[Wayne 962]

Okay, so I'm not an HVAC expert, but I have enough experience with them to know more than about 95% of the population. That hasn't helped me in this situation. Here's the details.

So, we have a Carrier 58MVP two-stage unit that can run at high heat or low heat. The heater was off the other day with a fault. It seems like the high heat mode (where it runs the blower motor fast) was registering a fault where the blower was running too fast (like when there is a blockage in the vacuum cleaner and the motor runs away). It keeps reading a fault saying "motor out of range" in the high range which means rpm is in excess of 1,300.

So, I tried changing the filter. That didn't work. I ran it without the filter. That also didn't work. Then I ran the motor in "test mode" with the intake door completely open (pulling air from the room where the FAU is located - completely bypassing the intake and the filter). That worked! So, I assume there is a problem with my intake.

The intake has a lot of dust and dirt in it. I plan on cleaning the vents soon (they are big, like 24" or so wide) to see if that will make a difference. I'm not sure it will. The length of the return (the distance from the FAU to the intake vent) is also very, very, very long - much longer than it needs to be. It wraps all the way around the outer walls of the house - it looks like I could shorten it by at least 10 feet or so if I need to. Also, the way it connects to the return vent in the house is super convoluted. If this were an engine intake it would be terrible. There is also a beam that is in the way of the flexible vent, that seems to be compressing it about 10-15%.

I took off the vent cover tonight (just the grille), and it seemed to work okay for a while, and then gave me the same error message (so it went from bad to intermittent, which I see as an improvement).

A real hack job if you ask me. Frankly, I'm not sure if it ever worked properly - there are some notes written on the cover of the FAU from 2008 indicating the same fault codes. Dunno.

So I have questions for people smarter in HVAC stuff than me:

- Having dust and dirt lining a 24" vent - is the dust and dirt enough to make a real difference? My gut says no, but on a marginally designed system, then maybe yest?

- Are the intake returns supposed to be "balanced" like some of the output runs? I didn't think so, but that might explain the super-long length of this intake run. Longer length = more resistance, so perhaps shortening it a bit will help.

I thought about disconnecting the intake hose under the house and letting it run just sucking air from under the house. If it's indeed a design problem, then that would fix it right away? Good strategy? I'm a bit leery of running it sucking air from under the house as it's real dusty there, and who knows what crap might be in the air down there.

This is the 2nd season with this problem - I didn't quite notice it fully last year because the heater works fine in "low heat mode" which it defaults too. I mean, if you set the thermostat to 68 degrees and it's now 68 degrees, then why mess with it. The difference is that now I noticed it was set to 68 degrees and it was 65 in the house - the heater had shut down for three hours (it resets after that time) and during that time, the temperature dropped. That's when I saw the error code.

Thoughts?

-Wayne