First, the question – why doesn’t anyone make a dew-point thermostat? Especially considering that many homes now have complex HVAC equipment including a heat-pump/AC, ERV, and (de)humidifier?

Based on temperature and humidity readings, a smart dew-point thermostat could decide whether an indoor DP of 55F is most easily achieved by running just the (de)humidifier, or the heat-pump, or both.

I am currently doing something like this using a node-red flow that controls a bunch of relays.

Hi Allison,
Hi Allison,

I have long been frustrated about why we are so obsessed with dry bulb temperatures. It’s almost meaningless in most hvac comfort situations. Our bodies feel the ENTHALPY of our surroundings, and we cool ourselves relative to dew point temps (or wet bulb temps).

So, why don’t smart thermostats control by dewpoint?

I have an idea for an invention: we devise a personal comfort data logger. This device clips onto your shirt and measures dewpoint temp, drybulb temp, RH. In addition, this device has provision for user inputs:

“I’m too hot”….I’m neither hot nor cold….”I’m too cold”
“It’s too moist”….humidity is “about right”….”It’s too dry”

As we go about our day moving from room to room doing our thing, the datalogger sends data to our HVAC controller. In an ideal application, each room is served by ductless air handler (preferably chilled water as cold source), and each AHU has a condensate removal pump.

But even in a typical home or building space, the system is informed better to maintain comfort. For a large building, why not assign 5 data loggers to 5 randomly picked people each day and take an average of polling the data, room by room?

At any rate, I’d just about kill for a smart thermostat that will let me create setpoints by dewpoint. Are there any that can?

Contrary to common misunderstanding, Phoenix, Arizona is INSANELY humid during our monsoon season. 105f @ 40% RH is absolutely brutal. Yes, it’s a “dry heat” for 4 months of our cooling season, but August is insane. I’ve been so frustrated, as I am rarely comfortable and my bills are always way higher. FWIW, we have a 2780 sqft, 2 story house with massive picture windows everywhere. The house came with a pair of 3.5 ton AC’s, which according to your other blog entry is very oversized (I timed it and today @ 101f @ 12% RH outdoors this evening, the unit will come on for less than 12 minutes at a time.

Thanks Roy. That’s exactly
Thanks Roy. That’s exactly what is most appealing about using dew point to control humidity. The ability to allow a building in the Southeast to get to 80-90F when it is unoccupied, especially for a week or two, without increasing the risk of moisture build up.

However, it’s possible that just setting the dehumidifier to ~50% would work well enough to avoid any potential issues assuming no major building envelop problems. I’m not really sure though.

Jacob: Allison must be
Jacob: Allison must be asleep, so I will try to answer your question about thermostats using dewpoint instead of relative humidity. I am not aware of any. There is no reason that thermostat manufacturers couldn’t do it, but I am guessing that their marketing people see no interest in it beyond the people on this blog. If you keep your space temperature at about the same temperature all of the time, then controlling to RH should do the same as controlling to dewpoint. But if you do have significant temperature setpoint changes, or you turn off the cooling system at times, dewpoint probably would be a preferable control variable.

Thanks Allison. Are there any
Thanks Allison. Are there any home thermostats that can control a whole house dehumidifier using dew point? It seems like most of the common products only use RH.

David, I disagree that wet
David, I disagree that wet-bulb temperature is hard to measure. It is probably the easiest humidity metric to measure since all you have to do is swing a wet temperature sensor (sling psychrometer). Also, wet-bulb temperature is not a direct measure of comfort. A line of constant comfort on a psych chart does not correspond to a line of constant wet-bulb temperature. A constant comfort line on a psych chart is much “steeper” than a constant wet-bulb line (see Chapter 9 of the ASHRAE Handbook of Fundamentals).

David, I did write about wet

David, I did write about wet bulb temperature once, but I wouldn’t say that’s my mission.  As you wrote above, wet bulb isn’t quite as accessible as dew point, and it’s not shown in my weather app.

Here’s that article I wrote about wet bulb:

A Sticky Psychrometric Situation: Wet Bulb Temperature

Jacob, there are some

Jacob, there are some available.  Check TruTech Tools (https://www.trutechtools.com).  I usually use a thermo-hygrometer or a temperature and relative humidity data logger and then convert to dew point with the help of an app on my phone. 

Also, it seems to me that the easier way to state your point is: if the dew point temperature is approximately the dry bulb temperature, than the relative humidity is high.

Also love the “desiccating 39%” phase, I’ll have to steal it.