High humidity in my wine cellar

WINE TALK
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I have had a two piece commercial Heatcraft cooling unit for my cellar in Seattle for many years. When the outdoor compressor failed after about 16 years, it was replaced about 5 years ago. When the in-cellar evaporator was found to have some refrigerant leaks after 20 years, it was replaced with a new one about a year ago. Essentially the same models as before. This setup has always over the years kept the temps at a rock solid 54 degrees, and the humidity in the low to mid 70’s…until the new evaporator was installed, that is.

The cellar has a cement floor, vapor barrier, high value insulation. The ceiling and walls abut heated living space, but with this set up and the insulation and vapor barrier, the temperature and humidity have never been a problem.

Last summer some months after the evaporator replacement, I returned after a few weeks away and the humidity was in the 90’s! There was some mold growth on the wood counter beneath the evaporator and a couple of shelves, and of course dampness on some papers I had in there. I got rid of the mold, and I ended up leaving the door open overnight and got the humidity down, but I have been fighting it ever since. There is no moisture coming through the cement floor that I can tell, the condensate drain does not appear to be backing up and no moisture anywhere that I can find. No signs of any wet spots on the walls or ceiling where there might be a small plumbing leak.

I bought a small countertop ?thermoelectic dehumidifier which I can only run when I am here to empty it, and I put large DampRid’s in the cellar and left the dehumidifier off when I was gone for a few weeks this summer. The humidity was in the mid 80’s when I returned, and even with the small dehumidifier it has not gone lower than 80-82%. There has not been any return of mold, but any papers on the counter get a bit damp feeling, and I am concerned about my labels. I would like to get the humidity back down into the low to mid 70’s where it was for so many years with the same set up.

I would happily buy a larger compressor dehumidifier, just to run when I am home and as needed, but I understand that they don’t work well at lower temperatures. Not sure if 54 F is too low for them. My HVAC firm that replaced the compressor and evaporator and maintained them over the years doesn’t seem to have a clue as to what might have changed with the newer non-refrigerant-leaking evaporator. Tonight I lowered the fan speed to see if that has any effect. We shall see.

Anyone have any thoughts? I suppose I could leave the door open again for a while, put up with the elevating temperature but count on the thermal load of the cold bottles to resist the wine warming up. BTW, I did that overnight at one point but left the evaporator running, thinking I could bring the humidity down but keep the room from warming up as much…the next morning all the wines in the Oregon Pinot Noir rack below the ceiling mounted evaporator were very wet with condensation. I had to wipe each one down to rid the room of all that moisture, which was no fun. At least none of my Burgs, which were in other racks around the room, were affected. Not sure why this happened, but I can guarantee my compressor and evaporator were working overtime with the door open overnight.

Any advice would be very much appreciated!

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It’s definitely going to F up some of your labels, which I guess shouldn’t matter but if anyone is being honest it does. You can mostly counter that by shrink wrapping bottles but that’s a chore and kind of wasteful.

There are a number of threads already on here. Search dehumidifier.

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It’s been a warm summer in Seattle, and humidity will rise as warm, moist air infiltrates the cellar and is cooled down. Maybe replacing any seals around your door to help keep the warmer air out.

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Hi, thanks for the response. I have been searching the threads, but haven’t found out anything yet that helps. Will keep looking.

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Hi Chuck, that’s a thought.
The room outside the door is pretty warm, as it contains a fridge and freezer. As I mentioned, the temps have always been and continue to be rock solid at 54. A couple of times in the past when I had to go without my cooling due to repairs (or going many years way back to a broken thru the wall Whispercool before we remodeled and went to a two piece system, and I removed the unit and I sealed up the hole in the wall and waited for a replacement) and the room took days and days to slowly climb in temperature to the low 60’s due to the insulation and door seal. So I have trusted the rooms “integrity”.
But maybe the door seal has worn over the years and is letting warm air in, and although the cooler is keeping the temps stable, the humidity is not staying stable.
I certainly haven’t checked the door seal, and will do so.
Thanks!

If the door seal is intact, then I’m still at a loss.

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Might be useless but I had a similar situation in a small wine fridge where a few bottles grouped together had water/moisture on them and some labels got ruined. Turns out one of those bottles was blocking some sort of fan/condenser and the air wasn’t properly circulating.in the unit . Once I moved that bottle the problem was solved. Maybe you have a similar situation?

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In my cellar the evaporator multi-fan unit is high on the ceiling at the ceiling/wall junction and the cold air is forced downward. Nothing is blocking it, and nothing has changed that would impact its air flow. The new evaporator is identical in make, size and shape as the old one…except that it does not have the “mini” leaks that eventually occurred in my old unit that led to loss of pressure in the refrigerant line.

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Chuck,
Since we don’t live in a very humid climate (I may be wrong there), and since the humidity in the cellar which used to be in the 70-75% range is, I assume, higher than in the adjacent rooms of the house, wouldn’t any leakage of warm air inward not necessarily be more moist than what is in the cellar? Or is it just that warm air leaking in will, in the process of being cooled down by a wine cooler (that unlike an standard air conditioner is not meant to lower the humidity I thought) release moisture into the room?

Btw, with your extensive experience, do you know of any HVAC or refrigeration companies in the Seattle area that have particular experience in dealing with the specific requirements of wine cellar cooling, that is, proper cooling with control of the humidity levels?

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Gents, I plugged this one into our AI Overlord ChatGPT and it popped out some actionable, practical answers. Below.

1. What Changed

  • For 15+ years your Heatcraft setup performed flawlessly: stable 54°F and 70–75% RH.
  • The only major variable is the new evaporator coil, which coincided with the sudden increase in cellar humidity (now stuck at 80–90%).
  • That strongly suggests something about the coil’s design, installation, or controls is influencing the moisture balance.

2. Why the Humidity Is Rising

A wine cellar system isn’t just about cooling — it also has to dehumidify. Here are the most likely causes:

  1. Evaporator Coil Temperature / Surface Area
  • Older coils may have run a bit colder, condensing more moisture from the air as they cooled.
  • Newer coils, designed for energy efficiency or with different refrigerant flow, may be running “warmer,” so they remove less water vapor.
  • If the coil isn’t spending enough time below the dew point, you’ll see temperature control without moisture removal.
  1. Fan Speed / Airflow
  • Too much airflow across the coil can warm the coil surface, lowering its ability to condense water.
  • Reducing the fan speed (as you tried) may increase dehumidification by giving air more dwell time on the coil.
  1. Defrost or Control Settings
  • Some replacement evaporators come with different fan cycling vs. continuous fan options.
  • If the fan runs continuously, it can re-evaporate water off the coil/drain pan back into the cellar when the compressor cycles off.
  1. Refrigerant Charge & Balance
  • Even though it’s a new coil, if the refrigerant charge isn’t dialed in properly, you could have suboptimal suction temps/pressures that affect dehumidification.

3. Why Portable Dehumidifiers Haven’t Solved It

  • Standard residential compressor dehumidifiers are designed for 65–90°F rooms, and they lose efficiency below ~60°F. At 54°F they’ll barely condense moisture.
  • Thermoelectric units are too weak for your space.
  • Industrial “low-temp” dehumidifiers exist (e.g. Santa Fe, Ebac, Quest), but they’re larger and pricier.

4. Options to Fix the Root Cause

  • Have HVAC tech measure coil temps and suction pressure. If the coil isn’t cold enough, the system needs adjustment.
  • Change fan control to cycle with compressor only. That prevents re-evaporation.
  • Reduce airflow (slower fan speed). You already started testing this, and it’s a smart move.
  • Confirm condensate drainage. Even a slight backflow or improper pitch can keep water in the pan, evaporating back into the room.

5. If Root Cause Can’t Be Fixed

  • Dedicated low-temp dehumidifier: Look at units designed for basements/greenhouses/wine cellars that work in the 50s°F.
  • Integrated humidity control: Some wine cellar systems use humidistats tied to the cooling cycle or add-on reheat coils to balance temp + humidity.
  • Temporary door-open trick: As you saw, it “works” but at a cost — condensation on bottles (thermal shock effect, warm humid air mixing with 54°F bottles). Not sustainable.

6. Bottom Line

  • The fact that everything worked fine for 20 years and failed immediately after the new evaporator points to a system design/installation issue, not a sudden change in your cellar environment.
  • The most likely culprits: coil temperature too high, fan operation re-evaporating moisture, or refrigerant balance.
  • Before investing in extra dehumidifiers, I’d push the HVAC firm (or a more wine-cellar-savvy contractor) to re-evaluate how the new evaporator is set up and whether it’s actually functioning like the old one did.
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Robert, remember we are talking relative humidity, not absolute. The amount of moisture that warm air can hold is much greater than cooler are. Therefore, the amount of moisture in let’s say 75 degree air at 40% RH might be far more than with 55 degree air at 75% RH.

The chat GPT response above is quite good.

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I was just going to say…

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Wow, that is some very helpful information, and gives me something, in fact a lot, to talk to my HVAC people about!!

I have been converted, and will now willingly, nay, happily bow down before my ChatGPT overlords and do their bidding!

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Does the fan run constantly? Usually you need a period of “off” on the fan to allow the moisture to drain. I am wondering if a new setting with the new evaporater changed that or the new evaporater doesn’t drain as effectively.

Whoever installed evaporater should come address the problem.

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Yes, the fan does run constantly, though the previous one did as well. But I am waiting to hear from our maintenance firm that installed it, the same model but obviously updated in the past 20 years (!), and we shall see what they have to say and whether they can switch this to cycle on and off with the compressor.

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First I want to say that humidity in the low 80s is not a concern. My cellar is about 80% and sometimes more for most of the summer and I’ve never seen condensation, mold, or issues with labels. 90s, though, yeah, that will be an issue. If your HVAC contractor is “clueless” about this then they don’t want your business, since it’s all about the evaporator (assuming you have sufficient insulation/vapor barrier, which apparently you do). Ideally you don’t want to oversize the evap – you want as low a fan speed and as low coil temp as possible. Assuming drainage works, those are really the only two factors that matter. Unlikely something you can adjust on your own though, they are interrelated with a whole bunch of other stuff like superheat, TXV valves, and so on. There’s a reason the very few contractors who do this stuff right can charge so damn much.

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Hi, thanks for posting.
My concern with the humidity in the low to mid 80’s when I returned to town last week after a few weeks away (with two large Damprids in the cellar) was perhaps because I was traumatized last summer when I returned to find the humidity in the 90’s and a bit of mold on the wood. And this time the pad of paper I write down what I remove for drinking again felt somewhat damp. I feared that without the Damprids it might have been higher.

Maybe if we don’t find the solution and it stays in the low to mid 80’s I won’t have to obsess over it. But it would be nice if either a slower fan speed, or setting the fan to cycle on and off with the compressor makes this new evaporator behave like the last one I had for 20 years, keeping it in the 70’s, and I don’t have to depend on DampRid or dehumidifiers to keep it in the 80’s!

The maintenance company are going to investigate this further.

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Couple potential issues with not running the fan all the time:

  1. You won’t have any air defrost. Your coil temp is probably right around or slightly below freezing in normal operation. If there is a situation that requires extended compressor operation, such as a lot of high-temp wine bottles to cool down (new purchases/restoration after power failure/etc.), there may be ice. You need the fan running without the compressor to remove this ice. (Freezers and some fridge-temp walk-ins have heater coils that cycle on/off to do this, but wine cellar systems won’t have this.)

  2. Depending on how your drainage is setup, you may need extended periods of positive pressure from the fan to help push out the condensation at the bottom of the pan.

The more I learn about this subject the less I feel I know. It’s really black magic. Kind of like tuning a classic car engine+carb. You need to find a wizard.

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More to think about and I will discuss these issues as well with the company.
Thank you!

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Just wanted to update here.
The technician came out and rewired so that the evaporator fan cycled on only when the outside compressor was on.
Also checked that the condensate drainage was not a problem.
The humidity went down to the 67-72 % range and seems to be holding steady there.
Temps holding at 54-55 degrees.
Hopefully I am not calling it a victory too early, but this seems to have taken care of the problem.
Yay!
Thank you for your suggestions.

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ChatGPT for the win!