LAARS system Ghost heating during summer

Customer in small home with a single loop system is getting heat to the baseboard during the summer. She reported the it was random, but almost daily. Also the owner moved in this past December so we don't know if this problem occurred during previous summers.

This is a LAARS EPB-110 with domestic hot water.

What I found todays is that it happens when the boiler is responding to a domestic hot water demand. To prove it to my self I flushed the single loop to get it cold. With the boiler sill warm there was no drift up the return or supply.

Then I ran hot water long enough to get the boiler running and heat started moving out the supply line at least as far as the first few baseboards. The heating circulator was not running, I verified there was no power to it.

Boiler internal diagram

Heating diagram from manual. Piped exactly like this excpt no bypass and single zone, no zone valves.

Any help will be apprciated

Robert