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Some additional thoughts. I'd guess not many CI boilers ever run at the rated efficiencies. Certainly not zoned ones as the Cycle Efficiency is the determining factor. IF, and most are, the boiler is oversized then cycle efficiency goes down even more. We have done examples here where actual efficiency drops into the low 70% on zoned, oversized CI on fin tube systems.

Some suggestions, upgrade the envelop first. Even a weatherstrip upgrade can make a difference, as infiltration can rob a lot of your fuel $$

Next do a room by room heat load calc, followed by a fin tube assessment. How low of a SWT can you actually run will now become clear.

Next look up some 20 year weather data. How often are you at design, or below in your area? Probably 80% of the heating system you are below design. So 80% of the time the CI boiler, properly sized is oversized and cycle efficiency starts to chip away at the operating cost. Even on a single zone system!

The higher the temperature required at the fin tube the more loss through the wall to the outdoors. Fin running at 140 loses less through the wall build up, than a 180F system. A simple delta T formula.

So a mod con, properly sized, installed and set up, addresses all the efficiency robbing concerns.

It may modulate down to the lowest zone load. ODR keeps the SWT matching the heat load fairly accurately. Ramp delay, anti short cycling can be used if it is a micro-zoned system where small loads fall below the lowest turn down. These all come into play even if it never condenses!

Below is BIN data is for upstate NY, but I suspect the trends would be similar for L.I, as far as % of design. I believe 0 or 5 above is design in this upstate area.

If you can get SWT down around 150 or lower, design day, the mod cons will really sing fig 3-6. Mod con efficiency goes up as they run at low turn down, fig 3-31. Widen the operating delta to 30° to lower the return even more. It's the return to the boiler that dictates the condensing operation.

Also a slide from Viessmann training regarding reducing SWT and ODR usage. With a CI boiler you are limited on how aggressively you can set the ODR and protect the boiler from extended cold run time.

That leaves the question of maintenance cost, life expectancy. That will be debated till the end of time.

I started a file of mod con cleaning from IG, with poster permission. I've seen boilers that hadn't been serviced in 8 years looking very clean when opened. And boilers opened after one year 1/4 full of mouse turd deposits.
I feel intake air filter and proper burner set up the first year and a check after a year has a lot to do with the combustion side condition.

So it comes down to number crunching first, or talking with folks that have made the mod con transition on CI fin tube systems in your area, for some case study data

System off, put hose on where you indicated, put hose in a bucket in a sink. Close ball valve.
At the same time, open hose bib, and open fill valve, running water thru the system to maintain at least 12 psi while monitoring the bucket. It's a bit of a juggling act, but you don't want the pressure to drop below 12 psi, and you don't want it near 30 psi.
When all bubbles have stopped in the bucket, at the same time, close hose bib and close fill valve. It's easier and more accurate to just kink the hose to control flow. Then you can let a little water out until you're back to 12psi or whatever you need your cold fill pressure to be.
Don't forget to open up your ball valve before turning the system back on.
Based on how it's piped, recurring air issues may be a theme