Running A/C Compressor during defrost is by design. Outside air is passed over the evaporator, which is cold, causing the moisture in the air to condense. This produces dehumidified air. That air is then passed over the heater core before being ducted up to the windshield.

Result is a more effective defroster.

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W. Brian Fogarty

'02 S55 AMG (W220)
'92 300SE (W140) - sold
'76 450SEL 6.9 Euro #1164
'76 450SEL 6.9 Euro #521

"Bond reflected that good Americans were fine people, and most of them seemed to come from Texas..." Casino Royale, Chapter VII