The high-altitude furnace manual does not match the gas heat-value number I found online.

Find out what each number means

Ask the technician to identify the manual revision that applies to your full model number. Then ask the gas supplier for the heating-value and gas information that the manual requires for your service location. “Heat value” means how much energy is in a stated volume of gas. The conditions used to state that volume matter.

As an example of why an online number needs context, an Xcel Colorado tariff document states minimum monthly gas heating values at specified standard pressure and temperature. A minimum service specification is not automatically the expected value at your furnace's installed altitude. This older document illustrates the distinction; get current location-specific information from the supplier for an actual setup. Xcel gas-quality specification, R24B.

We recommend asking the provider to write down the basis of the values used. That makes a disagreement answerable: which supplier value, which manual table, which elevation, and which existing approved configuration? Guessing a different pressure because a search result has a bigger number skips those questions.

Keep the physical checks with the technician

Do not attach gauges, open pressure taps, adjust the gas valve, change burner orifices, or alter a safety switch. A value that looks correct on paper does not establish how the furnace is actually operating.

The qualified technician should decide the necessary input, gas-pressure, combustion, airflow, and temperature-rise checks for that model. Ask for the conclusion in ordinary words: “Does this configuration follow the applicable instructions, and do the operating checks support it?” If a kit or adjustment is required, ask which factory-approved requirement supports it.

Other manufacturers can use different altitude arrangements. For example, Daikin's DM96TN instructions specify particular kit and fuel considerations and prohibit a blanket pressure reduction as the way to derate that furnace. The difference is why one Denver setting cannot be copied to every Colorado unit. Daikin DM96TN instructions, altitude section.

Your next step is to bring the exact manual and the online number to a qualified technician, then obtain the supplier's fitting value. Resolve the reference mismatch before any gas adjustment is considered.