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Corrosion on ductwork, flue height, A/C stop-start


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Hello,

I am new to the forum and my HVAC knowledge is very limited. While I have already searched for answers and read what I could, please excuse me if anything in the following is naive or a repeat of another thread. Also, if it is better to split this thread into several different threads with single topics, please let me know.

A bit of info:

My wife and I moved in to our house in May of this year.

We have central A/C in the house and it works well, save for one bit of odd behavior: When using the central A/C, the furnace will kick on the fan for a few seconds, then quickly stop/re-start, and then run without further interruption.

We changed out the old manual-dial thermostat for a programmable digital Honeywell.

I have recently tested the furnace on the "heat" program and it seemed to function properly.

Our inspector noticed a couple of things with the furnace. The first is the presence of corrosion on the exhaust ductwork leading out of the furnace and into the wall. The second is that the furnace flue on the roof is not tall enough and needs to be extended in order to meet code.

Here is a link to an online album since I can't upload photos right now: CLICK ME FOR PICS

I have an appointment next week with a local HVAC service company to inspect these issues and perform a general tune-up, but wanted to ask the forum beforehand to prepare for what the tech might have to say.

My questions are:

What do you think the cause of the corrosion is on the ductwork, and how should it be fixed?

Is the only reason to extend the flue on the roof to meet code, or are there performance benefits/other reasons to do so? (I think I've read that a short flue could be the cause of backdrafting and/or corrosion in the liner, if I recall correctly.)

Do you think the "start-stop-restart" behavior when running the central air is due to the A/C unit itself, or could it be something with the furnace or new thermostat? This does not happen when running the "heat" program, only the A/C.

Do you see any other potential problems in the pictures?

Thanks,

sleepy

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The corrosion is on the vent connector, that vents combustion byproducts from the furnace into the vertical vent system. It's because there is condensation forming inside the vent system. The picture of the furnace with the cover off shows it has been dripping into the furnace cabinet as well.

It could be because the vent connector from the furnace has two diameter increases and the gasses are loosing buoyancy. Also, a "Y" connection to the main vent would be preferred to a "T".

The picture of the exterior vent termination doesn't look too short - at least not from that angle in the picture. The inspector might be applying the rules for a solid-fuel vent/chimney termination.

A good tech should be able to figure out your issues and hopefully wont try to sell you work that you don't need.

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Thanks for the reply, Bill!

I'll do some research re: condensation in the vent system. Any idea on the start-stop-restart behavior of the central A/C? I know I was a bit vague on that part, but I'd be happy to supply more info if it would be helpful.

Thanks again,

sleepy

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I agree 100% with everything that Bill says. Nothing wrong with the height of that gas vent.

A couple of question - is that furnace located in conditioned space or is it in a cold area? What's the volume of the space where that furnace and water heater are located? Are there any exhaust fans or a clothes dryer nearby?

ONE TEAM - ONE FIGHT!!!

Mike

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Hi hausdok,

Thanks for the reply, and for corroborating Bill's view on the gas vent.

A couple of question - is that furnace located in conditioned space or is it in a cold area? What's the volume of the space where that furnace and water heater are located? Are there any exhaust fans or a clothes dryer nearby?

1. The furnace is located in an unfinished basement. We haven't been through a winter in this house yet, but I'm guessing the basement will be relatively cold in the coming months since we're in Michigan.

2. After taking some rough measurements, the basement is 26 x 31 x 8, so 806sqft and 6,448cuft.

3. There are no exhaust fans or clothes dryers nearby. The laundry is located on the second story. Also, the walls nearest the furnace and water heater are both exterior foundation walls.

Thanks,

sleepy

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As Bill stated the flue gas, from the furnace, is condensing and dripping back down the flue and leaking out at the connections. It looks like it may be due to an over sized flue.

With regards to the indoor blower starting,stopping and then restarting - this is not a good situation especially if the outside condensing unit is doing the same thing. It's not good for electric motors to bang on and off.

There is no way someone can troubleshoot this problem without actually looking at your system and testing the electrical circuitry.

Let us know what the tech finds.

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Hi Terence,

Thanks for the reply! I'll make sure to show the tech the on-off-on behavior when running the A/C unit.

One question on this...it is getting cold quickly here, down to the low 30s overnight, and we had a frost this morning. Is there any problem with running my A/C unit for the tech on Wednesday morning when the temp will probably be in the 30s to 40s?

Thanks,

sleepy

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Hi Terence,

Thanks for the reply! I'll make sure to show the tech the on-off-on behavior when running the A/C unit.

One question on this...it is getting cold quickly here, down to the low 30s overnight, and we had a frost this morning. Is there any problem with running my A/C unit for the tech on Wednesday morning when the temp will probably be in the 30s to 40s?

Thanks,

sleepy

A smart tech will use the electrical disconnect on the condenser unit outside to remove power from it, so it can't become damaged. Doing so will not affect the furnace or blower inside the house.

Make sure he does so.

Marc

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