You need a lawyer!, If you still owe the general contractor for work that has been completed. Then if there is no dispute as to the work done by the heating subcontractor, you can do the following:
1. Send a letter to the General, giving him X days to respond in writing as to why he has not paid the heating sub. Include notice that unless he either informs you as to good reasons for not paying or you intend to pay the sub yourself and to deduct it from what you owe the general. Send a copy to the sub and keep one for yourself.
2. If the general responds with reasons, then simply demand that he and the sub resolve the problem. If no good reason, then send second letter to both again stating your intent to pay the sub directly and to deduct the expense.
3. If still no action, upon the sub giving you a waiver / release of lien rights, you can pay the sub. But be prepared to have further problems with the general.
4. So make sure that you stay behind the work with your payments and that every sub and suppler has been paid before you give the general more money. Get receipts for materials and lien waivers from all subs.
You need a lawyer to keep track of all this.