I'm working to replace my home heating system from oil to natural gas (high efficiency condensate boiler). As part of this I've been working on getting quotes from various companies and the data is flowing back. However I'm getting conflicting information that I was hoping experts here can help me understand.
Very short background: house is in the boston area, partially finished (often used) basement with multiple load bearing joists that will interfere with running exhaust piping in the inside of the house. The back of the house, where the heating system is currently located, has a wrap around elevated deck that I'm told will prevent simple exhaust through the basement wall since it would be under the deck. We use the deck extensively with a large collection of plants, etc so vent exhaust during the warm months (hot water) may be an issue if it kills the plants. Heating is a forced hot water system (heat and hot water).
There are two possible venting options - up the current dedicated double insulated metal chimney (not brick - in a wood enclosure) used to vent the oil burner or go outside and run under the deck to the sides of the deck to keep them away from the plants.
One HVAC company has told me that venting under the deck (horizontal) could result in the vent pipe icing up over the winter possibly causing a blockage. Because of that they could not warrant the installation unless we went up the current chimney. This is a fairly high end - large well known company.
The other HVAC company has told me that venting up the chimney (vertical) might be problematic because if we had any issues with icing at the exhaust it would be inaccessible to us. They really really want to go under the deck.
Obviously such sharp contrasting information makes for a confused homeowner. I've tried to read a number of other postings and have read about the pitch of the vertical vent should be towards the device not towards the vent termination.
One complication - most of the vendors have indicated that using the existing chimney will pose a problem with respect to ensuring a waterproof terminator at the top since they would have to cut the existing vent stack off (once it comes out of the wood chimney box and clears the metal chimney cap) and replace it with some kind of homemade mechanism.
So a secondary question - is adapting a non-brick double insulated chimney by putting the pvc inside the existing metal chimney so unusual?
Any feedback would be hugely appreciated as I try to figure out what the "correct" approach for this job should be.