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- Sun load
The room is not broken. It is facing west at four o'clock
Drag the hour. The diagram shows where the sun is and which side of the building is taking it — and the note says what actually helps, in the order that costs least.
The path
A simplified summer day in San Angelo: the sun rises north of due east, crosses high and to the south around the middle of the day, and sets north of due west. The building below the arc is drawn in section, seen from the side.
What is taking the load
Morning sun hits east glass hard but briefly, and the building has cooled overnight, so it rarely produces a complaint. If an east room is uncomfortable by nine in the morning, that is worth a look — it usually means the room has very little air reaching it rather than too much sun.
- Blinds on the east side buy you the hour that matters and cost nothing.
- Check the supply register in that room is open and actually delivering.
- Do not size equipment around the morning. It is the afternoon that decides.
In the middle of the day the sun is high, so the load moves off the walls and onto the roof and the attic. In a single-storey building here that is the biggest single gain of the day, and it is the one people never think about because there is no window to blame.
- Attic insulation depth and attic ventilation matter more at this hour than any curtain.
- Duct work in that attic is sitting in the hottest air of the day. Sealing and insulating it is the cheapest real improvement in most buildings here.
- South glass with any overhang above it is largely shaded now — that is what the overhang is for.
Low afternoon sun goes straight through west-facing glass at the exact moment the building has been absorbing heat all day. This is when the thermostat stops keeping up, and it is when almost every call we get in July is made.
- Shade the glass. External shade beats internal blinds by a wide margin, and even a blind helps.
- Check the west rooms have both supply and a way for air to get back out — a closed door turns a balanced room into a hot one.
- Balancing more air to the west side for the summer is a real, adjustable fix on most systems.
- If the system genuinely cannot hold the building at this hour, that is a load calculation question — not a reason to guess at a bigger unit.
Once the sun is low or below the horizon the direct gain stops, but the mass of the building goes on releasing the heat it absorbed all day. That is why a west room can stay uncomfortable until nine or ten at night with nothing shining on it at all.
- This is normal, and it is a reason to pre-cool earlier in the day rather than to chase it in the evening.
- If the building never recovers overnight, that is worth a visit — it usually means airflow rather than capacity.
- Night is the right time to run the fan to even the building out, and the wrong time to change the setpoint by five degrees.
The drawing is a simplified summer day at this latitude, not an astronomical calculation, and it takes no account of your overhangs, your trees, your glazing or the building next door. It exists to make the argument visible, not to size anything.
What this does not do
- It does not size equipmentSizing comes from a load calculation that accounts for the fabric, the glazing, the orientation and the occupancy. This is a diagram, not a calculation.
- It does not know your buildingOverhangs, trees, awnings and neighbouring buildings change the answer completely, and all of them are good news.
- It is not an argument for a bigger systemA larger unit on the same duct work will still lose the west rooms at four o'clock. Shade and airflow come first.