Pool Main Drain: Worthwhile or Optional? Installation and Pipework
A main drain at the lowest point of the pool improves circulation, cleaning and drainability – but it can only be sensibly installed during new construction. This guide explains the benefits, the separate pipework to the pump and the integration alongside the skimmer.
What a Main Drain Does
The main drain – often also called the main outlet – sits at the lowest point of the pool floor and draws water from the depths. This is no minor detail; it influences the circulation of the entire body of water.
Colder, heavier water sinks to the bottom, as do fine suspended matter, sand and settled particles that collect on the floor. A skimmer, by contrast, only reaches the topmost layer of water. Only the interplay of surface extraction via the skimmer and deep extraction via the main drain moves the complete pool volume.
- Better mixing: temperature layers are broken up, and heated water is distributed more evenly throughout the pool instead of building up at the top.
- Cleaner floor: fine debris that settles on the bottom is continuously drawn off with the water and ends up in the filter.
- Fast draining: the main drain lets you pump the pool down almost to the floor – for example for refurbishment or a liner change.
The advantage is especially clear in combination with a heater: without deep extraction, the warm water stays at the top while the floor remains cool. The main drain brings this cold water up into the circuit and ensures an even temperature.
Safety note on draining: never drain a pool completely without good reason. Where groundwater is present, an empty concrete pool can float up due to buoyancy, and a liner can shrink and form wrinkles without the supporting water load. Drain only as far as necessary and route the water off via the waste position (to the sewer), not back through the filter circuit. While doing so, make sure the circulation pump does not run dry or draw air. If in doubt, coordinate any complete draining and the necessary buoyancy protection with a specialist company.
Installation Only During New Construction – Why Retrofitting Is Not an Option
The key point first: a main drain can practically only be installed during new construction. The built-in fitting is cast into the base slab or integrated into the pool floor, and the connecting line runs beneath the floor to the equipment room.
Retrofitting one later would mean opening up the finished pool floor and re-laying the line – as a rule not justifiable in economic or structural terms. The decision for or against a main drain is therefore made in the planning phase, not later during operation.
If you have an existing pool, you achieve floor cleaning instead with a pool vacuum or an automatic pool robot. This does not replace deep circulation and mixing, but it covers the pure cleaning aspect well. For new construction, however, the following applies: the option to retrofit later is missing, which is why the main drain should be planned in from the outset.
Separate Suction Line to the Pump
The main drain gets its own suction line running all the way to the pump. It is not joined to the skimmer line underwater, but instead routed separately into the equipment room and only connected to the skimmer line there – just before the pump. This is the only way to regulate both suction points independently of one another.
As for sizing: the main drain carries a large share of the circulation flow, so the line should be rather generously dimensioned. The outside diameter d63 (DN50) has proven itself; on smaller pools d50 (DN40) is also possible. A larger cross-section keeps the flow velocity low and the pressure loss small – this is easier on the pump and saves energy.
- Lay the line with a slight fall so that it can be drained and blown out for winterizing.
- Use bends sparingly and route them to minimize flow resistance; every bend increases the resistance on the suction side.
- Fit a union before the pump so that the pump and fittings remain detachable for maintenance.
All joints are made using the PVC-U solvent cementing process (solvent welding): degrease the bonding surfaces with cleaner, apply cement evenly, push the pipe in as far as the stop, and let the joint cure sufficiently before applying pressure.
Control via Ball Valve and Interplay with the Skimmer
Each of the two suction lines needs a 2-way ball valve, mounted in an easily accessible position before the pump. With these valves you set the ratio in which the skimmer and main drain draw water.
As a starting point, a ratio of roughly two thirds skimmer to one third main drain has proven itself. When heating up or in autumn you can increase the main-drain share to improve mixing and floor cleaning. For winterizing, the main drain can be shut off via the ball valve. Important: never throttle both lines at the same time to the point where the pump draws air or runs dry.
The skimmer and main drain complement each other; they do not replace each other:
| Feature | Skimmer | Main drain |
|---|---|---|
| Suction point | Water surface | Lowest point of the pool |
| Primarily removes | Leaves, pollen, grease and oil film | Fine debris, settled sand |
| Mixing | Top layer only | Entire pool volume |
| Retrofittable | Possible with limitations | No, only during new construction |
Safety: Note the Anti-Vortex Cover
A single main drain can generate powerful suction when the pump is running. So that no one can be sucked against the drain, an anti-vortex cover is essential – a flat, large-area cover that prevents concentrated suction at a single point.
In practice, two main drains set apart from each other, or a suitably approved cover, are often combined to ensure suction safety. Because safety requirements and applicable standards come into play here, the design and the choice of cover should be coordinated with a specialist company. Use only undamaged covers approved for the main drain, and never operate the pool with an open, unsecured drain.
A Clear Recommendation
Our clear recommendation: if you are building new, you should plan in a main drain. The extra effort during the shell construction is manageable, while the gain in circulation quality, floor cleaning and drainability is permanent – and can no longer be created after the fact.
For an existing pool, on the other hand, a retrofit is not worthwhile; here a pool vacuum or pool robot is the pragmatic route. In short: yes for new construction, generally no for existing pools.
Installing a main drain and running the suction line to the pump
- Determine the position During new construction, plan the main drain at the lowest point of the pool floor so that dirt and cold water collect there and are drawn off.
- Integrate the built-in fitting Cast the main drain into the base slab or integrate it into the pool floor and fit the approved anti-vortex cover. Structural and load-bearing work belongs in expert hands.
- Lay a separate suction line From the drain socket, run a dedicated line in d63 (DN50) with a slight fall to the equipment room – separate from the skimmer line, so that both remain individually controllable later.
- Fit the ball valve Install a 2-way ball valve in the main-drain line before the pump. It regulates the suction share and shuts off the line for winterizing.
- Join the skimmer line Connect the separately laid skimmer line – which also has its own ball valve – to the main-drain line only just before the pump.
- Unions for maintenance Fit detachable unions at the pump and fittings so that components can be replaced without destroying the pipework.
- Make the cemented joints Degrease the bonding surfaces with cleaner, apply PVC-U cement evenly, push the pipe in as far as the stop and hold briefly. Observe the manufacturer's specifications for joining and curing times.
- Cure and fine-tune Let it cure sufficiently before the first pressurization, then set the skimmer/main-drain ratio (starting value approx. two thirds to one third) via the ball valves.
Bill of materials
Example – please verify with a specialist. Parts and quantities depend on the installation; sizes shown are examples.
| Component | Size | Art. No. | KLN No. | Qty |
|---|---|---|---|---|
| PVC-U pressure pipe with socket | d63 | 76911 | 51515805 | as needed |
| 90° Bend, solvent socket (PVC-U) | d63 | 70980 | 00000641 | as needed |
| 2-way ball valve M1, solvent socket (PVC-U) | d63 | 128815A | 51511560 | 1 |
| Union Ocean, solvent socket / solvent socket | d63 | 170403 | 51512465 | 1-2 |
| Socket, solvent socket (PVC-U) | d63 | 70740 | 51513257 | as needed |
| Griffon cleaner / degreaser | – | 1 | ||
| Griffon UNI-100 PVC-U adhesive | – | 20023 | 51512566 | 1 |
| Main drain (bottom outlet) | – | – | – | ABS/V4A built-in part for the lowest point of the pool, with an approved anti-vortex cover |
| Skimmer | – | – | – | Surface suction, with its own suction line and ball valve in parallel with the main drain |
FAQ
Does a private pool absolutely need a main drain?
It is not strictly required; a pool works with just a skimmer as well. For new construction, however, it is clearly recommended because it significantly improves mixing, floor cleaning and drainability.
Can I retrofit a main drain?
Practically no. The built-in fitting and the line sit in the pool floor; retrofitting requires opening up the floor and is rarely economical. For existing pools, a pool vacuum or robot is the alternative.
Which pipe size is right for the main-drain line?
As a rule d63 (DN50), and on small pools also d50 (DN40). The larger cross-section keeps the flow velocity low, because the main drain carries a large share of the circulation flow.
How do I set the skimmer-to-main-drain ratio?
Using one ball valve in each of the two suction lines before the pump. As a starting value, use roughly two thirds skimmer and one third main drain; when heating or in autumn the main-drain share may be higher.
Is the suction at the main drain dangerous?
Without a suitable cover there can be a suction risk. With an approved anti-vortex cover, and two drains if necessary, operation is safe. Coordinate the design and cover with a specialist company.
This guide provides general technical orientation and does not replace professional planning. Safety-, electrical- and permit-relevant work must be carried out by qualified specialists.
