Ok all you HVAC gurus, I have a furnace problem and need some guidance. This winter our furnace started throwing a Limit Circuit Fault, which results in it running for a few minutes then locking out for 3 hours. So far the techs have replaced the limit switch, told me the returns are too small, the filter was restricting the intake air flow, and now they are saying the furnace I had another company replace 6 years ago is too large for the size of the exhaust vent, and we just haven't noticed until this winter that the heat wasn't working.

They quoted me $4700 for upgrading the vent pipe from 3" to 5". Now I don't doubt the company that installed the furnace 6 years ago didn't size it properly, judging from the other stuff they mucked up, but I find it hard to believe that my wife, my son and I, plus anyone who visited us during the winter months in the past 6 years didn't notice the heat wasn't working (we keep the temp 75-78 in the winter).

So does this sound like the proper solution to my problem, and does this sound like a reasonable price for the work?

Make, model and serial.

Picture of the over all install would be great. If you can't post one, you can send it to my email.

$4700 for a new vent system? Where does it currently vent?

Ouch, this sounds very ominous. I am skeptical of what you are being told. I am not a tech or HVAC contractor. Excam is your man.
We have a Coleman high effiecicy Propane furnace that started making a rumbling sound while running. Service guy came by to check. No rumbling noise when he got there.
Said the induction motor was hotish, drawing more amps than it should. Gave us a price to replace it and also the burners (4) The furnace has been great and trouble free. Is like 18 years old. He said it looked great otherwise.
Reliable and honest companies are worth so much. Crummy ones can be horrific.
Yours sound like the later maybe.
Let Excam guide you some.

That would be a big furnace or an awful long run to make a change that big in the vent pipe size, especially if it's been "working" for 6 years.... I'm no expert in residential furnaces, I'm an industrial combustion guy, but I'm sure Excam_Man can help you in a pinch.

As a note, over-sizing venting, reducing velocity, can come with a whole slew of other problems. 5" is a huge pipe...

Is there any chance that your existing vent stack is plugged up or has some obstruction in it restricting flow?

quote:

especially if it's been "working" for 6 years

Yeah, that's a red flag. But as Mark asks, did something get into the exhaust vent?

The cost for that new vent system is rather close to the cost for my complete furnace purchased in 2021. I would strongly suggest you spend the time to get 3 different companies to quote you for a repair. Because to be succinct you are getting bent over a table.

We just replaced a 24 yr old 90% efficiency with a 96% 2-stage for $5000.

.

Somehow I missed your email. I just come across it.
I "replied" to it, but it come back saying the address couldn't be found or is unable to receive mail.

Which is weird, since I hit "reply" and its the exact address.

quote:

Nuclear

How about an update?

Call a chimney sweep if it is a category 1 unit. This means either vented to a chimney or metal vent up....

3" is usually for very small Cat one units... but then it must me one because I've never seen 5" on the newer high efficiency models that just use pvc pipe vented horizontally out of the house...

Yes, I'd like to know more about how it is vented. I spent the first 25 years of my career relining gas furnace chimneys before they started going with these new fangled plastic vents....

To clear up the venting issue...

He has a 110k 80% furnace, which requires a 5" b-vent system.
It is currently being vented with a 4" expandable, flexible vent pipe connector. Can not see where it goes from there...

We had a somewhat similar issue on our old furnace. What the furnace guy did was install a return air vent to the ductwork in the basement, above the filter. Then we put a vent in the basement door, so the house could pull air down to the basement. He also installed a supply vent in the basement that we could regulate, so we could adjust how much pressure was being put in the ductwork.

In addition, that price seems very steep that you were quoted.

quote:

Originally posted by m1009:
What the furnace guy did was install a return air vent to the ductwork in the basement, above the filter.

And if it's in the utility room like most which get added to inadequate return systems, it's against code and a major safety issue.

I’m currently waiting for an estimate from the second company on replacing the furnace with a smaller 80k btu unit. The third guy to come out confirmed it should be a 5” vent instead of the 4” flex that goes up to the 4” vent pipe that someone incorrectly teed the gas water heat vent into.

At this point we are using the gas faux fireplaces to heat the house. They work, are properly vented, but god only knows how much I will pay for natural gas this winter.

quote:

Originally posted by Nuclear:
that someone incorrectly teed the gas water heat vent into.

Just so you know, they can be vented together when sized properly.
Depending on the total vent height and lateral vent measurements an 80k furnace + a water heater (33-40k), can still be over the maximum capacity for a 4" vent system.

I have a whole email full of info and questions...
I'll resend, if you have an idea of why it won't go through?

Excam_man, no utility room in our basement. Unfinished, it’s a ‘Michigan’ basement.