Need advice on tankless boilers

Do you still have the old steam radiators?

If Yes, are they compatible with a Hot water system?

How do you propose to connect the radiators to your new boiler?  There are many different designs that will work.  I believe that a parallel piping design with a reverse return is the best choice for an economical design.  You may also want to do home runs back to a central manifold.   Perhaps two or three manifolds for zoning  

Getting the proper system design is important.  If you get it wrong, you may never get the system balanced properly.  

And what @Hot_water_fan asked too?

Put a new steam boiler back in and put in a freestanding water heater, that's what I'd do.

It's what I'd do, too. Assuming that you haven't taken out the existing radiation and the various risers involved — just ripped out the basement piping.

Otherwise, as others have said, you've gotten way ahead of yourself.

First, you are essentially putting in a whole new heating system in an existing structure. That's always fun. Start with a room by room heat loss and a total building heat loss. Without that, you're guessing on everything.

Next, make a choice of heating system. In this case, it sounds as though you are looking for a hot water — hydronic — system. If so, your fundamental choice will be a mod/con boiler (not a tankless water heater — that's a completely different beast). That will inform you on the hottest water you can expect to use on the coldest days which, in turn, will inform you as to what radiators or whatever you use in the various spaces. Size them to provide the room heat loss from Step 1 up there. You may or may not be able to use some of the existing radiators.

Then decide on how and if you want zoning. This will help design the piping. Since you are going from scratch, I strongly recommend either using "home run" piping wherever possible, or reverse return piping where necessary. That will make it easier to balance and control the system. I would advise against using any of the existing risers from the old system.

Then, and only then, select a mod/con boiler sized to suit the actual building heat loss as calculated in Step 1. Do NOT be tempted to oversize it. Choosing the manufacturer of the boiler must be based — in order — on three considerations. First, what make or makes is your installer familiar with? Second, what make or makes have parts and service easily available in your area? And last — way last — what is the brand name on the front of the boiler. Installer familiarity and parts and service availability are really important with the newer boilers, as they have to be piped correctly - they are not very forgiving that way — and the controls, which are complex, set up properly, and second they will need parts and service much more than older boilers, never mind steam systems.

Since you have already taken out the chimney, you will be limited to boilers which can be power vented.

For domestic hot water, I also agree that an indirect fired by the heating boiler will probably be the most satisfactory. Since you no longer had a chimney, a direct fired oil or gas hot water heater is not an option. A conventional electric tank heater would be another option, depending on your electric rates.

Probably not. The OP said something about a government grant, which may prohibit it…

Of Course they can. But at what cost? You need to consider that the person that is asking this question has just spent a ton of $$$ in order to get those pipes and chimney out of the way. Did you actually read the original comment?

What if someone told you that the best thing for you is to put an oil boiler and a new oil tank back in your home after all you did to get a Gas Steamer? Or better yet tell you to get a coal boiler and convert it to oil heat with a 1940's oil burner …because that is the heat you had and you should go back to that, because it worked before.

How much thought would you give to that advice?

Your advice does not even come close to anything the original poster is interested in.

I understand that is not the same thing. You're better off with your system. And all your research you did and shared, to get where you are is awesome. But consider that your solution will cost over $80,000.00 to put the chimney back, have a steam fitter repipe the basement and connect to the new steam boiler. That is a 6000 Sq Ft building with a lot of radiators to connect. I just don't see that your advice is a cost effective option at this point. 

I agree with @EdTheHeaterMan — although keeping the steam should at least have been considered, it appears it wasn't. There could be a number of reasons for that. Our OP, @Tallmandave1, gave a few — including the comment that a government grant was involved, which can really affect analysis That ship has sailed.

I believe I gave a reasonably coherent (at least by my standards) set of suggestions as to how to proceed from here.

Your home is not 6000 sq ft. Paul! 🤣

So when are you putting in the oil burner? That's what I would do if I had your home!

Like I said in my first post I understand the old steam piping and radiation are gone.

Again, your getting ahead of yourself. Do you want a system that will work or always be a problem?

Do an accurate heat loss

Pick the water temp you want to run at.

Lower water temp=less fuel but more radiation=more efficiency=install cost is higher operational cost is less

Higher water temp =less radiation =less efficiency less install cost operational cost higher.

If you don't know install a mod con boiler and size everything for 150 degree water max

If the house needs 98,000 btus use a boiler that size. Why would you put in 300,000 btu worth of boilers?

So you have 7 bathrooms. How many people in the house?.

a 98,000 btu boiler will heat a very large indirect water heater probably an 80 gallon tank if you have that many people

Heat loss

pick water temp

size radiation

size pipe

size circulator

Skip a step and it won't work