HVAC experts please help!

[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

[Wayne 962]

Quick photo of all the dust and the convoluted path the air must take prior to reaching the return intake.



-Wayne

[Wayne 962]

The effective size of the intake is show with the red outline. The blue line is where the beam is located (on the outside of the hose, pushing it down). Seems like a real crappy design to me, but I'm not sure if this is the problem. I guess I should disconnect the vent hose to check.

-Wayne

[tooms351]

Is there a sensor somewhere in the intake that might be dirty? Is the fan possibly setting its speed from that sensor?

[cabmandone]

Edited because I somehow missed this! "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.
"

Make sure all of your return grilles have no obstructions and then make sure your supply vents are all open. IIRC, the MVP measures static pressure and adjusts the blower motor accordingly.

[cabmandone]

From the sequence of operation in the owners manual:
The blower starts at approximately 400-500 RPM. After
20 sec, the motor is turned off for 1/10 of a sec where a coast down
calibration is done to evaluate resistance of the conditioned air
duct system. The microprocessor then determines blower RPM
required to provide proper airflow for heating mode.

Again, I'd make sure that every supply register is open. Yeah your return duct being choked down is half assed and shouldn't exist. No I don't think the dust in the flex is causing the issue. The flex itself can be an issue if the bends aren't smooth (like pictured) or where unsupported and sagging. The furnace should be adjusting for the inefficiencies in the duct system through the coast down calibration.

One more thing: What is the code number you're getting? 44?

[billybek]

What does the forward curve fan look like?
Are the blades of the fan coated or filled with dust?

As mentioned check your returns and supplies and maybe have a the ducting changed out. Flex is nasty stuff for resistance to flow when it is installed well. Yours is not installed well.

[dad911]

AC also? Evaporator coil dirty? Buildup on the squirrel cage?

[cabmandone]

http://www.graycoolingman.com/uploads/1/0/6/6/10667336/58mvp-_2_stage_variable_speed_troubleshoot_guide.pdf
Open the troubleshooting guide and go to page 13 for Fault code 12. You can scroll from there to find the fault code and the troubleshooting steps recommended for fixing the issue.

You mention you used test mode. I believe this makes the system go through the stages of operation and then turns off. If you checked it only for a short period with the blower door off during self test mode, I'd put a jumper in, described in troubleshooting, and cycle the system. The jumper bypasses the thermostat and will allow the furnace to run for an extended period. I'd make sure you're in the high fire mode (I think you can jumper to high fire) and watch it with the door off to see if the code is set.

[black73]

LOL! Dude got all the money in the world and comes here for free advice rather than pay a competent AC Tech.

Looks like way too much dirt in the duct. Check for air leaks under the house pulling the dirt into the duct. Never run without a filter and do not pull air from a crawlspace. You are breathing that shat! Oh, and get your coils cleaned.
Hope that helps.

[cabmandone]

Originally Posted by black73

LOL! Dude got all the money in the world and comes here for free advice rather than pay a competent AC Tech.

Looks like way too much dirt in the duct. Check for air leaks under the house pulling the dirt into the duct. Never run without a filter and do not pull air from a crawlspace. You are breathing that shat! Oh, and get your coils cleaned.
Hope that helps.

He hasn't reached Bezos/Gates status yet I don't think.

This system is different in that it will gauge static pressure and automatically adjust the blower speed. It's labeled as infinitely variable speed IIRC which is why it ramps down to gauge static pressure and then ramps up again once the static pressure determined. It's a really impressive furnace... when it's running properly. Can be a royal P.I.T.A when it's not.

[wdfifteen]

It's the DME relay. It's always the DME relay.

[70SATMan]

I see a new book in the works,,,,

[nota]

I run 4 filters two on the door to the a/c closet one washable to trap dirt dust and hair one super disposable #9 the highest filter for smoke pollen ect

and the same on the bottom of the a/c unit one washable one super #9

and the fan cage veins still gets caked with dust and needs cleaning !!
I suspect the fan is your problem limiting air flow and the motor over speed
but with the build up in the vents a system cleaning is needed or the improved flow
will quickly be moving stuff on the fan again

that would be my first step clean the fan vents and evap coils for better air flow

never suck under house air inside open the system to inside air if you must

[stevej37]

I am no expert on this subject...but I had a similar problem last year with my 25 yo Heil furnace.
The tech came out and tested it and said it was time to upgrade...a few problems plus the heat exchanger was slightly leaking.
He took one look at my cold air intake and said he would fix the convoluted mess of ducting that passed around a couple floor joists.
When done...the new two stage Heil runs at about half the noise of the old and my heating bill is way down. I should have had it looked at years ago.

[sammyg2]

If the problem is truly over-speed due to low load, then that is likely choked on the intake as suspected.
But what about the blower? if the blades are very dirty to the extent that it affects the airflow over them (think ice on airplane wing) then that could be the problem also.

[Wayne 962]

Originally Posted by tooms351

Is there a sensor somewhere in the intake that might be dirty? Is the fan possibly setting its speed from that sensor?

Thanks everyone! I will try to answer all questions!

There appears to be a sensor. I don't think the sensor is bad, because when I let the unit draw air from the room (not using the air intake) it seems to flow air okay (in test mode).

-Wayne

[Wayne 962]

Originally Posted by cabmando

Make sure all of your return grilles have no obstructions and then make sure your supply vents are all open. IIRC, the MVP measures static pressure and adjusts the blower motor accordingly.

I found that a few of the supply registers were closed / clogged / covered with some items, so I thought for sure that was the problem. That didn't work though, so I don't think the problem is obstructions of the supply side? When I open the return grille, it seems to work *slightly* better. What I mean by *slightly* is that the error message sometimes disappears.

-Wayne

[Wayne 962]

Originally Posted by cabmando

From the sequence of operation in the owners manual:
The blower starts at approximately 400-500 RPM. After
20 sec, the motor is turned off for 1/10 of a sec where a coast down
calibration is done to evaluate resistance of the conditioned air
duct system. The microprocessor then determines blower RPM
required to provide proper airflow for heating mode.

Again, I'd make sure that every supply register is open. Yeah your return duct being choked down is half assed and shouldn't exist. No I don't think the dust in the flex is causing the issue. The flex itself can be an issue if the bends aren't smooth (like pictured) or where unsupported and sagging. The furnace should be adjusting for the inefficiencies in the duct system through the coast down calibration.

One more thing: What is the code number you're getting? 44?

The code is indeed 44 right now. Nothing else either. I bought some stuff at the hardware store to clean out the vents. The trouble is, I found some stuff down there that looks like it might have asbestos in it, so I FedEx'ed it to the testing lab in Los Angeles and I'll have them check it (by Thursday morning). Then if that comes back all clear, I will crack open the return pipe and tape a filter to it and suck air from the basement area and see if that makes a difference. I'll also clean all of the dust out too while I'm there.

-Wayne

[Wayne 962]

Originally Posted by billybek

What does the forward curve fan look like?
Are the blades of the fan coated or filled with dust?

As mentioned check your returns and supplies and maybe have a the ducting changed out. Flex is nasty stuff for resistance to flow when it is installed well. Yours is not installed well.

You can't really see the fan very clearly, but one can see about 10% of the edge of the blades and they are very clean. Sticking my hand in there (power off) I can't feel any dust on the blades. That indeed was one of the first things I checked...

-Wayne