D&D magic. 500 ways to blow crap up, not one damm way to heat your house.
Actually here is a spell I wrote for Relics and Rituals II: Lost Lore by White Wolf/Sword Sorcery Studios
Continual Heat
Heats igneous rock to provide
ambient warmth.
Transmutation
Level: Brd 2, Sor/Wiz 2
Components: V, S, F
Casting Time: 1 action
Range: Touch
Target: 1 fist-sized piece of igneous rock
Duration: Permanent
Saving Throw: Will negates (object)
Spell Resistance: Yes (object)
Description
The heat of molten flowing
rocks has long since died out in
many of the old lava tube chambers
of the Underfaust. In fact,
the underground passages are uncomfortably
cold despite the
blazing heat of the terrain above.
The necromancers of Hollowfaust
have made this forbidding network
of chill volcanic tunnels
their home, college, and fortress.
This is not a problem for the
undead that work and patrol underground,
but it presents a
problem for many of the living
wizards. Generally, the necromancers
bundle up in heavy robes
while traversing unheated corridors
and the occupied chambers
are often covered in tapestries and
carpets to insulate and retain heat.
Yet multiple fires would be impracticable
underground, and so
to combat the bone chilling cold
of their chosen home, the necromancers
developed magical means
to provide heat.
Spell Effect
This spell enchants a piece of
igneous rock to unlock some of
the heat that went into its creation,
warming the surrounding
area through convection. When
a fist-sized chunk of such rock is
subjected to this spell, it glows
red-hot and radiates heat sufficient
to warm a 20 cubic foot area
to 70 degrees. If the rock is touched
directly, it causes 1d4 points of
fire damage per round of contact.
Focus: A fist-sized piece of
igneous rock.