Undersized return duct and restrictive 1-inch filter replaced with a fully modulating 99% AFUE Lennox Signature furnace, MERV-16 carbon clean filter, and a cold-climate horizontal Lennox heat pump — delivering energy savings and pure air comfort
99% AFUE
Furnace Efficiency
MERV-16
Filter Rating
−30°C
Heat Pump Range
Dual Fuel
System Type
3 project photos
Photos documented for this case study
4 scope items
Equipment and work documented below
Surrey, BC
Completed Spring 2026
The project
A home with decades of restricted airflow, a 1-inch filter choking the system, and an aging Carrier furnace with a B-vent flue — completely transformed. Grove Mechanical replaced the undersized return air duct, upgraded to a Lennox Signature Series fully modulating 99% AFUE gas furnace with a MERV-16 carbon clean filter for hospital-grade air purity, removed the B-vent, and paired the system with a cold-climate horizontal Lennox heat pump. The result: dramatically lower energy bills, allergen-free air, and year-round comfort the home has never experienced before.
The original return air duct was significantly undersized for the furnace's rated airflow. Combined with a 1-inch filter that added excessive static pressure, the system was operating at severely restricted airflow — causing the heat exchanger to overheat, increasing fuel consumption, and shortening equipment life. Grove Mechanical rebuilt the return duct to proper Manual D specifications before installing the new furnace.
The aging Carrier furnace used a B-vent (natural draft) flue — a liability in any high-efficiency replacement project. The new Lennox Signature furnace uses a sealed PVC combustion air intake and exhaust system, completely eliminating the B-vent and improving both safety and efficiency.
The home's side yard offered limited clearance for a standard upright outdoor unit. Grove Mechanical installed the Lennox heat pump in a horizontal configuration on a custom gravel pad with anti-vibration isolators, meeting all code clearances while keeping the unit discreet and properly drained.
A complete mechanical room transformation: the undersized return duct was rebuilt, the 1-inch filter replaced with a MERV-16 carbon clean filter, the B-vent removed, and the aging Carrier furnace replaced with a Lennox Signature Series fully modulating 99% AFUE unit. Paired with a cold-climate Lennox heat pump, the home now has year-round comfort, dramatically lower energy bills, and air quality that removes smoke, allergens, bacteria, and VOCs. This is the standard every home deserves.
The return duct is the lungs of your HVAC system. If it's undersized, the furnace and blower are starved of airflow — causing the heat exchanger to overheat, increasing fuel consumption, reducing comfort, and shortening equipment life. A 1-inch filter compounds the problem by adding even more static pressure restriction. Rebuilding the return duct and upgrading to a deep-media MERV-16 filter cabinet is one of the highest-value improvements you can make to any forced-air system.
A standard 1-inch filter captures large particles but does almost nothing for fine particulates, smoke, allergens, bacteria, or VOCs. A MERV-16 deep-media carbon filter captures 95%+ of particles as small as 0.3 microns — including wildfire smoke, pet dander, mold spores, bacteria, and volatile organic compounds. It also has far lower static pressure resistance than a clogged 1-inch filter, which actually improves airflow rather than restricting it.
A fully modulating furnace (like the Lennox Signature SLP99V) adjusts its heat output continuously from 40% to 100% capacity based on what the home actually needs. Instead of blasting on at full power and then shutting off, it runs quietly at low fire for most of the heating season — maintaining a more even temperature, using less gas, and running far more quietly. Most homes only need 40–60% capacity on a typical winter day, so the furnace runs efficiently at low fire rather than cycling on and off.
Cold-climate heat pumps (like the Lennox XP21) use variable-speed compressor technology to extract heat from outdoor air even at temperatures as low as -30°C. In BC's mild climate, the heat pump handles most of the heating load efficiently — only switching to the gas furnace backup on the coldest days. This dramatically reduces gas consumption while providing efficient cooling in summer.
A B-vent (Type B gas vent) is a natural-draft flue used on older, lower-efficiency furnaces. It relies on the buoyancy of hot combustion gases to exhaust through the roof. High-efficiency condensing furnaces (90%+) produce cooler exhaust that cannot reliably draft through a B-vent, so they use a sealed PVC system with a powered inducer. Removing the B-vent and converting to sealed PVC combustion air and exhaust is required for any high-efficiency furnace installation and improves both safety and efficiency.
Lennox Signature Series SLP99V Furnace
LennoxSLP99V
Fully modulating 99% AFUE gas furnace — variable-capacity operation from 40–100%, ultra-quiet, sealed PVC combustion air and exhaust
Lennox Cold-Climate Heat Pump
LennoxHorizontal cold-climate heat pump — rated to -30°C, provides efficient heating and central AC
MERV-16 Carbon Clean Filter Cabinet
LennoxDeep-media MERV-16 carbon filter — captures 95%+ of particles ≥0.3 microns including wildfire smoke, allergens, bacteria, and VOCs
Rebuilt Return Air Duct System
Grove MechanicalCustom sheet metal return duct rebuilt to Manual D specifications — eliminates static pressure restriction and restores full system airflow
3 project photos from this documented installation
4 installed equipment and scope details
3 on-site challenges explained
Completed in Surrey, BC
A real result, not a stock showcase