900 Hams per month!

If you have natural gas now its going to be a tough sell to get your money back. Wood isn't cheap. How is the heating system now? Insulation, windows, siding? If you get an endless supply of free wood I can see it working for you but between the hot water and heat load of that building I'm guessing 15-20 cords/yr. That's a full time job.

I currently heat 3200 square feet on roughly 5 cord from the end of October through early April. I wouldn't expect you to be in the "15-20 cord/yr" range unless you choose to use a less efficient outside wood boiler.

You're definitely in the right place. Read all you can. I made the switch to wood from natural gas and my system payback is about 5 years. And that does not take into consideration how much warmer we keep the house with wood. But to make the dollars work out for wood you typically don't want to have to pay for wood. Especially when you have access to natural gas.

Tell us more about your house. How warm do you keep it? Is $900 a month normal for you in winter? That's a VERY high gas bill. Many here will tell you to start by looking at insulation, windows and sealing up the house. At 4200 sq ft I tend to think you need to look at insulation asap if you're really shelling out $900 a month for natural gas. My highest ever was $380...

Last remodel was in the 50's. Your allot more ahead putting any money you can into the insulation. That will be a very quick payback and it will make the house more comfortable to live in also.

It is typical for old houses to have no insulation whatsoever, and very minimal attempts to stop airflow through the framing. Insulation didn't become common until the 60's and 70's and then it was often 2" fiberglass.

Have you climbed into the attic to check things out? Sometimes even interior partition walls are even open to the attic, leaking air 24/7.

I'd say the easiest way to get rid of your heating bills is to make the renters pay!

Steam radiators aren't exactly the most efficient heating system -- I'd consider replacing them with monitor heaters that the renters with their own meters.

http://www.monitorheat.com/

By the time you replace the radiators with baseboard, buy, install a wood boiler w/storage and feed it I don't see how you would come out ahead.

The good news seems to be that if your building is in need of insulation you can make some quick ROI before addressing the heating. Insulation makes money. Big time.

Steam would be tough. Pex is a wonderfull thing in this situation, and personally I'd rather keep the CI rads and convert them to forced hot water. Of course, trying to remove a plug in a 100 yr old cast iron radiator could change anyone's mind. With the mass they work great for outdoor reset. I'd get rid of the thermostats and just use indoor sensors if I wasn't going to re-zone so the tennants pay their way. I had one building that I just divided the heat by 2 and the tennants paid- no questions. We both figured the additional meter charge would make it more expensive. Same with cable, internet, electric and so on. In fact that's how it is now with my parents living with me. Cheaper without the individual accounts.

Natural gas monitor space heaters would be great, and easier to meter individually. Personal experience with the space heaters is they don't heat through shut doors, so leave the doors open or expect a 60 degree bedroom and an 80 degree living room. Baseboards w/ individual boilers and water heaters would be much, much better but more expensive as well. What's your rent? Are you cheap or expensive? An extra $100/month would pay for the debt service on the installation and leave you with a better building.

I've got a 4300 sqft house and the first year I burned about 1600 gallons of oil. After some insulation, upgrades and a new insert + 2 cord/yr I'm down below 1100. That's how I figured 15-20 cords because I do believe 1 cord = 100 gallons of oil. 4 units means a ton of hot water, ect, so as far as a heat load on the building I don't think its that far off, and the 100 year old building makes me think I'm pretty close.

My last building in Rochester was built in 1900 and renovated in the 50s or 60s. I gutted it down to the studs. The only insulation I found anywhere was a little horse hair between the floor joists in the attic.