Fjx1 wrote:

2 years ago we had the front of the house stuccoed and the vinyl sided the rest
of the house. So now it's very tight. Didn't notice any problems. Now we
noticed basement felt too warm is summer. There's negative pressure by furnace
and water heater. This happpens when little attic fan goes on. Contractor came
back and put 14 vents in soffits. This didn't solve problem. Pilot stays on but
when I hold match near vent in blows down or out. Is there anyway to vent attic
better so fan doesn't do this?

This is a common, and very serious problem. As you are probably
already aware, the backdraft can be responsible for CO gas being
pulled into the living space.

I saw one installation where the flame from the main burner on the
water heater was being sucked into the room!

Most likely the soffit vents are simply not enough area to satisfy the
exhaust fan volume.
But...you may have another problem. The fact that an attic fan
(presumably the attic is fully closed off from the lower spaces)
can create negative pressure in the basement suggests that there
is a "bypass" opening from the attic all the way to the basement.
IOW, the attic isn't very well sealed.

Running the attic fan isn't only exhausting hot attic air;
it is robbing conditioned air from the living space.

Until you can get a good handle on what is happening in
this situation, I would not run the attic fan.

Jim