Gas furnace problem

Stan in Oly, WA

Well-known Member
We had a technician from a local HVAC company do an inspection/tune-up on our gas furnace a few weeks ago. I wasn't home when he was here. He told my wife that the heat exchanger is showing a weak spot---he said he could tell from the color of the metal---and is due to fail at some point in the near future. He told her the reason for this is because we have been using the 1/2" thick metal mesh filters which were previously used in conjunction with electrostatic filters (which we stopped using), as well as a standard disposable corrugated fabric furnace filter. This, he claimed, starved the furnace for air and led to overheating. Is this plausible? If so, what would be the immediate effect of the heat exchanger cracking during use? There is no emergency gas shutoff to deal with this situation.

If my tone makes it sound as though I'm suspicious, that isn't entirely the case. It's just that I would be more comfortable accepting the information if it didn't come from someone who has something to sell me.

Thanks,

Stan
 
I would NOT worry about the information.
A cracked heat exchanger will give off CO.
If the furnace was being starved of air, the "over temp" limit would be cycling the burner OFF.

Unless a heat exchanger is cracked would I even consider replacing the furnace.

John,PA
 
HVAC contractors wouldn't do ninety dollar inspection specials if they didn't expect to sell a few new furnaces. Any modern furnace will have overtemp protection to shut off the gas if airflow is restricted to the point that there's a risk of damage. If the furnace is 20 years old then I'm not so sure.

You can buy an excellent CO monitor for less than fifty bucks. Very cheap insurance against a bad furnace. Especially when you compare it with the price of a new furnace.
 
I would be very suspicious, and would likely get another opinion from a known reliable source. Just sounds a little far off base. I am curious how long he has been in the business. I have worked in HVAC motor mfg for many years, and had a good friend ask me to take a look at his condenser fan motor, as he was having to replace it yearly. The local HVAC guy was putting an open ventilated motor in a condenser fan which allowed elements into the windings. Each spring the motor failed. I helped him install a correct motor, with rain shield, and the motor was still running when he sold that home several years later. Someone was making a lot of money installing motors to fail, and he was either doing it purposely, or did not know enough about the business to be in the business.
 
When I had my hunting camp propane furnace serviced,
(no problems)
I asked for a few tips as I wasn't familiar with gas, having an oil furnace at home.
Guy said pretty much the same thing.
He said to buy a stack of the cheapest 99 cent filters and just change them often. Not to use the modern, filter everything, thick, pleated ones.
 
heat exchanger cracked several years ago. Smelled like natural gas leaking but couldn't pin down where. Checked all the lines; none leaking. Then saw the crack.
 
My furnace man after looking at my 35 year old natural gas furnace explained cracked heat exchanger worries this way. He says that because of the blower there is positive pressure on the heated house air side of the heat exchanger & negative pressure on the burner side of the heat exchanger because the hot exhaust air is being drawn up the chimney. So even if the heat exchanger is cracked, CO should not go from the burner side to the heated room air side. Rather air flow through a crack would be in the other direction, from the blower forced air side through the crack & out the chimney.

A more likely cause of CO in the house is a clogged chimney.
 
I agree the pleated paper filters are too restrictive. Lack of air flow will cause the heat exchanger to overheat. There is an overheat sensor that will cycle the gas valve to prevent the furnace from dangerously overheating. I doubt the overheating would cause damage to the heat exchanger. It will decrease efficiency though.

Really don't know what he was seeing that would indicate the exchanger is failing, they all discolor with use. The most common failure I've seen is rust through, typically from condensation from the evaporator leaking, or rain coming down the chimney.

A failed exchanger can allow CO to enter the house. Depends how the unit is installed. If it's inside the living space CO can be blown out of the unit instead of going up the chimney. Any home using gas heat should have CO detectors!

A leaking exchanger can also cause the fire to "spill out" the front of the furnace. Most modern furnaces have sensors that will shut off the gas if this happens, still not a good idea to store anything near the furnace! That is the most obvious symptom of a failed exchanger, the fire blowing out or being blown around in the fire box when the blower comes on.
 
Hogwash. Unless some idiot has removed and straight-wired it every gas or oil furnace has a high limit switch. It gets too hot it shuts down and when it cools off it restarts.

Invest in a co2 detector which should be there anyway and don't worry about it. BTW - if you watch the furnace flames when it starts and the flames stay the same when the blower comes on the hx is ok. The flames will change when the blower comes on if there's a crack.
 
Stan I would do as someone suggested and buy at least 1 good quility CO detector for each bedroom and let the replacment ride until you are certain one way or another.
Did or could the doubled filters cause a problem??? In your case I don't know because I don't know what the old electo filter looks like but my units have 2 filters because I built racks to do so. First filter is pleated Merv 6 and secound filter is pleated Merv 12 and they do not restrict air flow. Most electrostatics I have seen are aluminum mash that you wash off. That type will not restrict flow.
Ralph and NNP,this does not reflect on you in any way,but those are stupid statments coming from a supposed professional. I would love to come face to face with them to see if they care to wager $100 on insturments proveing their assertations. I will and prove both are full of $h1t useing tools and instruments desighned for HVAC professionals. If requested,I will cheerfully expound on filters and what you can do to save money plus extend equipment life.
 

I don't know about the filters in your case. But I have a friend/customer that has a very good oil furnace. several years ago I found it to have a crack in it's heat exchanger, which I replaced. When checking the system out I found that half one of the return intakes had been blocked off. There had been two stud cavities, side by side, that had return air registers, and one had been blocked off when some remodeling was done.
So yes even though there is an over temperature switch, over a period of time a lack of air flow can ruin a heat exchanger.

Dusty
 

I don't know about the filters in your case. But I have a friend/customer that has a very good oil furnace. several years ago I found it to have a crack in it's heat exchanger, which I replaced. When checking the system out I found that half one of the return intakes had been blocked off. There had been two stud cavities, side by side, that had return air registers, and one had been blocked off when some remodeling was done.
So yes even though there is an over temperature switch, over a period of time a lack of air flow can ruin a heat exchanger.

Dusty
 

This is true with the newer gas furnaces where there is a combustion fan on the exhaust side of the heat exchanger, creating a negative pressure within the heat exchanger. But in the older style furnaces that did not have a combustion fan but just used the draw of the chimney to remove the products of combustion, there is positive pressure within the combustion chamber, and if there is a crack/hole in the combustion chamber, then they will leak into the home.

Also on most oil furnaces there is a combustion fan on the burner before the flame, creating a positive pressure.


Dusty
 
You are right to be suspicious. Mom once paid for a new furnace and all the old one needed was a thermo coupler. She did call them back and had it removed. I also suspect another place sold her a NEW water heater when I'm betting a new element would of cured it. Sure the water heater was old but I'm pretty sure when Dad and I installed it was the top of the line Sears and was a stainless steel model. So I'm thinking the tank should of been still good.
 
I am not a furnace man, but do have some practical experience.
First, if you use any type of combustible fuel, gas, oil, or wood you need carbon monoxide detectors. One outside the bedrooms, and another in the same room with the gas or oil furnaces. If a wood stove the detector needs to be a ways from it to keep it from falsing when the door is opened.
Second, if you have a crack in a heat exchanger you will get carbon monoxide into the house air side of the exchanger during the time the burner comes on and starts warming the exchanger before it gets hot enough to turn the blower on. When it is running, if there is a crack on the upper (down wind) side of the exchanger it can draw the CO out due to a low pressure spot on the down stream side of the exchanger. to do that it would have to be in just the right spot though.

No matter what the condition of your furnace I recommend at least one CO monitor and preferably 2 in any home using burnable fuel for heat.
 
Bottom line, you are trusting your life to a bunch of people here that have no training or skin in the game.

I would pay to get it inspected by another party and install CO monitors.

What is your families lives worth.
 
We put our gas furnace in the 1960s. It looks good. But, I still put in a Carbon Monoxide alarm in. I like the sugestion of more than one.
I am going to get another one.
 
I to would be suspicious, seems the HVAC trade has picked up a lot of high pressure salesmen that want to install a new furnace anytime a house changes owners or they figure the warranty on the old unit is out. Not to make excuses for them but it's a very seasonal business that requires folks with some skills, here lies the trouble, you either hire folks in the fall and lay 'em off in the spring and have all the associated problems with turning the bulk of your employees every year OR you find work for them to do (whether it need to be done or not, just as long as someone else is paying). Add that to a commission based incentive program that often requires the employees to sell goods and services the customer doesn't need just to make a living wage and you can see the potential for abuse.
 
David,everyone,well almost everyone, advised caution and preventive measures while researching before throwing money at it. It doesn't require training to realize "told my wife" is a poor excuse for not writing it down. That is a red flag and I don't blame the man for questioning it. There is few if any contractors willing to insure the HEx is safe after has been condemend,think liability and lawyers. If a herpetologist tells you he identifyed a Black Mamba in the crawl space,there is not a sane human on earth willing to stake their fortune and repitation simply because they can't find the snake,except the lying snake man who charges $500 to come back to catch it while you are absent. And mail pictures to support his case and fee. In the event an exterminator captures a big ol chicken snake,the snake man go's into song and dance about not calling it a Mamba,you just misunderstood. The trick is old as man kind and known as poisoning the well and you did the same. If I pay a professional tractor mechanic to get my machines serviced and ready for hearvest,I expect him to write needed major repair out on paper,not tell my wife what to tell me. That's their MO so that if their hand is called later they can say the wife misunderstood. If he puts it on paper that I need a pump amd all new injectors there is no wiggle room after my neighbor fixed it with a couple of O-rings. I have enough training and skin to keep my trap closed when I don't know jack.
 
When I had gas furnace and water heater, I had a digital CO meter. They are very cheap and now have battery backup.

I have a portable LP heater for backup. I use the battery CO meter when I use it.

A person I worked with was scammed by a HVAC guy. I took my CO meter to her house and found nothing.

She had a natural gas furnace. I told her to call the gas company. The gas company came out and told her there wasn't anything wrong with her furnace.

Any person with a gas furnace should always be aware that CO is the silent killer and you should have a CO meter.
 
There's two ways to look at it... if you are the furnace service guy and see things not up to par and DON'T say anything, and the problem gets worse and there's CO death(s) you are likely to be sued because you will be brought up as having serviced the furnace.

On the other hand, there's scammers who will condemn a perfectly good furnace to get the $$$$ job of replacing it.

There used to be an outfit from a town a few miles from here that had some really official forms printed up (looking like they were a notice from the state or county) "condemning" the furnace.

Did a pretty good job of selling them a new one.

Of course, I can't say how many were really bad, or not.

Like others have said, though, a good CO detector or two are ESSENTIAL if you have a fuel-burning furnace in your living space.
 
I have found a a few furnaces with bad/crack, hole heat exchangers. I've always shown the customer the crack and even if it was fired with natural gas encouraged them to call the gas utility co.

Dusty
 
According to the tech there IS NOT a crack in the hx. The metal is discolored from heat according to him. It's a furnace. You reckon it ain't supposed to get hot? You reckon they managed to weld that sheet metal to make the hx without it getting hot? If he had said rust it would have been a whole nuther ball game but he didn't. If the furnace is 10 years old he can truthfully say it's due to fail sometime in the near future but it could last another 20 years. That's why you have them serviced every year and use detectors.

BTW - I don't like the super duper airflow reducing filters any better than your tech. It's his metal colour skills I question.
 

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