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Why do solar fittings leak?

Copper Fittings


Threaded Fittings commonly found in the North American Market in conventional plumbing stores are NPT (National Pipe Tapered) threads while Solar Thermal Hydronic Fittings are BSPT (British Standard Pipe Thread)) threaded. For the naked eye, NPT and BSPT threads looks the same which can create confusion among DIY installers and even professional plumbers and pipe fitters.

What is NPT Thread?


NPT (National Pipe Thread) seals are the most popular type of seal for pressure calibration systems in the U.S. and Canada. NPT male adapters have a taper thread that wedges into the female NPT adapter. They seal due to the "out of roundness principle" which means that the male stretches the female fitting until there is so much force that the connection can hold pressure. One of the challenges with this design is that if you connect stainless steel to stainless steel then over-tightening or poor lubrication can cause gauling and damage to the threads. Thread sealant is needed to seal but only 2 turns of thread sealant is required. Any more than that and the seal can leak around the thread sealant.

What is BSPT Thread?

BSPT (British Standard Pipe Thread) is similar to NPT except there are important differences. The angle across the flanks of threads (if you sliced the fitting in half long-ways and measured the angle from root to crest to root) is 55 degrees instead of 60 degrees as it is for NPT. Another important difference is that for many BSPT pipe sizes the thread pitch is different than NPT. Thus an NPT male will sometimes fit into a BSPT fitting or vice versa but they will not seal. This is a popular fitting in Europe, China and Japan but is very rarely used in North America except for solar water heating applications. Thread sealant (such as Teflon Tape or others) is needed to seal the male and female fitting together.


What is the Canadian and Solar Water Heating industry using BSPT fittings and Not NPT ones?

According to CANSIA 2017 Report, the market value of solar thermal heating industry in Canada (Hydronic and Air) was 4.1 Millions Canadian Dollars.

Solar Industry revenues

Solar thermal industry historical revenues in Canada (source CANSIA  2017 Report)

Even when the solar thermal Canadian industry was at its peak, the 37.4 Millions CAN$ market value in 2010 remains a fraction of the European or Chinese Solar Thermal Industry. Factories in Europe, India and China producing Brass or Stainless Steel fittings for the Solar Water Heating industry are not willing to produce NPT threaded fittings for a small market as the Canadian Market. 

Number 1 Reason for Leaking Solar Fittings: Forcing a BSPT Solar Threaded Connection to an NPT one

Many DIY Solar water heating installers and even some professionals assume that fittings they are buying along with their solar water heating kit have common NPT thread (like the thread we find at conventional plumbing stores). BSPT threaded solar fittings connect well with other BSPT solar fittings. The problem occurs when for example connecting a flexible solar Stainless steel solar pipe with BSPT threaded connection to an existing water tank that has NPT connection.

Also some online sellers and online shops do not mention on their Solar Fittings products pages and specifications that thread in BSPT and not NPT, adding more confusion to customers.

Before Making any purchase identify which fitting is BSPT and which one is NPT and most importantly when intending to connect BSPT to NPT fittings, use special NPT to BSPT adapters.

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