# Tule fog mornings: why your heat pump steams, and when frost is a problem > Why a heat pump steams on foggy Sacramento winter mornings, what the defrost cycle is doing, and the signs that ice on the outdoor unit needs a technician. URL: https://tulesacramentoheatpump.com/blog/tule-fog-mornings-heat-pump-defrost/ Seasonal Frost, a stopped fan and a cloud of vapor from the outdoor unit are a normal part of a Sacramento winter morning. Here is what the defrost cycle is doing, and how to tell it apart from ice that needs a visit. On a still winter morning in the Sacramento Valley, the outdoor unit of a heat pump can look like it is smoking. A plume of white vapor rises off the coil, the fan has stopped, and water is dripping onto the pad. Then, a few minutes later, the fan starts again and the unit goes back to quietly heating the house. That is the defrost cycle, and on a tule fog morning it is doing exactly what it was designed to do. ## Where the tule fog name comes from [Tule Sacramento Heat Pump](https://tulesacramentoheatpump.com/) installs, replaces and services ducted heat pumps across the Sacramento region, and the business takes its name from this weather. Tule fog is the valley’s winter ground fog, named for the tule reeds of the marshes that once covered much of the valley floor. It is the condition a heat pump here heats through most often. Key takeaways - Frost on the outdoor coil on a damp, near-freezing morning is expected; the defrost cycle melts it in a few minutes - Steam, a stopped outdoor fan, a whoosh as the valve shifts and water running off the pad are all normal parts of defrost - Ice that keeps building, a coil that never clears, or ice on the fan or top panel points to a fault - Do not chip or pour water on the coil; switch the system off at the thermostat and call for a diagnosis ## Why fog season frosts the outdoor coil ### Cold, but not very cold Sacramento winters are mild by heat pump standards. NOAA’s 1991-2020 hourly normals for Sacramento Executive Airport put the typical January 7 a.m. temperature at 41.6°F, with one morning in ten at or below about 32°F. The same station averages about 15 nights a year at or below freezing and essentially none at 20°F or below. The California Energy Commission’s heating design temperature for the airport is 31°F. Those numbers say a heat pump sized for the house, as the [heat pump sizing guide](https://tulesacramentoheatpump.com/guide/heat-pump-sizing/) explains, has enough capacity for a valley winter. They do not say it will stay frost-free. ### Wet air is the real factor The January mean dew point at the same station is 41.3°F, almost the same as the morning temperature. Air that close to saturation holds as much moisture as it can. In heating mode, the outdoor coil runs colder than the air around it so it can pull heat out of it, and on these mornings the coil drops below freezing while the air does not. The moisture in the fog then freezes onto the fins as frost. ### Why a damp morning frosts more than a cold one So the amount of frost depends on moisture as much as on temperature, and a damp morning a few degrees above freezing can still frost the coil. It is also why, on the valley floor, defrost behavior and outdoor-unit drainage matter more than extreme-cold capacity (a reading derived from the NOAA and CEC figures above). ## What the defrost cycle does ### The sequence When the control decides the coil has collected enough frost, it starts a defrost cycle: 1. The reversing valve shifts the system into cooling mode, often with an audible whoosh. 2. The outdoor fan stops on most units, so the heat stays on the coil. 3. Warm refrigerant flows through the outdoor coil and melts the frost from the inside. 4. Melted water runs off the fins and out through the base of the cabinet. 5. The valve shifts back, the fan restarts and heating resumes. The whole cycle usually lasts a few minutes. On a long fog morning it can repeat several times. ### Why you see steam The vapor is not smoke and not a leak. The coil is warm during defrost, the air around it is cold and close to saturated, and the moisture coming off the warm metal condenses into a visible cloud, the way breath does on a cold day. When the coil is clear and heating resumes, the cloud stops. ### What happens indoors During defrost the system is briefly cooling, so the air from the registers can turn cool for a few minutes. Many systems bring on electric heat strips in the air handler to cover this; a [dual fuel heat pump](https://tulesacramentoheatpump.com/dual-fuel-heat-pump/) uses the furnace instead. If the heat strips run only during defrost, that is fine. If they run whenever the thermostat setting jumps a few degrees, the electric bill can climb. The [aux heat and defrost guide](https://tulesacramentoheatpump.com/guide/heat-pump-aux-heat-defrost/) explains lockout settings, and the [electric bill guide](https://tulesacramentoheatpump.com/guide/heat-pump-electric-bill-sacramento/) shows how backup heat shows up in a SMUD or PG&E bill. Ice that will not clear? Describe what the outdoor unit is doing and when it started, and a contractor can tell you whether it sounds like normal defrost or a fault to look at. ## Normal frost versus a problem ### Signs that it is working - A light, even coat of frost across the coil that disappears after a defrost cycle. - Vapor and dripping water for a few minutes, then a normal-looking unit. - The house still reaching its setting, even if more slowly on the coldest mornings. ### Signs that it needs a technician - **Ice that keeps building.** The coil stays white between cycles, or the frost turns into solid ice through the day. - **Ice on the fan, the top panel or around the base.** A unit encased in ice has stopped clearing itself. - **Frost on only part of the coil.** Uneven frost can point to a refrigerant or airflow issue, though it does not prove either one. - **The outdoor fan never restarts,** or the unit runs in defrost for much longer than a few minutes. - **Backup heat runs constantly** and the system cannot hold the setting on a mild foggy morning. The possible causes include blocked airflow at the coil, a failed defrost sensor or control, a reversing valve problem, a low refrigerant charge or a fan fault. A symptom seen from the patio rarely identifies which one it is, so the diagnosis needs someone with gauges and a meter. [Heat pump repair](https://tulesacramentoheatpump.com/heat-pump-repair/) covers how that visit works. > What you can safely do > > Keep leaves, plants and stored items well away from the coil, and keep downspouts from pouring onto the unit. If ice is building, switch the system off at the thermostat. Do not chip, scrape or pour water on the coil, and do not open the cabinet. ## Where the defrost water goes Every defrost cycle sends water out of the bottom of the cabinet. Valley-floor stations publish a normal snowfall of zero, so the reason an outdoor unit sits on a pad or riser here is drainage, not snow. The pad should be level and high enough that melt water runs away from the unit and the foundation. Water that pools under the cabinet can refreeze overnight and build ice up into the coil from below. ### Fixing the pad when the unit is replaced If the old pad has sunk or the ground slopes toward the house, that is worth fixing when a unit is replaced. The [placement and noise guide](https://tulesacramentoheatpump.com/guide/heat-pump-placement-noise/) covers clearances, pads and where a unit can go, and seasonal [heat pump maintenance](https://tulesacramentoheatpump.com/heat-pump-maintenance/) is a good time to check that the base drains and the coil is clean before fog season. ## Before the next fog season A short check in the fall makes winter mornings less eventful: - Clean the area around the outdoor unit and rinse leaves off the top grille. - Replace the indoor filter, since restricted indoor airflow makes the whole system work harder. - Look at the thermostat’s backup heat settings, or ask how they were set at installation. - Watch one defrost cycle on the first foggy morning so you know what normal looks like for your unit. If the system is old and defrost problems keep returning, the [heat pump replacement](https://tulesacramentoheatpump.com/heat-pump-replacement/) page explains when a new system makes more sense than another repair, and the [Sacramento heat pump guide](https://tulesacramentoheatpump.com/guide/sacramento-heat-pump-guide/) covers sizing, utilities and rebates. ## Questions about this post ### Why does the outdoor fan stop while the heat pump keeps humming on a cold morning? That is usually the defrost cycle. The system reverses to cooling mode for a few minutes so warm refrigerant runs through the outdoor coil and melts the frost, and most units stop the outdoor fan during that time so the heat stays on the coil. The fan should start again, and the vapor should stop, once the coil is clear. ### How often should a heat pump defrost on a foggy Sacramento morning? There is no single number. Some controls defrost on a timer and others only when sensors detect frost, so the interval depends on the equipment, the outdoor temperature and how wet the air is. Several defrost cycles on a damp, near-freezing morning can be normal. A coil that stays white between cycles, or ice that grows through the day, is not. ### Should I pour hot water on a heat pump that has iced up? No. Do not chip, scrape or pour water on the coil: the fins bend easily, the water can refreeze around the base, and the fault that caused the ice is still there. Switch the system off at the thermostat if ice is building, keep the area around the unit clear, and have a technician find out why it is not defrosting. Want this answered for your house? Call with your current system, the rooms that bother you and your ZIP code. ## Keep reading - Learn [What is heat pump aux heat, and when should it run in Sacramento?](https://tulesacramentoheatpump.com/guide/heat-pump-aux-heat-defrost/)How backup heat strips, defrost cycles and thermostat controls work together, why the valley floor rarely needs strips for capacity, what they cost to run on each utility's rates, and when a furnace makes the better backup. - Learn [What should you know before putting a heat pump in a Sacramento home?](https://tulesacramentoheatpump.com/guide/sacramento-heat-pump-guide/)How a ducted heat pump heats and cools, why Sacramento sizes for the afternoon, what tule fog does to the outdoor coil, how your electric utility changes the math, and what ducts, panels, permits and rebates add to the job. - Learn [Where should a heat pump's outdoor unit go, and how do you keep it quiet?](https://tulesacramentoheatpump.com/guide/heat-pump-placement-noise/)How location affects airflow, drainage, service access and sound, what the City of Sacramento and Roseville permits allow when a unit moves, and the installation details that decide how loud a heat pump is. - Learn [How much will a heat pump add to your electric bill in Sacramento?](https://tulesacramentoheatpump.com/guide/heat-pump-electric-bill-sacramento/)How heating moves from the gas bill to the electric bill, what a therm of heat costs under each local rate plan, how summer peaks and fixed charges work, and the settings that keep a heat pump's bill from creeping up. - Learn [How is a heat pump sized for a Sacramento home?](https://tulesacramentoheatpump.com/guide/heat-pump-sizing/)How a load calculation turns your house, your town's design temperatures and your ducts into a capacity, why the old condenser's tonnage is a poor guide, and what Title 24 and the City permit allow.