How did you size it?
Ask what calculation supports the size, instead of matching the old unit's nameplate. Pacific Northwest National Laboratory (PNNL) and its code brief for oil-fired boilers both point to an ACCA Manual J load calculation, with equipment chosen under Manual S. Tell the contractor about insulation or air sealing done since the old unit went in, because that lowers the load.
PNNL says a significantly oversized furnace can cycle on and off instead of running steadily, wasting fuel and rusting internal parts through repeated condensation. With steam radiators, ask how the boiler will be matched to them rather than accepting a furnace-style calculation.
What efficiency are you quoting?
Get the exact model number and its AFUE (annual fuel utilization efficiency), the share of fuel energy that becomes useful heat over a season. PNNL notes that oil furnaces run slightly less efficient than gas, and that older natural-draft furnaces are usually 70% AFUE or less. For boilers, PNNL's replacement checklist calls for a sealed-combustion, direct-vent unit with a multi-stage or modulating burner, and says high-efficiency sealed-combustion oil and gas boilers exceed 90%.
To see what the rating is worth, the Department of Energy's furnace buying guide estimates a 95% AFUE furnace in the northern region uses 523 therms a year against 621 for an 80% unit, about 16% less fuel for the same heat. If the boiler also makes your hot water, ask how that will be handled in the new setup.
What happens at the chimney?
A new sealed-combustion boiler often vents out the wall instead of up the chimney. PNNL's checklist says the empty chimney opening should then be sealed, and if a water heater is still connected to that chimney, it may no longer draft well enough to clear its exhaust. The contractor should tell you the fix for that orphaned water heater, such as a chimney liner. Ask: "What's still connected to this chimney when you're done, and what will you do about it?" Our guide on checking a flue for backdrafting shows what to watch for afterward.
Condensing boilers produce water. PNNL says the condensate line should drain to a sewer or outdoors, with insulation or heat tape if it runs outside, plus a drain pan under the boiler as backup. Ask where yours will drain.
What about the oil tank and lines?
If you're keeping oil, have an oil-burner technician check the tank separately from the burner. The Pennsylvania DEP's tank checklist says to call one for unstable legs, rust, weeps or wet spots, drips at the filter or valves, stains at the fill or vent pipe, or higher-than-normal oil use. DEP warns that tanks rust from the inside, so fresh paint proves nothing. Ask whether the quote includes spill and overfill protection, which DEP describes as simple devices your oil distributor can advise on.
If oil is going away, DEP says to remove the fill pipe with the tank, or permanently seal both ends, and tell the oil company never to deliver there; oil has been pumped into basements through fill pipes for tanks that were gone. Philadelphia tank removal needs its own L&I permit; the heat pump guide above covers it.
If there's a leak, DEP says the property owner must report it to the DEP regional office (800-42-TANKS in Pennsylvania), and cleanup is often not covered by typical homeowner's insurance. DEP has a reimbursement program for releases from underground heating oil tanks of 3,000 gallons or less. The EPA notes home heating-oil tanks are excluded from federal underground tank rules, but state and local agencies can still regulate them.
Who pulls the permits?
In Philadelphia, L&I's Mechanical Permit page says replacing heating appliances in a one- or two-family house qualifies for an EZ permit without plans, a licensed contractor with current insurance on file with L&I must do the work, and anything that needs power also needs an Electrical Permit. The same page says boiler installations are regulated by the Pennsylvania Department of Labor and Industry, so for a boiler swap ask which approvals apply. Our Philadelphia furnace permit guide lists the fees and forms. In Bucks and Montgomery County townships, call the township building office before you sign.
What should the startup report show?
Don't accept "it runs." PNNL's checklist says accessible piping and radiators should be inspected for damage or asbestos before work starts, with problems reported to you. After installation it calls for:
- A combustion safety test, and a leak test on any gas line.
- At least a 20°F difference between boiler supply and return temperatures at design conditions.
- A warm-weather shutoff set high enough that you don't lose heat on cool spring and fall days.
Get the results in writing with the manual and maintenance schedule.
Before the old unit comes out, make sure your carbon monoxide alarms work. The Consumer Product Safety Commission says to have them on every level and outside sleeping areas, and if one sounds, get outside and call 911. See when a CO alarm means leave the house.