1. Home
  2. ⧑ Industrial Control Group
  3. Need Enthalpy-stat

Need Enthalpy-stat

Hi folks,

I'm not sure this is a good group for this question, but I can't find anyone in alt.hvac who even understands the question:

Does no one make a residential heat pump thermostat that also senses humidity and runs the compressor based on enthalpy, instead of just temperature? Not just a thermostat, but a "heat-index-stat," or "enthalpy-stat." Such a unit would not run the cooling until a higher indoor temperature has been reached when the humidity is low, but at a lower temp when the humidity is high. It would probably look pretty much like a conventional thermostat, but the set point would be in something like heat index degrees.

I'm not trying to control humidity and temperature separately. I've had people come up with ideas about connecting a humidistat with a thermostat. That's not the way to go. What I'm looking for senses temp and moisture, and decides when to run the cooling as a function of both. Enthalpy. I've found devices called "thermidistats" by Bryant and Carrier. Couldn't find a good writeup though, and they appeared to be just that. Mostly marketing drivel. Don't want one that senses outside humidity. It must sense indoor humidity to respond to high people load, or other sources of indoor humidity.

Ought to be one.

Thanks.

Reply to
Earl
Loading thread data ...

I think this could be achieved with a Dew Point (absolute humidity) hygrometer. You would set the desired Dew Point in degrees C/F on your whateverstat. The complication is that the heat pump dehumidifies as well as cools the air, which would make control difficult. If the dew point is too high, you want to dehumidify, but not necessarily cool.

--Gene

Reply to
Gene S. Berkowitz

Earl,

I can think of an industrial solution but it would be rather expensive.

1 - Get a moisture/dewpoint/humidity transmitter that gives a continuous, 4-20mA signal. These exist.

2 - Get a temperature transmitter, perhaps a 100 Ohm RTD, that produces a continuous signal.

3 - Program the DCS to use these two values together with a 2D table lookup to determine the degree of cooling. It is not necessary to define an intermediate "heat-index". Simply put the degree of heat/deadband/cool into your table.

Any system which uses a fixed setpoint, i.e. humidistat, thermostat, etc. will not produce what you are looking for.

Walter.

Reply to
Walter Driedger

Walter,

Thanks for your comments.

I was hoping there was such a unit in production -- don't really want to have to make one.

Seems like a good idea for any residence. With regular thermostats, in the summer people turn the set point lower when it's humid and they feel uncomfortable, and then when the humidity is lower it runs more, cooling to a lower temperature than necessary. It'd probably save energy, as it would run cooling as a more accurate function of discomfort than temperature alone.

The "heat index" idea was just a way to make it a simple single-setpoint number on the dial for lay people. It'd look a lot like a regular thermostat, but be calibrated in heat-index degrees. Seems as though a table could be used that had such a figure at the intersection of temp and humidity values.

You're right that fixed-setpoint humidistats and thermostats are not the solution, but that's the usual answer I have gotten.

Reply to
Earl

For use in a residence it would probably be better to use a semiconductor temperature sensor (they're available cheap), a humidity sensor (not entirely cheap, but not as dear as a current loop devices by orders of magnitude), and a microprocessor. You could power the whole thing from batteries, or from the thermostat transformer.

Sounds like a valid product -- when are you going to quit your day job?

Reply to
Tim Wescott

continuous,

Tim,

Thanks for your comments.

Quit my day job? :) I'm the worst person to try to figure out what this public will and won't buy. It seems to be getting less sophisticated all the time.

Reply to
Earl

I installed a primative enthapy controller 25 years ago. It was made by Honeywell and was a purely mechanical device (meaning a horsehair rH detector balanced against a bi metal temperature sensor) . It was used in a commercial application and controlled the operation of an outside air damper. ie: when the enthalpy of the outside air was greater than that of the indoor air then the damper opened (heating mode of course).

The new crop of uP based controls for residential conditioning like Carrier's Thermidistat only monitor indoor humidity. Indoor and outdoor temperature are of course monitored. Their intent goes more to improving comfort any energy savings is a side benefit. New ultra efficiant residential HVAC equipment dosen't dehumidify as well as the old crummy

6 EER units from days of yore. With the advances in motor control they can now build equipment that gets high SEER values when on the ARI/ASHRAE test stand but then, once in the real world, can also slow the indoor fan down let the coil's temperature drop and provide true comfort by removing the moisture.

I feel certain that honeywell must make a updated version of the enthalpy controller that I used so many years ago. I'll look tomorrow when I get to work...if I remember.

If you could find such a control, what would be your application strategy?

Reply to
Impulse

Ok, to do what you want you will still have to do a little control wiring Set your unit up using a two stage cooling thermostat and select which bulb (setpoint) based on output from the Entralpy controller

Enthalpy controls by Honeywell:

formatting link

Reply to
Jim Whiting

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.