Using Heat Metal to light webs on fire

Question

Suppose I use Web to trap an enemy. Could I then use heat metal on them to cause the webs to catch fire, thus damaging the enemy further?

Answer 1

No, heat metal will not burn the webs.

You don't specify, but given that neither the webs nor (most likely) the creature are made of metal, I assume you mean that you intend to trap a character in the web and then cast heat metal on either their armor or weapon, and you hope that the webs will then catch fire.

By a strict reading of the two spells, it appears that this will not work. The web spell says:

Any 5-foot cube of webs exposed to fire burns away in 1 round

The heat metal spell does not say anywhere that it produces flame or sets things on fire, so it would not trigger this clause of the web spell.

Your DM might rule that "glowing red-hot" means that the metal is hot enough to ignite flammable materials it comes in contact with, but that would be a dangerous and possibly contradictory precedent to set, since presumably it would also mean that casting heat metal on a character's metal armor would set their clothes on fire, and that is contrary to how the spell is usually ruled to function.

Lastly, I'll note that you probably don't want to set the webs on fire anyway. The restraining and difficult terrain effects of the web are usually more helpful than the 2d4 fire damage that you get from burning them.

(You might argue that in the real world, any iron or steel that is hot enough to visibly glow is also more than hot enough to ignite most flammable materials. However, metal armor at that temperature (at least 500 °C) would also probably deal a lot more than the real-world equivalent of 2d8 fire damage to its wearer, so the more reasonable interpretation is that metal in the imaginary world of D&D glows at a lower temperature than in real life.)

Answer 2

This doesn't work

Heat metal only works on metal things.

Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range. You cause the object to glow red-hot. 

Webs are not metal, nor are spell effects "manufactored".

If you try to heat a metal object that's touching the webs, it still falls short.

If you are simply trying to catch the webs on fire per this line:

The webs are flammable. Any 5-foot cube of webs exposed to fire burns away in 1 round, dealing 2d4 fire damage to any creature that starts its turn in the fire.

... it still doesn't work, because heat metal doesn't actually create fire, even though it deals fire damage. The webs have to actually be exposed to fire.