Post #895: A few words on room humidifiers

Posted on November 20, 2020

Source:  That well-known font of medical knowledge, the American Society of Heating, Refrigerating and Air-Conditioning Engineers.  This is from the 2016 ASHRAE Handbook—HVAC Systems and Equipment (SI), Chapter 22:  Humidifiers.

I started to look up the standards, if any, for room humidifiers, and stumbled across the graph above in, of all places the AHSRAE HVAC engineering standards manual.  I thought that was such an odd coincidence, given my last post, that it deserved special mention here.  HVAC engineers start their discussion of calculations for humidifiers by summarizing the significant health implications of maintaining proper indoor humidity.  The impact of humidity on flu mortality was demonstrated experimentally, thirty years ago, on mice.  And, sure enough, 40% to 60% is the zone that prevented the most flu deaths.

And now, a few words on room humidifiers:

I hate them.  All types of them.  Each in their own separate way.

If you already own and use a humidifier or two, the opinions in this posting are probably irrelevant.  Either you’re satisfied with what you have, or you aren’t.  But if you’ve never bought a humidifier before, you might find a useful tip or two here. Let me get you oriented.

1:  There are no standards, so buy big.

2:  The humidifier is an appliance that increases household work.

3:  Noisy, dusty, stinky, and/or expensive:  Pick one or more.

Think of all the worst properties of every appliance you’ve ever had, and they come together in the average room humidifier.  There are no standards.  They are built as cheaply as they can possibly be built.  You fill them by hand, in any of several awkward ways, frequently.  In some cases, you need to clean them frequently, again by hand.  Some require using distilled water, others require you to dump poison in the water.  Replacement items are expensive.  And so on.

Over the years, I’ve tried more-or-less every type of humidifier there is.  I have grudgingly settled on a fixed-pad evaporative humidifier with use of bacteriostat (bacteria-suppressing solution).  In particular, my go-to humidifier is this one.   For reasons that I’ll eventually get to in this posting.


1:  There are no standards, so buy big.

Pretty much every humidifier on the market will tell you how many square feet it is capable of humidifying.  And if you think about that for even thirty seconds, you’ll realize that, whatever that number is, it’s not well-defined.

  • What outdoor temperature and humidity do they assume?
  • What ceiling height do they assume?
  • How leaky or tight is the construction being humidified?
  • What level of indoor relative humidity do they guarantee?
  • Is that the max output — do you have to run it on high all the time to get that?

And so on.  Let’s face it.  Something that will effectively humidify 1000 square feet of housing in Virginia is going to be inadequate for 1000 square feet in Bismark or Cheyenne.  So that one-size-fits-all rating has to be taken with a grain of salt.

If you look at an actual engineering discussion of humidifiers (as in the ASHRAE manual), you soon realize that the whole subject is pretty tricky.   It’s all about the amount of water that you need to put in the air, per hour, to maintain humidity.  And that’s affected by lots of factors and lots of limits.

Just to give one example, in colder climates, the type of window glazing sets an upper limit on how well you can humidify an indoor space.  Set the humidity too high, and the water simply condenses on the insides of the windows.  If it’s freezing out (32F), and you have single-glazed windows, you’ll get condensation on the windows if you try to maintain anything above 30% relative humidity.

Arguably, the single largest contributor to your humidification load is the number of air changes per hour for your home.  The air in your home is being continuously replaced with outside air.  The current US standard is that all the air in your home should be replaced at least once every three hours.  Most modern building codes aim for roughly one change every two hours.  A typical value for an older home would be one air change per hour.

If you have an older home, the upshot is that you need to re-humidify the entire air volume of your home, once per hour.  In a more modern home, that might be once per two hours, or even three hours.

Let me now do a little calculation to show you just how much water that might entail, in the dead of winter, in this area.  In January, in Washington DC, the average outdoor air temperature is 36F and the average relative humidity is about 61% (per this source).  Let’s say you have an older home, 3000 square feet, with 8′ ceilings.  And, finally, you want to maintain 40% relative humidity inside your home, at 68F.

Under those assumptions, you need to put 0.75 gallons of water, per hour, into the air.  Or 18 gallons per day.  Just to overcome that one air change per hour.  This doesn’t count moisture that literally escapes through the walls of your house.  But it also doesn’t count the modest amount of moisture added to the house by cooking, showers, and so on.

If you have a tightly-constructed modern home, you’d need far less.  Or a smaller home.  But if you have a bigger, older house, of the type described here, you need somewhere around that 18 gallons of water per day to maintain 40% relative humidity in the dead of winter.

You’d need four or five of  my go-to humidifier, running at top speed (four gallons/day), all day long, to maintain 40% relative humidity.  (Whereas, if I went by the rating on the box, I’d only need three. And I want to avoid running at top speed, due to the noise.)

The upshot of that is:  Think big, and buy a hygrometer (humidity meter) or two.  If you’re planing to humidify your entire house, buy somewhat more humidification capacity than you think you’ll need.  You can always turn the humidifiers down to a lower speed, if you don’t need that capacity.  But you will be surprised how frequently you will exceed your humidification capacity in the dead of winter.

It is possible to over-humidify a home.  Particularly if you have a modern, well-sealed home.  But if you live in an older home, over-humidification is tough to achieve unless you really go overboard.

2:  The humidifier is an appliance that increases household work.

I don’t need to belabor this, I think.  In the example above, that theoretical 3000 square foot older house needed 18 gallons of water per day.  That you, the homeowner would have to carry to the humidifier.  That’s 150 pounds of water, per day, carried by hand.  Inside your house.  Every winter day.

That gets really old, really fast.  Needless to say, Rule #1 is to locate any large humidifier near a source of water.

The only other thing to mention is routine cleaning.  If you have an evaporative humidifier, at a minimum, even if you put chemicals (“bacteriostat”) into the water, you have to inspect it weekly.  And take it apart and clean it every couple of weeks or so.  Maybe soak the evaporative pads to remove the mineral buildup.  And so on.

Just understand that it’s not like your other appliances.  Your other appliances, they reduce the amount of work you do.  Humidifiers increase it.

3:  Noisy, dusty, stinky, and/or expensive:  Pick one.

There’s no such thing as an inexpensive low-maintenance humidifier.  At least, not that I’ve come across, for humidifying a large area.

Cool mist humidifiers break water up into tiny droplets using ultrasound or some mechanical (“disk”) means.  They are quiet and reasonably energy efficient.  In my experience, they rarely have problems with mold or bacteria.

But 1:  But you’ll probably want to use distilled water.  And that’s expensive.  And environmentally unfriendly, for the energy required to produce and ship distilled water.  If you break down and use tap water, they produce mineral dust.  This is not just unsightly, it’s arguably bad for your lungs.  Some units have “de-mineralization” systems, allowing you to avoid distilled water, but I question the effectiveness of those.

But 2:  They are typically small, single-room units capable of producing no more than a gallon per day.  So if you’re of a mind to humidify your whole house, count the number of these you’ll need before you buy.  And every one of them will need to have the water reservoir filled periodically.

But 3:  In my experience, the interlock mechanism that keeps the water in the reservoir is tricky, fragile, and effectively impossible to replace.  It’s typically a spring-loaded contraption with plastic parts, and so is destined for the landfill one way or the other.

Warm mist (a.k.a.) steam humidifiers simply boil water.  They are quiet and you can typically use tap water.

But 1:  They are not energy-efficient.  You are literally using an electric resistance heating element to boil water, which is the least efficient and most carbon-intensive heat source you could choose.

Just as an example, boiling 18 gallons of water requires about 45 KWH of electricity/day, which would work out to about $200/month at $0.15/KWH.

(I could use an extras-for-experts here, because all the other humidifiers also use heat, but indirectly.  “Cool mist” isn’t a feature of the ultrasonic humidifiers, it’s an unfortunate and unavoidable side effect.  Without getting into the physics of sensible versus latent heat, cool mist (and evaporative) humidifiers more-or-less suck heat out of the air.  They require your heating system to work harder to maintain a given temperature.  Whereas warm mist humidifiers put that heat directly into the water at the outset.  So it’s not that warm mist humidifiers are inefficient because they require heat input.  They’re inefficient because they use a particularly inefficient heat source (resistance electric heat) for that heat input.)

But 2:  As with cool mist humidifiers, you’d be hard-pressed to find one capable of humidifying more than one small room.  So, as with cool mist, you’re going to need a lot of these if you’re going to humidify a house.

But 3:  As with cool-mist humidifiers, the interlock mechanism that keeps the water in the reservoir is tricky, fragile, and effectively impossible to replace.  The only exception is “baby humidifiers” where the electric unit just inserts into the top of the reservoir.

Evaporative humidifiers simply blow air over wet pad.  The air picks up moisture from the pad.  Some of them (“console” humidifiers) might have that pad in the form of a moving belt that dips down into a water reservoir.  Others have the pads fixed in place.  Some have to be filled (e.g.) by carrying pitchers of water to them.  Others have detachable reservoirs that can be filled at the sink, similar to cool mist and warm mist humidifiers.  You can use tap water with these humidifiers.  These are made in large sizes capable of humidifying several rooms.

But 1:  The use of a fan makes these noisy, particularly on their highest settings.  I’ve never had one that I would call “quiet”.

But 2:  These are prone to mold and bacteria problems.  You (IMHO) MUST use chemicals (“bacteriostat”) in the water reservoir to suppress mold.  You MUST inspect the pads weekly for any sign of growth.  You typically have to replace the pads at least once per heating season, at a cost of a few tens of dollars.  Barring that, you may need to take them out and soak them once per season to remove mineral build-up.  And it’s not a bad idea to empty these out and scrub the interior at least once per heating season, just to be on the safe side.

Summary:  What I use, and why.

Source:  Amazon.

I use the simplest, high-volume evaporative humidifier that I have found.  Because:

a)  I’m too cheap to buy massive quantities of distilled water.  That, and the size of the space to be humidified rules out cool mist humidifiers.

b) The inefficiency of resistance electric heat, and the small size of warm mist humidifiers, rule them out.

So I’m left with evaporative humidifiers.  The lesser of three evils.

The model I prefer has a few nice features.

First, few moving parts.  In particular, it has removable reservoirs for re-filling, but they have no mechanical seal mechanism.  Instead, they just sit on the humidifier the way a water jug sits on a water cooler.  And you “flip” them into place the same way you would put a jug onto a water cooler. It’s one of those bulletproof systems that is clever for its lack of cleverness.

Second, if forces you to look at the evaporative pads every time you refill the reservoirs.  So you can’t accidentally overlook mold growth, or skip required cleanings.  The pads are right where you can see them, all the time.

Third, all the electronics simply lift off, which makes it easy to scrub down the plastic base and sides.

When it comes to humidifiers, they are all made just as cheaply as they can be made.  In that situation, the less there is to them, the more robust they’ll be.  This is the least humidifier I could buy that gets the job done for me.