In theory - every W you draw to power something, turns into heat energy, one way or another. Even light from lightbulbs.

I'm heating my apartment with my last gfx card, it pays for itself ++, but with the cost of litecoin going down it will eventually end.
If I had no other use for this card, except for mining - I would probably sell it, because it can devalue fast. But if the card is so old, that you can't sell it - I guess it's ok to use it a crypto-heater.

Also videocards were not designed to run 24/365
Even at modest settings something is bound to go off, be it a fan, or something worse. If you have no warranty then this is worst idea ever.

Finally - electricity is usually the most expensive source of heat.