Hey @#12, for an indoor temperature, is there a published formula somewhere for determining the "feels like" temperature? More importantly for my purposes, is it any different than the enthalpy? I know I've asked a similar question before but would you ever prefer air with higher enthalpy (in the summer) because it has a lower "feels like" temperature?

I believe the wet bulb temperature is the critical factor and for water in air, this is identical to the enthalpy.

The wind is a factor in the "feels like" temp but this factor is almost nil indoors.

 

Short answer: No

https://en.wikipedia.org/wiki/ASHRAE_55#Adaptive_model
https://en.wikipedia.org/wiki/Thermal_comfort
http://www.srh.weather.gov/jetstream/downloads/heatindex_rh_f_20x12.pdf

This is more than a 3 Google question. I'm looking for Lines of Equal Thermal Sensations, Trane Air Conditioning Manual, page 14, Fig. 2-8 where a graph of wet bulb vs dry bulb shows a straight line of, comfortable, slightly cool, and slightly warm perception. I think you're looking for something like, "Indoor Heat Index" which is the same as, "Outdoor Heat Index with no wind factor". What I did was get my Psychrometric chart and draw the results of the Equal Sensation graph on it. It's all straight lines, bub. Heat Index and Enthalpy are labeled in different units, but the comfort curves are essentially the same. You can wander around between the slightly cool and slightly warm boundaries, but there is no strange distortion of the enthalpy compared to comfort.

What was your question? Published feelz formula? No. Only a spreadsheet or graphical representation, AFIK.

 

... there is no strange distortion of the enthalpy compared to comfort.

That's what I was asking about. The app I'm working on does the psychometric calculations and makes some HVAC decisions. I've been using enthalpy as the primary decision factor and just wanted to confirm again that there's not some other distortion I need to use to get to "feels like". I consider the matter settled.

About the only thing I think is relevant above and beyond air enthalpy is the effect of outdoor temperature on the walls of your home, and the radiance you feel from them. An air temperature of 65°F might feel great when it's 90°F outside, but you'd feel much colder if it was 0°F outside and all your walls and windows have frost on the inside.

The other thing I didn't anticipate – because it never occurred to me – is that home thermostats just control temperature and when you set it, that set point presumes some humidity level. Duh. So if you're trying to control comfort (= enthalpy), a swing in humidity requires a different thermostat setting to maintain a steady comfort level. My fancy new smart thermostat includes a humidity meter, so it's not a big deal to correct for that. Just need to make a bunch more calculations than I anticipated.

My app has been in full control of my home HVAC for the last month or so and I've learned it's not so simple to control comfort! I've added a geofence to change the thermostat when the user leaves or arrives home, and a nighttime cool-down. That's eliminated the need for most of the manual overrides. I'll never eliminate all of them unless I can code in a mad-wife detector that knows when it had better cool the house down RIGHT NOW!

 

About the only thing I think is relevant above and beyond air enthalpy is the effect of outdoor temperature on the walls of your home, and the radiance you feel from them. An air temperature of 65°F might feel great when it's 90°F outside, but you'd feel much colder if it was 0°F outside and all your walls and windows have frost on the inside.

I tend to agree with your theory or analogy based on my home here in Utah, can't imagine what it's like where you guys are.

For me or my house it's brutal. Cinder Block with an exterior brick, ceiling insulation maybe 4 inches of fiber, couple that with Winters down as low as 15˚F or summers as high as 110 in dog days rather humid outside air.

Heat loss and Heat gain in my mind once the differential gain or loss to outside air increases so does your feeling of comfort when the equipment fails to remove or add to inside air fast enough.


kv

 

"The ASHRAE-55 2010 Standard introduced the prevailing mean outdoor temperature as the input variable for the adaptive model."
Reference #2, Thermal Comfort.
Subsection: Adaptive Comfort Model

Home thermostats control sensible heat. The SHR (sensible heat ratio) is controlled by the fan speed (air flow through the cold coil). The SHR is the ratio of sensible heat to latent heat removed by the air conditioner. Energy used for sensible heat is usually around 70%. Latent heat is related to the humidity, and therefore enthalpy. Air leakage is a major factor in SHR needs. There must be air leakage or you wouldn't like the result. How much? Ultra tight homes need intentional outdoor air injection. Outdoor temperature effect can have a lag of 9 hours in a concrete block wall. Maybe you should consider temperature measurement of the interior side of the exterior walls and the ceiling. Sensor must be insulated against interior air movement, which is everywhere if the ducts are designed correctly.

Boy have you bit off a job! I wouldn't be surprised if the math takes you 3 days.

 

I'm shivering at night and wearing a shirt and socks to bed. The wife has kicked off the covers and turned on the ceiling fan because she is too hot!

Hot flashes? What do you do? I just bundle up and deal.

 

I have suspected for years that heat has a density independent of the temperature. And I still believe it.

As far as comfort goes........it's subjective. I have a ritual that I go thru two times a year.....adjusting heat/air vents. My helpmate runs at about 5 degrees cooler than I do. Or I should say wants too.

Our temperature difference increases yearly too. Of course I will be getting colder the rest of my life.

 

Boy have you bit off a job! I wouldn't be surprised if the math takes you 3 days.

No, it won't be that bad because I've already got the psychrometric calculator built in to the app. I'll have to pick some humidity level, say 45%RH as the reference for the thermostat target setting, calculate the enthalpy for the target, and then calculate the dry bulb target temp for the actual humidity that will give me the same enthalpy. Actually that all just means using wet bulb as the real target.

For now I'll ignore radiation effects because it's largely taken care of by having a summer mode and a winter mode, with each mode having different thermostat targets. You wouldn't need two modes if the model considered every factor. Using just two modes may break down at extreme temperatures, which range here from -30° to 115°F. Maybe -5° to 95° in a typical year.

On a related note, my friend just scored a nice dehumidifier at a garage sale for FREE. My wife cleaned the filter, fired it up and it works fine! It's running in my basement right now. My air-conditioner-as-dehumidifier experiment has been a partial success. I've just turned it off now that the new unit is in. The capacity was a little low and I think an air conditioner is less effective than a dehumidifier at getting the humidity as low, at low temperatures such as in my basement. Could be too much air flow over the coils? I was thinking about ways to bring that down. The air conditioner will likely last a lot longer, though, and certainly works well.

 

Could be too much air flow over the coils? I was thinking about ways to bring that down

Too much air velocity.
Reduce the fan speed to accomplish a lower SHR. I derived that formula 35 years ago, but I forgot it and don't have time to look it up today.
What? You say the cold coil started frosting up before you got the fan slow enough to make it the right kind of dehumidifier? That's because it's designed as an air conditioner. If you want to push that limit, you install an evaporator coil rated for more tons than the compressor. A larger cold coil reduces the SHR by letting the air move across the cooling fins more slowly (has more time to drop water out of the air) and increases the efficiency of the whole air conditioner by sucking the last drop of cold out of the Freon before it goes back to the compressor to get recycled back into liquid refrigerant. ps, standard humidity goal in Florida is 50%. Your mileage may vary.

The capacity was a little low and I think an air conditioner is less effective than a dehumidifier at getting the humidity as low,

That might be true, but remember this: The dehumidifier is removing latent heat and replacing it with exactly the same amount of sensible heat plus all inefficiencies in the dehumidifier. The result is that the air conditioner will run more to suck out all the watt-hours you poured into the dehumidifier minus the water you dump out of the bucket. If you followed those last two sentences, you are conversant in Air Conditioning.

You should get a datasheet for a matched pair (condensor and evaporator) air conditioner and look at the variability of the SHR and which way it changes as the humidity in the house gets lower. It will either be very informative or it will make your head explode.

Edit: Adding pure heat (no moisture) after the cold coil or anywhere in the building improves humidity removal because a) adding heat to the "conditioned" air stream makes the relative humidity of the air stream decrease and b) the air conditioner will run more to remove the heat you added and thus, remove more humidity from the house just because increased run time.

 

Edit: Adding pure heat (no moisture) after the cold coil or anywhere in the building improves humidity removal because a) adding heat to the "conditioned" air stream makes the relative humidity of the air stream decrease and b) the air conditioner will run more to remove the heat you added and thus, remove more humidity from the house just because increased run time.

I wanted to say something but I knew I couldn't say it as well as you did here, so too as you did in the above.

kv

Edit: I've been out of the business for so long my comments are rather worthless, my apologies now.

 

I would like to say, this is a cool "Thread" @wayneh great idea. Thank you @#12 for contributions, puts me in the way back Machine. You would think I would have gotten more out of my degree in Heating and Air-conditioning but I didn't. I never wanted to contract or design, just repair.

kv

 

I have suspected for years that heat has a density independent of the temperature.

That is true. Humidity is called latent heat because it has the same effect as temperature on a (water cooled) human.
For people in the air conditioning industry, this is obvious.

 

I derived that formula 35 years ago, but I forgot it and don't have time to look it up today.

The formula wouldn't help me much since I have no tools to measure air velocity. I'm wondering though if it makes any sense to put a baffle on the front, like truck drivers do to their rigs in the winter, to throttle the flow. I can't imagine a simple way to slow the fan but that would be ideal, I think.

 

For people in the air conditioning industry, this is obvious.

For chemical engineers, too.

 

Actually that all just means using wet bulb as the real target.

If you have a psychrometric chart in memory, enthalpy, humidity, and wet bulb are merely a look-up table.

For now I'll ignore radiation effects

There are three factors: conduction, convection, and radiation. Ignoring one of them is self-defeating.

You wouldn't need two modes if the model considered every factor.

This brings up a 4th factor, acclimatization. Humans need different ambient enthalpy to achieve comfort because they acclimatize as the seasons change.

I would like to say, this is a cool "Thread" @wayneh great idea.

Right. Nice double entandre. (wrong spelling)
This Thread is hopping back and forth between the obvious and the esoteric. Hundreds of millions of buildings have achieved human comfort by using the dry bulb temperature and the SRH designed into the CFM (air flow) per ton. Dissecting out the components for enthalpy control is a viable option now that humidity sensors and MPUs exist.

Right now, wayneh is using the smartest A/C person he will ever meet, but I know what was in the education 40 years ago. I know it bloody well, but what wayneh is doing wasn't even considered 40 years ago: local, real time, calculation and control of enthalpy.

I can't imagine a simple way to slow the fan but that would be ideal, I think.

It's called, "fan curves". You can call for a different fan speed if the motor has more than one speed. Recent introduction of fan motors containing internal 3 phase drivers makes infinite adjustment possible, but you are still limited by the physics of the size of the evaporator coil per ton of cooling effect.

 

If you have a psychrometric chart in memory, enthalpy, humidity, and wet bulb are merely a look-up table.

I don't have just the table, I have the formulas you would use to make the table or chart.

There are three factors: conduction, convection, and radiation. Ignoring one of them is self-defeating.

I get your point but it's the de facto standard to control only dry bulb in a home. I'm adding an automated way to adjust for humidity, since that's the second largest factor. Radiation and acclimatization are the factors next in line to automate but, for now, I'm capturing both of them in the simple summer/winter mode switch.

It's called, "fan curves". You can call for a different fan speed if the motor has more than one speed. Recent introduction of fan motors containing internal 3 phase drivers makes infinite adjustment possible, but you are still limited by the physics of the size of the evaporator coil per ton of cooling effect.

My $100 A/C I'm talking about has two fan speeds and I'm using the lower one. Are you suggesting there might be an easy way to reduce it further, and that doing so might help it be a more effective dehumidifier?

 

My $100 A/C I'm talking about has two fan speeds and I'm using the lower one. Are you suggesting there might be an easy way to reduce it further, and that doing so might help it be a more effective dehumidifier?

No, it isn't easy. It's awfully expensive to use a $600 variable frequency drive fan motor.
That would make it a better dehumidifier, until you reach the frost limit of the coil.
I have been assuming you were designing a Universal Controller for everything from a window shaker to a sky scraper. (Because that is the range I work in)

 

I have been assuming you were designing a Universal Controller for everything from a window shaker to a sky scraper. (Because that is the range I work in)

Baby steps. Today my house, tomorrow the universe.

 

Baby steps. Today my house, tomorrow the universe.

Making the universe thermally comfortable is something I'd like to see!

 

What I did was get my Psychrometric chart and draw the results of the Equal Sensation graph on it. It's all straight lines, bub. Heat Index and Enthalpy are labeled in different units, but the comfort curves are essentially the same. You can wander around between the slightly cool and slightly warm boundaries, but there is no strange distortion of the enthalpy compared to comfort.

Well today it broke down. Here are the conditions:
Outside: 17°F at 70%RH. Enthalpy: 7.55 BTU/lb (We've been near zero since Christmas, so this is balmy.)
Inside: 61°F at 10%RH. Enthalpy: 4.09 BTU/lb

So my app is telling me to open the windows, to exchange that higher energy-content air into the house and purge the warmer-but-lower-energy air. I'm not doing that! If I actually had a smart window, I would have awakened to quite a surprise.

I do believe there would be a gain in sending the outdoor air into the furnace instead of the indoor air. I mean, thermodynamics don't lie. But the outdoor air clearly feels and is colder than the indoor air despite its higher energy content.