A horizontal pass boiler is usually more efficient while running, because it
extracts more heat from the flame. They are usually retain more heat when
not running, because there is less natural draft to cool it down.

wrote in message
oups.com...
You mean that the pin type boiler becomes less efficient as the heating
season passes, but returns to its normal efficiency after cleaning.
The 3 pass keeps a more consistent state of efficiency between
cleanings if I understand you correctly.

This doesn't seem like such a big deal if you service your boiler
regularly.