The contractor started smiling as the homeowner and I were talking in the basement. The homeowner had commented about some tank in his daughter’s bedroom closet. The contractor could see my interest peaking. I was geeking out because you don’t run across the ancient hot water jobs much anymore. He enjoys it when I get that way.
This house had a classic gravity system that was installed before there were circulating pumps in the piping and expansion tanks in the basement. The huge piping was all about minimal pressure drop for the water moving to and from the huge cast iron radiators. There were two supply mains lines off the top of the boiler that each split to two supply branch lines with the old school tee (shown in photo A, above). Ask for one of those at your local supply house.
Of course, the tank upstairs in the closet was the original version of an expansion tank (see photo B), but also established the static fill pressure for the system. The purpose of static fill pressure is making sure all the radiation is full of water. In the oldest systems, if water is in the tank above all the radiators, then the radiators are full of it.
However, the kid that I’m training to take my place when I retire, who will be referred to from now on as “the kid”, wasn’t on this job. He’s from northern Kentucky, and like his brethren from the Commonwealth, besides bleeding blue for UK basketball, they don’t like to cross over any of the Ohio River bridges. People from northern Kentucky get nervous when they are north of the Mason-Dixon Line, like they may not make it back to their beloved blue grass. People from Cincinnati don’t seem to notice when they are south of the river, but we have to be careful ordering iced tea. If you don’t specify, you get sweet tea. I like my tea straight up.
Back to the teaching point about static pressure. It is what I call a principal principle, meaning it is a basic law that is foremost in importance. I borrow this from the Keystone species theory in biology, which I learned from watching PBS. It basically states that certain species have a disproportionately large effect on its ecosystem relative to its abundance. Without starfish, tidal pools are barren. With them, tidal pools thrive. To turn it to wet heat: wrong static pressure = lousy heat.
To say it another way, static pressure is the first thing to check. Without establishing the correct static fill pressure, you are potentially spending time troubleshooting problems that aren’t there. Believe me, I know this from personal experience. Nothing like being the guru on the job, only to have the service tech mention an hour into looking all over the apartment building for a reason why the third floor wasn’t heating, that the gauge was only reading about eight pounds. Since that day I have preached the gospel of static fill pressure.
This is how I explain it in class. Figure 1 shows how one pound of pressure will lift water 2.31 feet, or to flip it around, a column of water 2.31 feet high exerts one pound of pressure at the bottom. We usually have radiation on the second floor. So, figure 2 shows how twelve pounds of pressure, the typical setting for a fill valve, lifts water 27.72 feet. Figure 3 shows how that 12 pounds of fill pressure lifts water into the second-floor radiator, with about 4 pounds to spare. If there is radiation on a third floor, increase the cold fill pressure to 16 pounds.
Thirty-five years ago, I thought the whole system was at 12 pounds. I was using the logic from a previous job, changing tires. If there was 75 pounds of pressure at the air compressor, there was 75 pounds of pressure at the impact gun. It took me awhile to realize 12 pounds at the boiler only equaled 4 pounds or so at the top radiator. At the guru looking like an idiot job, the eight pounds of static fill pressure only lifted the water 18.48 feet up in the building, which was enough to get water to the second-floor radiation of that apartment building, but not enough to get to the third-floor radiation.
If you’re ever unsure about the accuracy of your gauge, go to the highest radiation in the system and open the bleeder. Water should squirt out with some force. If it doesn’t, adjust the static pressure up with the pressure reducing (fill) valve. If it doesn’t adjust up, it is probably plugged and needs to be replaced.
Next month, you will learn about a local contactor’s approach to technician training. I don’t think many contractors take it to this level. What is the first thing to check on a hot water heating system?
Patrick Linhardt is a thirty-five-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.