Unfortunately this question is not easy to answer and is a very tough decision to make. There are many types of equipments and since they differ a general suggestion can not be followed. However there are a few popular suggestions that you can follow while trying to make a decision to replace or stick with the old one. If the cost for repairing your old furnace or boiler goes more than $500 then it is smart to replace the unit rather than repair it. Moreover if the unit is more than 15 years old then it is better that you replace it as a replacement option is going to be more feasible than repairing and using the same unit in the long run.
Another most important factor that can help you to consider for a replacement is when your unit is not working efficiently. You will be delaying your replacement decision because your old furnace is still functioning well but what you’re not aware of is the amount of loss you are incurring paying for the extra energy bill every month resulting from inefficient functioning. Most of the newer models consume lesser energy that switching to it can save your more than 60% from your energy bill. These rules also apply to other appliances like air conditioners and air pumps. Most of the models that are now available in the market can function with 90% efficiency. Installing a programmable thermostat can help you save even more. Other factors that calls for a replacement or a repair is when the unit start making too much noise or when you are experiencing a humidity problem.