Chimneys: if 6" liner added, does it need sweeping first?

[TANSTAF1]

I had a CSIA chimney sweep look at my chimneys. The reason for having him come is that my chimneys have not been swept since I've lived here (25 years) and who knows when before that.

I have never used the one with the fireplace so whether it had been swept or not is moot. But I am thinking of adding an insert which will require a liner. The (exterior) chimney is large - I think he said 11x16. (I do not have the estimate yet which might provide details.)

Some bricks had fallen off the top layer of the interior chimney. The interior one has a gas furnace and hot water heater. He looked at the top and I got the impression the next row of bricks below the ones that fell off did not need to be replaced. He must have determined it would be better to add a liner since without my requesting one, he got the BTU's for the furnace and heater and calculated that 6" one would be sufficient. The furnace is a 1976 Burnham America with an IBR rating of 83,500 BTU's and an AGA one showing 120,000 BTU input and 96,000 output. It replaced an oil furnace. I would think that one that old is not high efficiency (which from reading here can do more damage to a chimney). If I divide the 96000 output by the 120000 input I get 80%. Would that be the efficiency? Seems too high.

The heater is a 40,000 BTU 50 gallon one that was installed this past May by National Grid. Strangely there is no efficiency rating on it and we were more concerned with the tank life warranty to focus on that, but presumably National Grid would know if it was high enough efficiency to need a liner to meet code.

I am finally getting to my question which is as respects these chimneys I would think the gas furnace one does not need to be swept before the liner is installed as it would not put out anything combustible that would have seeped into the mortar joints, correct?

What about the fireplace chimney? I am still undecided as to whether we do an insert. We may just continue to not use it. Or we will just use it as an open fireplace (without a liner as I am as yet unconvinced that it needs one for use just 2 or 3 times a year) in which case we will have it swept first.

But if we do an insert I plan to insist on an insulated liner. If so, do I need to have it swept first? I think a 2" clearance is needed to combustibles which I guess could include creosote that has seeped into the mortar joints and I think a liner could sway within the chimney and come closer than that.

 

[thechimneysweep]

Your Sweep is right to suggest a liner for your gas flue: if your flue is unlined, the nitric acid present in the condensation from the gas exhaust is attacking the bonding agent in the mortar joints, especially at the top, where condensation is heaviest (evidence the loose top course of bricks). If left unlined, this chimney will eventually decompose into a stack of bricks with mushy sand between the layers.

The Master Sweep I apprenticed with 30 years ago taught me a rule of thumb for determining liner size for multiple gas appliances: the cross-sectional area of the liner must be at least the size of the flue collar on the largest appliance, plus half the flue collar size of each subsequent appliance venting into the same flue. The formula for determining the CSA of the flue collars and liner is radius squared x pi (3.1416), so the proposed 6" liner (28 sq.in.) would be large enough to vent a furnace with a 5" flue collar (CSA 20 sq.in.) plus a water heater with a 4" flue collar (CSA 12.5 sq.in.), the required flue size for that combination being 26.25 sq.in. CSA.

Before installing an insert and liner in your fireplace, you want to clean the flue, smokeshelf area and firebox as clean as you can. This will help stop creosote odors from entering the house when you fire up the insert, and also eliminate the chance that any left-behind deposits might ignite due to exposure to the heat from the insert and liner over time.

Just last week, The Sweet Child ((broken link removed to http://www.chimneysweeponline.com/emphow.htm)) inspected the aftermath of a chimney fire where a gas insert and liner had been installed in an unswept fireplace chimney. Even though the gas insert and exhaust liner create much less heat in the fireplace structure than a wood-burning system would, the creosote caught fire, and melted the aluminum gas liner to slag.

 

[JJEGLBS]

Just last week, The Sweet Child ((broken link removed to http://www.chimneysweeponline.com/emphow.htm)) inspected the aftermath of a chimney fire where a gas insert and liner had been installed in an unswept fireplace chimney. Even though the gas insert and exhaust liner create much less heat in the fireplace structure than a wood-burning system would, the creosote caught fire, and melted the aluminum gas liner to slag.

In my case, we rushed our insert installation last year and probably should have had the chimney cleaned. The question is: what can I do know with the insert (and 6" liner) already in? It's an uninsulated liner. I was thinking of adding loose vermiculite, or maybe Thermix.