low pressure at vents

[juster]

It is true that changing the size of the duct work closer to the end of the run is required for proper disrabution. Usually in order to keep the pressure the same all the way through the duct.
This is usually an engineered task done by the engineers at the time of design. No one should require their duct work changed unless you have been making adjustments to it.
Not sure why you would need to change the furnace from what was installed by the builder unless there was a problem with it. Like it was not set up properly or your set points were off. I assume you are stating that you have an electric furnace to heat your house. 30kw. Depending on the type of house you have, ranch, bungalow, two story, furnaces can take longer or shorter times to heat your house. The heating cycle should not be on all the time in order to heat your house. The AC could be on much of the time depending on your desidered set point (thermostat setting) outside air temperature and humidity. Because what you are stating is somewhat vauge and there are many variables. At first guess I would look at the control circut and thermostate to insure they are all working properly. Having a tech in trained to tune up your system may be all you need. A 4 ton unit cooling a 2400 sq ft house should be more then plenty in fact it could be to big a system. One should never over size your system as removing humidity is very important. If your system is over sized it will cool the house to quickly without removing humidity and you will end up with a cool damp house. Not good for your or your house. Have a tech tune up your system. I would call around first to different contrators getting their feedback on how they can help you before calling one one in.

[juster]

If you say you just upgraded something is defently weird. Does your furnace have a direct drive blower or is it belt driven. If it is belt driven it could be a loose belt. If you have a direct drive motor on your blower then the motor could be set at the wrong speed. Direct drive blowers can be set to run at different speeds for different conditions. Like one speed for just moving air around your house and another for heating or cooling. This set up is usually done at the time of instalation. If you have a manuel for your unit check out the install instructions for more insight in setting the initial fan speeds. Another factor to the fan speed scenario is that if your furnace is set to run on two speeds then your themostatl may have to be wired to trigger the different speeds when different conditions are happening. Like, just air cirulation or heating or cooling. Some furnaces that have the diffent speed motors are set up to just run on one speed but as mentioned above this is set up at the time of instalation.

The lack of air flow is not normal. I am supprised that the HVAC companies have not given you any insight to what your problem is.

Forgot to ask, since you had your new furnce and AC installed did it ever work * or did this just start * If it started later on then when did you notice the *
Is anyting blocking your * Say a filter that has fallen *