electrical calculation please

DFord wrote:
Humidity is always "relative" to the temperature. As air is cooled, it can't hold as much moisture as it did when it was warmer. Conversely, as cool air is warmed up, it can hold more moisture. It's relative humidity goes down as it's warmed up as long as more moisture isn't added.

With that background, you can understand that bringing in cold outside air into your house in the winter lowers the relative humidity indoors. It gets as dry as a dessert making static electricity a problem. Dehumidifiers shouldn't be needed in the winter unless your space is really tight and no "infiltration" occurs.

Example:
If you bring in 35 degree air at 100%RH, and warm it up. it becomes 70 degree air at 29%RH.

Seems like you're referring to me, though I'm not certain it's in regards to the dehumidifier in the basement or the dehumidifier I just bought for our RV.

For the RV, I got it because our unit is pretty sealed (2015 Rockwood Mini Lite 2306). Too sealed, perhaps. We do run the roof vent while showering with the window in the bathroom open and the vent fan while cooking, but still, water starts condensing, mostly on the single pane windows but also above the bunks, behind the bed, in the cupboards or anywhere that's cool with low air circulation. Our last camping trip it was about 45 °F during the day and 35 °F at night. It rained most of the time, so the outside humidity was 100%. We had a lot of condensation inside the camper.

I do understand relative humility, but that last trip if you run the calculation from 100% at 45 °F it's 50% at 65 °F, where we kept the camper. Much better, but not great when you have a wet shower and you're running the oven to cook dinner. Plus it's cold to vent a lot of air. Hence the dehumidifier.

I'm also planning on making a DIY air/air heat exchanger like the one RickW made on this site in 2012. Even though I can't see Rick's pictures, I got a good enough understanding from reading, the pictures from a user later in the thread, and general understanding of heat exchangers. This will be for venting for humidity, cooking/showering, and general fresh air when it's cold out. Obviously it's much easier to bring in fresh air when you can recover 75% of the heat of the air that's leaving.

EDIT: Described the removed link. Odd the forum is set to purge links to itself for new members, but okay. Likely nobody thought to whitelist this site in the auto-filter.

DFord wrote:
Humidity is always "relative" to the temperature. As air is cooled, it can't hold as much moisture as it did when it was warmer. Conversely, as cool air is warmed up, it can hold more moisture. It's relative humidity goes down as it's warmed up as long as more moisture isn't added.

With that background, you can understand that bringing in cold outside air into your house in the winter lowers the relative humidity indoors. It gets as dry as a dessert making static electricity a problem. Dehumidifiers shouldn't be needed in the winter unless your space is really tight and no "infiltration" occurs.

Example:
If you bring in 35 degree air at 100%RH, and warm it up. it becomes 70 degree air at 29%RH.

Agree with DFord. It also depends on where you live. In Florida I needed a dehumidifier in the RV of a winter. The windows were like a waterfall and you felt damp. Where I live in Kansas I need a humidifier of a winter in the house. It can get so dry the skin on your hands crack and lots of static electricity.

Humidity is always "relative" to the temperature. As air is cooled, it can't hold as much moisture as it did when it was warmer. Conversely, as cool air is warmed up, it can hold more moisture. It's relative humidity goes down as it's warmed up as long as more moisture isn't added.

With that background, you can understand that bringing in cold outside air into your house in the winter lowers the relative humidity indoors. It gets as dry as a dessert making static electricity a problem. Dehumidifiers shouldn't be needed in the winter unless your space is really tight and no "infiltration" occurs.

Example:
If you bring in 35 degree air at 100%RH, and warm it up. it becomes 70 degree air at 29%RH.

Hmm, well, hopefully this isn't taken as spam or promotion, given it's my first post here. Signed up because I kept getting taken here in results, and figured I might like hanging out here.

Anyway, I wanted to add my two cents as I've been researching dehumidifiers myself. Primarily for our TT, as we had condensation while camping and we plan to camp through the winter. Specifically, I wanted something light and small, yet effective. The peltier based ones sold all over the place like the one linked where usually about a pint a day. That's about what a person will exhale in 24 hours, so for two people cooking and showering those were right out.

Compressor models didn't seem good either. Heavy, noisy, and they typically aren't effective while cold (below about 65 °F). So after a bunch of looking around I came across a desiccant wheel dehumidifier. 14 lbs, 18" x 11" x 7", and rated at 13 pints per day. No compressor, the only noise is a fan. Seemed perfect for an RV dehumidifier, so I bought it.

For the OP, there's both pros and cons for this (rare) style of dehumidifier:

Pros: It's smaller than most dehumidifiers. 13 pints/day is likely more than enough to maintain a sealed up 12' x 14' room. And there's no compressor to break or refrigerant to leak out, the latter seeming to be the biggest killer of compressor based dehumidifiers. After reaching set humidity, it shuts off, and runs the fan for a couple of minutes every so often to "mix up" the room air and get a more accurate humidity reading.

Cons: It costs $180. It's power hungry. I measured 480 watts and 12 pints/day at 70 °F and 50% RH (pretty in-line with the specs), which works out to 1.0 pints/kWh. That's pretty pathetic efficiency, given that a 30 pint energy star compressor based dehumidifier is roughly 3.9 pints/kWh. The 480 watts means it's also a mini space heater, so if heat is a concern it may be out (it does have low mode at 280 watts, but with correspondingly less humidity removal rate). And if it ran full time on high (seems unlikely, given a sealed room above grade), that's 345 kWh/month, or $42 at a reasonable electric rate of $0.12/kWh.

Sorry this post is so long, I tend to ramble. Please note I'm not saying this is the best dehumidifier for the job! However, it seemed to me that inefficient as it is, given a sealed room and presumably low runtime required to maintain a low humidity, it may be a useful option to consider. The biggest pro is there is no compressor, the biggest con is the inefficiency. I cannot speak to lifetime, as I just got it. As always, do your own research!

EDIT: To replace removed link with a basic description not containing any product names.

DFord,

Thanks for the advice. Still thinking....

Amazon wrote:
REMOVES 18 OUNCES PER DAY: Optimal function in room sizes up to 2200 cubic feet (220 sq ft)

That's 1.125 pints/day. It doesn't sound like much. Before you put it in, buy a humidity indicator and let it sit in the room a couple of days. Then bring in the humidifier and let it run a couple of days (make sure to empty the catch pan so it won't shut off) and see if the reading on the indicator changes any. 55% humidity is enough to get a musty smell. You won't know if it works till you try it but don't expect much.

I bought a 20 pint/day Frigidaire dehumidifier from Lowes a few years back. I never thought it was working very well (I ran a hose to the floor drain in my basement so I didn't know exactly how much it was doing). The second season I could tell it wasn't doing anything. It had lost it refrigerant charge - everything was running but nothing was happening. I took it back, they gave me another one just as good and let me keep the first one. The second one didn't do any better than the first one. I sat them aside and bought one Harbor Freight was selling - $100 delivered. It worked great but after many years, I saw a recall notice (fire hazard) that covered it and they sent me another one that I'm still using today. I scraped the old ones out and got about $45 all total for the 3 compressors. The name brand units at Lowes weren't worth bringing them home. If you buy one be sure to get an extended warranty to cover it more than what the mfg does. They don't repair them so if it acts up (as they often do), you end up with nothing to show for your money. I've got the tools and experience to repair them but they're not made to work on.

DFord wrote:
Dehumidifiers are commonly rated in "pints per day"

At only 20watts, I'd doubt if it could remove a pint in 24 hours.

How about sharing the brand and model of what you're looking at and we can give you a much better idea of what it can do and what you can expect. Without that information we're all just wasting our time guessing.

Ok, here goes: humidifier

I don't plan on using this as a whole house unit. A/C seems to take care of that. Also, the TT has 2 max air vent covers and I leave the vents open 24/7 and we have not had a problem.

Where I planned to use this unit is in my workshop / storage area under the house ( house on pilings ). It is approx 12x14. There is no standing water issue outside, it is built on slab, no leaks. Just trying to eliminate a musky smell / dampness that I think comes because the room is closed up 95% of the time. No window. No A/C, just portable heater.

BTW My man cave is adjacent to this space and same size. Small pool table. It has a window A/C /heat pump. It is used much more often and there is no musky smell there unless it is closed up for a week or so. I also have an exhaust fan I run there at times for air exchange.

So there it is, you wanted more info. What do you think?

ScottG wrote:
Too weak to have any effect at all. I recommend you save your money.

agreed. if you have a humidity issue you likely will need to remove at least a few gallons of water/day. the device you mention is likely a peltier device, lucky to remove a cup of water/day.

Go to a big box store and look for a portable compressor driven unit. small ones hold 3-5 gallons have an external drain. Now, they will draw about 1-2A on 120V when running, 1A is about 120Watts, so at $.10/kwh, it will cost about 10-20 cents/day to run it for 8hrs/day.

I have a small thermoelectric dehumidifier that I keep in my convertible since it always leaks a bit. Works well in that tiny space, can get about 1/2 cup a day of water out of it.

Too weak to have any effect at all. I recommend you save your money.