1. You have upflow furnace. Filter is at the return.

#1 is a supply

#2 is a b-vent, where products of combustion vented out,

#3 is a condensate drain

Not a fan of a bypass humidifier in your case.

95% score, an A!

#1 is the supply plenum

#2 is the exhaust vent from the combustion area of the furnace. Typically but not always made using type b pipe.

#3 is a condensate drain coming out of the air conditioning coil (think of it as a heat exchanger for AC), that sits on the stack above the furnace and below the supply plenum.

I'm not sure where the furnace gets combustion air (fresh air to replace what goes up the exhaust vent) unless there's an opening through the ceiling around where the exhaust vent passes, if the area above is open to the outside. Are there any other openings in that small closet area?

This is 80% furnace. Combustion air comes from the house

Well the furnace isn't going to get combustion intake air through a solid door when it's closed. Dazhou, your primary concern is to do what Elmer has asked before thinking about a humidifier, or you could DIE. The Furnace closet is a confined space and needs combustion air to burn with the gas so where does it come from? I did mention you could die, so if you can't find a fresh air supply this is what you need to install, not a humidifier.

Looks like the furnace closet is in a confined laundry room, is there a gas water heater in the same room? If for some reason the Furnace pulls combustion air from a confined laundry room you just doubled your chances of death because the dryer and water heater or Furnace and dryer could suck all the air from the room and cause a backdraft of the water heater and/or furnace sucking poison gases back into your house.

Here is a short video showing a simulated backdraft when a small exhaust fan is turned on. The smoke you see in the video simulates Poison carbon monoxide gas and has no color.

I know you have no exhaust fan in the laundry room but turning on the dryer is the same thing.

This is why you need to worry about enough combustion air in the furnace closet/laundry room so you don't die. You really have a dangerous combination and need to address it ASAP!

1. Correct

2. Combustion air is kept separate from internal heated/cooled air

3. Correct

You can install the humidifier as you have proposed providing you have access to a cold water supply (presumably from the washer next to it). Check on the fresh (combustion) air supply for your furnace - it is important although I'm guessing you've had no issues previously so it's probably there somewhere.

Actually, what BT said.

"Combustion air comes from the house"

The furnace appears to be In a closet/cupboard with a sealed door. I'll disagree.

The picture with the words 'looking up' seem to me to show that this closet the furnace sits in is not sealed. So combustion air is likely being drawn from there if that is the case.

That said, it wouldn't be a bad idea to put louvers on that closet door going to the furnace.

In terms of adding a whole home humidifier for such a small house? I think it's not worth doing. You could pick up a small room humidifier plug it in when needed. This house is only 900 sq. ft.

FYI: humidifiers need constant maintenance. You 'NEVER' stop paying for one. So if you can't do the maintenance or pay to have it done... don't buy the contraption... it's a waste of $$$.

A table top humidifier will need maintenance, you have to add water to it. Scale and lime will kill it too one day. But you can replace a table top humidifier much easier and probably cheaper than one attached to your HVAC system.

My guess is if you do the whole house variety you'll be lucky to get to last beyond 3 years without maintenance on the device. Maintenance = work... in a world where people have trouble remembering to replace their air filter. Tell me I am wrong, go ahead.

HVAC is what I do for a living.

You are right ray but maintenance is only the half of it . In the author's furnace configuration it's a worse case scenario for problems. If the CO poisoning doesn't get them the starchybotrys mold will if a humidifier is is not installed perfect and with a better control than the cheap humidistat which comes with most humdifiers .

Running a pipe down to the return is a direct pathway for condensation to drip down into the wood under the furnace and a breeding ground for black mold. I'm just assuming it's only wood underneath (I'd put money on it) and any humidifier configuration could destroy their house. Two much humidity, short heating cycles, and leaks will allow condensation to migrate to that wood box. Even the condensation from the A/C if the evaporator coil catch tray was leaking could get down there causing mold (and rot).

After calling in a professional to verify combustion air intake is available and no chance of backdraft have a professional pan the return box to contain any unwanted condensation. Only then would I install a humidifier so I can over humidify and destroy my wood floors and furniture, and also grow mold in the rest of the house. If you're reading this dazhou, the point is even a simple thing like putting in a humidifier can have huge consequences by overlooking minor problems you're not familiar with.

Another thing, is the plenum (#1) sheet metal or fiber duct board under the insulation? Good luck hanging a humidifier on duct board.

Have you measured the humidity? If is above 40% then you should not need to install a humidifier. If is not then a portable like the Elechomes SH8820 Humidifier on Amazon for $100 should be sufficient. It gets many positive reviews and you can thoroughly clean it at the end of the winter.

You should also investigate if you have any infiltration around windows and doors. The air leakage from the outside cold air is the biggest cause of low humidity in the winter.

Kevin, It doesn't look like duct board to me -- I should know I work with it all the time LOL, It looks like the plenum was wrapped with insulation. Duct board is rigid and typically installed silver side out, then open side is sealed with tape.

The insulation shown in the picture is anything but rigid.

That said, you'd have to run water line and a bunch of other stuff, this is not a DIY job. I think the OP is just asking questions and for good reason.

For a 900 sq. ft home, it's not worth it.