Hi all,

I was so happy when NStar connected us to the new gas line on our street, am planning to switch from oil to gas for heat and HW, for fuel savings and more even heating performance.
Hoping to take advantage of any high efficiency rebates and zero interest loan.

Our house was built in 1900, colonial style, 3 floors ~2300 sq ft. Lots of original double hung sash windows with storms.
Very little insulation in the walls, not so great in the attic, and definitely some air infiltration.
Near Boston, Outdoor design temp i think is 9 degrees

Current oil fueled boiler is Utica SFH4150WT 175K showing 152,200 BTU/hr on plate.
Current hot water is 50 gallon electric heated.
Boiler is about 20 years old, we used 1350 gallon of oil in past 12 months.
We have 7 large cast iron radiators, 60 ft of cast iron baseboard, and 30 ft of newer fin baseboard.

I thought it would be a snap to move to a high efficiency gas boiler and set about getting 3 quotes before the dirt was settled over the gas line...
Quote 1 was for a Rinnai E110C combi unit to go with "on demand" HW.
They sent me a Manual J that said my heat load was 68,849 but it lists 2 "rooms" each at 1320 sq ft each? seems like a shortcut estimate

Quote 2
They looked at size of existing boiler and quoted 3 options (all 150K BTUs?)
not sure if they are even planning to do a Manual J
Buderus Logamax Plus GB142 w Megastor 53 gallon IHW
Viessman Vitodens WB200-Ww Megastor 53 gallon IHW
Burnham Alpine ALP150T w Megastor 53 gallon IHW

Quote 3
Spoke to an experienced plumber who has worked in a lot of houses.
He had concerns about the high efficiency mod-cons performing well with the large volume of water. He thinks they would not run at the high efficiency levels in a high mass system, and recommended:
Burnham ES2-5 which has a large cast iron boiler and can be vented out of current chimney (w addition of flue liner) and a Heat-Flo 50 gallon IHW storage.
Also recommends Outdoor Reset. He said we'd have to do a real manual J room by room measures so he could properly size the boiler, but he thinks the ES2 family would be a better fit for our radiators.
Problem is that burner is only 85% rated, so not only does not qualify for any rebates, but would not qualify for the 0% loan.

So let's assume that whomever does the proper sizing (I stopped by my library and have been reading Audel HVAC Fundamentals: Volume 1: Heating Systems, Furnaces and Boilers and started my own spreadsheet based on Chaps 3 and 4 !) - in the real world, can the new high efficiency mod con burners be installed and tuned to support high mass? I am pretty sure I want the Indirect HW.
Also, we are hoping to get better performance that we have now, where it takes awhile for the radiators to get warm, then they are really hot for while, then it cools down for an hour or two and then heats up again - thermostat just sits at 66 all days....

Hadn't thought all the details until we got into this process and then i started reading tons of stuff on line and then the Audel books...

We will try to mitigate insulation issues, but have siding on top of asbestos shingles so can't fill walls from the outside, a lot of issues like that...

Just trying to understand the pros and cons of any choices, thanx for any advice or ideas.