How are you going to calibrate it? There's much more to an air tightness tester than meets the eye. The pressure reduction applied to your house must be accurately determined, and it's not just one pressure, several readings are taken at different pressures. The flow rate of the fan must be known to measure the air being exhausted at a given pressure. How do you propose to do this? The unit I saw used to test my own house had a fairly sophisticated instrumentation system with it that was connected to a laptop running software to convert the measured flow rates into actual leakage values.

All that said, one thing you can do with a DIY system is de-pressurize your house and then go round with a smoke pencil to find the main sources of leakage - even if you don't know the rate, knowing the location of the leaks is half the battle.

Paul in Montreal.