David AndersenNov 21, 2022

Due to a change in my home heating needs, I have spent the last week trying to optimize my hot water radiator system. If there's one thing to know if you have a modern condensing boiler, it's this graph: efficiency is higher at lower return water temps + running at low % of max.

My boiler was poorly configured and ran way too hot, leading to frequent on/off cycles.

Now it runs for hours at 20% of max and the house is incredibly uniform in temp with far less gas used. Yay!

Show threadDavid AndersenNov 21, 2022
(I say this because my system was installed by some absolutely fantastic local folks who I would use any day, but the installers have an incentive to make sure you're never too cold & behaves like old system, as opposed to actually optimizing for the temperatures at which you keep your house. It was well worth going into the contractor mode of my boiler and spending some time setting up a good outside temperature response curve. Easy, too.)
Show threadDavid Andersen
This curve happens because a "condensing" boiler uses the cool return water to condense the water vapor out of the exhaust stream, recovering the heat in it. The cooler the water, the more energy it can suck out of that exhaust. It's how boilers went from 80-85% efficient to 95%+. But they're only that efficient when the return water temperature is low.
Nov 21, 2022 at 12:54amWeb