Marble Hill: the hollows are colder than the forecast says

Cold-air drainage, and why it defeats a generic load calculation

On clear, still winter nights, air in contact with the ground cools, becomes denser, and flows downhill. It pools in hollows, valley floors and drainages, and it can stay there until the sun clears the ridge.

The consequence is that a house sited in a hollow can experience overnight temperatures several degrees below the nearest official reading, and sometimes below a ridge-top house a few hundred feet higher. Frost forms there first and lifts last.

A load calculation performed against a generic regional design temperature misses this entirely, and the resulting equipment is quietly undersized for the coldest nights of the year — which is precisely when it matters. See load sizing.

What we look at on a Marble Hill site visit

• Position within the drainage. Valley floor, mid-slope or bench, and whether cold air has anywhere to flow past the house or pools against it.
• Obstructions downhill. A dense treeline, an embankment, a road cut or a neighbouring structure below the house can dam cold air on the property.
• Solar exposure. East-facing hollow sites can lose morning sun to a ridge and stay in shade for hours after sunrise, extending the coldest period of the day.
• Outdoor unit siting. Placing the condenser in the coldest, dampest pocket on the lot is a real and avoidable mistake. It increases defrost frequency and reduces available capacity at the worst moment.
• Drainage and standing moisture. Hollows collect water as well as cold air. The unit needs to sit above whatever the ground does in a wet February.

Defrost behavior matters more in a damp hollow

Frost accumulates on the outdoor coil when it operates below freezing in humid air, and every heat pump reverses periodically to melt it off. Hollow sites are both colder and damper than the surrounding terrain, so they produce more frost and more defrost cycles.

Two specification points follow directly. First, demand-initiated defrost — which senses actual coil conditions — substantially outperforms simple timed defrost in these conditions, because timed defrost either runs cycles that are not needed or misses ones that are. Second, the unit needs to be raised sufficiently and drained properly, because meltwater that refreezes around the base pan on a lot that never gets full sun becomes a block of ice by mid-January.

Equipment selection for this microclimate

The combination of a lower effective design temperature and higher frost loading argues for the same conclusion the rest of this county reaches, with more urgency:

1. Cold-climate inverter equipment with published capacity at 5°F, not a standard single-stage unit. See equipment selection.
2. Design temperature established for the site, not taken from a regional table.
3. Demand-initiated defrost, and outdoor unit siting out of the coldest pocket where the lot allows a choice.
4. Elevated mounting with positive drainage away from the base.
5. Balance point verified against the site design temperature, and the strip bank sized to the resulting shortfall rather than to the whole load.

Common questions about Marble Hill

Where these numbers come from

• NOAA U.S. Climate Normals — station data behind the 13-18°F design temperature range
• NEEP Cold Climate Air Source Heat Pump Product List — capacity actually retained at 5°F, by model

Every contractor in this network holds a current Georgia Conditioned Air or Electrical Contractor license. You can confirm any license yourself before you sign anything — Georgia Secretary of State license lookup.