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oil/fhw/baseboard - Slow heat on 1st floor
The 1st floor of house can take a very long time to reach the desired temp (ie 67deg) on colder days. The thermostat appears to be calling fine and the baseboard will heat up but after 6-7 hours the temp may only rise 1 or 2 degrees. The 2nd floor and finished basement both heat up just fine (sep zones). The zone valve is opening (also tried swapping the valve heads between zones - no difference). I would suspect the circulator pump except the 2nd floor does not have the issue. Any thoughts?
Boiler is 6 years old.