Chimney Liner


Old 02-04-15, 01:28 PM
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Join Date: Jul 2005
Location: USA
Posts: 70
Chimney Liner

I am looking for some education here.

I have a rental property with an old oil fired hot air furnace that has been giving me trouble for the last few years. It finally gave out last week, so I had to replace it. There was gas at the property, so I decided to change over to a gas fired hot air heater, rather than an oil fired furnace. I spoke with two HVAC guys in getting prices.

One told me that a "normal" efficiency heater would vent into the existing chimney, but, since I was changing from oil to gas, I would need to line the chimney. His recommendation was to go with a 92% efficiency furnace (Goodman 72,000 BTU Upflow) and vent out the back wall rather than using a cheaper 80% furnace and lining the chimney.

The second one told me that, since my chimney was already lined with terra-cotta, the liner was unnecessary. He explained that the liner would only be needed on a brick and mortar chimney (that does not have a terra cotta flue).

It should also be noted that the chimney is located on the rear exterior wall of the home, and that the water heater is and always has been gas fired, and vents to the same chimney.

Because the difference in price was not that great, after considering that the Utility company would give me a $300 rebate, I decided to go with the 92% furnace.

However, I have a couple of questions:
1 - Who was correct?

2- Why would the chimney need to be lined simply because I am switching fuel supply? If there were leaks in the joints that could allow Carbon Monoxide into the home, the change in fuel should not change that fact. In other words, had I stayed with oil by buying a new oil fired furnace rather than a gas furnace, it is my understanding that the chimney would NOT have to be relined. That doesn't make any sense to me.

3- If the chimney did need to be re-lined, could I run an aluminum liner, or would I be forced to use stainless steel?

Thanks in advance!
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Old 02-04-15, 01:48 PM
Join Date: Oct 2008
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The existing lining would not have been terra-cotta. Probably fire clay.

The applicable code is NFPA 211. Installing a more efficient furnace could result in the lining diameter being too large to produce enough draft to avoid condensation which, in turn, might require a second lining inside the first. If the replacement furnace was the same efficiency as the first, then the existing lining would be OK if, after inspection, it were determined to be in good shape.
Old 02-04-15, 03:22 PM
Join Date: Oct 2008
Location: New England
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I'm not a pro on this topic, but reducing the diameter size as gilmore explained is necessary to eliminate that condensation. Gas produces a lower temperature exhaust and need to get out of the chimney before it cools. However, you now have another problem, which is the orphaned gas fired hot water heater. When sharing the flue during cold weather the oil fired appliance would warm or dry out the chimney. Now that you are direct venting the new furnace, the gas hot water heater needs one of those smaller liners. I'm not sure if it is a code issue or just a mfg requirement, or just best practice, but those sales people should (should) know all of this.


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