actually needed. The Basement Watchdog BWC1 is a reliable external controller for this kind of setup.
For a battery backup pump, set the float higher than both primary triggers so it only activates during a power outage or if both AC-powered pumps have somehow failed. This setup has kept my basement dry through several heavy storms since I built it, including one nor'easter that ran both pumps simultaneously for a few hours.
Using discharge hose instead of PVC
If you're not ready to commit to a permanent PVC install, or if your pumps are already set up with garden hose adapters, you can run the same layout with flexible discharge hose. The check valves, tee fitting, and flow logic are all identical. The difference is you'd use barbed hose fittings at the tee instead of slip PVC, and hose clamps to secure each connection.
The tradeoff is durability. Corrugated hose is fine for temporary or seasonal use, but it doesn't handle long-term stress nearly as well. For anything permanent, use PVC for the main discharge run and only use flexible hose at the very end if you need a short flexible section to connect to an exterior drain or other oddball fitting.
If you want to go from PVC to a garden hose thread at the discharge end, a male hose thread adapter will get you from 1-1/2" PVC slip to a standard 3/4" garden hose connection.
Common questions
Yes, as long as each pump has its own check valve before the tee fitting. Without check valves, the two pumps will fight each other, with the stronger one pushing water backward through the weaker one. The check valves act as one-way gates so each pump contributes to the shared discharge without interference.
Most residential sump pumps do not have check valves built in. Some higher-end models include them, but it's not standard. When you're connecting two pumps to one line, you need to add an external check valve to each pump individually regardless -- the shared discharge creates backflow conditions that an internal valve isn't designed to handle.
Most residential sump pumps discharge through 1-1/2" PVC. Some smaller utility pumps use 1-1/4" at the pump outlet but typically include an adapter to step up to 1-1/2". For a dual pump setup sharing one discharge line, stick with 1-1/2" throughout. Going smaller than the pump's rated outlet size restricts flow and makes the pump work harder than it needs to.
Two pumps gives you redundancy and capacity. Wire them to separate circuits and one tripped breaker can't take the whole system down. Put the second on a battery backup and even a power outage doesn't leave you with a wet floor. On the capacity side, two pumps running together can move twice the water during a heavy storm when a single pump might fall behind the inflow rate.
There's a flow arrow on the side of every check valve body. That arrow needs to point away from the pump, toward your discharge line. In a vertical install the arrow should point up. Installing it backward blocks flow completely.
Set the float switches at different heights. The primary pump's float should sit lower so it handles normal duty. The second pump's float sits higher and only kicks on when water is rising faster than the first pump can handle, or as a backup if the first pump fails. External float controllers like the Basement Watchdog give you more control over trigger heights than the built-in floats on most pumps.
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