Seeking hot water system that can use existing vents

traceyr8

I’m looking to install a new heating system. Currently I have a combi boiler (for heat and cold) and I want to use a tankless hot water only (and change the heating to a heat pump unit on the roof).
In our apt we have two vents: Z vent for heat to go out and another one for fresh air (see pic). I believe all are stainless steel (images with stickers below)

Basically I’m interested to know if the unit from Lowe’s below can work without changing the vent configuration - I called AO Smith and they said i needed to use 3” vents but a Contactor we brought over said we could use existing vents for this model, so I’m confused) - or if anyone has a recommendation for something else? We live in a small 1,200 foot condo in the second floor of a brownstone and this would be in our apartment and not a basement)
https://www.lowes.com/pd/A-O-Smith-Signature-Premier-6-6-GPM-160000-BTU-Indoor-Natural-Gas-Tankless-Water-Heater/1000601675

kcopp

You have a lot going on there.

The 3" Stainless steel goes into what?

Is the 4" sheet metal an intake?

Most likely you will need to run new venting AND have an intake

A wall hung combi would work well w/o going to heat pumps 100%

It would take up less room and be quieter that the Laars Endurance.

traceyr8

The 3in vent is the exhaust. It’s going to the roof.
The 4in is fresh air, also coming from the roof, and it’s likely sharing fresh air intake with other units in the building.

We thought to change our AC (on the roof) to include in it a heat as well (heat pump) since our ac will need to be replaced pretty soon too.

For the hot water, we thought to use a tankless gas but we are trying to minimize the need for new venting if possible (there is one apt above us and we rather don’t make holes all over). So we are trying to find a tankless gas that could work with our existing vents