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Heating, Air Conditioning, Fridge, HVAC/Carbon monoxide concern w/Furnace


Dear Jim:

I am writing because in the last 24-36 hours my 20 year old Lennox gas-fired furnace started making a strange noise, intermittently, not constantly.  This started after I took out the furnace filter and replaced it back in the unit.

I had the furnace checked about a month ago by two different HVAC companies.  One said my heat exchanger has a small crack and the furnace needs replacement, but did not detect any C02 in the air on the first floor.  This happened last fall also, with a different company.

The second company this year said he did not think replacement was needed yet.  I am concerned because this morning I felt a little headachey, which is not necessarily C02 related.  I am not home now and still feel headachey.  I have a carbon monoxide detector and it did not alarm overnight or while I was home this morning.

If you can give me some idea of if I may have jarred the furnace so now it is making this noise, which sounds a bit between a whistle and a buzzing,or if this could be CO2 related, I would greatly appreciate your opinion of the situation.

I had planned to wait until February 2015 for replacement.  Do you think that is wise or should I replace sooner given this new noise?   Thanks for any help you can give in this matter.


Hello Sharon, As far as the noise goes I would not think that has any thing to do with the heat exchanger. You won't be able to hear any noise if it has a crack. From what I have read the headache should stop once your out of the house for a while. Good to hear you have a CO detector, in your situation I would test it to make sure the battery is good.

I do have a concern about tech who said that there was a crack. If he saw a crack he should have disabled the furnace. He might be trying to sell you a new furnace. I call those guy's sales tech's.

I would suspect changing the filter might have something to do with it. Was the old filter really dirty? If you still have the old filter put it back in and see if the noise goes away. If the old filter was really dirty you would have less air flow in your return duct. With the new filter the air flow will increase. Thus you might hear some whistling.

Let me know what happens.

Heating, Air Conditioning, Fridge, HVAC

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Jim S.


I can answer questions on gas or electric furnaces, ac units and heat pumps. I don't work on boilers,chillers,or home appliances.


I have been in the HVAC trade for over 35 years. For the past 15 years I have owned my own business. I am semi-retired so I can answer questions in a timely manner.

I attended trade school for 5 years on weekends and evenings. This was very hard to do, but well worth it!

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