Thank you all for your wonderful advice. We are weighing all opinions. Comfortdoc, I must say your thoughts are compelling! I am definitely leaning towards the mod/con wallhung. Much appreciated! Thanks for being there, guys! We all value your advice! Marti
I will take exception to the above opinions.
Figure the difference between the two as 10% efficiency, so the heat cost savings per year would be about 10% of your annual heating bill. My guess is that your heating bill at todays oil prices would be the equivalent of 900 gal oil. @ $3.00/ gal that is $2700 ( I know you have gas and it is cheaper so my arguments are even stronger); so the annual savings is roughly $300. If you do a present value calculation, at 6% rate, my estimate is that it would take 16 years to recover the $3000.
Accordingly, the miniscule increase in comfort notwithstanding, I vote for the cheaper one.
There is more to be saved than just the 10% indicates.
The source of combustion air is not figured into the rated efficiency numbers. However, this is a big additional savings for the 90% unit.
The modulating burner is also a savings. When it is operating at partial load, even at higher water temperatures, the efficiency goes up. Here is a chart from Viessmann showing this.
Also, since mod cons are low mass with mechanically induced draft, there is so much less heat lost during the off cycles. With a cast iron block on a chimney, whenever the boiler finished firing, there is heat left in the cast iron and water content of the boiler. This heat will go up the chimney during the off cycle. When the boiler fires back up, it has to reheat the block first. (This isn't as big of a factor for Buderus cast iron boilers compared to others since they use a low water content, relatively light block. 230 lbs, 2.4 gal water for the 74k BTU models)
There are reports from people at heatinghelp that document much more than a 10% savings after switching to mod/con. Some even have 30-40%.
To the OP, the Buderus GB mod/con uses an aluminum block. As someone already mentioned, aluminum blocks need a bit more attention to water quality - it isn't necessarily a deal breaker, but something to be aware of. There are also other good mod/con choices. My favorite is the Lochinvar Knight. Stainless Steel heat exchanger, modulates lower than Buderus GB, designed and built in the USA in Tennessee, and less expensive.