"Mike rock" wrote in message
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On May 4, 9:49 am, "RBM" wrote:
"Mikepier" wrote in message

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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.

The coil could be coated with mineral deposits, insulating it, or you
could
have a bad mixing valve .

What mixing valve are you referring to, the one on the boiler? I see a
mixing valve, but it is shut off, which means I'm assuming 100% hot
water out of the coil.
If you are referring to the shower mixing valve, I should point out
the same problem happens when washing clothes,dishes etc.

You have a cold water pipe feeding into the coil on the boiler, and a hot
water pipe exiting the coil. Somewhere at that location should be some type
of mixing valve, so you don't get 180 degree water flowing through your
domestic pipes. If it is off and not allowing any cold water to mix, the
problem is with the coil.