Finding a contractor.
First, call the listed number in your phone directory and ask whomever answers if they install radiant floor heating, condensing boilers etc. If they hesitate or don't know, move on.
Once you get a positive response, ask how many radiant floors or Mod/Con boilers they have installed in the last year; or the last 5 years. If they hesitate or don't know, move on.
Ask if the contractor provides a heat load analysis before installing a new boiler. If they hesitate or don't know, move on.
Ask your prospective contractors about your boiler or radiation choices. If they have a website, their pictures are a testament to their professionalism. If they try to discourage you - for questionable reasons - from buying Mod/Con boilers or radiant floor heating systems, move on.
If you find a person who affirms their knowledge and experience, ask them to come out and look at your project. Though I often give people ballpark quotes over the phone, it is a practice that is legitimately frowned upon by the industry, and it does not reflect poorly on the contractor if he prefers to look at the job before he talks about price. I sometimes charge a modest fee for a consultation, which I deduct when a contract is signed. In this regard, it pays to keep in mind what "free" estimates are worth.
Now that you have qualified your prospective contractors (three should be enough) it is time to invite them over. If you plan to have your spouse help you in your decision, it is a good idea to have them with you when the contractor shows up. Do some homework. Make notes about your current heating system both good and bad, e.g. bathroom is always cold, fuel bills are too high, noisy air sound at boiler, etc. Think about what you want from your next heating system and tell the contractors about it.
I would admonish you to stay away from glad-handed salesmen from your local gas provider, department store or big box hardware. They are not qualified to specify, install or service the equipment you are interested in. They will sell you a product, mark up the job and sub-contract the installation to a licensed - low-bidder - contractor that you will probably not speak to until the job goes in. Much of my work involves following around such contractors and correcting their mistakes, both trivial and serious. Unfortunately I have worked on many boilers that have operated inefficiently for decades.
Contracts do not have to be fancy but should include language you understand and agree with. One-year parts and labor is standard in the industry. Licenses and permits are fine, but the majority of contractors and inspectors have precious little experience with new boiler technology. Their job is to make sure codes are enforced to assure safety. Unfortunately the code usually lags behind the industry a decade or more. Homeowners would be ill advised to depend on a code inspector to guarantee a sound design, application or installation.
I am personally more interested in a contractor who has been to any factory training for condensing boilers or radiant floor design seminars. Next, I look for contractors with organizational affiliation with certification, such as NATE or RPA, a current Certificate of Insurance and a solid, recent referral from a General Contractor or satisfied owner. Don't forget the mechanic's lien waiver when paying the final bill.
As contractors pay for their membership to all affiliations, I do not put great stock in a general paid affiliation such as the BBB, or PHCC.