why does my mini split in heater mode raises RH?

[zizration]

hello , if anybody can help me with this i'd be stoked. i am running a mini split ac for cooling during the day and it works pretty good overall. at night when my temps drop i was hoping to use it as well. the only thing is i ran it last nite and it jacked up my RH to +80 % !!!! not good. any advice is greatly appreciated..

 

[queequeg152]

ur plants rasise the humidity not your ac equipment.

when you are heating you are not cooling the air... cooling the air is what removes moisture hence rain, hence oceans, hence life.

 

[zizration]

well yes and no to above. overall i'm going to have to disagree a bit and i realize i wasn't very clear in my original post.



when my lights go off and the ac turns off the room temp drop down to 64f from 74f and RH moves from 58% to 65% . i was hoping that by turning on the heater in my mini split it would raise the temps and maybe the RH would drop a bit. ?

It does warm the room to proper temps, but it also raises the humidity waaaay up, even with my exhaust fan running and dehumidifier running . as soon as i turn off the heater the temps drop back down and eventually the RH drop back to 65% again.



any thoughts?

 

[resinryder]

When you introduce heat to an already humid environment it raises the humidity. Just like a sticky summer day in the South.

 

[MrBungle]

when you turn on the heater you are warming everything up including those wet pots ... when you raise the temperature of water it begins to evaporate raising the humidity.. if you want to control night time humidity run a dehumidifier at night

 

[queequeg152]

When you introduce heat to an already humid environment it raises the humidity. Just like a sticky summer day in the South.

no its the opposite.

when u add heat to a closed system with a static amount of water vapor... your RELATIVE humidy drops.

the total grain of water in the system stay the same.

relative humidity peaks at the coolest part of the night unless some weather is rolling through or something like that.

this is why dew forms at night and not during the day.

 

[resinryder]

no its the opposite.

when u add heat to a closed system with a static amount of water vapor... your RELATIVE humidy drops.

the total grain of water in the system stay the same.

relative humidity peaks at the coolest part of the night unless some weather is rolling through or something like that.

this is why dew forms at night and not during the day.

So if I have 55% humidity in my room at lights off, turn the heat on, my humidity will drop?
I'm not doubting what you posted at all and understand it, but the warmer my room gets the higher the humidity seems to get.
My room is sealed and have seen the humidity rise when doing what I just described. So guess I'm now wondering why

 

[queequeg152]

well yes and no to above. overall i'm going to have to disagree a bit and i realize i wasn't very clear in my original post.



when my lights go off and the ac turns off the room temp drop down to 64f from 74f and RH moves from 58% to 65% . i was hoping that by turning on the heater in my mini split it would raise the temps and maybe the RH would drop a bit. ?

It does warm the room to proper temps, but it also raises the humidity waaaay up, even with my exhaust fan running and dehumidifier running . as soon as i turn off the heater the temps drop back down and eventually the RH drop back to 65% again.



any thoughts?

i see what u are saying, and u are correct in that is not what one would expect.

the person above is correct in that higher heat will earn u more evaporation etc... but it should now produce this effect.

its possible u have a faulty humidity sensor... thats certainly what i would check first. if its digital it could possibly be damaged in such a way that it does not move off of say... 50% humidity and it is simply applying temperature correction values to the fixed humidity reading off the sensor.

problem is these sensors are not like dry bulb wet bulb readings... i dont really know how they respond to temp... the correction could be positive or negative i dont really know.

its also possible that there is a great deal of un drained water sitting inside your minisplit cassette... perhaps some of this is re evaporating from the drain pan.

i seriously doubt this could explain a huge jump in RH however.

try the salt test on your thermostat/humidistat to confirm its working properly. alot of these digital humidistats are highly unrelaible and drift non stop.

the thin film capacitive ones are especially shitty.

you really want somethign like a hair hygrometer... they can be calibrated with a little screw driver and are entirely mechanical.

 

[40AmpstoFreedom]

Well transpiration is much less at night although more the higher the heat so I do not believe it is the plants at all (this is why you apply teas on the plants before lights come on).

Are you soil or hydro? I wonder if it makes your soil evaporate more?

Maybe try placing a hygrometer net to the ac while heat is on and have one away from the air and see if it is coming from the ac.

I know when you leave an AC on and it turns off after bit and the fan is still running the water build up on the coil will be evaporated into the air over time and this will definitely cause a big RH spike like 20%+ but I do not know how the heat system works in your AC. I am not an AC tech but i wonder if something similar is happening with the Heating coil (if there even is one?)


EDIT:

Ok I think I found the answer and I was sort of on right track. So since your AC was running all day I think water built up in it and then you flip the heat on and it casues to evaporate quite quickly into your room.

This thermostat setting is to blame

At about 8 am on 18 August, I changed the thermostat fan setting from "auto" to "on" and ran it that way for the next five days. As you can see in the graph above, the relative humidity (RH) also spiked and averaged 70% during the time the fan was running continuously.

Why did the RH and dew point go up? While the air conditioning cycle was running, the indoor coil got cold. Water vapor from the air passing over it would condense on the coil. When the compressor turned off, the coil would warm up. The water remaining on the coil normally would just sit there until the next cycle, with some of it evaporating and sitting in the air inside the coil housing. But with the fan running continuously, the air passing over the coil as it warmed up would evaporate that water and put it right back into the house.

Keeping the relative humidity at 70% is not a good thing. That's about the threshold where mold can really take off. If we left our thermostat set to the fan-on position all summer, I'm sure we'd have a mold factory in our condo. So if you've been told to run the fan continuously by someone who sold you an electrostatic filter, UV lights, or some other indoor air quality device, you may not have been given the complete story on how such products affect the IAQ in your home.

The higher relative humidity also meant that our home was less comfortable. We average about 58-59% with the fan set to auto, and even that's not ideal. The main reason for that is the short runtimes of our air conditioner, which I wrote about last week.

air conditioner thermostat fan on dew point 600

The graph above shows the dew point data by themselves with the mean values before, during, and after the fan-on period.

One thing that surprised me was how quickly the humidity rose when I changed the thermostat. Within minutes, I noticed the RH reading moving up. But getting back to RH levels below 60% took a couple of days after I switched it back to the "auto" position.

There is definitely an HVAC tech around here that can confirm whether or not I am right, but I think this is correct.

 

[queequeg152]

So if I have 55% humidity in my room at lights off, turn the heat on, my humidity will drop?
I'm not doubting what you posted at all and understand it, but the warmer my room gets the higher the humidity seems to get.
My room is sealed and have seen the humidity rise when doing what I just described. So guess I'm now wondering why

im strictly speaking of a theoretical environment with purely static amounts of water... like a mason jar or somethign.

if u saturate a jar with vater vapor(100% humidity), and then heat it up 10 degrees or so keeping the same pressure... the same amount of water is there but u now have a lower humidity.

this is all i can find that is not a confusing psychrometric chart.

go to .1 on the vert axis, then move horizontally to the right.

ive never seen a chart like this tbh. but i think its accurate.

 

[zizration]

the mini splits don't actually dehumidify in heat mode from what i have read.. as well this mini split doesn't have dehumidify mode, so it may not have that sensor at all? i am adding a second dehumidifier i guess. running in dry mode keeps RH nice around 58-59% but w lights off drops temps down to 62F..

 

[zizration]

i am growing in pro mix..

 

[40AmpstoFreedom]

Read my post I had edited it I think I found the answer. Also if you are bringing in air from outside the dew point raises at night as well which also contributes to it. Sealed or not sealed?

 

[zizration]

it isn't fully sealed, but at night my intake goes off..

 

[queequeg152]

I'm not doubting what you posted at all and understand it, but the warmer my room gets the higher the humidity seems to get.
My room is sealed and have seen the humidity rise when doing what I just described. So guess I'm now wondering why

tbh im not quite sure why that would happen it is undoubtedly due to the plants and water contained in the room, but i would think this would take like an hour to happen.

if you are adding purely sensible heat it SHOULD drop the relative humidity. are u sayig that the humidity marches up right with the temp? or does the increase in humidity lag behind the temp?

its important to recognize that more heat = higher vapor pressure in water so its going to evaporate more.

plants also transpire during the night time, but im unsure as to how much... and as to how much this is effected by temperatures.

but if you heat a room quickly with purely sensible heat... it should drop the RH first, then the effects mentioned above could take over and drive more moisture out of the plants and soil... this should lag behind though as the soil and to a lesser extent, the plant tissue will take a while to come to temperature equilibrium i would think.

 

[40AmpstoFreedom]

it isn't fully sealed, but at night my intake goes off..

Depending on size of outtake it could definitely still be pulling in but my money is definitely on the build up of condensation during day time cooling and then switch to heat causing it to evap.

 

[zizration]

for some reason I'm not able to quote.



but 40 amps ..


yea i agree the build of condensation idea makes sense,.i should get a damper for that intake line for lites out and see if that helps as i do exhaust all nite. thnx good ideas. I'm bummed as i was hoping i could just use this 24 hrs day for nice temps control. i will try auto mode and see if that helps at all...

 

[queequeg152]

the mini splits don't actually dehumidify in heat mode from what i have read.. as well this mini split doesn't have dehumidify mode, so it may not have that sensor at all? i am adding a second dehumidifier i guess. running in dry mode keeps RH nice around 58-59% but w lights off drops temps down to 62F..

u have to drop temperatures to extract moisture from air, so yes ALL heat pumps and ALL heaters will not be able to actually pull moisture out of the air.

when your minisplit is running however...its a massive dehumidifer, it just also removed sensible heat along with the latent heat(humidity).

a regular dehuy has the condensing coil infront of the evap so u get a huge temp drop across the coil assembly and hence get more water condensing on the coils.

your minisplit has that hot coil outside essentially "pumping" heat from inside out.


if you are looking to optimize moisture removal when the ac is on, you need to slow down your minisplits fan such that the temp drop across this coil is lowered ... but not so low that the coil freezes.

this WILL reduce the total capacity of your system by a small amount. u probably would not notice it tbh.

check your manual. almost all these units have various speeds for dehumidification and always on air circulation.

 

[zizration]

...

if you are looking to optimize moisture removal when the ac is on...

I'm not having problems with my RH when AC on.



it is at nite with the heater on that my RH spikes...

 

[Snook]

dehumidifier set at 60%.. problem solved.


EDIT: Oh yes, pipe the condensate hose to a container and use the condensate for watering..