hi, i have been reading this site for the past couple of days. this is a great forum. this is sort of an introduction/please point me the right way thread. i hope it is in the right place. thanks for any advice... i have an old house with the original radiator system. i would like to add a wood boiler in parallel to my natural gas boiler, and i guess from reading i should have a storage tank system. i have found a lot here about various add ons like control valves and storage tanks and extra pumps and controllers but i would like to find more on how to do a cost benefit analysis, or how you size stuff like the boiler. i would like to read about any similar projects here. the heated portion of my house is 4200 sf on two stories with high ceilings plus a 2000 sf full height unfinished below grade basement that is incidentally heated to about 5 c less than the house by the main 3" radiator supply and return pipes that circulate between the floor joists (there are two zones that split off near the boiler and each services both floors on about half the house). apart from the windows, the house seems to have good insulation for its age (85) although i have no idea how to rate it since it is powdered gypsum. it has a 200,000btu slant/fin galaxy cast iron boiler that is rated about 80% efficiency but is 20 years old. the boiler heats the house pretty quickly and the radiators work like a hot darn, but i have been told by a local boiler serviceman that the heat exchangers are probably scaled up (we have pretty hard water here) and that i should be replacing it. i am thinking of turning it into a backup. the house has a lot of large original single pained windows that are in good condition and impossible to replace. i looked at adding wooden storm windows and i think i could do a wood boiler cheaper, and chopping wood sounds more appetizing than fitting and packing away that many big storm windows every year. so here i am. last winter consumption was about 263.5 gj of gas for the radiators, plus some electricity (hot water and the kitchen area about 250 sf has baseboards as it originally was heated with a wood stove). our thermostat tracked usage and the furnace was usually on 8-10 hours a day mid-nov-early march so by my math we were using at least the rated capacity of the boiler, but i have no idea at what efficiency. the boiler heats to 160 and returns around 135-140 so i wonder if i am losing something there. i live in a zone where we get about 6500 heating (f) degree days a year. pretty mild winters down around -5 c usually with a couple of sustained cold snaps at -20 c or so. lots of room in the basement, and the chimney for the boiler was rebuilt 20 years ago and one side has never been used.