Only Expansion Tanks in the Building

It started with a typical call and a typical request from a favorite contractor. He wanted pricing on a pair of high efficiency boilers which is pretty routine, but in this case would be replacing a pair of 16-year-old high efficiency boilers, which isn’t very routine. One of the existing was having a few issues, and the homeowner wanted to replace both, since she could afford to.

We were wrapping up the call when he said that his service tech couldn’t find the expansion tank. Since it is a hot water job, it has to have some type of expansion tank. When water in a closed loop hot water heating system heats up, it increases in volume. In rough terms, the volume increases approximately 2.5% for a 100-degree F temperature rise.

If you start at an ambient temperature of 70 degrees F in a system that holds 100 gallons and raise it to 170 degrees F on a cold day, the volume of water increases to 102.5 gallons. If the expansion tank is there it accepts that extra volume. If it’s not there—or waterlogged, or undersized—the system pressure increases to the point that the relief valve opens. Since it’s a safety, we don’t want that happening on a regular basis.

Hydronic Mystery

Modern hot water systems use expansion tanks that are usually, but not always, in the boiler room. Since the job had high efficiency boilers already, I figured the expansion tank would be in the boiler room. But the contractor’s tech said it wasn’t there. Now I was interested and had a mystery to solve, just like a good mystery podcast, or a TV series based on a mystery podcast like Only Murders in the Building.

Steve Martin and Martin Short are some of my favorite comedic actors and star in the recent series (hence the title of this column). Podcasts and crime dramas always start with some obvious suspects that seem to be the culprit… until they aren’t. Not until the end do you find out the murder was done by someone you never suspected.

The Usual Suspects

The first usual suspect in the case of a missing expansion tank is an open tank above the highest radiator in the house, which could be the attic. The earliest examples of hot water heating systems used this technique. As the volume increased because of temperature rise, the water level of the open tank would also rise. If the coal fire got out of control and water temp went way up, the tank had an overflow that sometimes went out through the roof to the gutter, or down a pipe to the basement laundry tub.

Seems pretty simple, but works quite well. Unfortunately, homeowners didn’t like forty gallons of water up in the attic, because when the tank eventually leaked or ruptured, it made quite a mess of the lower floors. The tank moved to the boiler room in the basement soon after the turn of the previous century.

But not all the tanks. One of the things I look for on a hot water job is the expansion tank and I didn’t see one recently. It was a six-story apartment building built mid-century with the boilers in the basement. I asked the tech if he knew where it was. He told me that he had never seen one like it before and that it was in the attic. I wasn’t expecting it to be up there.

Hot on the Trail

The contractor and I decided to meet on the job to look at the piping for the new boilers and look for an expansion tank of some kind somewhere in the house. He took me to an outside entrance to what looked like the basement of the house. However, this boiler room was a fraction of the size of the house, had the two high efficiency boilers but no expansion tank.

The two boilers were hung on the wall near the door, while the piping to the various zones was across the room in the opposite corner. There were also two steel doors up on different walls for access to two different crawl spaces. Maybe the expansion tank would be in one of the crawl spaces…

These are two of the cleanest, driest crawl spaces I have seen, but there was no expansion tank in sight. We took a closer look at the piping to the various zones. It was a two-temperature system, using a three-way mixing valve to lower the water temperature going to a radiant floor zone. We could match a supply line to a return line for the various zones based on the pipe size except for one ¾” line that ran through a wall into one of the crawl spaces. It fed off the supply manifold.

I swung the steel door open and climbed into the crawl space with the ¾” line to investigate. None of the pipes were insulated, so I naturally put my hand on them. (Heating guys always feel the pipes. We can’t help ourselves.) The zone pipes were warm to the touch, but the mystery ¾” line, which fed off of the same supply header as the zone piping, was much cooler.

Either it fed a zone that hadn’t called for heat for a while, or it could be the connection to an expansion tank. I crawled along for twenty feet until the pipe disappeared through another wall. I reached a dead end. The wall seemed to be part of the original house, so I crawled back out to report the findings so far.

We had to get to the other side of that wall and see if that ¾” line would lead us to our missing expansion tank. Unfortunately, the homeowner wasn’t home. We would have to come back another day. So just like a mystery podcast, this concludes this episode and you will have to wait until next month to find out if this suspect is the culprit, or just another distraction to prolong the story.

Patrick Linhardt is a thirty-seven-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.