amount of domestic hot water is reduced

I have a Burnham V73T oil fired hot water boiler with a tankless hot water system. I have to purge the 3 zones once a year due to gurgling in the baseboard. I recently purged the baseboards and shortly after that I noticed that the amount of domestic hot water is reduced, 1 - 1 1/2 showers. I don't know if the reduced hot water is related to my purging, or just a coincidence. Could purging of the zones have caused the reduction in domestic hot water? What are potential causes of the reduction in domestic hot water in a tankless system?
I have now looked at the hot water controls and they are set as follows: Hi=190, Lo=170 and diff is set at 10. Looking at the manual for this burner these appear to be the settings for a steam boiler, not a water boiler which is what I have. The manual states that the settings for the water boiler should be: Hi=210, Lo=190 and diff is set at 25. Would there be a reason that the installer set these lower limits? Is there any potential harm by raising these limits? Is my assumption correct that my heating costs will go up with these higher settings? Significantly?
Thanks,
Bob

There are several possible causes of the problem, most associated with winter temperatures.

My municipal water source is surface water, which drops to about 35 F in the winter. That puts a greater demand on your tankless water heater.

You might check whether your boiler is running when you run out of hot water. Your boiler should have enough hot water in it to give you one shower, but if it doesn't come on when the temperature drops, you will run out of hot water.

I had a problem when the tempering valve failed in a way that let cold water mix in at all times. That didn't matter in the summer but gave me cold showers in the winter. I took the control stem out of the valve until I got around to replacing it.

When your circulator is running, it takes heat from the system and reduces the capacity to produce hot water. There is usually a boiler control to prevent the circulator from running when the boiler temperature drops below some setting, usually around 160 F. If the circulator continues to run and the boiler can't keep up, you will get cold showers.

My Honeywell Aquastat temperature settings got out of whack to the point where the actual temperatures were much higher than the settings. I tried for a while to apply "Kentucky windage" to keep it operating correctly but finally had to replace it.

One thing you might find with your tankless heater is that you use a lot of fuel in the summer just to keep the boiler hot. I found that I was using so much $2.00 per gallon oil that it was costing more for oil than it would cost for electricity. I installed a 40 gallon electric hot water heater in series with the tankless coil and shut the boiler off in the summer. The water is heated with electricity in the summer and with oil when the boiler is running in the winter.

any more help out there ? please !

I figure I should start with flushing the water heater but the installer only put a hose bib on the input. Being winter in Boston if I can wait until spring/summer I will install a hose bib on the output then and flush the coil. Don't want to risk having something go wrong and having no heat. How often does flushing resolve this issue? How often do bolts snap when removing the tankless water heater?