I need cleaning instructions for the Holmes HM5125 ultrasonic humidifier

You didn't specify the brand or model, but for generic advice, try using a strong solution of white vinegar and water and soak overnight or for several days (if very scaled up).

If you were using hard water in the humidifier, try using a commercial humidifier cleaner such as BestAir products (or similar - there are many to choose from): Amazon com BestAir 3US 6 UltraTreat Ultrasonic Vaporizer Water Scale...
(Make certain it's certified for steam and/or ultrasonic use, if your using that type of humidifier).
Follow directions on box. You will have to let soak for a period of time (hours/days) to allow chemicals to do their thing.

If you are using "soft" conditioned water, just refill with soft water and allow to sit for several hours, then discard and refill. The hardened minerals will re-disolve and will be removed when you flush the unit. For hardened minerals clinging to the sides, etc, just use your fingers to break up, possibly a brush or Dolby pad and break them loose.

(Soft water has just as much minerals as hard water, but in hard water there are mostly calcium based and turn rock hard and in soft water they are sodium based (sodium bicarbonate actually) and have a weak structure).

It's important to keep a humidifier clean of deposits, not just to keep the unit running, but to avoid air quality problems (bacteria will cling to scale buildup, which make excellent breeding grounds for them).

If you use "hard" tap water, clean (at a minimum) once a week. Use white vinegar and water - 50/50 mix. If your using "soft" conditioned water, clean once a month. Same with distilled water.
You could also consider using a small amount of bactericide in the water to prevent bacteria from growing. (Okay for filter type humidifiers, but DO NOT use in ultrasonic or vaporizer type humidifiers). 1/4 cap full should suffice. BestAir 3BT 6 Original BT Humidifier Bacteriostatic Water Treatment 32 oz

It all depends on a couple of factors.

First is the type of humidifier your using. Portable or whole house connected to the furnace. Steam, ultrasonic or filter. Your unit's instructions is the best guide as to how often you need to clean it and this can vary from every day to once a heating season.

My recommendation is to watch and clean the unit as soon as you see scale forming. Then in the future, reduce this to just before you see scale forming.

The second is the type of water you are using. Hard tap water is the worst and usually will require every day for small portable steam or ultrasonic units to once a week for filter models. For central whole house humidifiers, once a month (perhaps as little as every two weeks if your water is really hard, >10 grains hardness). There is no fixed formula for this, the idea is to perform on-going, preventive maintenance.

Soft or conditioned water will contain the same amount of minerals as hard, but the softener has replaced the calcium minerals with sodium minerals. (Sodium bicarbonate actually).The result is the humidifier will still form scale but it much easier to remove. With soft water scale, you can just break up the scale with your fingers and the filters can just be submerged in clean water and the salt scale will re-dissolve.

Third, with distilled water, you may only need to clean any of these units annually. (No minerals to for scale).

With any filter humidifier, consider using a commercial bactericide to prevent the growth of bacteria when the water sits for long periods (>14 days). Not so critical with soft water as the salt will retard the growth of bacteria, but very important with hard or distilled water. Don't use this product in ultrasonic or steam humidifiers as it will be carried into the air along with the water vapor, and can be considered as toxic.

With hard water, use a 50/50 solution of white vinegar and water to dissolve the scale buildup. This works best if done on a regular basis. If left go too long, it become impossible to clean properly. You need only use regular water when using soft or distilled water in your humidifier.

Air Innovations Clean Mist Ultrasonic Dual Tank Humidifier ...

www.qvc.com/Air-Innovations-Clean-Mist-Ultrasonic-Dual-Tank-H...

QVCAir Innovations Clean Mist Ultrasonic Dual Tank Humidifier. Air Innovations ..... I did not realize that you needed a special filter if you have hard water. If you do ...

Sharper Image Dual Tank Warm &Cool Mist Ultrasonic ...

www.qvc.com/Sharper-Image-Dual-Tank-Warm-&Cool-Mist-Ultras...

QVCSharper Image Dual Tank Warm &Cool Mist Ultrasonic Humidifier ..... I've only had toclean the membrane once since I purchased it, and that's really not a big ...

How to Repair an Ultrasonic Humidifer ' eBay

If the water has been sitting in the reservoir for days without use, it may develop a film on the ultrasonic element which inhibits the creation of the mist to come out the top of your humidifier. I would lift off the tank and empty the reservoir. Then VERY GENTLY with a piece of paper towel, wipe the ultrasonic element (looks like a disk the size of a quarter at the bottom of the reservoir) to clean off any film. Then replace the tank and let the reservoir fill up the tank and try it again.

Good luck
thebqe

Vinegar and water will help prevent that mildew smell from forming. One cup of vinegar to 1 gal of water.

You can soak everything but the motor in bleach or vinegar water. Rinse well with HOT water.
The filter is disposable, cleaning doesn't help with them.

I am assuming a lime buildup from evaporated water. Use vinegar or drip coffee maker cleaner.

According to Holmes, one of the causes of the Reset light staying on is scale buildup on the heating element.

Remove Scale: Allow to cool before maintenance.
1. Repeat steps 1-5 in the Daily Maintenance section.
2. Clean the base by partially filling the base with one
8-oz cup of undiluted white vinegar. Let solution stand
20 minutes, cleaning all interior surfaces with a soft
brush. Dampen a cloth with undiluted white vinegar
and wipe out entire base to remove scale.
3. Rinse with clean warm water to remove scale and the
white vinegar solution before beginning to disinfect
the tank.
4. Wipe the heating element clean with a soft cloth after
it has had chance to cool.

If the scale has really built up it might take longer than 20 mins for the vinegar to get rid of it and you might need to refresh with new vinegar.

Your Manual can be found at this link:
http://www.holmesproducts.com/Manuals/Holmes/HM5250-UC_English.pdf

Good Luck.