Air заслонка it is closed not completely.
The heat exchanger is polluted, or bringing pipelines are squeezed.
Management of a heater is changed or has drooped.
Submitting and drain hoses of the heat exchanger are polluted or cracked.
Check up a tension of cables; отладьте a course заслонки, if necessary remove extraneous subjects.
Clean or replace the heat exchanger; check up bringing pipelines.
Adjust or again hang up.
Check up, if necessary replace hoses.