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If blower works in ac mode and fan on mode but not in heat mode, the issue is going to be the relay which energizes blower on low speed is sticking open and not closing.Many of these relays are integrated into board and if yours is, you will have to replace the board.
If you can provide model number n furnace I can tell you if relay can be replaced idependantly or not and if you have a volt meter we can verify.
Model should be on a label just above burners on inside panel.