Tape is Tape NOT!

I must admit that when Glenn Walter of Shurtape offered to do a presentation on duct tape application for my class I thought “Oh HOW exciting!” (Yawn) After all, how much can there be to know about duct tape? It can’t even connect to the internet! But then I really hate to turn away help and I like having people from the industry come talk to my students, so we set up a class. One thing I have come to understand about life in general: whenever I think there is nothing to know about a particular subject, it is only because I know so little I don’t understand how ignorant I truly am. When Glen started his presentation I quickly realized that there is whole lot to know about duct tape. What follows is a cliff notes version.

Duct tape consists of a backing and an adhesive joined together. There are four types of backing: cloth (the old standard), film, foil, and foil-skrim-kraft (FSK).  There are two large categories of adhesives: acrylic and rubber based. Most techs know that codes require UL 181 tape. However, did you know that there are two general categories of UL 181 tape: 181-AP and 181-FX.  AP is rated for use on rigid ductboard and FX is rated for use with flex. Note that there is no specific UL rating for tape on metal duct systems – UL 181 is for ducts made out of manufactured materials. You may have also seen tape with a UL 723 rating, which is a smoke and flame spread standard. It makes a difference WHICH UL rating your tape has. Just because you see UL does NOT necessarily mean your tape is correct for the job. Even two tapes with the same UL rating can have different application specifics. Also note that the UL rating is not based on the backing or adhesive, but the tape performance in specific tests. Shurtape has 181-FX tapes with foil, film, and cloth backing with both acrylic and rubber adhesive. They are all approved for use with flex duct, but each has a specific application where it is better. For example Shurtape’s film backed tape with acrylic adhesive is rated for application between -20°F and 220°F. Their other FX rated tapes won’t stick at -20°F.

Most codes now require mastic for sealing metal ducts, but many people still use tape underneath the mastic. Did you know that many duct tapes are not rated for that use? Mastic does not stick well to film tapes. Not all cloth tapes play well with mastic because the chemicals in the mastic and the chemicals in the tape adhesive may not be compatible. The cloth tape might turn loose, then having the tape underneath the mastic just creates a place where mastic is not fully adhered to the duct. Foil tape usually does fine under mastic.  You should check with the tape manufacturer before using a tape underneath mastic.

Finally, the tape application is not complete until you apply pressure. For foil backed tapes, a plastic tool with a stiff rounded edge is preferable for working the tape into the duct. The pressure is needed to fully activate the adhesive on the tape. The adhesive undergoes a chemical process, similar to glue setting. Duct tape is not fully adhered until this process has been started through pressure application and completed with time. Believe me, there is a LOT to know about duct tape. If you would like to learn more, contact Glenn Walter of Shurtape at gwalter@shurtape.com

Check System External Static BEFORE Sealing Ducts

I recently read a thought provoking article by David Richardson in the July 7 Air Conditioning Heating and Refrigeration News. In it he argues that sealing an undersized duct system can cause problems. The system cooled BEFORE you sealed the ducts, and now that you have worked on it, the system cannot move enough air to stay operating. I know I have seen many systems with marginal ductwork that still managed to operate, even if inefficiently. The duct leaks could be allowing just enough extra air flow to keep the system operating. The combination of leaks on both the return and supply sides of the system serve to reduce the static pressure difference against which the fan must move the air. If the static pressure difference between the return and supply is already at or past the limit the manufacturer publishes for their system, sealing the ducts will increase the already high static pressure difference and push the system over the edge. This is why Mr. Richardson advises taking a reading of the total external static pressure difference BEFORE sealing the duct to avoid this trap. If it is too high, the duct system will need more attention than just sealing – it will need some duct modifications as well. Better to know before you do the job. What is too high? I don’t like to see anything higher than 0.8” wc – that is usually the top end on most residential systems. Most residential systems will operate comfortably around 0.5” wc. Some will go all the way to 1” wc, but at a cost. With ECM fan motors you are burning electricity to shove air through restrictive ductwork. With PSC blowers you lose airflow at high external static pressures. This costs electricity in another way. The suction pressure drops, the compression ratio increases, and system capacity decreases – causing extended run times and inefficient operation.

Southface

The Georgia State Consortium of Air Conditioning Instructors met on October 25 at Southface in Atlanta. Southface is a non-profit agency devoted to energy efficiency and sustainability in buildings and communities. In keeping with their ideals for sustainability, Southface does not allow bottled water, box lunches, or  syrofoam cups in their facilities. I must admit, I was a little put out with all the rules. “Who do these people think they are?” But then I realized, they are actually practicing what they preach. They know who they are. A sponsor, Technical Training Aids, graciously provided our lunch wthin the confines of all the rules and it was great. We gathered there to hear about what Southface could offer and tour their facilities. They provided an overview of the new residential energy efficiency code and its application in Georgia. All new residential construction in Georgia is required to pass a blower door test and all new duct systems must pass a duct blaster test for tightness. Those two requirements are the big ones for HVACR contractors. Further, the people performing the test must be state certified to perform it. They get certified by taking a relatively short course that lasts less than a week and then passing a performance test. So at least in Georgia, installing tight duct systems is no longer an “add on” for the better homes, but a requirement. Although there will certainly be many contractors that will regard all this as a pain, at least one in Athens sees it as an opportunity. Anytime he sells a high efficiency changeout, he quotes a duct seal and wrap upgrade. A seal and wrap consists of removing the old duct insulation, sealing the duct, re-installing the old insulation, and insulating over the old insulation. The duct blaster is used to measure the duct leakage before and after the job to document the improvement. So even if you don’t work in Georgia, you might consider promoting duct sealing and insulation. If your competitors don’t do it, that is a way to differentiate yourself from them. Most homeowners can easily understand that leaky ducts cost them money. For many systems, sealing and insulating the ducts will improve system efficiency even more than installing a new high SEER system.