90%+ Boiler vs 80% boiler

what kind of heating system is it attached to? in order to get the 90+ efficiency the boiler has to be able to run with low return water temp si the system needs to have enough area in the emitters to work with low water temps. something like radiant floors or some cast iron radiator systems. fin tube baseboard or convectors usually can't do that.

if the outdoor reset isn't set yup on the mod con that also will reduce efficiency.

water heating with an indirect water heater usually is not don with temps that keep it in the higher efficiency area.

what is the outside exposure of both buildings? is it the same?

Don't know how large the buildings are but 800,000 btu/month in the winter is not that much gas. Are these houses?

The Series 2 Ultra was more than a few (15?) years ago. Does the Ultra have supply and return sensors on the manifolds? Minimum and maximum firing rate? Were combustion tests done on both boilers? Whats the net BTU ratings on both boilers? Are they similar?

If it is a zoned system, 11 zones? The Ultra on a properly adjusted ODR should be more efficient.

#1 it can modulate to loads, if it is zoned?

#2 ODR might let it run in condensing mode for some of the season, depending on the fin tube lengths.

I don't see the Boiler Buddy doing much of anything on the Ultra.

If the cast boiler is cycling it's brains out on a multi zoned system, that is where so buffer might help.

Putting both buildings in identical load conditions with 11 units, if they have their own thermostats would be nearly impossible. Even if t stats were locked to 70°, occupancy use would vary substantially.

Maybe the bills are mixed up :)

The boiler buddy may give you longer cycles — but unless you can get the operating return temperature below 140 F or so — and lower is better — you'll do no better with a high efficiency boiler than with a regular one.

And doing that depends on whether you have enough radiation (you mention fin tube — so that would be total length of fin tube) to heat the structure adequately at low temperatures. Heating systems are SYSTEMS — not random collections of components — and how well they work depends very much on the whole system being considered as a whole, not just a boiler here and some radiation there! You may simply not have enough radiation installed to make a "high efficiency" boiler work at high efficiency.

A heat loss of the buildings will provide the answer along with a radiation survey. Anything else is a guess.

I'm thankful for this post because it tells me in even stronger terms to just stick with a cast iron boiler for the rest of my life (if I do ever move away from my steam house)

a modcon is the obvious choice for radiant or a gravity conversion. it can also work very well with panel radiators if they are designed for it. with baseboard or convectors there is more to consider like does a modcon avoid spending thousands on fixing a chimney or does the radiation allow you to run low water temps for a significant portion of the year.

even if the swt ultimately has to be high temp, gravity conversions still need to deal with return water temp protection on conventional boilers and that isn't particularly simple or inexpensive. it isn't super complicated but there is some complication and additional maintenance when combined with the ability to have odr built in generally tips the scale to a modcon unless there isn't a place to get the powered vent out of the building or something like that.

I think you are missing the point of a condensing boiler. It will condense and run 90% plus on some, maybe most of the year. But we don’t have enough specifics on you application

Location, heat loss of the building, amount of fin tube, boiler sizes)

A short cycling cast boiler on a mild day may be running low 70%, while the mod con on the mild days 90%.

A WAG of 25 btu/ft X 4500 sq ft =112,500 load on design day.

Ultras are available from 80-399,000

Assume someone sized it correctly, a 155,000 Ultra would be adequate. It turns down to 30,000

I doubt the cast iron has any turndown, so on a low load day it is 10 times too big, maybe more if it is oversized.

Care to share anymore data? At this point the comparison is just which type you like better, no data to base a best decision.

If efficiency is the goal, which seems why you started the post?

One of the minor arguments for condensing boilers is that they can run in condensing mode "most of the year" even on systems which need hotter water some of the time.

This is perfectly true. It is also misleading as to how much one is going to save, as perhaps I may be permitted to point out that "most of the year" the boiler won't be running that much, if at all, anyway.

Mind you, I am not opposed to a mod/con boiler powering radiation sized to the heat loss of the house and sized to operate on the low water temperatures, and controlled with an outdoor reset with indoor trim capabilities and variable capacity pumping and all the bells and whistles. Not at all. You can save as much as 10% of your fuel use that way, which can be significant.

Will it pay for the more expensive boiler and more expensive maintenance? Um… I am open to being convinced, but I can't make the numbers work.