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HVAC Supermodel
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Discussion Starter #1 (Edited)


This heat exchanger came from an $800,000 house that the installing contractor botched, they not only botched this house but all others in the development. This 12 year old 80k BTU 3 ton furnace had 2 8x20 returns, both using only the wall cavity one of which was completely blocked with insulation. The supply duct was 10x11... This heat exchanger didn't have a chance. When I showed up for a maintenance I fired up the furnace and felt a massive amount of heat coming from the blower compartment, you could fry an egg on the firewall (downflow). At that point I was sure the heat exchanger was blown, pulled the blower and found the cracks. Anyway, just wanted to show the homeowners here what happens when you don't make an informed, non-price driven decision when it comes to your HVAC system, cracks like these are bad for your health!
 

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Good information.

I have often described my (new home) installation, as 'installed by a drunk'.

The ductwork, both supply and return, had many leaks. One, a rather large leak, hidden on the return trunk in the basement, must of had us breathing CO for 20+ years.

Another gem was, where a run elbowed around cinder block making it short of the wall oval elbow (terminology?) by several inches. I had to chisel out some of the block to bring the ends together.

Worst of all, was when we first fired up the furnace and smoke filled the basement. Seems there was a short of power wires against the frame. The tech (owner) fixed the short, but didn't change the melted wires. Seems he just tucked them into the inside corner of the furnace. Didn't discover it until system changeout 20 years later.

V
 

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I'm Your Huckleberry
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Unless your heat exchanger was cracked or your flue vent pipe was leaking, no CO would make it into your home via only leaky ducts. Certainly your system would be less efficient with leaky ducts but CO would have nothing to do with it not too mention I highly doubt you would have lived 20 years breathing CO.

And it is very unethical and very unprofessional, far less than amatuer, to chip away at anything which not so loosely translated would also be described as deforming your home, to connect ducts. Buy the duct and extend it.

Just saying.
 

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CO from a furnace, is commonly drawn in through a leaky return duct. Seldom does a cracked heat exchanger put CO into the supply duct. The heat exchanger on an 80% is a negative vessel, with a positive pressure on the outside of it. So CO seldom comes from the crack it self.
 

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Unless your heat exchanger was cracked or your flue vent pipe was leaking, no CO would make it into your home via only leaky ducts.
I should have added that when the furnace was running, I was getting some back-drafting from the WH. The leaky return ducts plus the hidden gap didn't help.

Certainly your system would be less efficient with leaky ducts but CO would have nothing to do with it not too mention I highly doubt you would have lived 20 years breathing CO.

And it is very unethical and very unprofessional, far less than amatuer, to chip away at anything which not so loosely translated would also be described as deforming your home, to connect ducts. Buy the duct and extend it.
When the run was installed, the contractor chipped the end of the block, but not enough. There was no way to replace the piece of duct as it was captive between the top edge of the block and the ceiling beam. The gap was less than 6", so the only way to do it was to chip a little more away.

Just saying.
Like I said, the system was installed by a drunk. I agree with your statements, but it is (was) what it is (was). I have fixed all of the leaks.

On another note, what's a huckleberry?
 

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I'm Your Huckleberry
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CO from a furnace, is commonly drawn in through a leaky return duct. Seldom does a cracked heat exchanger put CO into the supply duct. The heat exchanger on an 80% is a negative vessel, with a positive pressure on the outside of it. So CO seldom comes from the crack it self.

I am not following or understanding any of this. The blower is first, then the heat exchanger and then the supply so how does the return, the very low negative pressure air flow on the far back of the furnace outwiegh the high velocity and volume just after the furnace on the supply side? Where is this CO being sucked back in to the return from if the blower is pushing it out of the supply side, return leak or not? We are talking a cracked heat exchanger with a return air leak and a cracked heat exchanger without a return air leak.

Can you explain, please?
 

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The heat exchanger, if it is cracked, will not put CO into the air directly. It may blow back through the burners out into the furnace room, where the return duct leakage, and or loose/open air filter bracket will draw it in. And then blow it into the supply plenum and duct.

The heat exchanger of an 80% is under a negative pressure, so there is no pressure to the blow the CO out into the supply plenum or duct.
 

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Wire Chewer
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Ouch that's nasty... did not realize it was under negative pressure though, but guess it makes sense, so the draft inducer motor is after the heat exchanger so it's sucking air from outside, through the burner then heat exchanger, then pushes it outside. I'm sure it's still a huge efficiency loss though.

This is why CO detectors are important. If by chance it did let CO in, you'd be informed about it. Though most people just go take the batteries out... lol

Duct tape should fix that good as new. :whistling2::laughing:
 

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Wire Chewer
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I am not following or understanding any of this. The blower is first, then the heat exchanger and then the supply so how does the return, the very low negative pressure air flow on the far back of the furnace outwiegh the high velocity and volume just after the furnace on the supply side? Where is this CO being sucked back in to the return from if the blower is pushing it out of the supply side, return leak or not? We are talking a cracked heat exchanger with a return air leak and a cracked heat exchanger without a return air leak.

Can you explain, please?
I'm guessing it works like what I said, but maybe someone can correct me, I'm not a hvac expert.

A picture is easier then trying to explain it. I'm sure it's more complicated than that, as I think there is actually a secondary heat exchanger, and probably a bunch of regulators and stuff to ensure the maximum clean combustion.
 

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That pic is an older style furnace which relies on natural draft, beenthere is talking about the new style 80% er's that use a small fan to draw the exhaust through the heat exchanger.

Undersized ductwork and oversized furnaces are very common, I hate telling people that their 4 yr old purchase is almost worn out. Furnaces, just like all our other appliances just can't take the abuse like the old stuff. They must be installed correctly!
 

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I'm Your Huckleberry
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The heat exchanger, if it is cracked, will not put CO into the air directly. It may blow back through the burners out into the furnace room, where the return duct leakage, and or loose/open air filter bracket will draw it in. And then blow it into the supply plenum and duct.

The heat exchanger of an 80% is under a negative pressure, so there is no pressure to the blow the CO out into the supply plenum or duct.

I finally realize what you are saying concerning the draft inducer motor pulling air through the heat exchanger being negative pressure. Of course, I was thinking you were saying the return air pressure would suck against the draft induced pressure and pull CO out and into the air stream directly.

So we are saying the same thing, the CO has to be brought out of the furnace first and sucked back in through the return air (leak) and then be distributed through the supply.
 

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I finally realize what you are saying concerning the draft inducer motor pulling air through the heat exchanger being negative pressure. Of course, I was thinking you were saying the return air pressure would suck against the draft induced pressure and pull CO out and into the air stream directly.

So we are saying the same thing, the CO has to be brought out of the furnace first and sucked back in through the return air (leak) and then be distributed through the supply.
But, it is generally not drawn out through the crack. The CO may have already been coming out before the heat exchanger cracked.

A lot of good heat exchangers have been condemned because of a gas fired water heater not drafting properly.
 

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HVAC Supermodel
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Discussion Starter #18
But, it is generally not drawn out through the crack. The CO may have already been coming out before the heat exchanger cracked.

A lot of good heat exchangers have been condemned because of a gas fired water heater not drafting properly.
I always do a visual HX inspection to confirm. I see what you're saying about the negative pressure thing but with larger cracks such as these I usually get a CO reading, why is that?
 

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I always do a visual HX inspection to confirm. I see what you're saying about the negative pressure thing but with larger cracks such as these I usually get a CO reading, why is that?
What is the return like? Open slot for the air filter?

Next time your on one. Take a CO reading in the return in the blower compartment, and see if it isn't pulling CO in through the return. then also measure the CO in the return duct a few foot from the furnace. Will probably read 0 in the duct.
 
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