Model #u120020d12 what is this thing?

That model number doesn't show up on Sunbeam's site. The only thing you get is a Cool Mist filter. But, this owner's manual in the attached link might be close.
http://demandware.edgesuite.net/aamb_prd/on/demandware.static/Sites-sunbeam-Site/Sites-sunbeam-Library/default/v1425041848701/documents/instruction-manuals/SCM3755C.PDF

This humidifier is AMAZING. I’m a huge fan of using essential oils (lavender, lemongrass, spruce, rose, sandalwood etc) and I picked this up from a thrift shop, added some essential oils to the water and my house smells wonderful.

Essential oil diffusers are virtually the same thing design wise except cost a fortune. This is my best diffuser! I paid $10 for it and it works so good, throws off a ton of mist which I love (it’s so dry here) and I will enjoy owning this unit for years to come. Only thing is to gently clean the little ionizer part with the brush gently and keep it clean. Also..this unit unlike newer ones does NOT shut off automatically so keep an eye on it. Running it dry might cause it to burn out.

Here is a link for parts to your humidifier http://www.aprilaireparts.com/aprilaire-440-humidifier-parts.html

I just had the same thing happen. I called the company and they are replacing it for free...

I would advise you to replace your old humidifier. The reason for this, is that at 17 years old, you will probably find that parts are scarce, and fixing one thing only breaks another. Also, the old humidifiers, by design, are a perfect breeding place for mold and algae, and switching to a new flow through type is advisable.

currently through the information that i have seen right now, the regulators are out of stock. the best thing that i can suggest is try the manufacturer or local hardware stores. now if your in canada you can always try canadian tire. i will keep looking just in case you aren't able to find one locally.

you need filter model hwf100 for that unit. now you can try at any local hardware store or if your in canada you can try at canadian tire ifyou want. but if you want to order them offline you can go to www.appliancefactoryparts.com. type in your model number, clickon the picture scroll down alittle and the filters are the first thing to see. if you have any other questions please let me know.

clogged solonoid? clean filter and orifice

You can call 1-800-955-7075 give them the make and model #.The part # should be 4217,10x113/8 frame for $14.56 plus shipping