question about negative pressure

[mattmctree]

i have just moved into a new house and noticed that when the furnace comes on the bedroom doors will close if they are almost closed but not latched. my house is old with a new addition. there is only one return for the whole house its a floor grate in the centre of the house.  i have a gas dryer , a gas stove and a gas furnace. its a single pipe high efficiancy > is this something i should be concerened aboout? i dont  do a ton of gas related stuff at work

[Porcupinepuffer]

It wouldn't be a negative pressure issue. It's air flow leaving the supply from the room and pushing back to that main return to keep the cycle going. Once the door is shut, you actually end up with a positive pressure within the room since the air is now forced under the door to get back to the return. This system you describe is pretty common in old houses.
The one pipe high efficiency is fine.

There's also a chance you may have moved in to a haunted house.

[Admin]

Is there a gas water heater?  Usually when you 1 pipe a furnace you need an additional outdoor air supply brought inside the mechanical room.  This will bring in fresh air as the furnace is exhausting and should prevent the negative air condition.

It could also just be an airflow issue like Porcupinepuffer says.

[mattmctree]

No its an electric water heater. I may double pipe the system in the future just because. Also my gas dryer piping was plastic flex and was coiled up in the ceiling. I for everything out and ran 24 guage c vent. Tapped of course. They had the venting touching the wainscoting and the only route I could do was to cover all combustible materials with sheet metal and cement board. Would this meet code?