Ok, I thought you were just using PEX to run to and from the solar collector, not as part of the collector itself. You should use copper for that purpose, because it has much better heat transfer capability, which by the way is another reason to use PEX for hot water lines in your house as the water will stay hot longer even if the pipe isn't insulated. Plus, as a solar collector the pipe should be in firm mechanical contact with the heat absorption plate for good heat transfer, which means copper as it is easy to solder to the plate.
I am not an engineer, but about 11-12 years ago I toured the Wirsbo factory, and saw how they spent a great deal of time and money testing this stuff, including the fittings before releasing it for general plumbing purposes. They have used it very extensively for radiant floor heating, but in the U.S. this was something new for plumbing, and they last thing they wanted was another PB (polybutylene or gray plastic pipe) fiasco. In case you don't remember, the PB industry went bankrupt because of the sudden and unexpected failure of the pipe and fittings in 1000's of homes, and it was taken off the market about 8 or 9 years ago as I recall.
Based on their accelerated aging and pressure tests, etc, they expect Wirsbo PEX to have a minimum life expectancy of 100+ years. That being said, not all PEX is equal.
Essentially, there are three methods of cross-linking the molecules within the pipe to create this incredible pipe, with 2 of the methods being more expensive and one being much cheaper. Wirsbo (now called Uponor) uses the peroxide method where the cross-linking occurs as the pipe is being extruded and while it is still in a semi-solid state. The other good method is the irradiation method, where the pipe is bombarded with electrons (I think) as it is being extruded. Both these methods result in app 85% of all molecules being cross-linked. The irradiation method is primarily used (at least it used to be) on pipe made in Israel and imported into the U.S.
The less expensive method is called the silane method. I don't know how it works, but supposedly it results in only about 65% total cross-linking of the molecules, which is still far superior to any other plastic pipe on the market. This is the vast majority of the pipe that is being sold under the Quest and Vanguard and other names for home plumbing purposes, and would most likely be the pipe that is sold in the home centers.
The best example of something that isn't crosslinked is one of those common opaque plastic water cups. These cups are made of polyethelene, but aren't cross-linked. Take one of these cups, and you will find that it is very easy to tear the plastic into linear strips down the side of the cup, but it is virtually impossible to tear the strips horizontally. The reason for this is because all the molecules are lined up parrellel to each other, which makes them separate easily in one direction, but not in the other. What cross-linking does is rearrange the molecules so that instead of being parrellel they are every which way, including side to side as well as for and aft, so the polyethelene now has the same strength in all directions.
In fact, PEX is essentially the same plastic as the standard black poly water pipe that is still commonly used for underground water service. The BIG difference is in the cross-linking. This is why anytime the black poly pipe fails it always splits in line with the pipe, not across it. I have used PEX for years, and have never, ever seen it fail.
One very neat thing about pex pipe is that it can be fixed if it is kinked. Try this as a test. Kink a piece of pipe by bending it back sharply on itself. It will kink just like any other piece of pipe, and remain that way. Now take a heat gun and heat the kinked section up evenly until it becomes very soft and then let it cool. The kink will be completely gone, and the pipe will be as good as new! If it is the opaque pipe it will turn almost clear when it is soft, while the blue and red pipe just get real soft. You will notice that the pipe just gets real soft and squishy, it doesn't melt like normal plastic does. It will burn if you get it too hot, but it doesn't melt and drip like wax, which is what the PB pipe does if you heat it.
I am not an engineer, but about 11-12 years ago I toured the Wirsbo factory, and saw how they spent a great deal of time and money testing this stuff, including the fittings before releasing it for general plumbing purposes. They have used it very extensively for radiant floor heating, but in the U.S. this was something new for plumbing, and they last thing they wanted was another PB (polybutylene or gray plastic pipe) fiasco. In case you don't remember, the PB industry went bankrupt because of the sudden and unexpected failure of the pipe and fittings in 1000's of homes, and it was taken off the market about 8 or 9 years ago as I recall.
Based on their accelerated aging and pressure tests, etc, they expect Wirsbo PEX to have a minimum life expectancy of 100+ years. That being said, not all PEX is equal.
Essentially, there are three methods of cross-linking the molecules within the pipe to create this incredible pipe, with 2 of the methods being more expensive and one being much cheaper. Wirsbo (now called Uponor) uses the peroxide method where the cross-linking occurs as the pipe is being extruded and while it is still in a semi-solid state. The other good method is the irradiation method, where the pipe is bombarded with electrons (I think) as it is being extruded. Both these methods result in app 85% of all molecules being cross-linked. The irradiation method is primarily used (at least it used to be) on pipe made in Israel and imported into the U.S.
The less expensive method is called the silane method. I don't know how it works, but supposedly it results in only about 65% total cross-linking of the molecules, which is still far superior to any other plastic pipe on the market. This is the vast majority of the pipe that is being sold under the Quest and Vanguard and other names for home plumbing purposes, and would most likely be the pipe that is sold in the home centers.
The best example of something that isn't crosslinked is one of those common opaque plastic water cups. These cups are made of polyethelene, but aren't cross-linked. Take one of these cups, and you will find that it is very easy to tear the plastic into linear strips down the side of the cup, but it is virtually impossible to tear the strips horizontally. The reason for this is because all the molecules are lined up parrellel to each other, which makes them separate easily in one direction, but not in the other. What cross-linking does is rearrange the molecules so that instead of being parrellel they are every which way, including side to side as well as for and aft, so the polyethelene now has the same strength in all directions.
In fact, PEX is essentially the same plastic as the standard black poly water pipe that is still commonly used for underground water service. The BIG difference is in the cross-linking. This is why anytime the black poly pipe fails it always splits in line with the pipe, not across it. I have used PEX for years, and have never, ever seen it fail.
One very neat thing about pex pipe is that it can be fixed if it is kinked. Try this as a test. Kink a piece of pipe by bending it back sharply on itself. It will kink just like any other piece of pipe, and remain that way. Now take a heat gun and heat the kinked section up evenly until it becomes very soft and then let it cool. The kink will be completely gone, and the pipe will be as good as new! If it is the opaque pipe it will turn almost clear when it is soft, while the blue and red pipe just get real soft. You will notice that the pipe just gets real soft and squishy, it doesn't melt like normal plastic does. It will burn if you get it too hot, but it doesn't melt and drip like wax, which is what the PB pipe does if you heat it.
