Your accordion should be stored vertically, resting on the bass end. That is why there are usually four little pads or "feet" for the accordion to rest on. Most accordions have the reed valves vertical when the accordion is vertical. If the accordion is stored flat with the keyboard horizontal, half the reed valves will hang down upside down. When the accordion is kept in this position, the valves (or leathers) as they are sometimes called--being usually strips of special accordion reed valve leather glued at one end, to prevent back air flow when the reed on the other side of the plate is not sounding--will permanently sag open, causing an unpleasant snuffling or rattling sound when the accordion is played. Repairing this condition involves removing scores of reeds from the blocks and straightening the leathers. New special "accordion wax " usually has to be poured around the reed plates. Some accordions, 'way less than 1%, have reed blocks that are positioned with the reed leathers parallel to the floor when placed on the "feet". This includes a few types of button accordions and some tone-chamber accordions. In those unusual cases the accordion should be stored horizontally. Rarely accordions have the reeds attached with something other than wax, but the labor involved is usually about the same.
Your Accordion Is Not So Hot
Accordions should never be left in the sun or in a hot place like inside a car in the summer. For instance, an accordion was displayed in a store window with the sun shining on it with no ventilation. The reed wax holding the reeds in place melted, and many of the reeds fell loose inside the accordion. This caused a lot of the notes to not play, and a whoosh of air to be heard instead. This caused an immediate and severe depreciation in the monetary value and salability of the instrument.
Accordions in Closet
If you keep your accordion in the basement, humidity is sure to make rust on the reeds, corrosion on the mechanical parts, a depressing musty smell, swelling and warping of the wood parts, and damage to the bellows due to deterioration of the bellows cardboard and leather. Best for it to stay in closet.
Bring Your Accordion, Just in Case
When placing the accordion in its case, put the keyboard in first, gently and carefully; don't pull up on the edges of the keys as this may bend them up. Do not grab or crush the bass buttons, and make sure the accordion is sitting firmly on the bottom with the bass strap not caught against the back of the accordion case. The buckles of the shoulder straps should not catch against the lining of the case, damaging the lining. That is why I often fold the straps under the key bed with a loop sticking up on the right. This also can add support under the place where the key bed joins at an angle to the shell of the accordion. When carrying it, the side of the case with the handle should be the side away from your body in case the case lid accidentally pops open, but with old cases beware of little nails coming loose from the case edge and shredding your polyester suit.
Give Me Some Room to Blow Down the Accordionist
Never play an accordion on or near enough to the ocean to allow salt mist to get inside, as this is certain to make the accordion completely beyond repair and restoration. First of all, the reeds will be so pitted and corroded by rust and corrosion on the reed plates, that they will be untuneable, not to mention the destruction of all metal mechanical parts. Also don't play in the rain, unless someone pays you a lot of money, because you will soon need an expensive repair.
The Accordion Which Came In from the Cold
When you bring your accordion inside from the cold, and play it in a warm room, the reeds and reed plates will cause water to condense on the cold reeds from the warm air. This is one major cause of rust on the reeds. It would be better to let the accordion warm up slowly before playing it, so that the temperature inside is equalized with the room temperature before ever letting air in from the room. The greater the difference between the air in the room and the accordion, the drier the air needs to be before you start to play.
Also note that:
The cold accordion may have some bad sounding reeds until the reed plates warm up, as the metal plates will contract, sometimes causing some of the vibrating reeds to brush against the plates.
The colder the temperature, the more sharp the pitch, as the cold reeds are stiffer and vibrate faster than warm reeds.