Mystery: Rising humidity with longer AC runs
breckenu
| Posted in Mechanicals on
My attic placed 4.0 ton AC unit (2010 Bryant 14 SEER) causes a significant rise in indoor humidity on hot days when the compressor runs near continuously. See the graph below from my ecobee t’stat report. My other 2 ton unit does not raise the humidity even when running continuously, in fact it lowers humidity. Both units maintain the set point of 75 deg but run near continuous to do it when outdoor is near 100 degF. What would cause the humidity to rise with long compressor runs? BreckB in Dallas, TX
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If the duct system it's tied to isn't well balanced, takes in ventilation air by design, or has duct leaks in the unconditioned attic it could be pulling in outdoor air whenever the air handler is running.
Plotting the indoor relative humidity is fine, since the indoor temperature stays within a narrow range, but plotting the outdoor air's relative humidity is only partial and potentially confusing information. Without the temperature at which that humidity is relative it's a somewhat meaningless number. Converting the outdoor temperature + RH to dew point would be more useful, since dew point is a measure of the absolute (not relative) humidity. The outdoor dew points tend to rise with outdoor temperature, even though in relative terms it may be falling.
Unless the thermostat is controlling the compressor and blower directly there may be some false assumptions. If the ducts are undersized or the filter is clogged and the evaporator coil is getting close to the frost point it's possible that the compressor is cycling off even though the air handler continues to run. With the compressor off some of the moisture accumulating on the coil is re-evaporated into the air stream, causing the indoor RH to rise until the compressor kicks on again.
That type of cycling could explain the periodic large swings in indoor RH every ~45 minutes during what is otherwise a long/continuous call for cooling, with continuous blower operation.
I know you said the setpoint was constant, but what does the temperature plot look like? Is temp actually being held constant?
I too am suspicious the compressor is cycling.
You can download the ecobee logs as a .csv and convert the info to dew point if a more detailed analysis is warranted.
To add to what Dana said, check if the condensate drain is plugged. A full pan under the coil could also be contributing to the humidity problem.
Bill
Measure the condensate removal rate to help diagnose it.
> Both units maintain the set point of 75 deg but run near continuous to do it
Both running at the same time? Or a 2 ton running alone can keep up?
I think your replies are on the right track. Below is the temperature graph for the same time period on this 4 ton unit. Also I have an outside air intake duct with an Aprilaire ventilation control (like Aircycler). When outside temps reached 99 to 102 degF in the last week, my indoor humidity rose above 70% according to the ecobee t'stat report. So I completely closed the ventilation damper to prevent any outside air and the days after that the RH% dropped into the 50-60's% as you see on the chart.
Is it possible the TXV valve is cycling during continuous running to prevent coil freezing, and then the fan would blow across a warmer and web coil? Would it help to reduce the variable speed ECM fan setting jumper from 400 cfm/ton to 350 or 315/ton? There is also a "cool size" jumper that is set at "Hi" now.
My 2.0 ton and 4.0 ton units both run near continuously on 100+ degF days from about 2pm to 6pm. This is a 3000 sqft one story on slab home.
It's odd to see cyclical ripples in your temperature when your compressor is running constantly. It looks to me like something is cycling. Maybe a safety. Sometimes the condensate pump will have a set of contacts to shut off the compressor if it overfills, maybe something is freezing up and causes a cycle on the low pressure switch.
>"Also I have an outside air intake duct with an Aprilaire ventilation control (like Aircycler). "
Make that the prime suspect then! Combining the ventilation function with cooling or heating functions is always problematic. Turning the ventilation down (or even off) is likely to fix at least part of the humidity issue. That type of ventilation is by definition unbalanced, usually pressurizing the house, driving cooler-drier conditioned air out via air leaks.
>"My 2.0 ton and 4.0 ton units both run near continuously on 100+ degF days from about 2pm to 6pm. This is a 3000 sqft one story on slab home."
Even with the parasitic load of ducts & air handlers being in the unconditioned attic 6 tons of cooling seems like quite a lot for a 3000' house, even at 110F. Duct leakage and air-handler driven infiltration from unbalanced ducts might account for that. The flex ducts can be checked and recommissioned, stretching them as tight as possible for maximum flow, getting rid of any tight radius bends or kicks. If the duct "system" looks like a ball of mating snakes that could be an issue.
An Energy Star duct system would have no more than 3 pascals (0.012" water column) room-to-room pressure differences under all air handler speeds, doors open & closed. This is hard to measure with cheap manometers, but even the ~$150 hand held manometers used for measuring static pressures across duct systems have a resolution down to 0.01" water column. Any rooms that measure higher than 0.01" fail Energy Star, and any that measure 0.03" or greater are worth remediating with some form of jump duct. If there are doored off rooms with supply registers but no returns odds are high that "the great outdoors" has become part of the return path. Door threshold cuts usually can't be made big enough for an adequate return path, but using a partition wall stud bay as a jump duct with a grill close to the floor on one side and closer to the ceiling on the other can work without too much loss of privacy. There are other types of jump duct measures if that isn't going to work for some reason.
I forgot to confirm that the emergency condensate pans are dry under both AC units.
Here are the graphs for my 2.0 ton unit - temp graph and RH% graph showing no rise in humidity during the hot part of the day.
Use a meat thermometer in the supply closest to the air handler and report back the results.
What is your house construction like? Are you in full sun or have a lot of windows with solar gain? Is the ducting and air handler inside or outside the thermal envelope?
My wild guess is you have leaky ducts in the attic.
Have you had your home tested with a blower door and duct blaster?
I know every house in TX has its ducts in the attic but that does not change the fact that it is a bad idea.
Every bit of air that leaks from the supply ducts into the attic will depressurize your house pulling in hot and humid air thru every gap.
Is the 2 ton unit also in the attic?
Walta
To answer the questions:
- Both units in the attic. ~R-30 fiberglass blown / painted radiant silver / ridge vents / continuous soffit ventilation. Ducts are 21 yrs old, flex, R-6.
- Meat thermometer @ ceiling supply vents: 4 ton: 51 degF 2 ton: 59 degF
- House has triple pane, lowE, alum frame with therm break. Large roof overhangs - all windows shaded from direct sun by roof or trees.
- 37 solar panels on south facing roof - further shading roof.
I suspect as some of you have, the R-22 charge in the 4 ton is off causing TXV cycling which produces a warm evap coil on and off during continuous running with fan distributing the moist air to the house. I will have it checked by my AC guys soon.
Mystery solved: My HVAC service tech discovered the 21 yr old attic plenums, both return and supply, are made from the old 1 inch thick fiber board and taped together. The plenums seams and the attachment to the metal furnace and evap box had separated in many places and causing the intake of humid attic air. Also the evap coils were dirty since they had not been cleaned in 6 years. I will soon be having the plenums changed out to insulated metal boxes and sealed up properly. The refrig charge was ok. Thanks to all for your insight on the problem.
Thank you for letting us know what you found.
While they are up there working having some sealing work done on the other joint would likely pay for itself in the first year or two. The correct way would be to clean the joints in the duct work and apply a foil tape or mastic to the joints.
If you can find someone that tests their work before and after with a “duct blaster” unit, they are more likely to do a better job.
Walta