Pre-insulated Pipe

“Dr. Ben,
I like that you maintain the mindset that drainback is the best method for solar thermal systems.  What are your thought’s on using pre-insulated stainless steal corrugated pipe in drainback systems?  Does the corrugation create too much resistance and strain on the pump?  I know there is an issue of ensuring there is no sagging in the pipe and there is more time associated with ensuring that but the time saving of not having to cut, clean and sweat copper is huge.

What methods/products do you recommend for the exterior pipe?  As you have stated in your blogs, one of the big advantages to db is the reduction of continual service/maintenance.  I want to make sure that the insulation will last the test of time.”

Nate Pembleton, Owner
Asheville Solar Company

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Nate,

Good to hear from you.

Your question about pre insulated pipe has two answers.  First, I don’t care whether the pipe is SS or copper.  I know SS is very popular in Europe – that’s where this product comes from.

However, there are two problems with this product.

First is the insulation is the soft foam type.  It can melt and become a sticky goo on the hot return pipe of a DB system that produces steam occasionally.  I used to have a 1 ft section of copper pipe with rubatex melted all over it.  You couldn’t scrape it off with a knife.

I don’t know of any soft foam insulation that I can recommend.  I recommed isocyanurate (ISO) foam insulation, 1″ thick for solar water lines, with an aluminum jacket outside and the default paper jacket inside.  Aluminum is the only outside jacket that will last forever (SS will, but very expensive).  Fiber glass or mineral wool insulation of the same R-value is also acceptable.

Second is the flexible nature of the SS pipe.  It is not really that different from the soft copper coils that installers used in the ’70s and ’80s to get away from straight pipe.  The problem occurs when the pipe must be run any distance horizontally.  It will sag and cause a trap for the collector lines.  Installers have tried using wire hangers every several feet, and one used a 2 x 4 support to keep the lines from sagging.

Some dealers used a hybrid – soft copper pipe on the near vertical runs, and hard copper where sagging could be a problem.  You might be able to get away with rubbery insulation on the collector supply line up to the roof, then do the ISO outside.  I would do the return line all the way to the tank with ISO.

The better insulation pretty much kills the preinsulated piping idea, since it is the wrong insulation.

If you want a really trouble free system, the extra time, money and effort getting the piping and insulation right is worth the effort.

Rubbery insulation will rot and crack under ultraviolet and shrink away from the collector sensor, causing no end of control problems.  You get into a “it ain’t broke, but don’t work” dog-chasing-tail situation.

I recommend the expensive, hard route.  The trick is to make that a “feature” when you sell it to the customer.

Good luck!
Dr. Ben