The water heater started dripping after the sprinklers went in: closed systems, thermal expansion, and the tank a DC rowhouse needs

This call comes in a few weeks after a retrofit, and it is almost never about the sprinklers. The owner of a rowhouse in Bloomingdale or Hill East says the water heater has started weeping from the little pipe on its side, or the pressure gauge on the riser reads higher every morning than it did the night before, or a faucet spits for a second when it first opens. All three are the same thing. The house has become a closed system, and nobody put in the part that a closed system needs.

What changed when the backflow preventer went in

Before the sprinkler work, your house plumbing was open to the street. When the water heater ran, the water inside it expanded a little, and that extra volume simply pushed back through the meter toward the main. Nobody noticed because the street absorbed it.

A standalone sprinkler system in DC gets a backflow preventer, or at minimum a check valve, where it ties into the service. We wrote about why and about the annual test in the backflow testing article. Depending on where the device sits, it can close off the whole house, not only the sprinkler branch. The same thing happens when DC Water or a plumber installs a check valve at the meter, or when a pressure reducing valve goes in to tame high street pressure. Once there is a one way door between you and the main, expanding water has no exit.

What thermal expansion actually does

A 40 gallon heater warming a tank of cold District water from the 50s to 120 degrees grows that water by roughly half a gallon. In an open house that half gallon leaves. In a closed house it compresses what little air is in the lines and then starts raising pressure fast, because water does not compress. Pressure that sat at 60 psi can spike well past 100 during a long shower recovery cycle, then fall back once someone opens a tap.

The relief valve on the heater is set to open at 150 psi, so it does its job and dumps a cup of water onto the floor. That is the drip. The cycling also works on everything else in the house. Toilet fill valves start running, faucet cartridges weep, the washing machine hoses age, and the sprinkler riser gauge shows a creeping morning reading that looks like a supply problem but is not. We covered how to read that gauge in the riser gauge article, and a trapped expansion spike is one of the rows in its table.

The fix is a small tank

A thermal expansion tank is a steel cylinder about the size of a basketball with a rubber bladder inside, mounted on the cold water line near the heater. The air side is charged to match house pressure, and when the water expands it pushes into the bladder instead of into the pipes. The plumbing code requires one, or an equivalent device, wherever a backflow preventer or check valve closes a system that has a water heater on it.In a DC rowhouse it usually hangs off a tee above the heater in the cellar, takes an hour to install, and is the cheapest part of the whole sprinkler project.

Sizing follows the heater. A two gallon tank covers most 40 and 50 gallon heaters at District street pressures. A larger heater, an indirect tank off a boiler, or a house with a pressure reducing valve set high can call for the next size up. The installer sets the air charge to the actual static pressure read at a hose bib, which is why the tank should be installed after the sprinkler tie in is finished and the pressure is known, not before.

Whose job it is

Here is where owners get caught. The tank is plumbing work, so a sprinkler contractor can say it is outside the sprinkler scope. The plumber was not on the job, so the plumber never saw the backflow preventer go in. The owner is left with a drip and two trades pointing at each other. We handle it by naming the expansion tank in the sprinkler bid whenever the design closes the house, and by installing it under the same permit set, because the plumbing permit for the tie in already covers it. If you are reading a bid from someone else, the bid article lists the questions to ask, and this is one of them.

How to check whether your house is closed

1. Look at the pipe where the water enters the cellar. A backflow preventer is the brass assembly with two test cocks and a pair of shutoff handles. A single check valve is smaller and may have no handles at all.
2. Look for a pressure reducing valve, a bell shaped brass body with an adjusting screw on top, anywhere on the main.
3. Look near the water heater for a small tank on the cold line. If you see any of the devices in the first two steps and no tank, the house is closed and unprotected.
4. Hang a gauge on a hose bib overnight. If the morning reading is 20 psi or more above the evening reading, expansion is building pressure.

When the drip is something else

A relief valve that drips steadily, not in cycles, usually has a worn seat and needs replacing. A heater that drips from the bottom seam rather than the relief line has failed. And a riser gauge that reads high at every hour of the day, not only in the morning, points to street pressure, which is the pressure reducing valve articlerather than this one. The overnight gauge test sorts these out before anyone is called.

Why it matters for the sprinklers themselves

Daily pressure cycling is hard on a sprinkler system that is supposed to sit still for decades. It works the seals in the backflow preventer, which is the part most likely to fail its annual test anyway. It fatigues CPVC joints over years. And it can push a residential head past its rated pressure during a spike, which is the kind of stress that shows up as a weeping head long after everyone has forgotten about the water heater. A tank costs less than one service call, and it protects the whole house, not only the heater.

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