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Need help with options for converting from oil to gas. Steam, IWH, and furnace involved.

agurkas Member Posts: 238
We bought a house in one of the west suburbs of Boston (next to 128 and Rt.2). About 2/3 of the house is a 1938 Cape and the other 1/3 is a 1978 addition. 2800 sq. ft. does not include unfinished basement.


Original '38 part 1st floor has steam AND forced air (cooling and heating).

'78 addition is basically a 12X18 kitchen with 8' ceiling and 30'X21' living room with vaulted 14'+ ceilings. Those two rooms are heated and cooled by forced air only.

All windows in '38 part will be brand new (replacing all the decrepit original windows), '78 side has double-pane Andersons from that period.


2 full baths (one is shower only and another is shower and bathtub).

We have a fairly young Weil - McLain WGTO-4 Series 3 oil boiler that can be converted, but it would drop efficiency even further to something like ~80% at best.

This boiler has indirect water heater attached, which is kind of oversized (per inspector) at 80 gallons.

Furnace, serving the '78 addition is borderline "replace ASAP" per inspector, so not dumping money in converting it.

Chimney for the boiler just got an insert, since it had a crack.

National Grid will provide rebate for either a boiler or furnace, but not both.

So here are options I was thinking about to reduce initial cost AND keep annual operating costs lower:

Option A: Convert W-M unit to gas and get new high efficiency furnace through National Grid.

Option B: Convert W-M unit to gas. Remove IWH from steam boiler. Leave it off until winter. Get high efficiency combi boiler via National Grig (Rinnai E-series? Navien?) and heat water for the house for it and connect it to the new air handler with hydronic coil that would replace current furnace (Lenox cooling coil is above it).

If I was thinking purely cost of equipment, probably Option B would be the lowest initial cost with all the rebates and tax credits and it would be cheaper to run long term, since I could turn off the W-M steam boiler in non heating season.

Would love some input. Need to educate myself better before I start having plumbers, steam folks, and HVAC companies over. I also assume steam person should be last to come to balance the steam side to the heating system?


  • Spence
    Spence Member Posts: 316

    Just to start your process: there is no reason, provided your conversion burner specialist really knows steam, that you cannot reach a combustion efficiency of the mid to high 80s.

    Indirect tanks are generally more efficient and easier to size properly for your needs than are the combination units.