Altitude, sizing and why bigger is not better

The altitude part

Caldwell's city center measures about 2,374 feet above sea level on the USGS elevation service. Nampa, Middleton and the rest of the valley floor are in the same band.

That is above 2,000 feet, which is where the fuel gas code starts treating appliances differently. The reason is simple: air gets thinner with altitude, so a cubic foot of it carries less oxygen. An appliance that meters gas as though it were at sea level is putting in the same fuel with less air to burn it.

Idaho adopts the 2018 International Fuel Gas Code, with amendments, so that provision applies here.

What it looks like when it is wrong

An appliance set up for sea level and run at this elevation tends to show it:

• A lazy, yellow-tipped flame instead of a crisp blue one.
• Sooting, inside the burner compartment or around the flue.
• Nuisance lockouts that nobody has ever got to the bottom of.
• A furnace that has never quite performed since the day it was fitted.

The last one is the tell worth paying attention to. If a system has been mildly disappointing for its entire life rather than having developed a fault, setup is a reasonable thing to suspect.

The sizing part, which matters more

Oversizing is the more common problem and it is counterintuitive, so it is worth explaining properly.

A furnace or air conditioner that is too big for the house heats or cools the air near the thermostat very quickly, satisfies it, and shuts off. Then the house drifts and it starts again. That is short cycling, and it produces:

• Temperature swings. Hot then cold rather than steady.
• Poor dehumidification in cooling. Removing moisture takes run time. A unit that runs in short bursts never gets to it, which is why an oversized system can leave a house cold and clammy.
• More wear. Starting is the hard part for a compressor and a motor. More starts, more wear.
• Rooms that never catch up. Short runs do not give air time to reach the far end of the ductwork.

Bigger is not safer, and fitting the same size on every job is not sizing.

How it should be done

Proper sizing is a calculation, not a rule of thumb based on square footage. It accounts for insulation, window area and orientation, air leakage, ceiling heights, the local design temperatures and, here, the elevation.

If you are replacing equipment, it is reasonable to ask whether a load calculation was done and to ask to see it. A quote that sizes by matching whatever was there before is repeating whatever decision somebody made decades ago, which may well have been wrong then.

When this is your actual problem

Sizing is worth investigating when:

• The house has never been comfortable, as opposed to having got worse.
• It cools but feels damp.
• The system runs in short bursts even when the weather is extreme.
• One end of the house is always wrong.

Sizing is not your problem when something clearly changed: it worked last week and now it does not. That is a fault, and the furnace problems page is the better starting point.

Related

Repair or replace covers what a replacement quote should contain, including the load calculation question. Weak airflow covers the distribution side, which is often mistaken for a sizing problem.