Hi, we installed a tankless hot water heater that is direct vented and removed the old gas fired hot water heater. Seven years ago we did a similar thing with our main furnace which was direct vented. Currently there is a gas fired boiler using the chimney to heat our Florida room, which isn't used all the time.

All I can find on lining a chimney is when you abandon a hot water heater. In this case we are abandoning a furnace - which uses more BTU's than the hot water heater. Now that the boiler is the only unit on the chimney is it still necessary to line the chimney.

If anything the chimney probably needed lining when the main furnace was removed 7 years ago?

A direct vent water heater usually will not use the chimney at all since they are designed to be vented directly outside. Post the exact brand and model number of the unit.

The tankless is no longer using the chimney. 7 years ago the furnace was removed the chimney, recently the HW was removed from the chimney, and now only a hot water boiler for radiant floor remains in the chimney. If the boiler is using the chimney (alone) does the chimney need to be lined?

Mini-Boiler 58,000 BTU (using chimney)
Furnace 120,000 BTU (removed from chimney 7 years ago)
Hot Water heater 40,000? BTU (removed 1 month ago)

Tankless HW installed 1 month ago (never used chimney - is direct vented side wall)

Thanks.

It is possible the chimney will need to be lined. If the boiler is high/medium efficiency then I would check into the manufactures install instructions to tell for sure. You did not provide exact model or brand that I asked for so I can be of no further help.