To apply for the scheme or for more information, please contact. Expect to pay between £2,000 and £4,000 or more, depending on the complexity of the installation, for a back boiler to combi conversion job. Interior plumbing & heating ltd. If you're looking for a boiler replacement, there are several different types of systems that we can install into your home these include. We are your 24/7/365 emergency plumber.