Storage, Humidity and Dehumidifiers

This page sits alongside the guides to laying an Aceca up for winter and to driving one through it. Those cover the car. This one covers the building it sits in, and the air inside it.

Three materials, three decay paths, one cause

As this archive records on the pages covering the car's construction and its body, an Aceca is hand-formed aluminium panelling over a composite framework of steel tube and ash timber, all carried on a large-diameter tubular steel ladder chassis. Damp air therefore has three separate things to work on, and they do not behave alike:

• Steel rusts, and it shows. The tubes and the chassis corrode visibly, in a way an owner can find with a torch and a probe. Rust is the honest failure of the three.
• Aluminium corrodes differently. It does not scale and flake in the same way, and where aluminium is in contact with steel in the presence of moisture, and worse still salt, there is a galvanic risk that a plain steel car simply does not have. That risk is a reason to keep the joints dry rather than a prediction of what any particular car will do.
• Timber absorbs, holds and hides. Ash takes up moisture from the air and holds it far longer than a metal panel holds condensation, which evaporates in an afternoon. A frame can be quietly wet for years and show nothing on the surface, until doors stop fitting or a joint gives.

The point to hold on to is that all three paths begin with the same thing: moisture in the air of the garage. Fixing the air is the only single measure that answers all three at once, which is why humidity control matters more on this car than it does on a steel monocoque where rust is the only enemy and at least announces itself.

1. The target, and where the sources disagree

There is no one agreed figure for classic-car storage, and it is worth seeing the spread rather than being handed a number:

• Auto Classica Storage and Ole Dissing give 40 to 60 percent, reasoning that rust all but stops below 60 percent, while wood, leather and rubber can dry and crack below 40 percent.
• CarCapsule gives 40 to 50 percent, citing corrosion research from the US National Institute of Standards and Technology.
• Cotes gives 35 to 55 percent.
• Munters gives 45 to 50 percent.
• Dantherm gives 55 to 60 percent at 18 to 20 degrees, aimed at professional storage facilities rather than a domestic garage.

This archive does not propose to resolve a disagreement between specialists. What is worth noticing is that nearly all of those ranges overlap between 45 and 55 percent, and an owner who holds the garage in that band is inside every one of them except the professional-facility figure.

The Aceca point

On a steel car the lower bound is a nicety. On a car with ash in its body structure it is not. Cotes' 35 percent floor would be too dry for a car with timber in it, and a desiccant unit running unchecked in a small, reasonably sealed garage can get there. That is the argument for the humidistat: the object is to hold a band, not to dry the building as far as the machine can manage. Timber that is repeatedly dried hard and then allowed to take moisture back up moves, and movement in a body frame is what eventually shows as doors that no longer shut cleanly.

2. Compressor against desiccant

Domestic dehumidifiers come in two kinds, and the difference matters most in an unheated garage in January.

Refrigerant, or compressor, units

These chill a surface so moisture in the air condenses on to it, in the manner of a fridge. They are efficient in warm air, and less so as the air cools, because there is less moisture to condense and the cold coil is more likely to ice up. Where exactly they give up is disputed. Motoring Research says they are ineffective below 15 degrees. Ole Dissing says refrigerant units work down to 5 degrees and suit northern European conditions. A third account puts the loss of effectiveness at around 18 degrees, with the unit cutting out near 5. Take from that a rough shape rather than a threshold: the colder the garage, the less a compressor unit will do.

Desiccant units

These pass air over a moisture-absorbing rotor and are largely indifferent to temperature, working down to near freezing. That is the whole case for them in a British garage that is never heated. The trade-off is electricity: a desiccant unit uses more power for the same amount of water extracted. In exchange it gives out warm, dry air as it works, which raises the temperature of the space a little and helps in itself, since warmer air holds moisture rather than dropping it on the car.

For completeness on the energy question, Ole Dissing states that a refrigerant unit returns roughly two and a half times its energy input as heat.

3. Heating is not a substitute

Warming a garage does lower relative humidity, and owners often reach for a heater first because they already own one. Ole Dissing states that heating is around two and a half times less efficient than dehumidification for lowering relative humidity. There is a second problem beyond the electricity bill: heating the air does not heat the car at anything like the same rate. A cold mass of metal in warm, moist air is exactly the condition in which condensation forms on the car itself, which is the opposite of what was wanted.

4. The garage and the floor

The building

A brick or stone garage and a steel-framed fabric shelter behave quite differently. Masonry has thermal mass, so its temperature moves slowly and lags behind the weather, which can work for or against you: it damps out daily swings, but a cold wall stays cold into a mild damp morning and collects condensation. A thin-skinned shelter follows the outside air closely, and its underside is a ready surface for condensation which then drips on to whatever is below, cover included. A cover that is repeatedly wetted from above is not protecting the car; it is holding water against it.

The ground

The floor is the part most often ignored, and it is frequently the largest single source of moisture in the space. A sealed concrete slab is a very different proposition from a slab with no barrier beneath it, and both are different again from a plastic sheet laid over grass or hardcore, or from bare earth. Moisture rises from bare ground continuously, day and night, in every season, and no dehumidifier can win against an unlimited supply. Putting a proper sealed vapour barrier between the ground and the air inside is usually the cheapest worthwhile improvement anyone can make to a storage space, and it makes everything bought afterwards work better. This is a principle rather than a specification, and the detail of how to do it is a builder's question, not this archive's.

The Aceca point

Timber standing over damp ground takes up moisture that nothing on the surface reveals. Paint looks the same, the alloy looks the same, the chrome looks the same, and the ash quietly climbs to a moisture content it should never reach. A car stored for years on an unsealed floor can be immaculate to look at and compromised in the structure.

5. Running it

No honest schedule can be given from a table, because it depends on the size of the space, how well it is sealed, what the floor is doing, the weather and how often the door is opened. Anyone publishing run hours for a garage they have not measured is guessing. The answer is to measure: put a hygrometer in the garage, ideally near the car rather than by the door, watch it for a few weeks across different weather, and let a humidistat control the machine so it runs when the air needs it and stops when it does not. Measurement replaces the schedule entirely.

6. What to look for in a machine

By capability rather than by name, since this archive recommends no products and links to no retailers:

• A humidistat, so a band can be set and held rather than the space being dried as far as the unit can manage. On a car with timber in it, treat this as essential.
• Automatic restart after a power cut, so a tripped breaker in November does not mean an unprotected car until you next visit.
• Continuous drainage, meaning a hose outlet rather than a tank that fills and stops the machine. A full tank is the commonest reason a dehumidifier is doing nothing.
• Low-temperature operation, stated by the maker, if the garage is unheated. This is the point at which the desiccant question above becomes a purchase decision.

7. Pouches and crystals

Moisture-absorbing pouches, tubs and silica-gel bags are useful in a closed cabin, in the boot and in the luggage area, where the volume of air is small and the object is to stop the interior smelling damp and the trim taking up moisture. They cannot dry a garage. The air volume is too large, the ingress too constant, and the absorbing capacity too small. Treat them as interior housekeeping, not as humidity control.

CarCapsule suggests swapping them roughly every sixty days from October to April in a humid climate, and notes that silica gel can be dried out in a low oven and reused rather than thrown away.

8. The battery

Disconnecting the battery stops any parasitic drain from clocks, alarms or wiring faults, which is worth doing on its own account. It does nothing about self-discharge, so a disconnected battery still gradually flattens over a winter, and a flat lead-acid battery sulphates and loses capacity permanently.

A modern maintenance charger, variously sold as a smart charger or a battery conditioner, is a different device from the trickle charger many owners still have on the shelf. An old trickle charger pushes a more or less constant current whether or not the battery wants it, which over months can boil off electrolyte and damage the battery. A maintenance charger monitors voltage and delivers a top-up only when the battery needs one, so it can safely be left connected for long periods.

On safety, charge in a ventilated space, keep the charger and its leads clear of fuel and of anything that could fall on them, use a unit the maker states is suitable for unattended long-term use, and check on it early rather than leaving it for four months untouched. One detail particular to these cars: the Aceca left Thames Ditton positive earth, so establish which way round your car is wired before connecting anything to it.

9. Rarity is the real argument

Around 328 Acecas were built in total. Everything above is technical, and the technical case for controlling the air is sound enough on its own, yet it is not the reason that matters most. What winter damp takes off an Aceca cannot be replaced in the ordinary way. There is no shelf of panels, no stock of ash frame sections, no second chassis to draw on. Every car that survives does so because somebody looked after the building it stood in. Seen that way, a hygrometer and a well-set dehumidifier are not fussiness; they are the cheapest conservation any owner will ever do.

Common questions

What humidity should an AC Aceca be stored at?

There is no single agreed figure, and the published guidance differs. Auto Classica Storage and Ole Dissing give 40 to 60 percent, on the basis that rust all but stops below 60 percent while wood, leather and rubber can dry and crack below 40 percent. CarCapsule gives 40 to 50 percent, citing corrosion research from the US National Institute of Standards and Technology. Cotes gives 35 to 55 percent, Munters 45 to 50 percent, and Dantherm 55 to 60 percent at 18 to 20 degrees for professional facilities. Nearly all of them overlap between 45 and 55 percent, which is the sensible target for an Aceca. Because the car has ash timber in its body structure, the lower bound matters as much as the upper one.

Can a dehumidifier be too dry for a car with a wooden frame?

Yes. Timber gives up moisture as well as taking it up, and air held very dry for months can shrink and check wood and dry out leather and rubber. Cotes' published floor of 35 percent would be too dry for a car with ash in its body structure, and a desiccant unit left running unregulated in a small sealed garage can reach that. Run the dehumidifier through a humidistat set into the 45 to 55 percent band rather than letting it dry the space as far as it can. The humidistat is not an optional refinement on this car.

Desiccant or compressor dehumidifier for a cold garage?

It depends on how cold the garage gets, and the sources disagree on where a compressor unit gives up. Motoring Research says refrigerant compressor units become ineffective below 15 degrees. Ole Dissing says refrigerant units work down to 5 degrees and suit northern European conditions. Another account puts the loss of effectiveness at around 18 degrees with cut-out near 5. Desiccant units work down to near freezing, which is why they are the usual choice for an unheated garage. The trade-off is electricity: a desiccant unit uses more power for the same extraction, though it gives out warm dry air as it works. Ole Dissing notes that a refrigerant unit returns roughly two and a half times its energy input as heat.

Should I disconnect the battery when storing the car?

Disconnecting stops any parasitic drain, but it does nothing about self-discharge, so a disconnected battery still slowly flattens and sulphates over a winter. A modern maintenance charger, sometimes called a smart charger or conditioner, is generally the better answer: it monitors the battery and tops it up only when needed, unlike an old-fashioned trickle charger that pushes a constant current and can cook a battery left on it for months. Whichever you choose, charge in a ventilated space, keep the charger away from fuel, follow the maker's instructions for unattended use, and remember that the Aceca left the factory positive earth, so connect with that in mind.