Frost Protection: Draining Pool Pipes and Fitting Unions Correctly
Frost damage to pool pipework almost always occurs where residual water cannot drain away. By allowing for drainability from the pipe-planning stage and fitting unions in the right places, you save time in winter and avoid burst pipes and components.
Frost protection begins with pipe planning, not in autumn
The widely used autumn checklist often comes too late: whether a system can be drained completely at all is decided when the pipes are laid. Plan for drainability from the very start, rather than improvising later with a bucket and wet vacuum at low-lying bends.
Three basic rules matter. First, all pipes should lead to one or a few defined low points where the water is collected and discharged. Second, separable connections belong wherever components have to be dismantled or drained separately in winter. Third, every high point without venting is a potential water pocket that bursts the pipe when it freezes.
For the usual pool circuits in d50 (DN40) and d63 (DN50), it pays to consider the pressure and suction sides separately. Both must be able to drain independently of each other in winter, because they have different low points and shut-offs.
Positioning slope and drain valves correctly
For residual water to drain on its own, the pipes need a continuous slope towards the low point. A gentle but steady slope of a few millimetres per metre is usually sufficient – what matters is that there are no counter-gradients and no dips that form water pockets. Avoid horizontal runs over longer distances.
At the low point, you fit a drain valve. A PVC-U ball valve has proven effective as a branching drain valve, tied into the pressure pipe via a tee. When open, a ball valve has a full bore through which the pipe can drain. Note, however: even in the open position, a ball valve retains residual water in the body cavity around the ball, which can freeze and burst the valve body. So do not simply leave it fully open; instead, over the winter move it to a half-open position (about 45°) so that the cavity can drain and the pressure can be relieved, align it frost-safely at the lowest point so that it drains by itself, or drain or remove it.
- Provide one defined low point with a drain valve for each circuit section.
- Position shut-off valves (skimmer, main drain, inlet nozzles) so that sections can be isolated and drained individually.
- Route buried pipes with a slope towards the plant room or a soakaway point.
The drain valve is often made one size smaller than the main pipe, for example d50 (DN40) on a d63 (DN50) pressure pipe, connected via a reducing bush.
Unions on the heat pump, heat exchanger and solar
Heat exchangers, heat-pump condensers and solar absorbers usually cannot be drained completely via the pipe slope, because water remains standing inside them. This is exactly where separable unions belong – ideally on both sides, that is, in the flow and return of the respective unit.
With a union on each side, you can loosen, tilt and empty the component in winter, or remove it entirely, without having to cut open any solvent-cemented joints. This protects precisely those frost-sensitive components such as titanium or plastic heat exchangers, which rarely survive freezing undamaged.
For the solar absorber on the roof, draining is particularly important: inlet at the bottom, outlet at the top – after shut-down, the absorber drains through the lower connection when the top is vented. Unions on both solar connections make separation easier and prevent residual water from freezing in the mats and splitting them open.
Note: Fit unions with cleanly degreased bonding surfaces and a suitable PVC-U solvent cement. The union’s O-ring seal is not cemented, so that it remains detachable.
Blowing out residual water and emptying components
Even with a good slope, residual water remains standing in bends, fittings and equipment bodies. This is most safely blown out with compressed air. Connect a compressor to a connection point via a pressure reducer and push the water out section by section towards the opened drain valves. Keep the pressure low as you do so – as a rule of thumb, not above about 1.5 bar and never at full compressor pressure, so that PVC-U cemented joints, seals and thin-walled components are not overloaded.
Safety when blowing out: Wear safety goggles and keep your face and body out of the blow-out direction of open pipe ends. Under pressure, plugs, covers, end caps or loose components can be flung away and cause injuries. Secure or remove loose closures before blowing out and increase the pressure only slowly.
The individual components are additionally drained one by one:
| Component | Winter protection measure |
|---|---|
| Filter tank | Pull the drain plug, leave the sand/glass in the tank, leave the lid open |
| Filter pump | Remove the pre-filter basket, open the drain plugs, let the pump housing drain |
| Multiport (6-way) valve | Move to the winter or intermediate position so that the spider gasket is relieved |
| Heat exchanger / heat pump | Separate via unions and drain completely |
| Solar absorber | Drain at the bottom, vent at the top, blow through with compressed air |
Leave the drain valves and the filter drain open over the winter, so that no condensation or seepage water can collect; set ball valves to a half-open position so that the body cavity also drains.
The frost monitor as an active alternative to draining
Where a system is meant to keep running through the winter – for example with covered pools or certain heat pumps in frost-protection mode – a frost monitor takes over the safeguarding. It measures the temperature and switches on the circulation pump as soon as things become critical; moving water freezes considerably more slowly than standing water. The switching threshold is usually a few degrees above freezing point.
A frost monitor is not, however, a replacement for draining, but a supplement: in the event of a power failure or pump defect, it no longer offers any protection. For free-standing outdoor pools, complete draining remains the safe solution.
Important: The electrical connection of a frost monitor and its integration into the control system belong in the hands of a qualified electrical contractor. The requirements for the protection of electrical equipment at the swimming pool are governed by standards – have the wiring, fusing and equipotential bonding carried out and inspected professionally.
Draining a pool system for frost protection
- Switch off the system and lower the water level Switch off the filter pump and heater and disconnect them from the power supply. Lower the water level below the inlet nozzles and the skimmer so that these pipes can drain.
- Shut off and separate sections Close the skimmer, main drain and inlet lines at their shut-off valves, so that the individual sections can be drained independently.
- Drain the filter and pump Remove the pre-filter basket, open the drain plugs on the pump and filter tank and let the water drain completely. Set the multiport (6-way) valve to the winter position.
- Open the drain valves at the low point Open the drain valves at the low points of the pressure and suction lines and let the pipes drain via the slope.
- Separate the heat pump and heat exchanger Loosen the unions on both sides of the heat pump or heat exchanger, drain the unit and – where possible – store it in a frost-free place.
- Drain the solar absorber Drain the solar circuit at the lower connection and vent at the top, so that the absorber mats drain completely.
- Blow out residual water Using a compressor at reduced pressure, blow the remaining water out section by section towards the open drain valves, especially in bends and fittings. Wear safety goggles and do not stand in the blow-out direction of open pipe ends.
- Document the frost protection Leave the drain valves and drains open over the winter. If a frost monitor is used, check that it works and note down the measures taken.
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 |
|---|---|---|---|---|
| 90° Tee, solvent socket, equal (PVC-U) | d63 | 72632 | 51513925 | 1 |
| Reducing bush (PVC-U) | d63 | 1 | ||
| 2-way ball valve M1, solvent socket (PVC-U) | d50 | 128814A | 51511559 | 1 |
| Union Ocean, solvent socket / solvent socket | d63 | 170403 | 51512465 | 2 |
| Union Ocean, solvent socket / solvent socket | d50 | 170397 | 51512461 | 2 |
| Griffon cleaner / degreaser | – | 1 | ||
| Griffon UNI-100 PVC-U adhesive | – | 20023 | 51512566 | 1 |
| Frost thermostat / frost protection thermostat | – | – | – | Switches on circulation a few degrees above 0 °C; connection by a qualified electrician |
| Compressed-air compressor | – | – | – | For blowing out residual water in sections without sufficient fall; use with a pressure reducer and keep pressure low |
FAQ
Is it enough just to cover the pool in winter?
No. A cover protects the water surface, but not the pipework and equipment. All water-carrying pipes and frost-sensitive components must be drained or actively protected against frost.
At what temperature does it become critical for the pipes?
It becomes dangerous as soon as the water in unprotected pipes freezes, that is, just below 0 °C. Frost monitors therefore usually switch on the circulation a few degrees above freezing point to have a safety margin.
Can I simply add antifreeze instead of draining?
This is not advisable with pool water. Antifreeze does not belong in the pool water and must be disposed of properly. The clean solution is to drain the pipes and separate the equipment via unions.
How do I get water out of sections without a slope?
You blow out such sections with compressed air at reduced pressure towards the nearest open drain valve. This is why water pockets without a drainage option should be avoided as early as the pipework stage.
Why fit unions on both sides of the heat pump and solar?
With a union in the flow and return, the unit can be loosened, tilted and drained completely in winter without having to cut open solvent-cemented joints. This protects the frost-sensitive heat exchangers and absorbers.
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.
