My in-laws have an oil fired hot water baseboard system with a
tankless HW heater coil rated at 5 GPM. Presently the aquastat is set
for the boiler to come on when the water temp drops to 165 deg and
shuts off when it hits about 185-190 deg.
When someone takes a shower, the water temp initially is very hot, but
then after about 5 minutes the HW starts to run out. Any ideas what
could be causing this? Could the coil be dirty such that it impedes
the heat transfer?
The boiler heats the house with no problem, its just the HW part that
has an issue.