Before buying a whole new system just because the coil is failing, I would
suggest buying a separate water heater. Install the water heater in series
with the output of the tankless coil so that hot (or at least tempered)
water goes to the heater and thus requires much less energy to bring the
water in the heater to the necessary temperature. This also has the added
benefit of letting one turn off the boiler during the months that it is not
needed to supply heat to the baseboard radiators since, when not heating the
house, a boiler is a very inefficient way to produce domestic hot water.
--
Peace,
BobJ

"Ed Pawlowski" wrote in message
...

"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.