The problem you are facing is that when the circulator pump goes on to bring
hot water up to your radiators, it also brings down the cooler water that is
in your system. This lowers the temperature of the heat exchanger in the
tankless coil and therefore the temperature of your shower water.

OH..........that makes sense.

If you do
not want to make an investment now, one thing you could do simply turn down
the house thermostat to 50° a few minutes before you take the shower. This
will shut off the circulation in the heating system and allow the boiler to
heat the water in the tankless coil to near 180°.

Wow, we had been doing the opposite with limited success -- thanks!

Ultimately what you should have is a separate water heater, gas if you have
it, electric if you don't. This will work independently of your heating
system, whether boiler/radiator or furnace/fan. It will also be much
cheaper to run in the non-heating season as you do not have to keep your
boiler on just to make hot water. (By the way, if you keep your current system
with the tankless coil, you can
use the output of the tankless coil to feed your water heater. Therefore
when your boiler is running, you will be feeding warm--not ice cold--water
into the heater and thus save a lot of cost of running the water heater).

That makes sense. I actually like the old cast iron radiators when
they are working properly. Radiant heat feels a little kinder on the
system than hot air blowing - we have enough of that ;-)


Peace,
BobJ

thanks BobJ