The house I recently bought has a humidifier built into the central heating system. However, I was told they can cause the galvanized steel ductwork to rust over time.

I have been using a portable humidifier. However, during really cold weather when the furnace runs more, it needs refilling almost daily.

My question is: doesn't the humidity from the portable unit also get into the ductwork and will cause the same rusting? I would really like to use the built-in, for the sake of convenience.

Any advice?

Galvanized steel is rust resistant (not rust proof), it does rust if there is standing water it is less likely to rust with running condensation on it, or in moisture laden air. EXAMPLE: My well pressure tank is constantly covered with condensation during the humid days - there is no rust on the top or the sides, but near the bottom where the condensation collects into standing puddles the steel has started to rust. Now this tank has been standing there since the mid 1960's and the rust is not that bad.

Galvanized steel is (usually) coated in zinc. Galvanized steel comes in grades and I would assume that your ductwork would have been matched up with the Central Air system, or the other way around. Ductwork is always under attack from moisture, thus it is graded to withstand that kind of attack.

Either way you get moisture into the ductwork. Even on a humid summer day with the A/C running you are going to have that issue as well. To prevent that from happening after the condenser shuts down (to stop cooling the air) the fan will blow for a minute or so longer to blow out the moisture laden air in the duct work.

I would assume that the built in humidifier with your heater would do similar running the humidifier on and off depending on the cycle of the furnace.

Without knowing the age of the thing and the set up you have going on, I would only assume that the humidifier turns on and off blasting warm moist air near the start of the cycle and shutting off adding moisture near the end of the furnace cycle. That would blast out the moisture laden air into the rooms and not leave too much standing inside to condensate on the walls of the ductwork to prevent rusting and damage to the ductwork (and house since ductwork is usually in walls, floors and areas where there is a problem with ventilation)

I do know that a furnace usually heats up air for a minute, then turns on the fan to blow out that warm air, then before shutting off the fan it stops heating air, the fan continues blowing until the temperature in the heating chamber drops to X degrees (X being what ever the tolerance for the system is). Most furnaces (with ductwork) run the fan on and off in balance with the heating element maintaining a safe temperature in the heating chamber while not allowing the chamber to get too cool when in the process of heating.

Now with your portable humidifier it is running constantly, dumping water vapor into the air which could (in theory) collect moisture laden air in the ductwork which would lead to condensation which would lead to standing water and could cause the ductwork to rust out more quickly.

I would assume that your portable humidifier runs constantly and has a small element that either heats up a small amount of water (making steam) or uses sonic vibrations to vaporise water. Something like that would continue to go through X amount of water (X being the amount it is manufactured to reduce to vapor) no matter how moisture laden the air is. Its like boiling water on the stove, it could be a very humid day and still your water will boil away if you are not careful.

Based on all of that I would say that it is "safer" to use the built in humidifier over the portable one.

However, I would urge you to look at the furnace and get the name of the manufacture, the model number and see if there is a 1-800 number. If the system is new enough you may be able to find it on the Web and/or address your question to the manufacture (via phone or email)

In my years in the business, I have never seen duct work rusted do to a properly installed humidifier.

By properly in stalled, I mean level, wired to only come on when the heat and fan are on. Set to operate at the proper humidity level for the temperature. Humidifiers needs the heat from the furnace to evaporate the water and the fan to blow it out of the furnace. Warm air will hold more water then cool.

I have seen duct work rusted from air conditioning. Duct gets cold, sweats in the basement or craw space, collects water and rust. Metal ducts need to be properly insulated and vapor sealed.

Thank you both! Very informative and helpful! Guess I'll be humidifying the convenient way now. Thanks, again.

Humbly,
DAG

Any helpful tips before firing it up???

Yes.

Get a professional H/VAC man out there to check your whole system out especially if you don't know the last time it was used.

That should really be a yearly thing - that is if you don't know about the system or just don't care to do the work of checking and changing filters. If you have heating and colling, then it is advisable to have a check up starting each season before you switch to heating or cooling.

David