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metal liner for chimney needed for new furnace?

I am obtaining estimates to replace the furnace (forced air) and A/C for our house. One of the estimaters told me that with the newer more efficient furnaces, it will be be required to have the chimney lined with metal since metal conducts heat better that a clay pipe liner. He said that without the metal liner, the furnace exhaust would condense prior to making it out ot the chimney, causing eventual deterioration and collapse of the chimney. Is there anyone out that that can comment on this? Is the metal liner needed for permitting? I live in Morris Township NJ.

Re: metal liner for chimney needed for new furnace?

Did all of the estimates include this liner or just the one estimate?
With some furnaces, you can use PVC pipe for the exhaust and intake air. Was this provided as an option for you? A metal liner in the chimney would not be necessary for this type of exhaust. They would not necessarily have to route the pipe out the chimney either, since these are usually fan driven units, they can be exhausted out a side wall. Ask these kind of questions to the folks giving you the estimates and see if your local inspector will give you any information.

Re: metal liner for chimney needed for new furnace?

Newer forced draft furnaces will not work properly emptying into an over sized chimney there is not enough heat or flow to get the proper draft. It must have a pipe 3" or 4" in diameter. High efficiency furnaces can be vented through PVC pipe and can be, and it is recommended, vented out a side wall. Both have restrictions on the number of elbows and length of pipe.

Re: metal liner for chimney needed for new furnace?


The estimator was correct.

Older furnaces and boilers were engineered to have a stack temperature of 500-600 degrees, so they would heat the flue pipe & clay chimney liner to induce a strong draft to get the combustion products up & out of the chimney top.

This, however, was very fuel wasteful; all modern furnaces & boilers have re-engineered their units now to a stack temp <300 degrees.

As McDaniel points out, this is not enough to heat the clay/brick & induce a draft, thus a steel liner is required.

There is a separate class of 3-pass condensing boilers & furnaces that don't use the chimney at all; they can condense the burned fuel in the combustion chamber & heat exchanger to the point where all that's needed is a pvc pipe to drain the slightly acidic dirty water down a nearby drain.

Such condensing boilers & furnaces (usually gas-fired) are much more expensive, require more elaborate parts & equipment & tend to need more periodic adjustments than more conventional units.

Re: metal liner for chimney needed for new furnace?

I am looking at the 80% efficient units, which are exhausted through the chimney, not the super-high efficiency types which exhaust out the side of the house. The problem that was presented to me by the estimated was that the exhaust would begin to condense inside the chimney thereby causing the chimney clay liner joints to deteriorate and the chimney to then deteriorate. The second estimater knew nothing of any requirement for a steel liner.

It's a gas-fired furnace.

Re: metal liner for chimney needed for new furnace?

Given the type of furnace you are looking at, the two issues presented earlier are the reasons for the liner. The chimney being too large for a proper draft and the lower exhaust temperature.

Re: metal liner for chimney needed for new furnace?

Considering that you are looking at the 80% efficient gas fired unit it's all the more reason it's critical having the correct sized liner. The one estimator was correct for the reasons that JLMCDANIEL and JacktheShack outlined in their posts.

The one thing I would add is to consider having a stainless steal liner rather than a galvanized one. Depending on the building code in your area it may be a requirement to have the stainless steel one anyways. The exhaust gas will be moisture laden and somewhat acidic which will eventually rot the galvanized steel liner. There will likely be an up charge but depending on how long you will be in your home it's worth it.

If the second estimator didn't have a clue about this my recommendation would be avoid that company.

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