"Mikepier" wrote in message
...
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.

Good chance it is the coil clogged or nearly so.

If the boiler is more than 15 years old, before dropping hundreds to replace
the coil, consider replacing the boiler it with a more efficient boiler and
a water tank that is indirect fired. I changed mine last December to a
System 2000 and my oil use so far is down 32%. I don'[t have summer use
yet, but I expect hat to be dramatic also. Ask the in-laws how often that
boiler kicks in during the summer just to maintain the hot water at
temperature. Quite often from my experience. With no heat on, I don't hear
mine go on at all because the 40 gallon tank holds it at temperature for
days if not used. Mine is from www.energykinetics.com

If they opt for a new boiler, be sure to check out state funded rebates and
0% financing available through the state.