If the sheet metal union has a contract with your employer, and you performed sheet metal work within the scope of your work, then the union could try to get the employer to terminate you and hire a union worker instead (assuming that the union has an HVAC tech among its ranks.
If that happens, then you can contact the NLRB and complain
that you have been discriminated against under NLRA Section 8(a)(3), and that you should have been offered an opportunity to join the union, rather than terminated in preference to some other union employee.
Hope this helps.
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