Microclimates on One Lot

Why this matters

The weather report describes the town. The equipment lives on one wall of one house, and the air at that wall can run several degrees warmer or colder than the forecast, get twice the wind, or sit in the one low spot that floods. A boiler whose outdoor sensor sits in the sun under-heats on bright cold mornings. A condenser tucked under a deck breathes its own hot discharge. A heat pump set under a roof valley grows a block of ice every thaw. None of these is an equipment fault, and every one gets diagnosed as one until somebody walks the lot.

Four things that make one lot several climates

Sun and shade. Direct sun heats surfaces far more than it heats air. A south or west wall in winter sun can sit 20 to 30 F above air temperature at its surface, while the air a few inches off it is warmed only a few degrees. That distinction decides which equipment cares: a sensor mounted on the siding reads the siding, while a condenser's coil sees the air it draws.

Reflected and stored heat. Pavement, patios, rock beds and masonry walls absorb sun and give it back as radiant heat and as warm air rising off them through the afternoon and into the evening. Glass adds a sharper version: a concave low-emissivity window pane can focus reflected sun onto a nearby surface, which is how vinyl siding on the neighbor's wall gets warped. Equipment set over a large paved area on the west side draws warmer air in the late afternoon, which is when cooling load peaks.

Wind exposure. Prevailing winter wind, and the way houses channel it through a side yard, change three things: how much heat an outdoor coil sheds in defrost, where snow drifts, and whether a combustion vent or a dryer exhaust gets blown back against the house. A heat pump on the windward corner of an open lot works in a colder effective environment than one in a sheltered courtyard.

Water and cold air moving downhill. Water collects at the low points of a lot, and on calm clear nights so does cold air, which is denser and drains into hollows the way water does. A low corner can run several degrees colder than the rest of the lot by morning. The other water source is the roof: a valley, a short gutter or a missing downspout extension puts concentrated runoff in a line along the eave.

What each driver does to the equipment

What shade does not do. Shading a condenser is often sold as an efficiency upgrade. Its measured effect is small, commonly a few percent at most, because the coil responds to the temperature of the air it pulls in, and shaded air near a house is usually only a degree or two cooler than air in the sun. What does cost real capacity is a placement that makes the unit recirculate its own discharge: under a low deck, in a tight alcove, or against a fence closer than the manufacturer's clearances. Shade that comes with a roof over the discharge is a net loss.

Reading air temperature versus surface temperature

Walk the lot with the right instrument. An infrared thermometer reads the temperature of a surface, which is what a wall-mounted sensor will feel. A shaded air thermometer, held a few feet off any surface and out of the sun (a sling psychrometer or a probe shaded by your body or a card), reads the air, which is what a coil breathes. Mixing the two is the commonest error in a microclimate walk: an IR reading of a sunny patio compared with a probe reading on the north side shows a difference that the condenser will never actually experience.

A worked example: one house on a sunny January morning

A customer complains that the house is cold on bright mornings and fine on cloudy ones. The boiler runs an outdoor reset control: it lowers supply water temperature as outdoor temperature rises. Its curve, read off the control's settings, runs from 180 F supply at 0 F outdoors to 100 F supply at 60 F outdoors, a slope of 80 F of supply over 60 F of outdoor, or about 1.33 F of supply per 1 F outdoors.

At 9 a.m. the tech walks the lot. The readings, with the instrument used for each:

                  NORTH (rear yard, shade)
      +-------------------------------------------+
      |  low corner: air 17 F (probe)             |
      |            north wall: air 20 F (probe)   |
      |      +------------------------------+     |
      | WEST |                              |EAST |
      | air  |            HOUSE             |air  |
      | 21 F |                              |22 F |
      |      +------------------------------+     |
      |  south wall siding 44 F (IR)              |
      |  reset sensor here, reads 32 F            |
      +-------------------------------------------+
                  SOUTH (street, full sun)

What the numbers mean. The air around the house sits at 20 to 22 F; the low corner at 17 F is the cold-air pocket. The south siding is 44 F in the sun, and the reset sensor, mounted flat on that siding, reads 32 F: warmer than the air by 12 F (32 minus 20, taking the north-wall air reading as the true outdoor temperature) and cooler than the sunlit siding, because its housing is partly shading itself.

What the boiler did with it. At a true 20 F outdoors, the curve calls for 180 minus (1.33 x 20) = about 153 F supply. At the 32 F the sensor reports, it calls for 180 minus (1.33 x 32) = about 137 F. The boiler is running about 16 F cooler than its own curve intends (153 minus 137), on exactly the mornings the sun is out. On a cloudy morning the sensor and the air agree and the house is fine. That matches the complaint.

The fix. Relocate the sensor to the north wall, under the eave, clear of the dryer vent on that wall and out of the cold low corner, at the mounting height the control's instructions give. Then the sensor and the north-wall probe should agree within a degree or two on the next sunny morning. If they do, and the house still lags, the curve itself is wrong, which is a separate adjustment.

What would change this answer. If the house had no shaded wall at all (a row house with only a south-facing exterior), the right move is a radiation shield over the sensor, or a sensor type the manufacturer lists for that mounting, rather than choosing the least-bad sunny wall. If the control had been reading low rather than high, the cause would point the other way: a sensor in the cold low corner, or next to a basement window leaking cold air, which over-fires the boiler and shows up as overheated rooms rather than cold ones.

Placing the outdoor unit on the same lot

The same walk decides where a heat pump goes if this customer later adds one. The west side reads warmest by a degree (21 F against 20 F on the north wall), which is noise for heating capacity. The deciding facts are elsewhere:

• The north wall takes the prevailing winter wind and the snow off the rear yard, so a unit there wants a raised stand that holds it above the drift line the customer describes.
• The east side has a roof valley that dumps onto the walk. A unit under that line will ice up with every thaw and freeze, however good its defrost control is.
• The low corner collects both cold air and meltwater, so a pad there sits in slush.

The west side, on a raised stand clear of the drip line, wins on drainage and drift rather than on temperature. Write the reason on the job, because the next tech will see a unit on the "warm side" and assume temperature was the call.

The microclimates the house makes for itself

Not every hot or wet spot comes from the weather. The house and its own equipment create small local climates, and they are the ones nobody thinks to look for because they move with the equipment's schedule.

• Exhaust and discharge. A dryer vent, a water heater or furnace vent termination, a pool heater exhaust or a generator exhaust puts warm, moist air in a plume a few feet wide. A sensor in that plume reads high only while the appliance runs, which produces an intermittent symptom that disappears by the time a tech arrives. Two condensers set side by side with one discharging toward the other's intake have the same problem in cooling.
• Irrigation overspray. A sprinkler head that wets a coil or an electrical disconnect leaves mineral spotting on the fins where the water is hard and keeps the enclosure base wet. Heads get added and adjusted by other trades after the equipment goes in, so check the spray pattern on any landscaped lot.
• Window wells and stairwells. A below-grade window well or an exterior basement stair traps cold air and drifted snow. Equipment set at its edge sees the colder pocket and the meltwater that drains into it.

The check for each is the same: ask what runs on this wall and when, and whether the symptom follows that schedule rather than the weather.

How to verify you read the lot right

Two checks close the loop. For a sensor, compare its reading with a shaded air probe at the same time on a sunny day and on a cloudy one; agreement within about 2 F on both days means the location is honest. For an outdoor unit, compare the air entering the coil with shaded air a few feet away while it runs; an entering air reading more than about 3 F above the shaded reading in cooling means the unit is breathing its own discharge or air heated off pavement, and the placement, not the unit, needs the fix. Start from those defaults and tighten them as your shop collects its own readings.

References

• The boiler control or sensor manufacturer's installation instructions, for sensor mounting location and height
• The outdoor unit manufacturer's installation instructions, for airflow clearances and snow-region stands
• See related: Reading the Place Before the Quote; Climate Zones and Design Temperatures in Plain Terms
• See related: Lot Drainage and Where the Water From Your Work Goes
• See related: Heat Pump Cold-Weather Operation Reference