What happens if you crack open a window and turn on an exhaust fan for an hour or so?

Those numbers aren't making any sense. Are you sure you are measuring them correctly.

Outdoor air at 45F and 60% R.H. when heated to 68F would drop the relative humidity to around 28% +/- according to my psychrometric chart and my antique vision. That's right at the lower limit of what most folks would consider comfortable (the ASHRAE comfort range is 30% to 60% R.H.) You could install a humidifier if your system includes ductwork or utilize room humidifiers if you have a ductless system. I suggest your check with a local HVAC professional to see what options works best with your HVAC system in your particular climate zone.

@sktn77a: "Those numbers aren't making any sense. Are you sure you are measuring them correctly."

Why not? Its raining now and the outdoor temperature is 45 degrees but my package heat pump is running in stage 1 and the temperature and humidity of the air emitting from a register is 90 degrees and 12%, respectively. The room humidity is 38% but can get far lower if the outdoor temperature is lower, forcing the heat pump to operate in stage 2 or stage 3 (backup heat strips). I've seen the air emitted from a register as low as 1% humidity and the room humidity get very low as a result.

Relative means "in relation or in proportion to something else". When referring to relative humidity the relationship of "something else" is the moisture vapor as a percentage to the total amount of moisture the air can actually hold at a specific temperature. Warm air can hold more moisture vapor than cold air can so RH is just the ratio.

A 5 gallon jug can hold more water than a 1 gallon jug, if they both were filled 50% full they don't hold the same amount of water each and it's only a percentage of what they can hold. The same principle applies when humidity is measured as a percentage, so think of 70 degrees as the 5 gallon jug and 40 degrees as a 1 gallon jug. If you want to know the actual volume of moisture in the air it's expressed as "absolute humidity" and expressed as grams per cubic meter of air (g/m3.

There are tables and charts to express humidity as relative or absolute. Your outdoor absolute humidity for 40 degrees at 70% RH is about 32 g/m3 give or take a gram, and your indoor absolute humidity for 70 degrees at 18% RH is again about 32 g/m3 give or take a gram.

To sum it up the actual humidity in the air outside and inside is actually the same amount. To add more air vapor inside to feel comfortable you must increase Absolute humidity to 54 g/m3 (or about 40% Relative humidity.) The comment about opening a window and cranking on a fan is not relative stupidity but absolute stupidity and will just cost you a bunch of money in heating. A pretender with NO knowledge commenting on HVAC is also a danger to others when a question could effect personal safety so beware of these people with misinformation.

It's interesting that a new build of 2500 sq feet can be conditioned with equipment producing only 25K BTUs.

With only one head on each floor, how does the conditioned air circulate to all parts of the floor space. I'm thinking about areas like bathrooms and at least one or more bedrooms where the heads aren't located.

" A pretender with NO knowledge commenting on HVAC is also a danger to others "

This person is usually best ignored but the irony of the comment was something I couldn't pass by.

Here’s the current conditions:
Outside - Temp: 45F, RH: 68%
Inside - Temp 68F, RH: 17%
Air Handler - Temp 90*F, RH: 10%

According to an online calculator for RH, the current outdoor air would have a RH of 30% if the temperature increased to 68 with no other variables.

Our HRV and ceiling fan in the living room do a good job circulating conditioned air between rooms. It is typically 3-4 degrees cooler in the bedrooms, which are furthest from the mini split air handler located in the open area of the living room, dining room, kitchen. But this works for us since we all like to sleep in a cooler climate. We have a small electric wall heater in each of the bedrooms and bathrooms as supplemental heat but they are only necessary when temperatures drop below 25 degrees, which isn’t a regular occurrence where we live. It’s not a perfect system but works for us. We can add a third head unit to the heat pump in the future.

Is the temperature and humidity of the air coming from the air handler normal? Seems quite dry to me.

I wrote above that the temperature at the air handler is 90 but in reality is fluctuating between 88-98. Humidity is constant at 10% though.

Hi, Gabe,

I think you've got issues with your relative humidity monitoring device. Based on my psychrometric chart and antique eyes, If you take outdoor air at the conditions indicated (45F, d.p. = 35F, R.H. = 68%) and heat it to 68F, you get a relative humidity of 32% That's within ASHRAE's prescribed comfort range of 30% to 60% R.H. albeit at the low end or the range.

As for measuring the relative humidity of the air stream leaving your air handler, it's both incorrect and not particularly relevant. If you vary the air temperature between 88F and 98F the relative humidity varies from 18% R.H. at 88F to 12% R.H. at 98F (again, according to my psychrometric chart and antique eyes.) I suspect your electronic meter is limited at the low end of the measurement range by the particular type of sensor installed.

Now for the non-quantitative part: If you think it's too dry, then humidify the air to achieve the desired comfort level, irrespective of what the relative humidity reading is.

For the sake of completeness, a couple of notes about prior posts:

"If you want to know the actual volume of moisture in the air it's expressed as "absolute humidity" and expressed as grams per cubic meter of air (g/m3." A gram is a unit of mass. Absolute humidity has units of mass of water/unit volume of air.

"To add more air vapor inside to feel comfortable you must increase Absolute humidity to 54 g/m3 (or about 40% Relative humidity.)" You don't add "air vapor," to increase relative humidity you add water to the air.

Normal convention is to stick to a single set of units--either metric or Imperial. It would not be appropriate to indicate a temperature in degrees F and an absolute humidity in gm/m3.

Thanks for correcting my minor and simple errors Charles. I've always used the term "calibrated eyes" for many decades, and never heard of "antique eyes' but I like it. I think I'll start using "antique calibrated eyes' from now on.

I don't know what age eyes meet the definition of "antique eyes," Kevin, but I find myself using the term more since I passed the "senior" milestone applicable for discounts at my local grocery store. That was age 60 and I passed that some time ago.

I appreciate the analysis. I was also suspect of the RH reading at the air handler but my inclination was the monitoring device had limited range. So, I just looked up the specs and found that the device’s range is between 10-99%. I think the device is accurate to within its specifications and the 10% reading I’m getting between 88-98F at the air handler is because RH is actually at or below 10% across that temperature range. If this is true, the air handler is drying the air, which as you pointed out would be around 30%, but the air handler is reducing the humidity to 17%.

So why is the air handler removing so much water and is this normal?

Here are the device’s specs:

Heating the air doesn't remove any moisture, it just decreases the relative humidity which was described by Kevin above.

Very interesting.

Agreed, thank you. I’m still unsure why the air is not closer to 30% RH at 68F based on the humidity of the outside air.

We have a lot of maple wood in the house, including some very large slabs and stairs, and I want to prevent any issues.

" I’m still unsure why the air is not closer to 30% RH at 68F based on the humidity of the outside air." You're trusting the indication of an inexpensive, uncalibrated electronic device. Take a simple dry bulb temperature and a simple wet bulb temperature utilizing a sling psychrometer and use a published psychrometric chart to determine relative humidity.

" sling psychrometer "

How many homeowners out of 100 do you think owns one of these, CR Homes? Start at 0 and if you want to count up, don't go far.

Yeah, just as soon as i finish using my slide rule, ol’ timer

You might recall using a sling psychrometer in 5th grade science class. They don't need batteries or calibration. They consist of a dry bulb and wet bulb thermometer which can be slung around to get the wet bulb reading. If you're questioning the accuracy of a digital thermohygrometer, it's a simple way to check if the reading is in the ballpark (I keep both in my briefcase of troubleshooting tools.) You can buy a serviceable one for less than $20 and you can find a psychrometric chart on the internet for free.

Who doesn't have a calibrated digital thermo hygrometer? I have 3 and costs were between $80 to $300.
A calibrated traceable hygrometer I bought for around $80 years ago was calibrated and certified to be within it's accuracy range for humidity of +/- 2%, or a 4% swing and only for a year, and could be substituted for a sling psychrometer which has the same accuracy range. But it's old and calibration service is more than just buying another one for $100+.
The one you used has a 8% accuracy swing and not for the full RH range that isn't certified to maintain advertised accuracy even out of the box, so it's far from being even close to accurate. If you want a digital full range hygrometer calibrated and certified to be within a 2% swing with a fast sampling speed you're looking at $400 or more not to mention calibration service fees to maintain calibration.
What you need is Humidifier if your house can't retain passive humidity sources such as showers, laundry, dish washing, cooking ect. Your HRV does not transfer humidity but a ERV does with limitations. I've searched for humidifier options for a HRV and found none. The only other option is to install a humidifier in the HRV fresh air outlet, or a portable humidifier. A steam humidifier would the best option if you decide to install one after the HRV and would require a water source, electric and controls to only allow operation when the HRV is running using a simple sail switch and a humidistat connected in series.
I use a small Honeywell portable humidifier in my shop and they call it a vaporizer, whatever but it's just a heating element in water. These types IMO are better because they grow almost no mold or Bactria and why I suggested a steam generator for you. Just the kind of guy I am.

" just as soon as i finish using my slide rule "

I guess I qualify as an old timer. A slide rule was essential in my college chemistry and physics classes for solving quantitative problems and equations. I remember "lubricating" the slide before exams by pulling it out and running the edge along a bar of soap. That insured smooth sliding and eliminated jerking movement that could prevent getting the slide exactly where you needed it to be for a particular calc. It was incredibly useful and a big time saver. This was before the invention of portable calculators, of course.