Welcome! Here are the website rules, as well as some tips for using this forum.
Lukewarm Hot Water: Burnham Triangle Tubes, Burnham Boiler, Beckett Burner, Honeywell 8124 Aquastat
Everything was perfectly fine and then we woke up two days ago with luke warm hot water. We have hydronic oil-fired heat (beckett + burnham) and on-demand, tankless, oil-fired hot water (beckett burner, burnham boiler + burnham triangle tubes).
I thought it might have been the mixing valve at the boiler. Cleaned that out. Mixing valve was actually pretty clean but there is a decent amount of mineral buildup inside the pipes. Not enough to restrict water flow.
The heat works fine. What we are doing right now to make showers bearable is crank the heat tstat. When the beckett burner is burning, the water goes from warm-ish to just warm enough to be bearable to put your head under the water in the shower.
My guess is that since mixing valve is fine and hydronic baseboard heat is fine (we are not out of oil, burner was just serviced, working fine etc.) and I didn't touch anything (no changes to system)... and that this is a sudden change, it's probably a thermocouple or temp gauge somewhere (or maybe relay in Honeywell controller). Something inside the boiler isn't telling the controller that the domestic hot water isn't hot enough therefore light the fire. Or, the temp sensor may be working fine but the relay (or whatever it is) inside the Honeywell isn't lighting the fire.
I don't know much about these things. The hydronic baseboard heat is working fine: 007 circ pump kicks on... then a few seconds later the burner fires, then the house gets warm. There is one 24vac transformer on the boiler (which is working fine) and the heat side of the boiler works fine, it's just the potable hot water part that isn't staying warm...
I do the annual services on the boiler myself. I own a soot saw, ash vac and a bacharch draft gauge. I clean, change out the filters, check draft, look at the fire, inspect the fire chamber insulation, etc. I've never touched the domestic water side of the system... probably out of ignorance... I just figured if it ain't broke... I know there's a little bit of mineral scaling and buildup (we have a whole-home water filtration system sitting in the box waiting for me to install... it's "on the list"). Other than that I'm at a bit of a loss. It seems/feels electronical somehow. A temp sensor or a switch --because when we trick the system into turning on the fire (crank in-home heating tstat) the domestic hot water is just slightly cooler than what it was at a few days ago. If it matters, I believe that the Honeywell is set to 180, 160 and 15 diff. I have not touched those settings in 5+ years.
Thanks in advance.