Hi MichaelJCliff76
I think there are several questions mixed together here.
Any two or more people can enter into a contract - in this sense of the word, a contract is simply a legally enforceable agreement where both parties agree to do something. In the case of a boiler service contract, the customer agrees to pay you money regularly, and you agree to mend their heating if it breaks down, subject to certain conditions.
The next question is how to express that agreement and all the conditions clearly and without leaving too many areas of uncertainty - for that, you would need a lawyer - preferably one with experience in this field.
Finally, there is the question of whether this would be a wise contract to enter into for either you, or your customers. For example, for it to be attractive to your customers, you are going to have to guarantee a certain call out time - say within 24 hrs. Are you a big firm or a one-man-band? What if you go into hospital? Or, at the worst case, you die - people who have paid for their contract will expect their boiler fixed, whether you are ill, dead, or on holiday. As others have said - a service contract is actually a form of insurance - the payments from everyone whose system is trouble free pay for the occasional customer whose system keeps going wrong. Thats fine if you have 100,000 customers, but not so great if you have 20 - which might include 5 unlucky systems or lemon boilers.
I can't see it working unless you have a large business with several thousand customers, a dozen or so engineers, and some fairly heavy duty admin back up.
:welcome: to the forum michael...76 
