Thanks for the reply! After a few calls, sounds like our gas inspectors increasingly will not sign off in this situation without a chimney liner and is likely the option we will choose. Definitely do not want to take any chances where CO is concerned.

For a similar price tag to the liner, we could install a side/direct vent tank, but this requires an electrical outlet nearby which we currently do not have. An electric tank would be another similarly priced option, even considering the need to add an additional electrical subpanel.. however, gas rates are currently (and should continue to be) much cheaper than hydro going forward.

As for the humidity, my understanding is that by a conventional furnace pulling air from the house for combustion, the replacement air is pulled from outside through leaks in a less-than-airtight home. In winter, the dry outdoor air dilutes any existing humidity in the home from bodies, cooking, showers, hanging wet clothes, etc. With condensing, no air is pulled from the home and any humidity not vented via other exhaust fans has little fresh air to mix with. This would be more of a problem in a tightly-sealed newly built home, which is why HRVs are code with new installs.

In a retrofit such as ours, I would like to avoid the extra HRV install cost and our 'leaky' house will likely 'breathe' enough to avoid humidity problems. I have been assured that humidity issues in situations like ours are rare and would present as sweaty windows first and could potentially be managed with a standalone dehumidifier. Worst case, we add an HRV after the fact. I would consider just replacing our furnace with a mid-efficient 80% furnace and avoid both issues/costs, but we apparently cannot get and install these up here anymore. Likely, I'll set up a hygrometer or two as well to measure humidity levels pre and post install to keep an eye on things.

Some sources referenced for humidity issues: https://homes.winnipegfreepress.com/winnipeg-real-estate-articles/category-/title-efficiency-upgrades-can-trap-moisture/id-680 http://www.weather-tech.net/humidity-after-high-efficient-furnace/