I have a 15 year old cast iron mid-efficiency boiler (HydroTherm 125,000 BTUH input) heating a small ~950 sq ft 1 1/2 story house with cast iron radiators. The boiler stops firing on very cold days on high water temperature (boiler shutoff set @ 205 F), but keeps the house warm since I've removed wooden decorative enclosures around main floor radiators. I've been told the large pipes feeding the radiators were installed originally for natural convection circulation, before circulating pumps were invented.

The Vitodens 200 DHW supply should be adequate for our 2 people, one bath house, but I'm worried whether I can get a condensing boiler to run in condensing mode. The last time I had a Viessmann contractor look at our house he said that cast iron radiators need to be matched up with cast iron boilers. This had a somewhat logical ring to it, but I'm a technically oriented guy and I require a more technical answer. His comments killed my ambition to pursue a condensing boiler.

I'm trying to revisit the suitability of the Vitodens 200. But, I don't want to spend all that money and find out that I can't get the return water temperature down low enough to operate in condensing mode.