Hello Forum, I've got a 2-flue chimney, with fireplaces in my basement, and directly above that on the 1st floor. 1st floor is in use as a wood-burning fireplace. The basement fireplace has been closed up, and the flue has been used for the past 40 years as exhaust for the boiler. I'm replacing the boiler with a direct vent unit which will be vented directly to the side of the house. I will no longer need that flue for the boiler. We can close up the hole in the side of the chimney. Can I re-open the basement fireplace that used to be served by that flue, and use it? I don't mean do the work myself; I'd have a professional do it. My question is more based on code (I'm in NY state) and the safety of such an arrangement - are dual-flue chimneys up to code in the modern era, or are they just grandfathered in if they are still working? It would matter because we would be re-opening a fireplace that's already closed, so it would probably require a permit. Thanks in advance, Nelson

Side wall venting of an oil fired 80%, which is most common, appliance is called "power venting". There are two manuf of power venters. Field Controls and Tjernlund. In my business I represented Tjernlund for 20 years and have a lot of them installed in the Northeast, where 80% of oil heat is located. As I noted previously, back in the 80's I represented a company who made a condensing oil fired furnace. They had other excellent products but the condensing unit put them out of business. Power venters are excellent products BUT, they have to be installed and serviced by a real tech. The biggest problem with PV is that the servicing company will clean the furnace or boiler and not touch the PV. Everything that goes through the furnace goes through the PV. When properly serviced they are an excellent exhaust system for oil appliances Power venters on oil boilers which also provide hot water are an occasional problem if located where the fumes from combustion are venting close to your deck or by a window which you want to open. The flue gases get blown out of the building but then waft along the wall to your BBQ and, well, if you can believe it, some people don't like the smell. If it is a winter only operation the odor issue is much reduced as the patio is not used and the windows are closed. Of the two manuf of PV, while I am biased, I think Tjernlund makes the best. I can defend that technically, but wlll offer one point. The Field's unit blows the gases straight down against the side of the house where the Tjernlund blows them straight away from the building. A properly set up condensing oil furnace is a very efficient appliance. That said, they are service intensive and have not been embraced by the oil companies. This is after 30 yrs on the market. If the techs don't like them, then you had better not like them either. My biggest problem with oil heat is that there has just not been any technology advances in the business. Why after all these years is there not an effective reliable modulating burner? Oil has 140kbtu, vs 100k for NG and 92 kbtu for LP. My feeling is that the modulating technology common in gas appliances makes up the difference in heat potential in the benefits of modulation. For the OP I just can't see trying to save a chimney that has deteriorated to the point that it is separating from the building. That is the result of decades of condensation and acidic consumption of the flue itself. There is no way that chimney passes the requirements of NFPA 211-Chimneys & flues, NFPA 54-Gas Code or NFPA 31-Oil Installation, all of which have an inspection requirement which this flue simply will not meet. Lining a structurally sound chimney makes sense. Lining a compromised chimney is throwing good money after bad, imho!

In general, taller chimneys have stronger drafts. Stronger drafts help to reduce creosote formation. Many other factors are involved, however. Exterior chimneys are much more prone to creosote deposits than interior ones because they cool off much more rapidly. Insulated chimneys, with or without a metal liner, stay hotter longer so resist creosote formation better. Christopher_H is correct that using properly seasoned wood is the best way to keep creosote to a minimum. The only way, however, to tell whether wood is ready to burn is to use a moisture meter on a freshly exposed surface. A moisture content of 20% or less is ideal, but this will still sizzle. Even at 20% moisture there are two pounds of water to boil off in 10 pounds of wood. Smell is not at all reliable. Many of the products sold as creosote preventing are completely useless. None of them eliminate creosote. Instead, they change its form so it is dry and flaky and more easily removed than the dangerous glazed kind. The best way of avoiding creosote is to burn hot fires with seasoned wood. It is also important to have the chimney cleaned and inspected every year.

A stainless steel liner will not corrode and is a much better option for a gas burning appliance than traditional clay tiles. Just make sure that the one installed is registered for use with gas as that fuel produces some corrosive bi products. I would also steer clear of anyone who proposes packing voids with bricks, etc. a very poor practice and probably a violation of building and fire codes.