It's understandable why there is confusion over "variable length", "adjustable length" and "expansion length". And the emergence of European chimney suppliers has not made it any easier.
We call it a "variable length" - in Europe it's called "adjustable length". When we call it an "adjustable length", Europeans call it an "expansion length". So to keep it simple, we will only deal with "variable" and "adjustable" lengths:
In straight runs the Variable Length serves two major functions: to make up for odd lengths of pipe in short runs, and to provide for joint sealing. It adjusts to a needed fixed exact length and its internal joint must be sealed by working sealant under the seal ring, as well as between this ring and the flange. It is not considered water and pressure tight and IT DOES NOT PROVIDE FOR THERMAL EXPANSION.
Typically speaking, the Variable Length comprises a sliding inner section, flanged at one end only. This sliding piece is sized to fit closely inside a standard pipe section. A seal ring is provided to prevent the inner section from slipping after assembly.
No known Variable Lengths are UL listed to UL103 or UL2561. Some are listed to UL1738 for condensing applications.
The Adjustable Length serves THREE major functions: to make up for odd lengths of pipe in short runs, to provide for joint sealing AND to compensate for thermal expansion between fixed points.
Adjustable Lengths have an outer pipe clam shell jacket inside the sliding inner pipe. The gasket band assembly is preassembled at shipment on the inner pipe. It only needs to be loosened for attachment to an adjacent piece of pipe and then tightened for a leak resistant fit.
No known Adjustable Lengths - other than Powerstack - are UL listed to UL103 or UL2561 for 1000°F/1400°F and 60 in.WC of pressure. Adjustable Lengths are not used for condensing applications.
Unlike other brands, the Powerstack Adjustable Length, it is considered water and pressure tight and is the ONLY one of its kind UL listed to 1400°F and 60 in.WC of pressure AND for condensing applications.
The Expansion Length is a universal solution for both high pressure / high temperature applications, such as steam boilers and generators, and low pressure / low temperature applications, such as condensing systems and CHP units.
The Powerstack Adjustable Length is a lower cost alternative to bellows for positive or negative pressure applications up to 60 in.WC at a maximum temperature of 1400°F. The Variable Length can also be used as an adjustable component on such systems when standard lengths are not suitable for the space required.
A Variable Length is always a lower cost solution for odd lengths of pipe in short runs.
Few manufacturers recommend Adjustable Lengths unless they have no other options. They are more expensive and do not provide better temperature or pressure ratings than Adjustable Lengths. They are never used for condensing applications where there is not concerns about thermal expansion.
Until now, ENERVEX has only used Adjustable Lengths. Mainly because it's a better solution with a UL listing for 1400°F and 60 in.WC of pressure and can substitute some expansion bellows. But also because it is guaranteed not to leak in condensing applications.
We recommend specifying EPS Adjustable Lengths whenever possible. No other manufacturer can meet our specifications. And we provide a 5-Year No Leakage guarantee.
However, when quoting jobs where other manufacturers are specified, we recommend using Variable Lengths, unless the customer values our 5-Year No Leakage guarantee and the fact the product is UL listed. Our Variable Lengths are NOT UL listed and are NOT covered by our 5-Year No Leakage guarantee. Only the Adjustable Lengths are.