Understanding & Making the Right Choice of Swimming Pool Pipe & Fittings for Your DIY Swimming Pool.

BSP (British Standard Pipe) sizes do not represent what it says on the tin. If you have come from a previous FAQ we have published Understanding BSP Threads You will have quickly realised they do not represent the actual imperial measurement they are stated in. A standard domestic pressure pipe used in the vast number of UK swimming pools is stated as BSP 1.5″ but actually measures across its outside diameter  (OD) near enough 2 inches (48.3mm accurately measured in metric). BSP refers to the old measuring of the internal diameter (ID) and what would be referred to as its nominal flow rate in that size of steel pipe.

From our own help desk conversations many clients make the mistake of stating they have 2″ pipe on their installation when they placed a ruler and with Rack of Eye (Rock of Eye) guessed the measurement as 2″/50mm. When 1.5″ BSP swimming pool pipe is 48.3mm a close call for the inexperienced eye.

We at Pool Experts will always recommend swimming pool pipe & fittings manufactured to BS/EN1452. The 3 main classes of swimming pool pipe are class C-9bar, class D-12bar and class E-15bar. Although class C at 9bar is way above the standard running pressure of most swimming pool equipment 1 to 2.5bar maximum, because of the slightly thicker wall in the class D and not a large difference in price C to D we choose class D as our go-to product.  Other advantages to follow**. We do however supply Class C as some lighting products and return jet assemblies friction fit better in the thinner wall of Class C.

**Another reason for Pool Experts choosing class D is the term “Derating”. This is where the pressure rating of uPVC pipe reduces with the temperature of the water being pumped. Not to go into too much technical details but basically the heat adversely affects the chemical composition of uPVC and can weaken it structurally. This can also affect joints and although chemically bonded to each other, the affects of the cold and warm water cycle in the expansion and contraction can be a real issue in excessive water temperature. The main 12bar rating is measured at 20ºC. For a water temp of 30ºC the derating value is 0.8 so the pressure rating reduces to 8.8bar. Similarly for 40ºC you can expect a derating value of 0.6. The higher the temp the higher the Derating.

For 1.5″ class D rigid swimming pool pipe we would recommend a maximum flow rate of 11m³/hr with a flow rate of around 1.5m/sec. Class D 1.5″ pipe can be pushed upwards to around 17m³/hr but be warned this higher velocity creates higher friction loss and increased head with unacceptable noise and the potential of equipment damage if maintained.  Some consideration needs to be given if you have a long run on the suction side of your installation, i.e. the pipework before your pump, the shorter the better. The 11m³/hr quoted above should be reduced to around 8m³/hr if you cannot avoid the long run. You can also consider using 2″ pipe for this section of pipework. The pipework is under negative pressure and therefore creates turbulence that can increase inefficiencies and noise that go hand in hand.

The same goes for 2″ class D, with a desired maximum flow rate of 17m³/hr and a flow rate of around 2.0m/sec, this pipe can be pushed to 23m³/hr but with the same consequences.

There is a flexible version of uPVC pipe that is rated around 6bar. The rules if Derating still apply to this product and because of this lower pressure rating we do not recommend its use for long runs of pipe or as the first choice for inground installations.  Another main disadvantages are joints can be very problematic, they need careful preparation and once welded even with quick drying cement need to be immediately left and not moved as this can create voids in the weld leading to premature failure. A further disadvantage is in the way the pipe is manufactured. The internal surface of the pipe normally has a spiral construction described as Striations, not as pronounced as the outer surface but still there. The uneven internal surface of the flexi pipe creates turbulence within the water flow and can create quite heavy inefficiencies. You want laminar flow, this is the most efficient way to move water. This is why rigid pipe is more often than not the best option by far for swimming pool construction.

Advantages are, short runs to join mismatched alignment or repairs when it is impractical to get a straight piece of pipe correctly located. Top Tip, where a small alignment is necessary consider using rigid pipe with heat applied to bend into position.

uPVC (PVC-U) Class D normally is supplied grey in colour, not white and unless stated this often contains a mixture of uPVC and ABS plastics. In our experience uPVC offers a little more flex and reliability over the ABS versions if buried and any ground movement was to occur in your installation. To minimise any risk of cracks and splits occurring Class D swimming pool pipe manufactured from uPVC only should be used if buried or entombed in concrete bases and walls.

You will find many companies out there selling different classes C, D, E etc with PN specification thrown in for good measure. PN stands for Pressure Nominal (derived from the French Pression Nominale) PN10 is a 10 bar rated pipe, PN16 is 16 bar and so on. If manufactured from uPVC all the same rules apply.

British Standard Pipe Thread Sizes, BSP Swimming Pool Pipe

When ordering pipe for UK pools, a new construction or even the refurbishment of a pool constructed 30/40 years ago, unless you have an accurate measurement that states otherwise, swimming pool pipe should always be ordered in Imperial measurements. Standard UK swimming pool pipe for domestic installations is 1.5″ and 2″ where a larger flow rate is required. The specification for Swimming pool pipe used in the vast majority of UK pools (BS/EN1452) can be found on one of our previous articles BSP Swimming Pool Pipe. 

BSP stands for British Standard Pipe, developed from when Britannia Ruled the Waves in manufacturing terms. BSP pipe and thread sizes bear no resemblance to the imperial measurement they are described in. BSP (British Standard Pipe) size refers to the nominal bore diameter of steel pipe coming out of British manufacturing from all them years ago. In swimming pool terms a standard pipe diameter is described at 1.5″ BSP, the outside diameter (OD) of the pipe is actually 48.3mm nowhere near its description size of 1.5 inch. The other main pipe size for domestic swimming pool pipe is 2″ represented by an actual OD size of 60.3mm.

From our own help desk conversations many clients make the mistake of stating they have 2″ pipe on their installation when they placed a ruler and with Rack of Eye (Rock of Eye) guessed the measurement as 2″/50mm. When 1.5″ BSP swimming pool pipe is 48.3mm a close call for the inexperienced eye.

Confusing to say the least but there are many conversion fittings available when the pool equipment being fitted comes with metric pipe or a metric thread set. These days this is particularly apparent for the fittings supplied on a good number of heat pumps. With a short length of 50mm pool pipe, you can normally fit a simple 1.5″BSP/50mm socket to socket on a solvent weld to convert onto what is used in most UK swimming pools.

Understanding BSP Pipe Threads

In swimming pool installations, the most common type is the parallel BSP thread, sometimes referred to as BSPm that is the male type with threads on the outside of the pipe and BSPf the female versions with threads on the inside that the male fits into. With this design, the thread maintains a consistent diameter along its entire length. Very occasionally tapered BSP threads, known as BSPT, are also used, particularly in metal pipework. Unlike parallel threads, tapered threads gradually reduce in diameter towards the end. There is a misconception that PTFE tape should be used on every type of this thread. NO, only the tapered versions, more on this later **

BSP thread sizes are specified in imperial measurements, such as ½”, ¾”, 1”, and upwards. This can be confusing in countries where pipework is measured in metric units. However, many metric swimming pool pipe installation still use imperial thread sizes for their connections. For example, a fitting designed for 50mm pipe may correctly have a 1½” BSP thread. Although this may appear unusual, it is standard practice.

BSP threads have a flank angle of 55 degrees. This is the angle formed between the two sides of the V-shaped thread profile. To establish a seal on a male to female fitting because the threads do not perform a seal in their own right, an appropriate sealing component, such as an O-ring, a flat washer or a bonded sealing washer, is usually necessary. Without a suitable sealing arrangement, BSP threads alone will not normally provide a reliable watertight joint.

**Tapered BSPT threads work differently because the taper allows the mating threads to tighten against one another and form a seal. A small amount of PTFE tape or another suitable thread-sealing compound is generally applied to help create a secure, leak-resistant connection. Top tip, do not overtighten BSPT or apply large multiple layers of PTFE tape, it is so easily done cracking the outer plastic of the thread housing. For some really technical stuff you can read the wikipedia page on this subject.

BSP and NPT Threads: Why They Are Not Interchangeable

NPT is a tapered thread system, whereas BSPm and BSPf are parallel. Although both standards use imperial size designations, their thread profiles and specifications differ. Consequently, they should not be treated as interchangeable. The difference in thread design makes it difficult to achieve a dependable seal when the two are connected. Attempting to compensate by tightening the fittings excessively can damage the threads or crack the plastic components.

For this reason, BSP and NPT fittings should not be mixed. Always identify the thread standard correctly and use compatible fittings and sealing components to ensure a safe, reliable connection.

Swimming Pool Covers

In most cases, yes — covering your pool is strongly recommended. There are two key reasons why pool owners choose to use a pool cover: safety and heat retention.

1. Safety Around Children and Pets

If you have children, pets, or frequent visitors, covering your pool is an important safety precaution. An uncovered pool can present a significant risk, particularly for young children who may be curious about the water.

A properly fitted pool cover acts as a physical barrier that helps prevent accidental falls into the pool when it is not in use. Many covers are specifically designed with safety features that can support weight and reduce the likelihood of accidents. While a cover should never replace supervision, it adds an extra layer of protection and peace of mind for families.

2. Reducing Heat Loss

A swimming pool cover with good thermal retention is another major factor. Pools lose a large portion of their heat through evaporation, especially overnight or during cooler weather. By covering your pool when it’s not being used, you significantly reduce this heat loss.

This provides several benefits:

• Lower heating costs because the water stays warmer for longer

• Improved energy efficiency for heated pools

• More comfortable swimming temperatures when you return to the pool

Even simple covers can make a noticeable difference in maintaining water temperature. With an indoor pool a full cover can reduce condensation on windows, walls and the ceiling by up to 90%

Additional Benefits of Pool Covers

Beyond safety and heat retention, pool covers can also provide other advantages:

• Keeps debris out, reducing cleaning time

• Reduces water evaporation, saving water and chemicals

• Helps maintain water balance, meaning fewer chemical adjustments

Summary

Covering your pool is a simple step that can improve safety, energy efficiency, and overall pool maintenance. For most pool owners, a pool cover is a worthwhile investment that helps protect both people and the pool itself, and a good 3rd your pocket….

Pool Covers.  A must, keep the heat in and debris out. Covers4pools For pool covers, cover rollers and liners, ring for a quote ask for Chris Benson 01904 236344 Mention this is an enquiry from Pool Experts Bolton and for your own DIY build