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All answers having "Heat source" as clue:

3 Character Answers:

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GAS

IRE

OIL

RHO

SOL

SUN



4 Character Answers:

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OVEN

PEAT



5 Character Answers:

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MIAMI

SPICE

STEAM

STOVE



8 Character Answers:

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RADIATOR



10 Character Answers:

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GASFURNACE