Domestic hot water problem

I have a DF520 that works great except for the domestic hot water. The stoker is about 12 years old. When you turn the shower on the water is HOT but goes to luke warm in about 2 minutes. It was fine when we started the stoker in late Oct. but started this trick a few weeks ago. I'm not very happy with the tech. the dealer sends out so I'm hoping someone can help out. Thanks Matt

Mixing valve failure maybe?

coil scaled up?

A mixing valve problem or scale buildup in the coil is possible, but let's start with the settings. What feed/air setting are you using and what are the aquqstat setttings?

Yes what boiler temps are you running?
Does your DHWC go into a holding tank or direct down the line to the sink or washroom area etc?
Do you use a mixing valve in your setup or no? (I don't on my setup)
When you started noticing issues with the temp was there any changes just before that to the system at all?

Could also be a bad shower valve that is letting the cold get mixed into the hot.

It's not uncommon for a remote single-lever valve to cause this problem. Shut off the supply valves to every single-lever valve in the house, especially the washing machine and only let the shower valve on. If the shower returns to normal heat, open the others one by one, starting with the washing machine.

Yes there is a mixing valve for the domestic hot water, the original since the boiler was installed. There were no changes made to the system when this started. Now I will show my ignorance, I don't know what the settings are,feed/air or aquastat. The water temp. gauge is normally about 180-190 dgrees. Tonight everything worked good but will check all single lever faucets tomorrow. The thing that is odd , It doesn't do it all the time. Thanks Matt

the higher your boiler temperature, and the harder the water, the quicker the coil will become blocked and insulated with minerals, calcium. can be flushed out with muriatic acid, and will work like new.