Indoor winter humidity level affects required heat set-point for comfort?

D107

I have heard that keeping indoor humidity levels in winter around 40% will make for greater comfort--partly in the sense that it will seem warmer at the same set point. It occurs to me as I'm looking at putting in a new boiler, that, in addition, adding a whole house humidifier to our attic central AC might allow a lower set point and save heating $$. (Like in summer, we can feel cooler with lower humidity levels.)

SuperTech

Unless you can keep the humidifier in a closet, and pipe the steam line coming off the humidifier into the attic and the ductwork it's not going to be a good idea.

Humidifiers require water supply and drain lines that can't be installed in an area that is subject to freezing temperatures such as an attic. Fortunately a nice steam humidifier like the Aprilaire 800 can be installed like I described.

It won't just seem warmer, air with a higher humidity level provides a more comfortable warmth than dry air.

Your ducts are for A/C only? What provides heat to your home? A boiler and baseboard/radiators?

D107

I should have mentioned that my attic is now insulated with closed cell foam (about R38)--a conditioned space within 3-5 degrees of floor below. Ducts AC only. HW boiler in basement; main/2fl cast iron rads; basement steel baseboard. Is there any way to measure potential heat savings? I was thinking with 40% humidity--instead of the usual 20-25% in winter--set point might be dropped maybe two degrees--a guestimate of course.

SuperTech

Sounds good to me. That foam insulation is a blessing for us HVAC techs that have to brave the attics in the summer.
I recommend an Aprilaire 809 steam humidifier. Only with a steam humidifier will you be able to achieve your target R.H percentage. And I agree that you should feel at least 2-3 degrees warmer with 40% relative humidity. And you will have less static electricity in the house and nosebleeds related to dryness will be eliminated

D107

So the humidifier would be in the attic and piped into the air handler? Unfortunately the AC system doesn't reach the basement, small 1st floor half-bath or under-insulated side porch, but in general this would likely be a big improvement.

SuperTech

Typically I see these units installed on the side of a vertical air handler/furnace or on the wall next to the unit. The steam line and nozzle is usually installed in the supply plenum. A humidistat is provided with the unit, but if you have a thermostat like the Honeywell 8000 series that can be used to control it as well.

D107

Great idea. I wonder if the introduction of water into the duct system might create conditions for mold and therefore require an annual cleaning. And I guess the humidistat, if connected to a thermostat, can also turn the AC on and off based on humidity more than temperature.

EBEBRATT-Ed

@David107 adding a humidifier is a great idea but.............they require quite a bit of maintenance....not a free ride.

The good news is even if your ductwork doesn't reach certain areas the humidity will. Moisture transfers by vapor pressure not by air flow.

In the winter the indoor air is very dry. You may have trouble getting to 40% but the improvement is likely worth it

Your thermostat could fire up the ac on high humidity but you would need duct reheat to prevent overcooling ...you don't want to do that, it's just for commercial controlled spaces, labs, manufacturing etc.

A humidity high limit located 10ft downstream of the humidifier wired to shut the humidifier down on high humidity is a good idea and prevents duct flooding.

mikeg2015

Ive liked my Aprilaire steam humidifier. Good option for a standalone AHU unit. Mind the installation instruction of distance and clearances. You need a 240VAC supply for a bigger less tight old home. 120VAC for a modern home would be adequate.

Get a good thermostat or controller that will control humidity based on outdoor temperature. 40% RH is great over 30-35F but you need to drop the set point when it gets colder depending on windows and home construction. 10-15% is all I can do when its -10F.

Makes a HUGE difference having humidity. RH can drop to 5% on really cold Midwest nights if you warm the house to 68-70F.

Gordy

David107 said:

I have heard that keeping indoor humidity levels in winter around 40% will make for greater comfort--partly in the sense that it will seem warmer at the same set point. It occurs to me as I'm looking at putting in a new boiler, that, in addition, adding a whole house humidifier to our attic central AC might allow a lower set point and save heating $$. (Like in summer, we can feel cooler with lower humidity levels.)

On top of feeling warmer the other benefits are substantial. Comfort is key to a lavish life style.

D107

@mikeg2015 is this the model 800? Seem to be various models with similar numbers. Most seem to connect to the AC system, not stand-alone. We have three floors, and basement is always kept closed--not sure if the steam can pass through that, but two floors seem doable. Whole house 1800 sq ft; two floors = 1300 sq ft.

mikeg2015

Yes 800. There is a version that included a stand alone fan powered kit you mount in a wall.

You only need to install it on the main level. Humidity migrates naturally and stack effect drives humid air upwards to upper floors. Stack effect is how I’m winter the warmer temps indoors cause cool air to infiltrate in lower levels, usually sill plate and windows, and warm humid air to exfiltrate our leaks upstairs... usually windows, roof soffits. So in summer warm humid air leaks in upstairs. So upstairs AC need to deal with humidity and that extra load. Downstairs can be sized smaller than typical. Almost always oversized by a factor of 50-100% on older homes.

I have mine on my 1st floor heat pump system and nothing upstairs. Upstairs RH is still higher most of the time because of that air movement. I also bring in outside air continuously through a duct and filter box to offset air leaks a little. Probably 60-80cfm.