"Tman" wrote in message

High limit setpoint for the burner is 180. I observe the digital readout
swing between 180 and 170 as the burner turns on and off -- as expected
given the 10deg differential. There is a little bit of overshoot on the
high side, e.g. the burner is off at 180 but the temp swings up to say 185
before sliding down to 170. The boiler temp guage (tridicator) indicates
a reading that is roughly commensurate with the aquastat readout.

If this stimulates any ideas:

Water circulating through the radiator loops looses about 10-15 deg on the
trip. E.g. 180 deg out of boiler, 167 into boiler, typically.

Oh yeah, this has a tankless coil on it. I haven't hooked that up
(yet) -- I plan to in the next day or so. So the tankless coil is vented
to the air.

T

Try setting the differential to 15 or 20 degrees. IMO, 10 is very tight,
but even at 160 you'll be getting good heat transfer. Some new systems will
reduce the temperature of the boiler in warmer weather for just that reason.
They use an outside sensor to determine the water temperature to circulate.
I've not timed my burner, but I know it will go more than 10 deg.
differential. It will shut down the circulator if it drops below 120.

As for the tankless coil, I'd skip it. They are very inefficient when no
heat is needed. They put out a lot of hot water and can keep up with your
shower forever, but the chamber is poorly insulated and you lose a lot of
heat to the house. OK when you need heat, wasted when you don't. You add an
indirect fired tank and zone. How many times in the summer do you hear the
burner kick on just to keep that water hot? Too many. Think about that 400
pounds of cast iron being kept hot "just in case" 24 hours a day. Move the
hot water into a 40 gallon well insulated tank instead.

Bubba says your boiler is oversized. I don't know you climate, but I tend
to agree with him. My house is 2000 sq. feet and the output is 15,0000 Btu
less than yours. During the coldest weather we had in 27 years, it easily
maintained temperature, an 80 degree rise.