I have a Bosch Tankless that is about 7 years old. It has been "ok" but I would have liked the hot water to arrive faster. It takes about a minute to warm up. I collect the cold water in a bucket and water my plants.

NOW I have a problem with explosions in the pipes when I use it. This never happened before.
It only happens with the hot water. I am getting tons of black crud into my bathtub after atomic explosions in the pipes. In fact, at one point, the water totally stopped flowing.

Does the Tankless need to be flushed like a regular hot water tank?

If so, can I do it myself?

If so, how?

I had a plumber here who cleaned the unit (the pilot light goes out constantly) He didn't flush it. He said to do that, he needed a pump and some fluid. But
It didn't explode at that time. Only after it had been lit for an hour and I used it to
To fill the bathtub, did it explode again, stop flowing and then all that black crud started coming out. First time ever in 7 years.

BTW, my furnace thermostat battery replacement cost me $250. They used my batteries.

Tankless heaters do not have a tank to flush. The water flows right through the heat exchanger, its like a 24/7 flushing. The black crud is likely a washer or seal down stream of your tub. I'd pull the tub cartridge with the main closed and then cover the valve body with a towel and turn the main on for about 1 minute. Tell us what got caught in the towel. Your comment about a thermosta caught my attention. Tell us more about the stat like model and brand. No battery for a stat costs $250, who did that job?

Thanks for the info.
The plumber told me the coils in the tankless collect lime and the passage becomes
Smaller and smaller with time.

And I have no one to do flush anything but a plumber or to check anything out.
He didn't seem to think of anything such as you mentioned.
AND it wasn't just the bathtub that spurted junk - yesterday, so did the kitchen sink.
Today nothing happened when I turned it on so far, so maybe it is fixed.
When it spouted it looked like tiny black gravel or like the roofing stuff in my gutter.

Plumbing rarely fixes itself so expect more gravel. I douubt if its lime or calcium, they aren't black but failing washers and diaphrams are black.
The plumbers thought about the coil may be right but it would likely be decades before that happens.

Yes, you have certainly helped. I feel that I am going to have to figure it
Out without the plumber and then tell him what I think it is.

I am going to look up a plumbing chart and see what and where diaphrams
And washers are, and why they are needed. I never in all my years experienced
This. Do you think it was the result of having the tankless installed?

Any plumbing installation can cause debris to break free. If you have a bad washer it is u stream, maybe the main shut off itself.

I've seen a number of recent comments posted in various forums claiming that tankless water heaters do not need to be flushed. This is not true! They do need to be flushed, particularly in areas with hard water. I speak from personal experience. My tankless water heater needs cleaning once every couple of years. A buildup of something (calcium, minerals, whatever) occurs inside the unit that causes its efficiency to deteriorate over time. In my case, by the time the efficiency problem is perceptible I have week or two before the unit refuses to heat water. Once it is flushed out, it works like new for a couple of years before needing another cleaning. The first time it happened I was sure that there was something seriously wrong with the heater.

I flush the unit by using a small plug-in pump to circulate a solution of water and vinegar from a five gallon bucket through the water heater. You can search the internet for detailed instructions on how to do this. Rest assured that it can make a huge difference in your heater's ability to do its job.

By the way, in my case the solution in the bucket starts clear but after a couple of hours of circulation it is deep black and slimy.

I've seen a number of recent comments posted in various forums claiming that tankless water heaters do not need to be flushed. This is not true! They do need to be flushed, particularly in areas with hard water. I speak from personal experience. My tankless water heater needs cleaning once every couple of years. A buildup of something (calcium, minerals, whatever) occurs inside the unit that causes its efficiency to deteriorate over time. In my case, by the time the efficiency problem is perceptible I have week or two before the unit refuses to heat water. Once it is flushed out, it works like new for a couple of years before needing another cleaning. The first time it happened I was sure that there was something seriously wrong with the heater.

I flush the unit by using a small plug-in pump to circulate a solution of water and vinegar from a five gallon bucket through the water heater. You can search the internet for detailed instructions on how to do this. Rest assured that it can make a huge difference in your heater's ability to do its job.

By the way, in my case the solution in the bucket starts clear but after a couple of hours of circulation it is deep black and slimy.