Recently, I had the misfortune to see three mistakes about two-pipe systems in the same week. One was in a trade magazine where they show the use of a two-pipe system on a new install. Using a two-pipe system in itself wasn’t the mistake, but rather it was that it showed the use of an automatic, spring-loaded, thermal valve (Firomatic) on the return. In addition, there was a duplication of valves, which is also not a good thing since you truly can overdo a good thing.
The only purpose of a two-pipe system is to make a pump “self-priming;” that’s what every textbook and pump manual says. It won’t allow for more vacuum or any other fantasy, old wive's tale or urban legend that continues to circulate through this industry. Why is it that we believe these word-of-mouth legends and ignore the printed word?
The fact that a syphon is one of the most powerful laws of physics is constantly ignored in the oil business when using two-pipe systems. These unnecessary systems lead to higher installation costs and greater risks from oil line leaks. In addition, if you’ve read my article on Total Gear Suction Capacity (TGSC), you also know that a two-pipe system may have also made your filter too small for the job. If you’re not lifting oil, Figure 1, and running a vacuum at the pump, you just don’t need a return line— period, FACT!
You should never put any valve on a return line, it's just not needed. I have seen people put check valves, hand valves and every kind of valve on return lines, and to me, it's like someone standing there saying, “Shoot me, I'm stupid and don't have a clue of what I'm doing!” First of all, you must understand that all return lines have pressure on them, anywhere from 2 to 10 psi, to push the oil back to the tank. So you don't need to worry about “back-flow” and air because the pump will push through it and send the oil back to the tank anyway.
The second stupid excuse I hear about valves in a return is to service the pump. If you can't stop the flow of oil from the return, it's because it's a gravity job. The syphon is working and proves you don't need the valve or the return line.
The second mistake I saw was on a job that I was asked to look at by a major client. It’s always very discouraging and sad for me to see mistakes made by people who should be setting the bar, not lowering it. Oil heat is supposed to be the best, and we, as oil heat technicians, are expected to do it right 100 percent of the time, everytime.
On this particular job there is a 1,000 gallon tank sitting in the basement four feet over the burner. In addition, a part of the oil line ran underground and there was no oil safety valve (OSV). The burner was equipped with a single-stage pump, and of course, you’ve probably figured out my complaint—the second pipe (return) isn’t needed. If you have a 1,000 gallons of oil four feet over the burner, it’s gravity—pure, simple, an undisputed FACT!
It’s important to note that any portion of a line that is overhead does not need gravity protection from the OSV. But, an OSV protects against the head pressure of the oil traveling down the vertical column and acts as a dynamic pressure reducing valve, and that's where the PRV in PRV38 comes from. In addition, the part of the line that is buried does require that the buried line be protected by an OSV. So in this case, it's doing double duty.
In the course of the years, I have seen horrendous oil leaks caused by leaky return lines and the damage can be catastrophic. Recently, in fact, I just took on a case where the house was torn down due to a return line screwup. Here, they forgot to disconnect the return line. They pulled an underground storage tank and put in a new aboveground one. When they installed the new tank, they forgot to disconnect the oil line. When will we ever learn or can we? All I have to say about it is enough; let's do away with as many two-pipe systems as possible.
*George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. He can be reached at Firedagon Academy, 608 Moose Hill Rd., Leicester, Mass. His Web site is www.FiredragonEnt.com and his phone number is 508-421-3490.