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.

Hi Autumn hopefully your new home, however tight, includes an outdoor-air ventilation system. Basically, a dedicated fan that exchanges indoor air with outdoor air. Energy raters, home performance contractors and some HVAC professionals can evaluate and advise if not. For health more than humidity. There is no such thing as too tight. The tighter the better. Your descritpions sound like classic leaky, inefficient house to me. Dry, leaking winter air requiring excessive heating during the cold season and infiltrating humid summer air the wrongly sized AC, can't deal with in the warm season. Could be wrong just sayin. Strive for cooktops vented to the outdoors and if having home performance contractors out to evaluate, have them check bath vent fans. They should be exhausting at least 50 CFM, most dont. Keep the bath door closed while showering and vent until moisture is gone. Easy ways to control big sources. A lot of moisture comes from the ground which is harder to fix on existing construction. Hopefully you don't have a crawlspace and maybe a home performance contractor can make suggestions for the foundation. Exterior grading adjustments uphill of my old home have stopped basement water intrusion, keeping it much drier. For vulnerable foundations lacking appropriate finish grading, gutters should be used and maintained with reckless abandon. I feel most east coast climates require at least some summer air-conditioning, even in MI. Mini-split heat pumps are a good solution but don't settle for HVAC contractor sizing and brand recommendations. Try to find a third party to evaluate needs for load sizing and brand. One can probably lower energy bills by offsetting some of the gas-burning. Lower energy costs plus my friends in PA and uncle in WY appreciate your concerns over their well water quality from fracking threats.

This is not a DIY job. Multiple gas burning appliances attached to the same flue can under certain scenarios cause the creation of CO (carbon monoxide a colorless, odorless gas that kills). These appliances do not create CO on their own. They create CO2 (carbon dioxide). Notice the similarity of these two substances CO - CO2 The difference that separates the two is 1 molecule of oxygen. Because these appliances burn gas, they need oxygen to burn. They will take oxygen from around the area in which they are installed. If there is a flue gas vent problem what do you suppose can happen? (Clearly there is a problem because it's dripping.) If this problem causes a drop in available oxygen in the air, you have the makings of a deadly combination. Because combustion air of these appliances will not be able to get the oxygen they need for full, complete combustion the result is exhaust that consists of carbon monoxide, the higher the PPM count the deadlier it is. If you can't afford a new furnace with a newly run flue to the outside separated from the water heater flue, save your money until you can do it right. Note: Newer higher efficient furnaces 90% or higher can be vented with PVC plastic run horizontally (They are called condensing furnace). You can not do this with your current furnace.

If you have cracked bricks/mortar/cap at the top, and don't fix them, they will spall (if freezing climate), water will still enter and could possibly work it's way under the roof depending on how your chimney is set up. Don't understand your description about flue cracks, but they would insert a s.s. liner and then grout between it and the existing flue. I haven't seen that process alone be anywhere near $4200.