None of the pages that rank for this search are written for the person who has to fix the boiler. Here is what the state's own materials mean for an owner on the day a tenant reports no heat.
Lack of heat is named in the DCA bulletin as a defect to a vital facility, the category that unlocks the tenant's repair-and-deduct remedy under Marini v. Ireland, rent abatement, rent withholding, and constructive eviction, once the tenant has given written notice and adequate time. The bulletin also says that when heating equipment fails and the landlord does not act after proper notice, the local board of health may act as the landlord's agent and order the repair. In other words, if you do not choose the contractor, the town or the tenant eventually will, at a price you did not negotiate.
The cheapest version of this problem is the one where a licensed HVACR contractor is diagnosing the system on the first day, the tenants have a written update, and the file has receipts. The steps on the right are that version.
The request below goes to LA Trusted Partners, which matches New Jersey owners and managers with independent HVACR providers. LA Trusted does not perform the repair, set the price, or promise a response time; the provider who calls you back owns the diagnosis and the quote. For what that quote should look like, see the furnace and boiler repair cost guide.