Water hammer causing black water in sink/toilet/shower?


I installed a sprinkler timer on a silcock outside the house, to irrigate some new grass seed I planted. When the timer turns the water on, there is a brief episode of water hammer before the water starts to flow.
Ever since I put the timer on the spigot, there's been black water in the bathroom fixtures. At first I thought it was a disintegrating rubber washer in the toilet. But the black water has come out of the sink and shower too.
The black water goes away after a few seconds.
I imagine I need to flush my hot water heater. But does anyone know why this might be happening?
We are on city water. The timer had previously been installed on another outdoor spigot, and this had never happened.
What to do?
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On Sun, 22 Aug 2010 19:29:49 -0700 (PDT), Bryan Scholtes

It sounds like a coincidence but that still doesn't explain what's happening. Or maybe the hammer is freeing up dirt somwehere in the pipes. How old is your house.

I hear that Blackwater has changed its name to Z, or V. Do you live in Z or V?

You get water hammer when it turns ON? Not when it turns off? That doesn't make sense to me. Maybe it's just the water pipe moving backwards as the water moves forwards, if so it seems to me the problem would most likely be in the last pipe before the sillcock. Put your hand on the pipe while someone advances the timer or manually turns the water on.
Anyhow if it's really water hammer, they sell things that look like fat brass test tube, 3 or 4 inches long, in the middle of two female garden faucet connectors, a total of two or three inches long. I have one on the cold or hot water inlet of my washing machine, because when I got steel clad hoss to the washing machine, and didn't have mere rubber anymore, the steel kept the rubber from expanding and absorbing the shock. Maybe you could put one on the silcock just before the timer valve, and they also make them to be sweated to copper pipe.
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XE to be precise.
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*Is there an anti-siphon valve on the sprinkler timer?
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I am going to try Sioux Chief's add-on water hammer attachment for spigots. I've also ordered a new timer with a ball valve to try and avoid water hammer in the first place.
We'll see if the black water comes back - I haven't used the home plumbing today.
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The black water is back - I flushed the hot water heater and it was full of black sediment. And because I was turning the water supply to the heater on and off (to loosen sediment) my whole house was treated to another round of black water.
I disconnected the timer from the outside. My best theory is that water hammer was causing black water... but I'll never know, I suppose. Everyone seems to have an answer for black hot water, but there is no other explanation on the Net for black cold water.
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