We bought a house with a tankless heater two years ago and this is my first time owning one of these. It's in an upstairs attic and has been running fine until now.

Yesterday, I went into the attic to change the air filter on the furnace and noticed that the water heater drain pan was full. It's still puttinf out hot water just fine so this is the only indication something is off.

What would you be doing in my situation? Drain the water heater?

Maybe observe it even is operating? See if it's dripping slowly?

I did that first and did not see anything noticeable. I'm stumped.

I'd dry out the pan, and get you a little leak detector for the pan. When it goes off, you can check it out and see what's happening.

There are a lot of options for leak indicators, but we've got several that tie into our alarm system. Very cheap insurance and great for monitoring these kinds of things.

Not sure of the cause, but be aware you need to flush/service the unit every six months if you want it to last.

Need to figure out why water got in the pan in the first place. It was likely the water heater, but with all the rain we've had it could've been a roof leak that dripped in there.

Might be worth checking if your unit has a condensate drain and how that run off is managed.

My unit had a small pan to catch then redirect to drain. I have since modified for direct drain.

barnacle bob said:

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Might be worth checking if your unit has a condensate drain and how that run off is managed.

My unit had a small pan to catch then redirect to drain. I have since modified for direct drain.

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That's what I'm wondering, but the condensate would be a bigger issue when it's cold out right?

I'm going to look into this. Thanks.

We were hit hard a week and a half ago by the weather in Dallas. I checked all the attics after that and it was bone dry. I had that thought too.

every 6-12 months flush it. Kit:
https://a.co/d/1lTVO4p

If your pan is filling up, you probably have a clog in the drain line from the pan to the outside. For me, I would drain the pan to remove the risk overfilling. Then I'd snake the line to remove any clog. Then I would add, "pour a capful of bleach down the line every 6-12 months when I do the flushing" to the maintenance checklist.

Not to derail, but do y'all with water softeners still flush your tankless regularly?

I had the same issue a while back. I had a plumber come out and basically clean it and flush it out and all is well. You are supposed to have this done with these units periodically.