seller claims ignorance

What is the name of your state? pennsylvania

i bought a house in the spring. when i had a heating contractor come to service the heater i was told that he would not touch the furnace because it had been illegally modified and is unsafe. it is a gravity heater that a fan and motor were attached to in order to force heat to an addition that was built on the back of the house. the fan and motor are housed in a wooden box that was taped shut. the home inspector looked at the heater but did not untape the attached wooden box. he said he did not know what the box was for. the inspection report includes standard language stating that the inspection is not a code inspection. i told the realtor that this should have been disclosed on the sellers disclosure. the seller's response was that they knew nothing about the modification and that they never had the heater serviced in the 15 years that they lived in the house. there are a few stickers on the furnace from 5 years ago from a company that is no longer in business. To get heat into the addition I am told I would have to replace the heater and add duct work that will cost $3- $4000. Do I have recourse?

[HomeGuru]

Before closing, you should have inquired as to what was in the box. Then require the Seller to remove the tape so that the inspector could check out the components. You should also have inqured as to the age and when the heater was last serviced. If you knew that there was no service for 15 years, you should have required a heater inspection by a licensed HVAC contractor.
If the Seller really did not know about the illegal modification, it will be hard to prove that they are 100% at fault.