On install day, the furnace replacement process follows a set order. The crew protects the route to the furnace, shuts off gas and power, removes the old furnace, sets and connects the new one, then starts it up and tests it before walking you through it. A straightforward like-for-like replacement, where the new furnace goes in the same spot and vents the same way, is often done in a single day. Moving the furnace, switching from the chimney to plastic vent pipes, or reworking ductwork or gas piping can push the job into a second day.
Before install day
The site visit
A good replacement starts with someone looking closely at what's there now: the furnace and its fuel, the chimney or vent pipes, the gas line, the electrical supply, the ductwork, and the air conditioning coil if one sits on top of the furnace. They should measure the space too, including the doorways and stairs the old furnace goes out through and the new one comes in through.
This is also when problems that change the plan get caught. In older basements, paper-like wrap on the ducts or the cloth connector between the furnace and the ductwork can contain asbestos, and Massachusetts lists both as suspect materials. If testing confirms asbestos, a licensed abatement contractor has to remove it before the furnace work disturbs it.
Sizing and choosing the equipment
The old furnace's size is a starting point, not the answer. If the house has had new windows, insulation or an addition since it went in, the heat it needs on a cold night has changed. A heat loss calculation (ACCA Manual J) works it out from insulation, windows, air leakage and the local winter design temperature. ENERGY STAR's installation guidance puts it plainly: size equipment from the home's actual characteristics, not a rule of thumb.
Then come the choices. A standard 80 percent furnace can usually vent into a properly lined chimney, while a condensing furnace (90 percent and up) needs its own plastic vent pipes and a drain for the water it produces. If you have central air, the new furnace has to fit under the existing coil and move enough air for it. It's also a natural moment to weigh a heat pump against a furnace.
The written estimate and the permit
Before you agree to anything, get a written estimate that names the equipment by model and spells out the work around it: venting, gas piping, ductwork transitions, the condensate drain, the thermostat, permits and removal of the old furnace. If a chimney liner or electrical work might be needed, it should say so now rather than on install day. Giving the price before any work starts is how OniFix handles every job (here's how we work), and it's a fair standard for any contractor. For typical numbers, see what a new furnace costs in Massachusetts.
Gas furnace work in Massachusetts is permitted and inspected. The gas permit can only be issued to a licensed plumber or gas fitter, who files it with the town and later calls for the inspection. Oil furnaces go through the local fire department instead, and depending on the town and the job, an electrical or building permit may be needed too. The installer should handle all of it, not you.
Install morning: arrival and protecting the house
The crew should arrive at the agreed time and start with a quick walk-through: the route they'll use, where any new vent pipe will run, and anything you want them to be careful around. Floors get covered from the entry door to the furnace, and tight corners along the way can be padded. Then the old system is shut down: thermostat off, power cut at the furnace's service switch and breaker, and the gas valve feeding the furnace closed. If the gas has to be shut off to the whole house for piping work, the water heater and other gas appliances go off for a while and get relit afterward.
Removing the old furnace
The gas line is disconnected and capped, the vent pipe comes off the chimney (or the PVC pipes are cut free on a condensing unit), and the electrical supply and thermostat wires are disconnected. The supply plenum on top and the return duct are separated from the cabinet, and any water is drained, both from the trap on an older high-efficiency furnace and from the air conditioning coil's drain line.
The coil needs special care. It usually sits right on top of the furnace and often has to be lifted and supported during the swap without kinking the copper refrigerant lines. If those lines have to be opened, the refrigerant must be recovered first, which is work for an EPA 608 certified technician. Then the old furnace is carried out and usually hauled away for scrap.
Setting the new furnace
Ductwork connections
The new furnace is set in place and leveled as its manual specifies. It rarely matches the old cabinet exactly, so sheet metal transitions are made to join the supply plenum and the return, and the joints are sealed with mastic or foil tape, not cloth duct tape. The return matters most: manufacturers require it to be sealed to the cabinet so the blower can't pull air, and possibly combustion gases, from around the furnace.
Venting
A condensing furnace vents through plastic pipe, usually PVC: one pipe for exhaust and, on most installations, a second that brings outdoor air in for combustion. The pipes leave through a sidewall or the roof, with the terminals kept clear of windows, doors and expected snow. Massachusetts adds its own rules: a sidewall vent has to be at least three feet from the gas meter, and if it ends less than seven feet above grade, a sign reading "GAS VENT DIRECTLY BELOW. KEEP CLEAR OF ALL OBSTRUCTIONS" goes on the wall at least eight feet up, in line with the vent.
An 80 percent furnace vents into the chimney through a metal connector. Manufacturer instructions prohibit venting into an unlined masonry chimney, and many older chimneys need a properly sized metal liner. Going condensing raises a different issue: if the old furnace shared the chimney with a gas water heater, the water heater may be left alone in a flue too big for it to draft properly. Installation instructions require testing the appliances still on the chimney and resizing or relining the vent if they don't vent correctly. A liner is far easier to plan for at the site visit than to discover at noon on install day.
The condensate drain
Condensing furnaces produce mildly acidic water, a steady trickle in cold weather. It runs from the furnace's trap to a floor drain or, if there's no drain low enough, to a condensate pump that lifts it to one. The line has to stay where it can't freeze, because a frozen or blocked drain will shut the furnace down.
The gas and electrical connections
The gas connection is made by a licensed plumber or gas fitter, which Massachusetts requires. Every joint is then leak tested with a leak-detection solution or an electronic detector, never a flame.
On the electrical side, the furnace is wired to its circuit through a service switch, so anyone working on it can cut the power on the spot, and the thermostat wires go to the new control board. If the job needs new wiring, a licensed electrician does that part.
Startup and commissioning
A furnace can light and blow warm air while running well outside its design range, so a careful installer measures instead of assuming:
- Gas pressure. A manometer on the gas valve confirms the incoming and burner pressures match the manufacturer's numbers, so the furnace burns the amount of gas it's rated for.
- Combustion analysis. A probe in the flue measures oxygen, carbon monoxide and flue gas temperature, showing whether the burners are burning cleanly and efficiently.
- Temperature rise. The difference between the air entering and leaving the furnace has to fall within the range on its rating plate. Too high and the furnace runs hot and can trip its safety limit; too low on an 80 percent furnace and flue gases can condense in the vent.
- Static pressure and airflow. Static pressure is the resistance the blower pushes against, a bit like blood pressure for your ductwork. It shows whether the ducts can carry the air the furnace and air conditioner need, and the blower speeds are set accordingly.
- Safety controls. The flame sensor, high-limit switch, rollout switches and the pressure switch that proves the vent is clear are checked.
- Thermostat setup. The thermostat is configured for the new equipment and tested in heat, cool and fan.
The furnace then runs through full heating cycles. In cold weather, a full test of the cooling side may have to wait for a warm day, since air conditioners aren't meant to cool when it's cold outside.
Cleanup and the walkthrough
Packaging, old duct pieces and debris go out, and the work area and route are cleaned up. Then someone should walk you through the new system and show you:
- Where the filter goes, its size (printed on the filter's edge) and how often to check it; our guide to how often to change a furnace filter covers the schedule.
- How to set the thermostat, including its schedule and fan setting.
- Where the furnace's gas shutoff valve and service switch are.
- What normal sounds like. On a call for heat, the small inducer fan starts, the burners light with a soft whoosh, and the main blower follows a little later. When the thermostat is satisfied, the burners go out and the blower runs briefly to pull out the last of the heat. A condensing furnace may gurgle as water drains, and a faint smell during the first few cycles is common as manufacturing oils burn off.
Install day is a good time to test your carbon monoxide alarms, too. Massachusetts requires them on every level of a home with fuel-burning equipment. If one ever goes off, get everyone out and call 911.
Ask about the manufacturer's warranty as well. Some major manufacturers give their longer parts coverage only if the equipment is registered within 60 days of installation. Find out whether the installer registers it or you do, and keep the paperwork together.
After install day: the inspection
The job isn't finished until the inspector signs off. For a gas furnace, the town's gas inspector checks the piping, the connection and the venting; for oil, the fire department inspects. The installer arranges the visit, but someone usually has to be home to let the inspector in. Keep a copy of the permit with your furnace paperwork.
How to get your home ready
- Clear a path from the door the crew will use to the furnace, plus a few feet of open space around the furnace and water heater.
- Leave a parking spot close to that entrance; both furnaces travel by hand truck.
- Keep pets in a closed room, since doors will be open and people will be carrying heavy equipment.
- Mention any quirks along the route, like a sticky bulkhead door or a low beam over the basement stairs.
- Plan for the heat to be off for most of the day. In cold weather, keep interior doors closed to hold warmth, and keep any space heater well clear of anything that can burn.
If your furnace is near the end of its run, our furnace replacement page explains how OniFix handles a change-out, from the first look at your system to a price you see before any work starts.
Sources: ENERGY STAR, HVAC Quality Installation; 248 CMR 3.00, gas fitting permits and inspections; 248 CMR 5.12, venting of appliances; Mass.gov, smoke and carbon monoxide alarms; MassDEP, asbestos regulation FAQ.