Age, size and eff of old boiler. What fuel source?

The large addition, each floor is same size? If not, specify each different floor's living space.

To be built on slab or crawl space?

How would you describe existing system's comfort for heating and AC for cooling?

Post back.

IMO

First floor addition will be 2000 sq. ft. Old space is 800 sq. ft.
Second floor will be 1800 sq. ft. Old space is 800 sq. ft.
New basement space will be 900 sq. ft.Old space is 800 sq. ft

Addition will be built on concrete slab.

Not sure of age, size and eff. of old boiler or comfort of existing system. Working to renovate and update before we move in. The house was poorly maintained and everything is ancient. House was built in the 1930s. Fuel source is oil but everyone recommends we replace that as oil is very expensive around us and hardly used anymore.

You have nat gas service available to your home?

TD

I see no point in tearing out the radiators, they provide superior comfort to forced air. I would leave the rest of your house completely independent of the addition HVAC wise. Replace the boiler if needed and install forced air in the addition if hydronic isn't in the budget. How do you plan on balancing temperature between the 3 floors of the addition? If the same ducts are being used for both heating and cooling, you won't be able to pull it off with duct sizing alone. At a minimum you'll need seasonal manually adjustable dampers, but an electronic zone system, multiple systems, or a combination of the two would be preferable.

Yes, natural gas service is available.

We plan to have a multiple zone system.

I would keep the radiator heat in existing main house. I would update though to a high eff nat gas boiler. You should see a measurable payback over the more expensive oil.i would definitely put this in your budget.

You have to decide on whether to have radiant heat, boiler heat, or hot forced air in new addition. Keep in mind you still will have to have a ductwork system for AC unless you consider mini splits for each floor. So you have multiple options to look at for new addition. Nat Gas service though should be brought to the new structure and old structure. On new structure, I would want each floor zoned.

IMO

I renovated an 150 year old house in Northern Virginia about 10 years ago. So, I have an idea of the area and situation. I'm just a simple homeowner who wrote the checks. Let me pass on some experience.

Radiators are great. They provide constant, even warm heat without drying out the air. We were advised to keep the radiators if at all feasible. Ours were hot water and not steam radiators. We already had natural gas service. We did not have air conditioning.

We ended up replacing the ancient boiler with a new system. We added some radiant floor heat in some areas where it was necessary to remove the radiators. We also added a separate central air conditioning with all of the necessary duct work. You may want to strongly consider radiant floor heating throughout the addition. They can zone it and it will work off the boiler. We loved it. The heat and comfort were superior.

We still had to install central air conditioning with all of the duct work. It cost more to do it this way, but if you can work it out, you will be very happy. Keeping the radiators is well worth the effort.

>They provide constant, even warm heat without drying out the air.

Dryness of the air has nothing to do with the type of heating system, it has to do with infiltration. The more leaky a house is, the more dry it will be. Conversely some super efficient new construction is actually too humid and needs fresh air ventilation, even in the middle of winter. If a forced air system has leaky ducts in an unconditioned space, well, that promotes infiltration. But a properly designed, installed, and maintained forced air system won't have those problems.

Having said that, I agree with the rest of your post. Hydronic heat in general, whether baseboards, radiators, or radiant floors, provides superior comfort to forced air.