Humidity Complaint Investigation

Purpose

"The house feels clammy" is the complaint most often answered by selling something. This procedure exists so it gets answered by measurement instead: what moisture is entering the house, what the system is actually removing, and how much of the day it runs. Get that on paper and the cause names itself. Skip it and the shop sells a dehumidifier into a house whose real problem is a dryer venting into the laundry room, then owns the complaint forever.

The second thing it fixes is the unit of measure. Relative humidity moves with temperature, so two rooms that read differently on RH can hold identical moisture. Dew point does not move with temperature, and that is the number this whole procedure compares.

Scope

Covers the diagnostic visit on residential and small light commercial forced-air systems where the complaint is too humid or too dry: baseline readings, source inventory, mechanical fault check, latent performance measurement, airflow, pressure and infiltration, and the findings conversation.

Does not cover installing the remedy, which belongs to the dehumidifier and humidifier SOPs. Does not cover the full IAQ assessment visit, which is a broader survey and owns the combustion appliance zone test and the room-by-room grid. Does not cover mold sampling, which this shop refers out. If a single symptom already points at one system, the humidity troubleshooting trees in this library get there faster than a full investigation does.

Roles and responsibilities

Procedure

1. Walk in with a personal CO monitor already running and read the occupied space before touching anything. Clip it at breathing height before the door opens. Acceptance: 0 ppm in occupied space; anything above zero is a finding, not rounding. Hazard: carbon monoxide is odorless and a wet basement complaint often sits in the same room as a natural draft appliance. Stop rule: at a sustained reading in the tens of ppm everyone leaves the building, you call from outside, and the gas utility or fire service is notified before any humidity work resumes.

2. Take the outdoor baseline first, then the complaint room and one control room, all at 4 ft. Record dry bulb and RH at each, away from registers and off exterior doors. Acceptance: three locations recorded with the time, plus system state (running, off, or cycling) beside each. What wrong looks like: an indoor reading with no outdoor number to read it against, which makes a 60 percent day indoors look like a system fault in a 78 F dew point climate. Stop rule: no outdoor baseline, no diagnosis. Hazard: none at this step, it is instrument work in occupied rooms.

3. Convert every reading to dew point before you compare any two of them. Write the dew point beside each RH figure. Acceptance: a dew point for all three readings from step 2, in F. This is the step people skip and it is the one that decides everything after it, because a bedroom at 68 F and 65 percent and a hallway at 76 F and 50 percent are within a couple of degrees of the same dew point, which means one room is cold, not wet. What wrong looks like: chasing a high RH reading in the coolest room of the house. Stop rule: a complaint room whose RH runs well above the control room's while their dew points match within about 2 F is a cold room rather than a wet one, so record that finding and take it to room temperature and airflow instead of running a whole-house source hunt for it. Hazard: none at this step, arithmetic done at the kitchen table.

4. Inventory the moisture sources with a rate or a duration attached to each. Ask about showers per day and whether the fan runs, cooking, an unvented or poorly vented dryer, aquariums, indoor plants, firewood, a dirt crawlspace, and any recent water event. Acceptance: a written list where every entry carries a number or a frequency, not just a mention. What wrong looks like: a list that says "some plants," which cannot be ranked against anything. Stop rule: standing water anywhere ends the humidity investigation until it is dealt with, because no equipment out-runs an open source. Hazard: crawlspaces and basements with standing water are electrical risk, so nothing gets stepped into with cords or lights in the water.

5. Check the mechanical faults that fake a humidity problem before measuring performance. Confirm the humidifier is isolated in cooling season, the fresh air damper is where it should be for the season, and the condensate is actually draining rather than standing in the pan. Acceptance: humidifier water valve closed with the pad dry, damper position recorded, and drain flowing under a poured cup. What wrong looks like: a bypass humidifier still open in July, which is a moisture source hiding inside the equipment. Stop rule: any of the three found wrong gets corrected and the space given a day before performance is judged. Hazard: the condensate pan sits beside line voltage, so power is off before a hand goes into the cabinet.

6. Measure latent performance across the coil with the system in steady state. After 15 minutes of continuous running, record dry bulb and RH at the return grille and at the first supply register, and convert both to dew point. Acceptance, as a shop default: supply dew point at least 5 F below return dew point at those two points, with condensate visibly running. What wrong looks like: a 1 or 2 F depression, which means the coil is not getting cold enough to condense, usually from airflow rather than charge. Stop rule: under 5 F, do not conclude the system is undersized, go to step 7 first. Hazard: probes go through the register face and the test port, never past a blower guard, and the unit stays running with its panels on.

7. Measure airflow before blaming capacity. Read total external static against the blower's nameplate rating, then determine delivered cfm and divide by nominal tons. Acceptance: total external static at or below nameplate, and 350 to 400 cfm per ton, taking the low end in a humid climate because a slower coil runs colder and wrings out more water. What wrong looks like: a blower left on the high tap after a previous call, which cools fine and dehumidifies badly. Stop rule: outside the band, correct the tap or the duct and re-run step 6 before anything is quoted. Hazard: this reading is taken with the cabinet closed through the manufacturer's ports, and the door interlock is never defeated to watch a spinning wheel.

8. Rank the causes, name the top one out loud, and write it down. Read back the dew points, the source list, the latent depression and the cfm per ton, then state which cause is first and what the second one is. Acceptance: the customer can repeat the top finding back in their own words. What wrong looks like: a findings conversation that opens with a piece of equipment. Stop rule: with a combustion or standing-water finding open, nothing is quoted that visit. Hazard: none at this step, it is a conversation at the kitchen table, and that is where it belongs rather than shouted over a running air handler.

When the site does not match

Complaint room is a finished basement below grade: the control reading moves to the floor above, and slab and wall surface temperatures get taken, because a below-grade room can sit at the same dew point as the rest of the house and still condense on the wall. Nobody home long enough for a 15 minute run: leave a datalogger for 24 hours rather than judging latent performance off a short cycle. Heating season complaint of dry air with no humidifier fitted: steps 1 to 4 and 8 still run, steps 5 to 7 do not, and the finding is usually infiltration rate rather than equipment.

The record this produces

The reading sheet carries: arrival CO; outdoor dry bulb, RH and dew point with the time; complaint room and control room dry bulb, RH and dew point with location, height and system state; the source inventory with a rate against each entry; the three mechanical checks as pass or fail; return and supply dew point with the depression in F; total external static against nameplate and cfm per ton; and the ranked cause list with the top cause circled.

Two people use it later. The tech doing the retest after the remedy, who needs the same rooms at the same heights or the comparison means nothing. And the service manager, who can tell from the depression figure and the cfm per ton whether a proposal is supported by measurement or by a hunch.

Worked pass: 1,900 square foot slab house, August, clammy at 72 F

Customer keeps the thermostat at 72 F and says the house feels wet. Three-ton system, single return, laundry off the hallway.

Step 1. CO monitor reads 0 ppm in the living area. Pass.

Steps 2 and 3. Outdoor 88 F and 61 percent RH, dew point about 73 F. Complaint room 72 F and 58 percent RH, dew point about 56 F. Control hallway 73 F and 56 percent RH, dew point about 56 F. Same dew point in both rooms, so this is a whole-house moisture level, not one cold room.

Step 4. Source list: two showers a day with a fan that runs, no aquarium, and a dryer whose flex duct has come off the wall fitting and is discharging into the laundry room. That last one carries a rate: roughly one load a day, every day.

Step 5. No humidifier fitted. Fresh air damper closed. Poured cup clears the drain. All three pass.

Step 6. After 15 minutes running: return 72 F and 58 percent RH, dew point about 56 F. First supply register 58 F and 88 percent RH, dew point about 55 F. Depression is 56 minus 55, or 1 F, against the 5 F gate. Step 6 fails and takes its stop rule: go to step 7 rather than calling the system undersized.

Step 7. Total external static 0.42 in w.c. against a 0.50 nameplate, so the duct is not the constraint. Delivered airflow measures 1,410 cfm; 1,410 divided by 3 tons is 470 cfm per ton, above the 350 to 400 band. Blower found on the high tap. Dropped to the tap that gives 1,080 cfm; 1,080 divided by 3 is 360 cfm per ton, inside the band.

Step 6, re-run. Return 72 F and 58 percent RH, dew point about 56 F. Supply 53 F and 90 percent RH, dew point about 50 F. Depression is 56 minus 50, or 6 F, above the 5 F gate. Pass.

Step 8. Causes ranked: first the dryer discharging indoors, second the blower tap that was already corrected on this visit, third nothing further identified. Customer repeated back that the dryer duct is the main item.

Retest four days after the dryer was reconnected: complaint room 74 F and 45 percent RH, dew point about 51 F, down from 56 F at the same location and height. Two fixes, 5 F of dew point, and no equipment sold.

References

• ASHRAE psychrometric relationships for the RH to dew point conversion, or the conversion built into your measuring instrument, used consistently at every reading point
• Manufacturer's blower table for the specific air handler, for the tap-to-cfm relationship and the nameplate external static rating
• See related: the whole-house dehumidifier installation SOP and the humidifier installation SOP, which own the remedies this investigation may point to
• See related: the IAQ assessment visit SOP for the broader survey, and this library's humidity troubleshooting trees where a single symptom already names the system