Can my municipal water line siphon from my house water lines?

Question

Our local water tower is empty during this winter storm.

I heard my water heater "running" even though there were no faucets on in the house - I immediately thought I had a burst pipe.

After finding no leaks, I turned on a faucet, and there was actually suction through the spout.

Because my home is on a hill, my theory is that other citizens running their water actually drained my house lines. Is this possible? Shouldn't there be backflow valves or something to prevent this?

In addition, should I be worried about my pipes freezing now that they appear to be empty?

Do standard "winter prep" rules apply to mostly drained plumbing?

I have 40 gallon tank electric heater. I have unplugged it since discovering there was no water.

Answer 1

Yeah, it sounds like you got backsiphoned

Most houses in the US don't have any protection against their municipal water service going negative pressure and backsiphoning any garbage present at outlet points into them. Even if you have lawn irrigation, the vacuum breaker for that only prevents the irrigation system from contaminating things, not your whole water service. Likewise with hose bibbs; if your house is recent, your hose bibbs will have vacuum breakers on them, but many older houses lack these, so it was very possible that the contents of your neighbor's kiddie pool or garden sprayer got sucked into the water system. (Fortunately, most other domestic things have air gaps as part of their installation, but even those aren't foolproof due to the rise of fixtures with extensible heads.)

So, I'd turn off the water heater to avoid heating an empty tank, and make sure to flush your lines and faucets thoroughly once the water's back on, since there's no telling what could have been sucked in during the backsiphonage event. Also, listen to your water utility, as they may be putting out boil-water orders for a fair length of time after the water comes back on, or even telling you not to use it for drinking if something more...industrial has gotten into the works.

Answer 2

It should not be possible to fully drain your water tank unless you use the drain valve and tilt the unit. The lowest the water can go is the bottom of the cold water pipe. Notice the blue line I added to the image.

However if your heater is still heating then it could evaporate all of the water and cause premature death for your water heater. Notice both heating elements are above the minimum water line so you don't even need to drain all the water to kill your heating elements.

It would be wise to turn off your heater until utilities have been restored. There is no sense in wasting electricity to heat water which you cannot use especially during rolling blackouts.

Depending on how long you have stagnant water in your water heater then you should cycle out 50 gallons of water through your tank once services are restored or else you risk exposing yourself to legionnaires disease among other things. Before cycling out the water bring the temperature up to at least 140°F for a few hours, cycle the water out, and then bring the temperature back down.

I am not sure what you mean by "heard it running" but it could have been air passing through it via your aforementioned faucet suction.

If you can then turn off your main water supply line to prevent the backflow symptoms. If the water supply returns while you're not home then it could be catastrophic if you had an undetected pipe burst or if your home is below freezing temperature. Once water has been restored then slowly turn on your main supply and listen for potential leaks.