A/C & Humidity Sensor

[Tripod137]

Unlike my home central air, the prius has a humidity sensor. As we have all probably noticed, 80F and 15% humidity is a lot different then 80F with 65% humidity. I would like to work on the humidity aspect of my house, but would like to know what a good Temp/Humidity ratio is. How does the Prius know what temp/humidity ratio is good? Does it have a chart telling it what a comfortable humidity is based on the temp? or is it more basic and just aims for < 40% (or whatever the value is) humidity at all times?

And I wonder why they DON'T seem to put humidity sensors in central A/Cs in the first place?

[bbald123]

Do you have a cite that shows that the Prius has a humidity sensor?

The answer to your question is quite complex actually. What feels good to you in one season will be unbearable in another.

[qbee42]

It's referred to as the THI, or Temperature Humidity Index, also called Heat Index. Google that an you will find the information you need.

Tom

[Patrick Wong]

Originally Posted by bbald123

Do you have a cite that shows that the Prius has a humidity sensor?

See p. BE-92 from aircondi file for the location of the sensor; and p. BE-104 from airco file for a description. These are excerpts from the 2004 New Car Features manual, which I highly recommend for owners interested in "theory of operations". This can be obtained at techinfo.toyota.com

[bbald123]

Extremely cool. Thanks Patrick.

[Tripod137]

Originally Posted by bbald123

Do you have a cite that shows that the Prius has a humidity sensor?

http://www.autoshop101.com/forms/Hybrid18.pdf
has a picture of it.

[Tripod137]

Originally Posted by qbee42

It's referred to as the THI, or Temperature Humidity Index, also called Heat Index. ...

Thanks qbee, I'll check into that.

[ChapmanF]

Originally Posted by Tripod137

And I wonder why they DON'T seem to put humidity sensors in central A/Cs in the first place?

Actually, they do, at least as far back as 2004 when a friend and I changed out an old furnace.

Most new central furnace/AC or heat pump systems now are available with a "continuously variable speed blower" or some similar phrasing; the marketing slogan varies from brand to brand but when you look in the box it always means they've used a General Electric "ECM" as the blower motor. (I'm guessing as soon as GE's patent on the thing runs out there'll be other suppliers in the game, but for now it seems to be sole-sourced). That's a very interesting animal that's actually a 3-phase motor driven by a computer controlled variable frequency inverter in the same package (sound a tiny bit familiar to anybody?). Among the tricks it can do is control accurately to a specific airflow setpoint rather than just running blindly at a speed - it tracks the flow resistance of the filters and ductwork in real time, and does it without any additional sensors, just by knowing the fan curve and analyzing the electrical response of the motor. (It has nonvolatile memory programmed at the factory with the fan curve of the specific equipment it's installed in, which is why you can't just swap the 'identical' motor between different makes and models of central unit; apparently they don't publicize any way to reprogram it in the field.)

Most central units that have one of these blowers also have connections for a thermostat that can send more signals than just the usual white, yellow, and green. A suitable thermostat can signal the A/C to run either to bring the temperature down, or to reduce the humidity, and the controller will choose different relationships of airflow to compressor output accordingly. On a cooling demand, that's a relatively high airflow; on a humidity demand, it puts less air through the coil more slowly, which extracts more water without excessively cooling the house.

The Carrier "Thermidistat" was an early example of a thermostat that also measured humidity and sent those additional signals to the central unit. Now it's more common and some models of the Honeywell VisionPro and some Robertshaw's, etc., can do the same thing. A few years ago Carrier introduced an "Infinity" line of thermostats that communicate with the central unit over a data bus instead of just adding more and more different colored 24v signal wires, which is nifty but of course only works with their equipment. It's bidirectional and will actually display ECM parameters like your airflow and static pressure on your living room wall - so you can quantify the improvement when you get around to changing the filters.

Me, I don't have one ... there's a 27 year old Lennox downstairs. But something I could start thinking about, now that I've replaced the 24 year old truck.

-Chap