Calculating "optimal" humidity

The air in the house was getting so dry that I would wake up in the middle of the night with nose pain a few days ago during some very low temps.

So I got a humidifier. And if I was normal I would have stopped there. But something is wrong with me. I can’t be bothered to refil the tank manually. I jerry rigged a vicks electrode boiler into the tank of a toilet that auto-refills and gets flushed daily. water maintenance solved.

But I realise the electrodes will cake up with calcium in short order. So I found a whole house unit that has an auto-cleaning ability (does not use the yearly replacable filter). That’s on order now.

And I noticed condensation on window glass for the first time, so now I want to cap that humidity to just below the limit of condensing on the windows.

I’ve seen “tables” of optimal RH for a given “range” of temps. How are those calculated? I want a formula (temp in, max RH that won’t condense out).

I realise this is further complicated because everyone’s windows are different. If mine were perfectly insulated, there would be no condensation. But my cows aren’t spherical of uniform density in a vacuum.

So. Should I measure Delta T on both sides of the window? Or can I calculate based on the R-value? I realise that would only be the listed r value. The true R value probably deviates from what it’s listed at.

To be honest I’m more concerned about mold in the wall cavities. My windows are glass and plastic and couldn’t care less and the condensate hasn’t dripped into wooden trim.

Can someone point me to formulas I could use to calculate an optimum, or maximum humidity for the winter?

you want to run around 35% at a minimum…
I have Best Temperature and Humidity Sensors - Centralite scattered around the home…
Please note all this calculation about “humidity” is relative and depends on the humidity outside…
So one minute you could have 50% humidity next minute 40%…(with the same output from the machine)…so I focus on min acceptable value and try to keep it there…
(which unit are you using for humidification?)

there are techniques like putting a sensor outdoor, connected to indoor etc so that it can calculate the true humidity CALCULATING RELATIVE HUMIDITY … (its too much math for me :slight_smile: )

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I’ve seen that chart. And I’d agree on at least 35%. I’m learning 55%-60% is about the limit where it starts causing condnsate on window glass. But I think that’s entirely dependent on the temp outside.

Now I will question one thing you said:

How does RH inside depend on RH outside? Our house doesn’t have ventilation with the world outside. When all doors and windows are closed it’s fairly sealed. (That’s how we build them in the midwest USA. No intentional air exchange between inside and outside.)

RH (I think) is the output of a function that consumes 3 things: temperature, pressure, and quantity of water (g of water per kg of air).

If the air isn’t exchanging between inside and out, then nothing outside can add or remove water from inside’s air can it? likewise, outside shouldn’t be able to meaningfully affect temp inside if we still have electrical power and the HVAC is running.

Pressure is all i can think outside that would directly affect inside. If the barometric pressure were to say suddenly drop, that would raise the RH% inside and out. (Our houses aren’t sealed like an airplane/submarine thank goodness) If pressure dropped far enough we’d get clouds inside the house because RH would exceed 100%.

I thought about this more last night. I think that if I want to avoid condensation on window glass. I need to keep track of the ‘dew point’ inside the house, since that’s what water forming on the glass is, dew. A keep track of the temp of the inside of the window glass, and make sure that the window glass is always hotter than dew point.

The trick is am I really going to put a contact thermal sensor on the window, or is there a formula to predict indoor window glass temp based on data I already have access to via national weather service and indoor sensors. And because we open and close curtains randomly, I’m fearing I’m going to have to install that contact sensor!

Dewpoint has pressure as a function…so yes you are right…

another resource… aiming 35% avoided “excessive” condensation for me…fyi…

I think we’re used to seeing dewpoint from the news man and thinking it’s one dewpoint for the city, indoors or out. But actually your house has its own separate dewpoint inside if you are operating a heating/AC system or… I was about to say " or a humidifier" but I don’t think the presence of a humidifier or dehumidifier would alter the dewpoint. I think only temp and pressure do. and maybe particle (dust) concentration but I’d hope never to have so much of that to matter.

Boy oh boy, turning the crazy formulas into lua/python code in a reactor module is gonna be fun.

I’ve got a small closet in the basement I keep a dehumidifier in as well. Storage area for guns, ammo, fireworks, the ‘dry room’. The dehumidifier there uses a peltier cooler to condense water, and a pump runs once a day to get rid of it. It’s usually 8-10% or so drier than the rest of the house which doesn’t strike me as particularly effective, but it’s in the basement and I suspect moisture comes through the concrete. Or maybe through the drywall. It wasn’t built to be conditioned as I am using it. I’m thinking I might need to get that plastic that weed growers, cough, I mean, mushroom growers use and line it if I want to get that room drier.

Yes will be a fun project…do let me know how you get on, if you ever do it, pls…would be interesting.

Good luck with all those calculations :slight_smile:

Here’s what I ordered:
Desert Spring DS3200p with auto flush drain

This is the only whole house system I could find that does not use a replaceable filter. Instead it drags plastic discs through standing water. Each cycle through the water is a chance for the disc to wash itself off. And the water in the basement gets changed out once a day on a timer so that minerals don’t build up.

I’m hoping it’ll be pretty hands-free.

The only the biggest problem with this unit is that it’s only made by one company in freaking Canada. Not stocked in the US so I get to pay extra for shipping and it won’t be here for a month!

In the meantime I forget this beauty using the electrode boiler of a Vicks vapor rub humidifier and extension cord to bring electricity into the toilet!

It looks pretty insane but I have not had to touch it for 4 days now. When the water gets low it auto refills courtesy of the toilet float. And every time we flush it gets all the minerals washed out of the tank! Win win.

I’ve got a reactor plug-in monitoring the humidity in the adjacent room and it cuts power when I reach 50% RH. And then I have a scene running every 30 minutes to turn it back on until I figure out how to represent these humidity formulas in the Vera.

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that looks interesting…
what exactly does that do for you, can you explain pls.?

They’re both humidifiers. They both put humidity in the air.

I put the vicks humidifier in the toilet tank because toilet tanks automatically refill when they get low. So I don’t have to add water manually like some sort of trained ape every 12 or 24 hours. When power is supplied, it makes humidity and warmth. The copper wire is just some scrap from the garage to support it at the right height and angle.

The vicks humidifier is an “electrode boiler” like inmates make out of stolen electric cords and spoons. So it doesn’t create white dust like an ultrasonic. And it doesn’t have a big sponge “filter” that stays wet and attracts mold and has to be replaced every season when it fills up with calcium.

The vicks boiler probably kills any mold inside it because its literally boiling the water (steam sterilization). And it’s faster than other humidifiers. It can put a gallon in the air in 4 hours sometimes. Though it does produce mineral debris. But each time the tank is flushed, it drains all the water and with it most the debris.

Biggest downside is boiler humidifiers use more power. I’m seeing about 200w but it fluctuates and has gone as high as 1000 when I added salt to the water! But it’s winter. It’s cold outside. And wasted energy is heat, which we need more of anyway right now.

It’s cheap too. $16 at target with 5% back on the target card. That’s what they look like normally, I’m just not using the tank that comes with it. If I had a restroom next to the furnace, I’d try to make something like this permanent. But the Desert Spring unit is pretty similar. It has a water line and a drain line like a toilet tank. And it self-flushes. It just uses evaporation not heat, so it’s a little less fast, but probably less of a fire risk that way if it’s going to be running unattended forever.

I could imagine eventually the electrodes on the vicks will become so encrusted with minerals that they will touch and it will have a little arc fire inside until the breaker pops. It doesn’t have any circuitry or fuses. And the breaker may not pop at all if that arc doesn’t consume more than 20A, which is far more than you need to set the plastic housing on fire I’m sure.

I don’t think a fire would spread inside a porcelain tank filled with water though, so it should be good until the whole house Desert Spring DS3200-P arrives. With my luck that’ll be right when spring begins and I don’t need it.


is the downside that you have to leave the toilet door open so the humidity spreads?

Yeah. I suppose that’s a downside. Door open and tank lid off. And I run the fan of the HVAC system a few times a day to even it out around the house.

We don’t use that one very often though. There’s also some energy loss when we do flush it since the water warms up in the tank and gets replaced by cold, and humidity loss when someone uses the stink fan. But that toilet gets maybe two flushes per day and I only need this to last a month or so to get the whole house one delivered.

I was pretty sick a couple weeks ago when it was much much drier in the house and getting some humidity in the house really helped me recover.

I started thinking about putting a float valve in the vicks tank itself, but its not particularly tall, and all the float tank fillers were meant for toilets, and I’d still have to clean the minerals out so then I was thinking how could I rig up a flushing mechanism to the vicks tank and I realized i was building a toilet tank in my head, so why not just use the one we have. worked well

its a very clever solution! welldone :slight_smile:

now I just need to make an obnoxious ticktock elite life hack video about it!

In my research I also found out that some folk use a mixing valve to fill their toilet with warm water so that the tank doesn’t ‘sweat’. I’ve never seen that, but the cold water in my area is very cold in the winter. painful to use just cold for handwashing some days. I wouldn’t be surprised at all if the cold water lines got below the indoor dew point when the water is running.

Next house I build I will see about burying a few hundred feet of pex under the foundation to warm up tap water with ground heat.

Do those centralite sensors work with vera? They look like zigbee and I thought vera was zwave.

sorry I should have clarified, i use Ezlo hubs that has Zigbee and Zwave in them.

Great idea with the Life hack video!!! :slight_smile: