I will post a related question in the furnace section, but, strictly speaking in terms of energy conservation, is there a whole house humidifier type (for a gas forced air system c. 1990) that is the most energy efficient either in terms of cost of operating itself, and/or, most effective in reducing energy used by the furnace?

 

A whole house unit would require much less energy. You would have to have multiple portable humidifiers to have a comfortable level in a average sized home. Most portable humidifiers only provide about 500sqft, if not less.

I'm no pro on this, just using simple reasoning.

 

The actual cost to operate a humidifier is directly related to how much moisture you need to add.
The absolute least expensive is a pan of water on a register to a commercial tank or canister type that can take 20 kw or more of energy to boil water to make steam.

The real savings are in the operation of your furnace.
In cold climates the humidity levels are low in most houses.
Low humidity makes you feel cold because it causes the sweat on your body to evaporate which makes you feel cold and in turn makes you turn up the thermostat to feel warmer.
By adding humidity it reduces sweat evaporation which allows you to be comfortable at lower temperature settings which saves $$$.

 

Just a quick comment. The first step in weatherizing a home is air sealing. It saves on energy/heat and it reduces the amount of moisture that escapes from your home. I've worked on well sealed homes that have to run dehumidifiers in the winter and I'm not too far south of you. Too tight in my opinion, but the point is, the amount of humidity you need to add may be significantly less after you do a bit of air sealing. I don't want to drift too much so I'll leave you with that thought.
Bud