Drop in DHW temp....don't know why

I have an Econburn 150 that use to heat my home and domestic hot water. For the past few weeks, my domestic hot water hasn't been getting as hot and I run out of hot water quicker. This is the third year for me using the Econburn and I've never had this problem. The domestic hot water is heated through a simple side arm unit. I have no idea what could be causing this. Everything seems to be running as it should. Has anyone else experienced this problem? If so, how did you fix it?

Thanks

Two things I can think of: A thermosiphon loop has become possible (valve not closing fully somewhere?) or you have scale buildup inside the sidearm.

Thermosiphon is easy to identify - when the Econoburn is not running, is the boiler water line from the top of the sidearm hot for any significant distance from the sidearm?

Scale could be possible if you have hard water - other folks have reported this. A search would likely turn up some insight.

If the rest of your system is working as usual, I'd suspect the scaling.

Side arms in most cases are good for 2 maybe three years before they plug up. Sometimes less if your water contains a lot of mineral.