Furnace Repair in Bow Mar, CO

Bow Mar is a town of 945 people in Arapahoe County at 5,518 feet, where the gas derate runs 14.1%. A 90,000 BTU/hr furnace fires about 77,300 BTU/hr here and delivers roughly 61,800 BTU/hr at 80% AFUE. The fault that produces the most no-heat calls at this elevation is the one homeowners least expect: the draft pressure switch refusing to make.

What does a pressure switch do, and why is it altitude-sensitive?

It is the safety that proves the inducer is genuinely moving flue gas before the gas valve is permitted to open. A diaphragm senses negative pressure through a small port and closes a set of contacts at a stamped setpoint, often a few tenths of an inch of water column. That pressure is produced by a fan moving air, and at 5,518 feet the air it moves is thinner, so the same inducer assembly generates less differential than it would at sea level. The switch setpoint does not move to compensate. The margin between what the inducer can produce and what the switch demands is therefore narrower in Bow Mar than at the elevations these parts were bench-tested at, and anything eating into that margin causes a lockout.

What eats the margin?

A vent run longer, or with more elbows, than the installation instructions permit. A partly blocked or screened termination. Water in the sensing tube, or a cracked hose. An inducer wheel loaded with dust, which sheds draft capability gradually. Condensate standing in a trap or collector box on a condensing appliance. And an inducer motor slowing as its bearings age, which still spins and still sounds correct, which is precisely why the switch gets blamed and replaced first. Under Bow Mar's 14.1% derate every one of those faults surfaces sooner than it would a few hundred feet lower.

How should the switch be tested?

With a manometer on the switch port while the inducer runs, comparing measured pressure to the setpoint printed on the switch body, and with an ohmmeter across the contacts to see whether they actually close at that pressure. Those two readings separate the only three possible answers: the switch is bad, the draft is genuinely inadequate, or the sensing path is plugged. Jumping a pressure switch to make a furnace run is not a diagnosis. It defeats the single safety standing between a blocked flue and a house full of combustion gas.

Does a town of 945 get the same service as a large city?

The diagnostic is identical; the logistics are simpler. Bow Mar is small and ringed by much larger neighbors, with Littleton holding 46,368 residents at 5,351 feet and Englewood 33,082 at 5,371, so a Bow Mar call usually drops into a route already running nearby. What matters far more than the size of the town is that the appliance gets measured: draft, gas pressure, temperature rise, flame signal and combustion, on every visit.

How does Bow Mar's 14.1% compare with the ground around it?

Bow Mar stands above the towns immediately east and south of it and well below the ground to the west. Littleton at 5,351 feet derates 13.4% and Englewood at 5,371 feet derates 13.5%, while Ken Caryl at 5,761 feet derates 15.0%. Bow Mar's 167-foot rise over Littleton is worth 0.7 percentage points of furnace output, roughly 630 BTU/hr on a 90,000 BTU/hr input, so equipment specified across that line without recalculating is set up for somewhere else.

What happens if a lockout gets ignored?

Most controls retry a set number of times, then hold for an hour before trying again, so an intermittent draft fault presents as heat that comes and goes overnight and reads like a thermostat problem. Meanwhile the actual cause, whether a restricted flue, a tiring inducer or standing condensate, keeps getting worse. A furnace that locks out on pressure once a week in November will lock out permanently in January, and January is when Bow Mar's 14.1% derate leaves the least spare capacity in the building.

What is worth having ready when you call?

The furnace model and serial number, the blink or digital fault code as displayed before anyone resets it, and a note of whether the exhaust leaves through the roof or the sidewall. That combination usually tells us which parts to load before we drive. MoJo Home Services is family-owned, licensed, insured and EPA-certified, and has worked gas heat in Arapahoe County since 2009. The number is 720-807-4050.

Same crews, neighbouring towns

Nearby and on the same schedule: furnace repair in Littleton, furnace repair in Englewood, furnace repair in Ken Caryl.

Heating and cooling fail for different reasons in the same house — see AC repair in Bow Mar.

More: furnace repair · everything in Bow Mar.