After a recent conversation , I started looking into hard info that humidifiers make you feel warmer ( of which I could find none other then it seems to be the general consensus) but I did read an interesting article about humidity and thermal conductivity , and it started me thinking . The jist was the more moisture in the air the more energy the air can hold ( most already know ) but also the faster it can absorb heat . My question is could heating with humid air actually effect the sensible heat and improve your eff. . Disregarding of course the adage of you can keep the room cooler

I don't know much about humidity. Does it take more BTU's to heat humid air vs dry air?

Though water has much higher specific heat than air (heat required to raise 1 pound 1 degree), it makes little difference at humidity levels typical of indoor conditions. The reason is that at room temperatures even "humid" air is mostly air, very little water vapor proportionally. When we use temperature rise across heat strips of known power to compute blower CFM, no correction is made for humidity or lack thereof.

We do know that changing the humidity of air consumes lots of energy - wringing a pint of water out of air during summer cooling requires over 1000 Btus and in the humid southeast we figure that 25% or so of total AC capacity goes to this purpose (latent capacity). We slow blowers and tolerate reduced SEER / EER to get this done.

Conversely, humidifying air during dry winter days costs heat - to get a feel for that see how long it takes to boil off a pint of water on a stove top. Air from a rotating drum or similar humidifier without its own source of heat feels much cooler than ambient air, and a thermometer will confirm this. Wet Bulb temperature is much lower than dry bulb temperture when humidity is low.

We know that good dehumidification often allows slightly higher thermostat settings during summer AC cooling, and we know what a disaster an oversized AC unit is for comfort - system makes occupants feel hot and muggy or cold and clammy. Oversized systems don't run long enough for proper dehumidification

So does it follow that more humid air in winter (disregarding, for now, how it became more humid) allow a lower thermostat setting for same comfort?

For an answer, I surfed an ASHRAE standard: 55-2004. In it is a table showing that during summer comfort can be obtained as high as 82 deg F at 30% humidity, but that for comfort 60% humid air must be no warmer than 78. People in Phoenix are forever telling us "it's different here...it's a dry heat".

In winter there is a similar effect, though not nearly as pronounced: 30% humid air must be heated to a minimum of 69 deg F for comfort. 60% humid air can be as cool as 68 deg F, just 1 degree lower. I don't have time to check a psychometric chart right now, (kids need to be bathed and bedded down) but I'm quite certain that humidifying 69 degree air from 30% to 60% RH without adding additional heat will result in a dry bulb temperature quite a bit below 68 deg F.

Implicit in those figures is ASHRAE's requirement that indoor air always be maintained within the range of 30-60 percent relative humidity. For comfort air must be humidified to at least 30% or dehumidified to no more than 60% regardless of dry bulb temperature.

In other words, as usual, there is no free lunch.

I dont want to steer the conversation because Im looking for confirmation but

from what I read about 24 btu to heat a pound of dry air , the question is at what temp because unlike liquids which vary only at saturation temps gases have a sliding btu requirement, to heat 1 degree .As pressure changes it increases the requirement of btu to increase every degree

I plan on testing this theory when I am in Maui next week....

Dean

Since a pound is a measure of weight/mass, the change in volume occupied by that same one pound at different temps or pressures should have no bearing on the energy required to heat the mass.

Regards,
Pat

weird right , it is true with water ( allthough to less of an extent ) and can be eisily confirmed

first off  btu is only heating a gal from 69-70 not just any temp

second at home take a gal of water and a thermometer and a stop watch and a electric range, water will heat from 50-60 slightly faster then 60-70 and so on intill the obviouse huge delay at saturation

"A BTU is defined as the amount of heat required to raise the temperature of one pound of liquid water by one degree from 60° to 61°Fahrenheit at a constant pressure of one atmosphere."

This can also be expressed for you hvac guy simply
a funace with a cold heat exhanger is going to be way more eff then the same one once it warms

my apologizes your correct , point still the same

This is my source Springerlink Thermodynamic Journal

(1) Lehrstuhl für Strömungsmechanik Universität Erlangen‐Nürnberg Cauerstraße 4, D‐91058 Erlangen, Germany, DE

Abstract  The convective heat transfer from a cylinder to a humid air stream flowing normal to the cylinder was investigated experimentally at atmospheric pressure over a range of variables which is relevant to the use of hot‐wire anemometry: air temperatures between 30 °C and 70 °C and velocities between 12 and 37 m/s. For molar fractions of water vapour up to 0.27, the heat transfer increased with increasing humidity. The ratio of heat transfer rates in humid air and dry air is a unique function of the molar fraction of water vapour, independent of the air temperature and flow velocity.
Received: 28 November 1996/Accepted: 5 July 1997

Interesting - it lends truth to an adage about wet vs dry 'cold' - cold dry air isn't nearly as miserable as less cold wet air - I grew up in coastal Mass - there isn't much more miserable than a cold wind off the Atlantic in winter.

It is worth noting that the temperatures, humidities, and velocities cited in the German source above are way way above those encountered in normal indoor air

Ill try to post this

What Im curious about is does it make any Measurable difference in eff.?

and am I reading this right or should I stick to what I know?
While this doesn't show increased RH it does show higher wet bulb which is indicating the same thing
What I dont understand is Celsius for one and second the heat exchanger effectiveness .8 and change to .9 and change
is that measurable .
I personally swear by humidifiers for comfort and if this holds any water its icing on the cake but I aint smart enough to read all that stuff other then to get the jist but not anything quantitative

Since I started this thread Ive been reading just about non stop on the topic and what Ive come up with is increasing humidity does increase heat transfer coefficients
With the risk of sounding stupid increased coefficients is good right?
I realize increased efficiency is good and attempted to get a definition for cofficiency and started seeing double
La-men definition please

Increasing heat transfer coefficient should increase efficiency. It would do so by reducing the required temperature differential (how much hotter the coil has to be over the air stream being heated) If the coil doesn't have to be quite as hot then refrigerant in it can be a bit cooler and therefore condense at a lower pressure - that in turn would reduce compressor head and current and thus increase efficiency.

Trouble is, the effect is likely negligible within the range of humidities of indoor air, and the effect would be more than overcome by the additional energy needed to increase humidity (vaporizing extra water at 1000+ BTU per pound)

I know that's the hitch , my question back to you , is that 1000 btu's lost or does that energy now just exist in a different form ,ie equal enthalpy
assuming of course your not using electric resistance to boil the water , just bypass , you would be creating vapor at the eff of the geo , Right
I know this aint ground breaking stuff , definitely less then 1 percent, Right?

Correct - enthalpy is increased in the form of latent heat.

It isn't lost, at least until the air exfiltrates or is exchanged out of the house (unless an ERV is used for the exchange).

While ASHRAE recommends humidity in the 30-60 % range year round, running at the upper end of that range in winter introduces potential problems with condensation within walls and on windows. That can damage windows and cause mold growth in stud bays, depending on construction techniques and details - not good. Surf writings by Joseph Lstiburek (Building Science Consulting) for more on this.

The basic goal of what we do is indoor comfort at least cost. ASHRAE provides guidance that we should stay within. The occupant / customer has final say via thermostat and humidity setpoints. My basic conclusion (and I appreciate the opportunity to think through all this) is to humidify / dehumidify no more than necessary and then heat / cool for comfort. Increasing humidity above what is necessary for winter health and comfort is not a path toward comfort at least cost, that is, overall efficiency will decrease and cost increase with humidities higher than necessary during winter.