In late Summer / early Autumn and biting cold winter, we receive many call from customers asking why their piano has gone out of tune or reporting a change in the feel of the piano.

Modern pianos really need an environment with a relative humidity level between 45-65%. But, fluctuations within this range of greater than gross 5% will likely affect the tuning and potentially the precise regulation of the action and keyboard.

When there is a spike in humidity – especially after a dry heatwave and onset of a thunder storm, the humidity can reek havoc in your piano. Wood, felt and cloth in your piano will expand and contract with these fluctuations.

Tuning Stability

The strings are under great tension and the cast iron frame is supporting each end of the strings. Like a guitar or violin, pianos also have a wooden bridge where the strings pass (and weave) over so receive down-bearing. The strings run up-hill over the bridge. The bridge conducts the string resonance into the soundboard. The bridge is usually beech, maple or vertical laminations of maple and mahogany. The soundboard is usually premium straight grain quarter-sawn spruce. Ribs brace the underside of the soundboard and are also usually spruce wood. The ribs will be notched into the timber frame of the upright or inner rim of the grand.

The top side of the soundboard has the bridge and strings upon it, and is convex profile. The under side of the soundboard has the ribs beneath and is concave profile. This shape of the soundboard is referred to as the “crown”.

When there is an increase in humidity, the soundboard – despite immense downbearing from the strings under tension – can raise, increasing the crown. This increases string tension, and consequently the global pitch of the piano. The upper bass and lower tenor is where there is usually the greater increase in pitch, as this is where the strings are riding on the centre of the soundboard and over 2 bridges.

This rise in pitch means the piano has gone sharp. Also, where there are 2 strings in the upper bass to each note (bi-chord) and 3 strings to each in the tenor/treble (tri-chord), the slightest change in tension of each string will distort the unison and causes the note to sound out of tune in itself. This sound can be cloudy, phasing or even zingy.

The opposite happens during a dip in humidity. The soundboard crown will lower slightly and this will decrease the pitch, making the piano pitch flat. Again, the lower bass and upper tenor and treble may not necessarily go out much, and often the upper bass and lower tenor will be greater affected. The same effect of the phasing unisons is very likely as it is not to be assumed that the piano equally goes up/down in pitch.

Damage to Soundboard / Bridge

Extreme fluctuations in humidity – particularly very dry conditions can force the timbers to split. The planks that make the soundboard could split or crack along the grain or the join. The ribs could come away slightly. The bridge could split. The bridge pins are steel and will not expand and contract like wood, so even the best cured hardwoods can expand and contract – over time the bridge pins elongating their holes in the bridge and the top capping of the bridge could split. The bridge could come away slightly or lean over. The effects of such damage can be purely aesthetic, but as they are acoustic and structural areas of the piano they are constantly in a state of resonance and will be vibrating – so unwanted buzzing is the most likely outcome. The soundboard could loose its crown and the downbearing would also be reduced. Overally the tone, power, tuning stability and degree of resonance or sustain can be significantly compromised.

Old pianos are known to suffer these issues. Largely because they were built with damp homes in mind – not modern central heating. The animal-based glues would fail. The wood had a higher moisture content. Modern pianos often used PVA glues which provide better humidity fluctuation resilience and timbers cured and dried to a lower moisture level.

The wrest plank (tuning pin block) used to be a plank of hardwood, perhaps with a top and bottom capping, but would dry out and split along the grain, rendering tuning impossible and requiring total replacement of the plank (require de-stringing, removal of frame etc.).

Of course, such damage to your piano soundboard is very unlikely indeed if you maintain a safe level of humidity. Only the extreme environments can pose risk.

Sticking/sluggish Keys?

Very common in new pianos is to find after a spike in humidity is sluggish keys. This can even be after delivery of your new piano or when moving home. The culprit is the sudden upward change in humidity and moving a piano from one environment to another can be enough to bring on the sticking key.

I say ‘new pianos’ because a new piano will have key bushing cloth that is not worn but relatively snug on the pin.

The piano keys are essentially rocking on a balance rail. There is a front rail and a back rail (back touch). To keep the keys vertically aligned and in position, there is a thin pin travelling upward from the balance rail through the bottom of the key to beyond the top. The hole in the bottom of the key is perfectly round, but the top is elongated to accommodate the greater travel at the top of the key. There will be a hardwood capping on the top of key known as the ‘chaser’. The chaser is lined on either side with premium felt cloth. This is a ‘balance rail bushing’.

The front of the key also requires alignment, so there is an elongated pin travelling through the bottom half of the key from the front rail. This is known as the ‘batt pin’. Again, there is a cloth bushing lining sides the batt pin hole under the front of the key. This is the ‘batt pin bushing’.

Felt and cloth have a nasty habbit of absorbing humidity before wood. They also tend not to contract when the humidity retreats and rely on wear and compression to compact back down.

The keys are the most exposed part of the piano action system… the front key bushings especially! Absolutely, the most common cause for sluggish or sticking keys in a piano is a spike in humidity either from weather or moving from dry to humid environment, and the front key bushings expanding. The good news is that this is very easily resolved by the tuner technician compressing the cloth back using key easing pliers. It is best to do this once the humidity is back in range.

If you are unable to get a tuner tech. soon enough and you have a few keys causing trouble, you can try moving the keys laterally to see how much play there is. If the bushings are too snug, there will be very little lateral movement ie. less than 0.5mm. If so, you can quite safely hold the key front and press to the left and right, using the hidden batt pin as a compressor to the cloth. This will help free up the key.

Typcially, the sluggish keys are more obvious when the sustain pedal is used. This is because the weight (grand) or spring (upright) of the damper lever is isolated from the keyboard when the sustain pedal is depressed. Without the sustain pedal depressed, the weight/spring resistance of the damper lever usually forces an otherwise sluggish key back into position.

You can test your key travel by keeping the sustain pedal depressed while playing 2 keys side by side together up and down the keyboard and watching their return speed, using 1 of the keys as a comparison. The sluggish key will return noticeably slower than the others. And to diagnose if it is the front bushing, wiggle the key left/right – if it is resistant both ways, it most likely needs to be compressed (eased).

Some pianos come with a felt keyboard cover. It is not actually a dust cover, but in fact a strip made from felt cloth and is designed to be laid on the keys with the keyboard lid (fallboard) closed. The fallboard helps protect the keys from dust but also delays ingress of humidity and the felt cloth keyboard strip will be the first to absorb a spike in humidity. This will reduce the chance of your key bushings expanding.

The balance rails bushings can also be the culprit and also are treated through compressing with key easing pliers by the technician.

It is also possible that through wear and dry conditions, the key bushings become over compressed and require ‘sizing’. A process of deliberately expanding the bushing cloth with a careful application of alcohol/water by the technician will cure keys with excess lateral movement and excess slanting on the Y-axis.

If key bushings are worn and cannot be sized, then the cloth is replaced. This is a very worthwhile and relatively inexpensive process, resulting in a far more secure feel to the keyboard.

The wooden hole in the bottom of the key on the balance rail can also elongate and become rather noisy. The hole can be sized using a solution to expand the wood for a snug fit on the pin.

In prolonged exposure to humid conditions, the keys may actually also be less responsive – additionally to key bushings being snug – if the balance hole is too tight on the pin. The hole can be eased carefully to allow the key to drop down on its weight, without having forward-back travel.

Action less responsive? Uneven touch?

The back of the keys are resting on a thick strip of baize felt (back touch). The middle of the keys are resting on small box-cloth washers (balance rail punchings) and the front of the keys when depressed contact the front touch large baize felt washers (front touch punchings).

The height of the keys depends on the thickness of the backtouch felt and the number and thickness of fine paper and card washers that serve as shims, to correct the trajectory of the key from its back rest position to the front of the key top. The depth of touch is determined by the height and the number and thickness of the larger front touch paper and card washers.

The foundation of piano regulation (the calibration parameters for the piano mechanism) is the keyboard. Keys should be in line and level, and the depth of touch also.

The entire performance of a piano action stems from the 10-11mm of key travel. There are 5 key steps in piano regulation, known as the ‘action cycle’:

  • Blow (45-47mm | travelling distance from resting hammer to string)
  • Damper lift (20-25mm | individual pick up of the damper lever measured by hammer distance from string)
  • Set Off (1-4mm point of hammer disengaging from action and striking strings through exclusively momentum)
  • Drop (set off+1mm | grands only: influences hammer return height in repetition on balancier and supports the hammer roller during majority of blow and escapement for reduced escapement friction)
  • Check (15mm | essentially a ‘brake’ to catch the hammer after striking the strings to permit repetition)

All contact points between the key, action components and hammer are lined with cloth, felt or buckskin.

Action levers and ‘jacks’ and hammers are pivoting and joined by a centre pin. The pin hole is again bushed with premium felt cloth.

The felt backtouch could expand/contract as with the balance rail felt, card and paper punchings/washers – affecting the key height and consequently depth of touch. The front rail baize, card and paper punchings may expand/contract too, affecting depth of touch. Increased key height can alter the blow, drop (grands) and check – but also the repetition and after-touch. After-touch is the amount of key travel after escapement. Too little feels hard to control. Too much feels sloppy and compromises repetition.

On uprights, a common problem in dry conditions is ‘lost motion’. Contracted balance rail washers, whippen heel cloth, hammer butt buckskin and hammer rest felt together cause wasted initial key travel. This could be as much as 1mm of the very first part of the key travel, doing absolutely nothing. This is easily adjusted by the technician turning up the key capstan screws/bolts which are the point of contact between the back of the key and the bottom of the action.

Again, on uprights, the opposite occurs as frequently in humid conditions. Known as ‘hammer creep’, the same components have expanded with humidity and the hammers are not resting on the hammer rest rail, but are resting on the action component know as the ‘jack’. This means the action is already a fraction in play when at rest. The hammer is actually having an unwanted head start before playing the key. The action cycle is too keen on and will have excess aftertouch. Also, after playing a note softly, the same note may fail to play next time as the jack may have not returned under the hammer. Again, easily resolved by the tech. winding the capstan down until the hammer is supported at rest by the hammer rest rail but also there is no lost motion and the touch feels immediate.

Heavy touch & poor repetition?

Heavy touch and poor repetition are most likely caused by sluggish keys through the previously explained key bushing and balance hole topic of “sluggish keys”. But, ‘tight centres’ is a troublesome issue and can be eased through application or a special lubricant solution to the centre pin bushing. If this is ineffective, the centre pin is removed, the bushing reamed and a new pin fitted. This is caused by prolonged exposure to high humidity. Conversely, dry conditions can mean too little friction at the centre pin and also poor alignment, travel and noise of action parts – particularly hammer flanges. A larger gauge centre pin is needed in this instance.

Rusty, Musty and (caked-on) Dusty!

No-one would want their lovely piano to become an unsightly and smelly piece! But, high humidity means a damp piano. The wood absorbs moisture and on the surface develops mildew. The felt dust from the gradual wear (hammers, hammer buts/rollers and checks) falls onto damp wood and literally cakes onto the keys, along with the usual dust in the environment. This can built up in action parts too and compromise efficiency.

Tuning pins, strings, bridge pins, pressure bar, frame bolt and cabinet hardware become tainted and tarnished. The soundboard develops a film of dust – particularly grands – that cannot be simply blown away, and may not even wipe clean but require careful wet-wiping.

Pianos can come to us from previous owners who had lovely clean homes – but the piano just has that slight musty scent of a village hall… lots of cleaning up needed as well as corrective works to the mechanism.


Hammers are made of felt. Premium lambswool felt wrapped in great tension around a wooden core (moulding), glued and stapled. Shaped like a diamond, egg or pear – piano hammers are hugely responsible for the tone of the piano – being the percussive component that makes this otherwise ‘stringed’ instrument – ‘percussive’. The shoulders of the hammers are stabbed with fine needles by the technician to manipulate the tension in the felt to introduce elasticity for a rounded and sustaining tone, balancing layers of rigidity for power and brightness. The surface of the hammer must be square and level, striking each string uniformly and square on, for a clean immediate tone. The hammers do react to humidity fluctuations. The tone can become softer or brighter according to degrees of humdity. Also, the hammer shanks may twist very slightly, and this affects the hammer spacing – which means the slightly worn surface of the hammer (with string marks) may no longer be striking the strings, and the more cushioned are between the marks is instead, making that note sound very different to the rest. The hammer will also no longer be square on to the strings, so the left of the tri-chord (3 strings to each note) may be struck a fraction before the middle and right string. This will sound ‘enveloped’ and not clear or immediate. The tone will be confusing.

Invest in a hygrometer

Ideally a digital one with min/max memory so you aren’t obsessing over it! Probably the best £15 you ever spent and almost certainly going to save you money and more…

Monitor! Regularly check and note the RH (relative humidity).

The acceptable range ideally is 45-65%. Minimal fluctuations of no more than 5% is best for peak performance and stability of your piano.

Remember that warm/mild outside temperatures plus rain means high humidity is very likely, and you won’t have heating on to contra that. Winter rain is different – because of heating, and you will likely have doors/windows closed. In the winter, if it is especially cold and not raining, the very dry outside conditions combined with your heating will make it very likely that the environment for your piano will be too dry.

If the humidity is too high

Buy a dehumidifier and set the hygrostat to engage when the humidity exceeds 55%. This should keep the environment within 55% RH (relative humidity).

If the humidity is too low

Buy a humidifier and set the hygrostat to engage when the humidity falls below 50%. This should keep the environment above 55% RH.


If the humidity is always within 45-65% (the acceptable wide range) you may want to consider investing in either a dehumidifier or humidifier as necessary to ensure peak performance from your piano, as 10% gross fluctuation can disturb the fine regulation, tuning and tone of your piano still. Opt for 50-55% as the ideal range, so chose either humidifier or dehumidifier as necessary to achieve this. If the humidity is happy at 50-55% most of the year but in the winter it dips to below 45% – then definitely get a humidifier and use when needed. Vice versa if good in winter but humid late summer brings readings exceeding 65% (get a dehumidifier). If your home wants to average 45-50 or 60-65, that’s fine – just let the piano adjust for 4 weeks before correcting tuning (pitch) and/or servicing, or any corrections to it such as easing/sizing bushings.

Schedule Tuning & Servicing

Also, remember to have your piano tuned every 6 months and ask the tuner about servicing. The main reason tunings are 6 monthly is because of the climate variation between summer / winter, as well as elasticity of piano wire/settling down and impact on strings from use.

If the tuner does not offer technical services as a technician, ask for a recommendation or ask your dealer. Some tuners are excellent piano tuners and are not technicians. Some technicians are excellent technicians and do not tune. It is no reflection on their skill or competence in what they do, merely a different service offered.

Your piano should be serviced every 2-4 years depending on condition and use. Servicing can include cleaning out, removing dust, regulating, voicing, lubricating, adjusting pedals, checking for moth, reporting on signs of humidity (high/low) exposure – generally keeping your piano at its best. Even if you think your piano is fine, unless you know what to look for and how to remove components for inspection, you may miss something that could be crucial to the long-term health of your piano and its value.