TAMNOZ wrote:How do you know if you need a fresh air supply or not. I have a Keystoker with power vent.
Here's my stab at answering your question. There's no exact and concise 'one answer'
The extreme (I personally had this situation) would be if you lack proper measured draft and see it change to the good when a door or window in the furnace room is opened and supplying all the needed make up air.
On the other end of the spectrum, any air pulled out by the combined fans has to be made up from someplace else. The make up air will filter in through any leak anywhere in the house until the volume of air leaving the furnace is balanced out. In some situations, it might make a drafty house feel even draftier.
The thought behind a designed air supply is the air is supplied directly to the combustion fan. It cuts down on the cold air filtering in through unwanted places (drafty spots). It makes up air pulled out by the power vent and combustion fan.
If you have a baro, make up air for that should not be provided by an outside make up line. It would defeat the purpose of the baro. That's a whole other discussion.