Humidifier - Hot vs Cold Water Experiment (2023 Update)

[trader4]

For those of you that followed any of the discussions here about whether using hot water versus cold water in a furnace humidifier makes a difference in the amount of water that evaporates, I did the actual test on my unit. Here's exactly how I did it and the results.

The unit is an Aprilaire 700A installed on a gas forced air furnace. To do the test, I made sure to keep everything constant except the incoming water temp. My unit is connected to hot water, so to change the water temp, I turned off the water heater and used up the hot water through normal household use. I got the hot water tank temp down to 51F. The house humidity was 37% at the time of the test and the temp was 67F. I routed the humdifier drain hose to a location that I could put a gallon container under it and remove it without disturbing the hose. I placed a temporary container under the running hose to catch the water. I then started the furnace with the humidifier set high enough to be running constantly. I let the system stabilize for 5 mins to get to a steady-state. Then I quickly replaced the temporary container with another empty gallon container and started a stop watch. During the 5 min period, I also measured the temp of the water exiting the drain hose. At precisely 5 mins, I removed the container and used a measuring cup to determine the volume of water. I also shut down the furnace.

Then, I fired the water heater back up and let it run to get the water temp to a higher temp to see what happens with medium temp warm water. I then turned the water heater off again. That temp turned out to be 102F. I then repeated the process of starting the furnace, letting it run for 5 mins to stabilize, then measuring the drain water output for another 5 minutes. Next, I fired the water heater up again and allowed it to come up to normal temperture. And again, I turned on the furnace, let it stabilize for 5 mins, then measured the water output for 5 mins.

Finally, I measured the amount of incoming water into the humidifier by connecting the drain hose directly to the water tube where it would normally drop onto the distribution tray at the top of the panel.

Here are the results:

Temp Water collected in 5 mins Temp of Water Exiting Drain Hose

51F 66.5 Oz 78.4F

102 61.5 78.0

135 58.0 79.2

Water collected during 5 mins direct from tube at top of humidifier, ie incoming flow: 73.0 Oz

Now, doing the math, the amount of water evaporated during the 5 mins:

51F 73 - 66.5 = 6.5 Oz

102 73 - 61.5 = 11.5

135 73 - 58.0 = 15.0

Using hot water more than doubled the output. Or in percentage terms, using 102F water resulted in 77% MORE water being evaporated compared to 51F water. Using 135F water resulted in 131% MORE water being evaporated compared to 51F water.

So, there you have it folks. This was IMO a very reasonable and realistic mesurement and confirms why Aprilaire says you get higher output with hot water.

[HeyBub]

Thanks for the quantification. Though I think the basic question - does hot water evaporate more quickly than cold - was settled about ~60,000 B.C. with the taming of fire.

[Jim Elbrecht]

-snip-

Well done-- This is one of those deals where I *knew* the right answer, but was surprised with how *much* difference it makes.

I'm still procrastinating on putting a bucket under the output hose to see how much water actually gets past the water panel.

Speaking of which- I changed mine today because groundhog day is about midwinter in my part of the world. The old panel didn't look any different than the new one to my untrained eyes. There is a little lime on the outlet tray- but otherwise it looks new. Do most folks follow the Aprilaire directions and change the water panels twice a year?

Jim

[Smarty]

Heybub,

Thanks for posting this. As I reported in a very early reply on this subject, my humidity shot up very dramatically when I followed Aprilaire's suggestion to use hot water. The difference was extremely noticeable and measureable with both my humidistat and the use of a separate instrument.

I was totally astonished when a (self-proclaimed) HVAC expert of 30 years disagreed, and said that it would make no difference.

I'm glad to see that your measurements confirm my own experiences here.

Smarty

[Frank]

Interesting result but thermodynamically increasing one gram of water from 51 to 135 deg F only takes 23 calories of heat while converting the one gram of water to vapor at 212 deg F takes 540 calories.

One would expect a smaller difference in your results so there must be other factors involved. Without doing the detailed math, I would have guessed no more than 5% improvement.

I'm also surprised that the Aprilaire uses about 10 gal of water/hour. My French drain under the outlet is always wet but does not flood and I have no sump pump.

Humidity in house is fine so I would not bother to use warm water in mine. All I do is change the medium once a year.

[Chet]

Edited ">

Don't get technical :)

People love to argue over this and over whether warm water will freeze quicker than cold water.

They need to have their fun.

Chet

[dpb]

[e-mail] wrote: ...

...

Hmmm....along w/ Frank I'm puzzled by the apparent paradox in the thermodynamics here.

My first question would have to do w/ whether the assumption of constant input flow rate is valid not knowing the Aprilaire at all. Secondly the exit temperatures seem puzzling.

Would take knowing and/or learning far more about the specifics of the Aprilaire and the installation and the experiment than I care to quantify the results, however... :)

--

[HeyBub]

Well, there is SOME experience behind hot water freezing faster than cold.

In the pre-written-word days of refrigerators, placing a tray of hot water in the freezing compartment would melt the frost, allowing the ice tray to get closer, if not directly on, the cooling coils. Just sitting on an inch of frost was a fair insulator.

Now here's an interesting question: which boils faster, tap water or distilled water? And which freezes faster?

[trader4]

=A0 =A0 =A0 =A0 =A0 =A0 =A078.4F

=A0 =A0 =A0 =A0 =A0 =A0 =A0 78.0

=A0 =A0 =A0 =A0 =A0 =A0 =A0 79.2

The flow rate is determined by the water pressure and by an orifice with a small hole at the solenoid valve. It's hard to imagine that hot water would flow at a slower rate through it, which is what would be needed to skew the results.

What's puzzling about the exit temps? They are for practical purposes all the same. Which is what one would expect. If you start with cold water, then the water temp rises on it's way down the panel. If it's hot water, it cools on it's way down. Either way, with a lot of air blowing on the panel, it seems reasonable that it reaches a final temp somewhere before the end of the panel.

The only thing that is a little surprising to me is the amount of difference hot vs cold made. I would have thought that there could be a 25 to 50% difference. I think it has to do with the fact that evaporation in this case is a complex process, not entirely described by latent heat. All you have to do is have the air moving by catch some of the higher energy particles that are trying to escape the surface tension of the water.

It's essentially a panel with max surface area, through which air flows. It's fed by a water pipe that has a solenoid valve and a small orifice to meter the water flow.

[dpb]

...

I was commenting simply on the design of the experiment is extremely sensitive to the assumption that that is indeed a constant. As noted, I have no knowledge of the device itself, so if there were a float or somesuch, it's a possibility that the lower evaporation rates influence the inlet volumes.

Again, that is dependent on details I'm unaware of...if there's sufficient time/distance for a secondary heat transfer to occur then it really is a case of the red herring in the data--it has no really bearing on the experiment. Again, just commenting...

I had the "aha!" moment shortly after I posted and was coming back to amplify and, I think, answer the "why" question.

Looked up water vapor pressure as a function of temperature. The results you got and the vapor pressure and some derived values that will discuss on down are as follows. You can see the vapor pressure is quite sensitive to temperature as was your evaporation rate.

Normalized to 51F Normalized to 51F T(F) Evap(oz) VP(mm Hg) Evap VP VP**3 51 6.5 9.5 1.0 1.0 1.0

102 11.5 52 5.5 1.8 5.5 135 15.0 130 13.7 2.3 12.2

Assuming there's more than adequate heat available to evaporate the water, the problem is one of residence time and heat transfer. My conjecture is the vapor pressure is key in atomizing and thereby increasing the effective heat transfer (and probably also increasing the residence time some, to boot).

The normalized columns show the relative effectiveness of the evaporation as compared to the cold water as the temperature was raised (again, assuming there isn't any major bias in the experiment which I'm willing to accept is probably not too bad given the orifice as a rate controller for the key variable of input). It shows the effective evaporation went up by an even faster rate than the vapor pressure.

But interestingly enough, the magnitude of the numbers caught my eye and I just guessed at a power law exponent of 3 and voila! -- the agreement is quite nice...

I think this is the basis for underlying the phenomenon.

I didn't look for engineering correlations for predictions of evaporative rates, but they undoubtedly exist and Perry's Handbook would be a reasonable place to start. It would be interesting to see if such correlations did follow a power law but I'm not inclined to pursue it further to see--I really don't know why I got in before (other than the single measurement sensitivity in the experiment caught my eye being a thing I was heavily involved in in many real situations in an earlier life).

Anyway, maybe that will add some cannons to the fodder... :)

[makolber]

=A0 =A0 =A0 =A0 =A0 =A0 =A078.4F

=A0 =A0 =A0 =A0 =A0 =A0 =A0 78.0

=A0 =A0 =A0 =A0 =A0 =A0 =A0 79.2

Hi Trad...

nice job...

But I have the same questions as Frank and dpd..

1)The outlet water temperatues are almost the same so you have to agree that when the water temp is the same the evap rate is the same. So for at least part of the panal, the water temp and evap rates are about the same. In order for the overall evap rates to change by 2x, there has to be a large portion of water panel with a large temp diff.

2) I agree with the other poster, you assumed that the inflow is 73 and equal in all 3 cases. It seems logical but even if there is a small change in the inflow rate, it will make a big error in the results. I could imagine that hot water might not flow through an orfice the same as cold, but I don't really know.

3) I am surprised that the flow through rate is so high. For every 1 galllon that is evaporated about 5 to 10 gallons go down the drain. I was expecting the flow through rate to be more like 2 gallons in , 1 gallon out and 1 gallon evap. The longer the water is in the humidifer, then the less difference the inlet temp would make.... If the water flows quickly right through then I can see why hot inlet water might be more significant, but then the outlet temps would not be the same so I don't know...??????

4) Whats the air temp from the furnace that is flowing by the water?

If you verify that the inlet water rates are really 73 in all three cases, I will be willing to concede...

(and I thank everyone for keeping this discussion civil)

Mark

[Bubba]

Well trader, of course you knew I would respond but I will try to be civil. I am rather suspicious that after all this time you have "ALL OF A SUDDEN" performed the test out of the blue and come up with your results. You've been so against doing the test and now all of a sudden had a change of heart? I wouldnt be half suprised if you made up all these numbers BUT, I will give you the benefit of the doubt that you did it. My suggestion would be to DO IT OVER and figured out where you screwed up.

77% and 131% more output??? What, are you magic too. Can you wiggle your nose and produce a rabbit out of a hat? You've "hosed" something with your test. Very minimal improvement (output) would have been expected. Try Again. Bubba

[trader4]

=A0 =A0 =A0 =A0 =A0 =A078.4F

=A0 =A0 =A0 =A0 =A0 =A0 =A0 78.0

=A0 =A0 =A0 =A0 =A0 =A0 =A0 79.2

You know perfectly well that's not true. I never once said I was against doing the test. I said several times that I would do the test when I had the opportunity.

And just as I predicted many posts ago, you now dismiss the results of the test. Since you want to claim I hosed up something, let me point out a few facts. In another thread, when you wanted to know why I hadn't yet done the test, you stated that it would only take 5 or 10 minutes to perform. Clearly, you can't perform this test in that period of time. I let the furnace run each time for 5 mins just to let it reach a steady-state condition. Apparently that concept and I'm sure a lot more escaped you. That you stated this test could be done in 5 or 10 minutes, combined with your previous rants against engineering and science, should give everyone here a good indication of who actually took the time required, knew how to design the experiment and did a valid test.

It's clear that the amount of water evaporated compared to what comes out the drain is small, only 10 or 20%. So, if you just stared into a big old 5 gallon pail for 5 mins, you would never notice any difference and coclude that it doesn't matter. Only by taking the time, keeping everything constant, and taking careful measurements would you find out the difference.

[trader4]

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A078.4F

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 78.0

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 79.2

Yes, I agree.

I thought about that a bit more after replying to dpb too. The orifice is plastic, so it is possible that at a higher temp the plastic could expand and constrict the opening.

That's a good point. The higher the flow rate, the greater the distance down the media element the hot water will be present. And conversely, with cold water the longer cold water will be present. But the flow rate must be such that with evaporation it reaches the steady-sate temp of 78F somewhere before the end of the media. And from the other aspects we discussed, it must be fairly far down the media.

I'm now curious as to why they have the flow rate so high. You need a reasonable flow rate to keep the minerals from building up, but I agree that it seems a lot higher than you would expect. Something around 50-50, as you suggest seems like it would be sufficient. Curiously, Aprilaire says not to try to reduce the water flow using a valve. But, I have to say, it seems like energy is being wasted heating all this water, either in the furnace or the water heater. I think I may reduce the flow in mine to see what happens, how fast the minerals accumulate, etc.

Don;'t know, didn't measure that.

If I had thought about it at the time, I certainly would have measured it. It would have been easy to do. I agree that it's an interesting possibility and the only remaining possible variable. So, to eliminate that, I will test the flow rate at hot and cold.

[Bubba]

Would you like me to go way back a couple of months to where this first started and point out how long it took you to do this simple test that you have obviously screwed up?

Of course I am. Did you bother to take time out and look at the numbers you actually wrote down? 131% more output using hot water?? Dont you think if Aprilaire was able to acheive that, they would have that plastered all over their literature? "Aprilaire, the magical humidifier. It manufacturers its own humidity"

OK, big friggin deal. A half hour. You feel better now? I cant help it if it took you all day to do something simple. Typical EE. Drag it out for hours, drink some coffee, think, ponder, think some more, go take a walk, read a novel, scratch your ass, etc.

No, I think it proves that even someone with the supposed training of even an EE degree can screw up a purely simple experiment to the point that he thinks he just invented the wheel.

and clearly, you even screw that task up. Bubba

[Bubba]

Excuse me. Did trader just say that? The Aprilaire is wasting water and energy? Could it be? And you had to do that experiment that you screwed up to come up with that conclusion? Wow. Most intelligent people could simply look at the drain tube while the unit is running to figure that out. Im beginning to think you are hopeless trader.

Keee_Rhist! How many more months will it take you to accomplish that little test? Bubba

[dpb]

[e-mail] wrote: ...

I'd be more suspect of some pressure differential changes -- a slight pressure change w/ the temperature, for example could make a significant difference.

...

...

It certainly is an energy hog and a waste of water as well.

It would seem to me the only logical installation would be to both put a reducing valve in the line and to recirculate the waste back into the system rather than pouring it down the drain.

I'd certainly not go any place close to so far as to say it's "the only remaining possible" variable.

To evaluate more fully would take a looking at the specific setup and all in depth which obviously isn't possible over usenet. I say this not as an attack but as a point of emphasis based on 20+ years of involvement in industrial experimental design and testing wherein one soon learns real humility in discovering how easily one can overlook unexpected effects or hidden correlated variables that confound test results even in the (apparently) simplest of conditions.

I'm willing to admit the nonlinear effect and direction is probably real owing to the vapor pressure effect noted earlier; I'm far from agreeing the quantitative values are more precise than simply estimates at this point.

In regard to this last point I'll add the point that I intended to make in the previous posting that in all likelihood it isn't all actually evaporation but a significant amount is likely simply atomization and mass transfer added by the higher temperature raising the water vapor pressure.

--

[dpb]

dpb wrote: ...

Typo is confusing in the last sentence..."added" was intended as "aided"

--

[Tony Hwang]

Achieving the desired set humidity. Using cold water humidity in the house is properly maintained during winter month. Also using Aprilaire humidifier here.

[glenn36]

where is the locatin of the humdifier return or supply

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