Furnace Installation & Replacement
At the moment of installation it is already too late to make decisions about equipment sizing. That call gets made during design — from a load calculation, not from the label on the furnace being carried out.
"Same Size as the Old One" Is Not a Calculation
The fastest way to size a replacement furnace is to read the old one's plate and order a match. It is also the most common way to install the wrong equipment.
That original unit was probably sized by a rule of thumb decades ago. And your home has changed since — new windows, added insulation, a converted garage, a remodel that moved walls. The load it has today is not the load it had then.
A load calculation uses ACCA Manual J algorithms against your actual site climate, construction, and insulation to determine what the house loses on the coldest day of the year. It is required by code for a new installation, and it can often be reused for your Title 24 compliance later.
Sizing Limits We Design To
Capacity is measured against the calculated load — not against the outgoing equipment.
Furnace heating capacity
No more than 6 kBtuh above the calculated load
Heat pump heating capacity
No more than 12 kBtuh above the calculated load
Any heating capacity
Never below the calculated load
Variable or multi-speed turndown
Low stage at or below 80% of load
Why oversizing hurts: short cycles, uneven temperature between rooms, weaker humidity control, and a shorter equipment life. It performs badly on every day except the extreme one it was sized for.
Installation Is Not Finished When It Turns On
A furnace is a machine that controls fire, and every one of these checks is the difference between a unit that runs and a unit that runs correctly.
Temperature Rise Tuning
After roughly ten minutes of burner operation we measure supply minus return air and tune blower speed to the middle of the rating plate range — commonly 30°F to 60°F. Raising blower speed lowers the rise; lowering it raises the rise.
Gas Pressure Verification
Incoming line pressure must sit between 6.5 and 14 in. W.G., with manifold pressure at the burners set to a nominal 3.5 in. W.C. Under-firing and over-firing both produce incomplete combustion and send heat straight up the flue.
Flame Quality Inspection
A correct flame has a dark blue interior, light blue exterior, and at most a light orange tip — steady, not fluttering. It must never touch the heat exchanger. Impingement drops flame temperature, halts complete combustion, and builds carbon on the exchanger.
18%
Airflow lost to one skipped step
The Duct Flange Problem
Upflow furnaces ship with the bottom return air opening partially closed by sheet metal duct flanges that the installer is supposed to unfold and remove. When that step is skipped, the return opening stays restricted and airflow drops by roughly 18 percent. Nothing alarms. Nothing fails inspection. The furnace simply spends its entire life starved for air — which then shows up years later as high limit trips and a cracked heat exchanger nobody can explain.
The Commissioning Checklist
- Bottom return duct flanges removed and airflow opening restored to full size
- Temperature rise measured after ~10 minutes of burner operation and tuned to mid-range
- Manifold gas pressure verified at nominal 3.5 in. W.C.
- Incoming line pressure confirmed between 6.5 and 14 in. W.G.
- Burner flames inspected for steadiness, correct color, and no heat exchanger impingement
- External static pressure measured at the return and supply
- Condensate trap primed before first start on condensing furnaces
- Sediment trap fitted at the equipment where not factory supplied
- Line voltage polarity confirmed — electronic safety controls depend on it
- Permit pulled and closed with the local building department
We also leave you with a system manual documenting what was installed and how it was configured — so any future change, service call, or upgrade starts from a record instead of a guess.
Worth Comparing Before You Commit
If you are replacing a furnace anyway, it is the right moment to price a heat pump alongside it. Heat pumps are roughly three times as efficient, cool as well as heat, and carry rebates and tax credits that narrow the up-front gap. We will quote both against the same load calculation so you are comparing real numbers.
See the heat pump option →Before you choose equipment
Sizing and fuel type are the two decisions that matter most, and both are easy to get wrong.
What Size Furnace Do I Need? Why Bigger Is Never Better
An oversized furnace costs more, wears out faster and heats less evenly. Here is why.
Read the article →Heat Pump vs. Gas Furnace: Which Is Right for a Los Angeles Home?
Which one actually makes sense in a Los Angeles climate, and what the running costs really look like.
Read the article →How to Tell If Your Furnace Is Producing Carbon Monoxide
The warning signs of a carbon monoxide leak, and which ones mean leave the house now.
Read the article →Installation FAQs
How do you decide what size furnace my home needs?
With a load calculation built on ACCA Manual J algorithms, using your site climate, construction, insulation, and orientation. It estimates the heat your home actually loses on the coldest day of the year. We never size by matching whatever is currently installed, because that unit was very likely sized by an outdated rule of thumb.
Is a bigger furnace better?
No, and oversizing is by far the more common error. Heating capacity must meet the load and exceed it by no more than 6 kBtuh. An oversized furnace short cycles, distributes temperature poorly across rooms, and wears out sooner. Undersizing is rare and only shows up on genuinely extreme days.
What temperature rise should a new furnace be set to?
Within the range stamped on the unit's rating plate, which is typically 30°F to 60°F, and tuned toward the middle of it. We adjust blower speed to get there: faster blower lowers the rise, slower raises it. Running outside that range either overheats the heat exchanger or lets moisture condense inside it.
Do I need a permit to replace a furnace in Los Angeles?
Yes. Permits are required by law, and a load calculation is code-required for a new installation. Skipping the permit creates liability when you sell the home and can give your insurer grounds to deny a claim. We pull the permit and close it out with the local building department.
Why does my new furnace need a filter I have to supply?
Filters are generally not shipped with the equipment and must be sized and installed externally by the installer. Operating a furnace without filtration can void the warranty. Filter size is calculated from air flowing across the filter face at roughly 300 feet per minute, so a larger furnace needs a physically larger filter.
What is the most commonly skipped step in a furnace installation?
Removing the sheet metal duct flanges from the bottom return opening on upflow furnaces. If they are left folded in place, they restrict the opening and cut airflow by roughly 18 percent. The furnace runs, the customer notices nothing, and the system is permanently airflow starved.
Should I replace my furnace with another furnace, or switch to a heat pump?
Both are legitimate. A furnace makes sense when your gas infrastructure is sound and up-front cost is the deciding factor. A heat pump costs more initially but is roughly three times as efficient, removes combustion risk entirely, and qualifies for rebates and tax credits. We price both so the comparison is yours to make.
Get the Load Calculation First
Every good heating decision starts with knowing what your house actually needs. We will measure it, then show you the options that fit.