I had my boiler and water heater replaced with a 90% efficient boiler (or maybe 95 I can't recall) and an indirect water heater. The water heater became a zone for the boiler. I had all the hot water I needed and the indirect tank didn't need maintenance . The boiler used PVC pipe for flue so I removed the old boiler flue.

What Seabornman did is the way to go, but you won't need a water storage tank with the Navian. It's a cute little thing you hang on a wall, and won't need an indirect tank, but I do not know why they call it a boiler because it's not, It's a souped up tankless hot water heater. Sure it's efficient and cheap selling retail for about $2200, but it's an easy cash cow for the contractor paying the wholesale price with an easy fast install so why wouldn't they suggest it.

No way you'll get 20 years from the "not a boiler" tankless water heater heater, and you'll continue to pay for it in annual or biannual maintenance fees. I strongly suggest buying a good boiler with little worries or problems to contend with for many decades, like your old one. Here is an old Thread on houzz, Navian condensing combi, good or bad? and suggested reading all 120 comments to see what you're getting into. I also suggest reading the warranty, I think the devil wrote it and I can see going weeks without heat or hot water before Navian approves the repair. The best part is they won't pay the contractor more than they feel they should pay, so who pays the rest?

I was going to give the same advice as kevin9408. First is there anything wrong with the current Weil McClain boiler? A properly maintained 30+ year old still has years of life. If it has been requiring repairs then replace it with a stand alone boiler. The simpler the better in my opinion. Then I recommend replacing the hot water heater with a gas model. Again I like the simple atmospheric type, but it is your choice to spend additional money and install types which are more efficient.

3rd-ing Kevin's advice.

All good posts, Kevin thanks for the link, good read. I have another home on Cape Cod, Weil-Mclain HE-4, with a 40 gal HTP hw tank. Works great, lasts a long time. Corrosion seems to be the biggest issue with the boiler in question. It does have the Gold update, but I just replaced the circulator, and the blower is rattling. Maybe get another opinion.