"... oil changed every 300 miles,...."

I'm assuming misprint.....otherwise clinical issue.

Are you seeing evidence of "crusting" on your fixtures?

"Flushing" is easy enough.....but there should be a reason.

that was every 3000 miles!

No evidence of any issues with fixtures, this is just the "standard" advice from the manufacturer, I believe. I really can't imagine how you could give advice for the whole country, all circumstances. If just up to me, I would wait at least the full 5, but wanted to know what others thought.

I don't even know how much they charge. And what would you have to see that would make you think it was time NOW?

IMHO....flush at 3-years just for heck of it is silly. What, exactly, is the "flushing" your vendor is suggesting? I know they're going to blow a bunch of water through the unit but that's what happens every time you open the hot-water spigot anyway. If there's no evidence of deposits on your fixtures, therefore, no evidence of deposits anywhere, why would they want to do this?

Then, again, I think oil changes at 3,000 just for the heck of it is silly, too. But that's an entirely different discussion.

I'm with you, asolo-- it sounds like revenue for the installer, but they can justify it based on the manufacturer's recommendation. (And he's backed off the oil change thing, finally, but yes, I agree there, too! That was just for perspective on why I need evidence to make my case.)

Does anyone "in the business" read this and have an opinion?

There isn't an exact answer to your question, it will vary by how hard the water is, and how much you use.

What they are doing (or should be doing) is removing the lime buildup in the heater with vinegar. The acid will dissolve the hard water deposits that will eventually clog a tankless.
It's about a 10-15 minute flush with vinegar.

You can google it and see many videos on how to do it.

Three years is about right in your situation. I wouldn't just let it go.

Tankless heaters use small heating coils to expose the water to heat quickly. You don't want those passages to clog.

You can call around for a good price over the phone. They don't need to come out to give you a quote, this is a basic job.

Jake, are you saying this is potentially a DIY job? We are reasonably handy. As to water use, 2 people who are moderate users at most. I will call around, though-- thanks!

It is a DIY job.

For less than the price of a service call, you could buy the pump and hoses and have everything you need.

If a service valve was installed, the connections to the water heater and little pump in the pail are made with washing machine hoses or short garden hoses.

There are also cleaning kits for sale online - If you buy a kit, stick with the vinegar rather than any cleaner that comes in the kit. You know that the vinegar is safe and is what is recommended by the manufacturer.

Here is a link that might be useful: How to clean a tankless heater

Thanks, Jake-- I will look into it. I think with our particular water quality, it would be a while before any lime buildup would amount to anything, but I like the vinegar solution. Thanks a bunch!

Mine wasn't flushed for the first 7 years, until I did it June 2011 when changing a bad flow-sensor. Didn't do a very good job of it, improvising with a not-so-suitable pump. Intend to get a better pump with fittings, flush it (with vinegar) again soon, then continue yearly.