Workshop dehumidifier

Im happy with my Meaco DD8l.  Most important thing for a shed is that you get the dessicated type not refrigerant type. Dessicated works at low temps.
With the Meaco you can either set a humidity level where it cuts out, a timer, or it stops when the container is full.

Hard to tell the running costs.  It will run more the First few days until the timbers dry out.

I think a 1l would be too small

I have the  Meaco DD8 junior on my boat (37 ft x 12ft) so possibly double the volume of your shed.  Extracts 5l per day while living aboard in the winter.  Wouldn' want to be without it.

https://www.meaco.com/products/meaco-dd8l-junior-dehumidifier

Ventilation is your friend and it is free.

Dehumidifying an outdoor space is a bit of a waste of time. You are effectively trying to dehumidify the world.

Your shed is that halfway house between between a well sealed and insulated indoor room and a truly outdoor space. As such it isn't well ventilated enough to prevent condensation and mould and it isn't well sealed enough to dehumidify.

I learned this by accident when we moved into our last house which had a decaying post-war prefab garage. When we moved in, I had the same problem as you. After a severe winter storm, standing inside, you could see daylight in places but that cured the problem without letting any rain in.

So, before you invest in a dehumidifier, try more ventilation. And that ideally means air flow in, through and out (think of bike helmet ventilation) so something at each end. Depending on the age and value of the shed and your future plans, this could be something as simple as drilling holes in it high up under the shelter of the eaves, or installing proper angled ventilation grills that will be better at weather protection.

Something that I don't quite understand with these types of issues is: lots of ventilation is always suggested as a solution (and I've no reason to doubt it), but how do you know how much ventilation is too much? I mean, 'perfect' ventilation would mean that the air (and temperature) inside a shed/garage would be just the same as outside, right? But outside there's an awful lot of dew in the mornings.

Am I just dumb? 🙂

The absolute best solution would be a massive roof with no sides and a huge overhang, sufficient to protect from above and from the sideways incursion of rain.

As you say, that would offer equalisation of temperature a humidity without the excess moisture provided by precipitation. Not many places have sufficient space for such a building, and security is (as they say) 'less than ideal'.

So in a building that needs to offer a compromise of protection from the elements with protection from theft in a reasonable space, a well ventilated shed which can breathe is pretty much as good as it gets.

Ventilation is free, natural easy to achieve and not fighting a losing battle with dehumidifying the world.

How much is 'enough'. Start with less and work up I would suggest.

Worth getting a humidity monitor to stick on the wall to keep an eye on it.

My shed 14*10ft is usually ok. After almost constant rain for weeks in December followed by a sharp drop in temp there was a bit of condensation Ran the DD8 3 or 4 times and it's OK now.

Unlike a house there is no moisture generators in a shed. No breathing most of the time. No cooking. No clothes drying. So if it is wind and water tight it shouldn't need that much ventilation. But as said ventilation is free.

I just bought the same Meaco dd8l junior everyone likes in December. But so far it managed about 2 days before conking out and just showing an error light, it's been a month now and still no replacement from the manufacturer.

I'm sure it works well enough, but their customer service has been a bit rubbish.

Bought from meacodehumidifiers.co.uk which it turns out isn't actually the same as the manufacturer.

Something that I don’t quite understand with these types of issues is: lots of ventilation is always suggested as a solution (and I’ve no reason to doubt it), but how do you know how much ventilation is too much? I mean, ‘perfect’ ventilation would mean that the air (and temperature) inside a shed/garage would be just the same as outside, right? But outside there’s an awful lot of dew in the mornings.

Am I just dumb? 🙂

Dew/frost forms because the ground loses heat through radiation so the surface gets colder than the air. That doesn't happen in the shed.

And there's no rain. So all the daytime sunshine is warming up the shed, not doing work drying out wet ground.

Ventilation is your friend and it is free.

Dehumidifying an outdoor space is a bit of a waste of time.

You kind of need to be doing one or the other - if you want to use a dehumidifier then the shed needs to be pretty airtight and you'd ideally keep the door shut when you're working in there too.

So all the daytime sunshine is warming up the shed,

Warming the shed doesn't make moisture disappear unless it damp air can also get out.

If the sheds not that well sealed rather than a dehumidifier perhaps and extractor fan with a humidistat. Fit it at the opposite end to any ventilation the shed already has so it draws air through the space

Also worth bearing in mind how a dehumidifier works.

It is basically a refrigerator which attracts moisture by being the coldest thing in the room. Desiccant based units do operate differently but can be overwhelmed by operating outdoors, despite what this says -  https://www.meaco.com/collections/desiccant-dehumidifiers

In most sheds, metal objects (your bikes and power tools) are the coldest things in the room. Hence they attract moisture and eventually rust as a result. Other objects made of more absorbent materials such as fabric or wood tend to go mouldy. Even with a desiccant dehumidifier metal objects will still attract moisture.

A dehumidifier works well indoors where it doesn’t have to compete hard to be the coldest thing in the room. It is far less efficient in an out building because the air itself is cooler and more moisture laden and it is surrounded by stiff competition in the ‘who is the coldest’ rankings. When you introduce you entering the shed every now and again you suddenly bring more moisture and you are hot, so you offer no competition as the coldest thing in the room, but you do significantly add to the moisture level.

In short, your shed is a far from ideal place to run a dehumidifier efficiently. We have a Meco in our utility room that copes admirably and efficiently with taking the moisture out of the air when drying washing, but it is an otherwise warm and well sealed room. Meco are a pretty decent brand, but any dehumidifier requires the right operating environment in which to work efficiently.

By any standard we have just endured the wettest Autumn and early Winter most of us can remember. That certainly hasn’t helped with the moisture levels in most out buildings.

Thanks everyone for the advice.  I'll look to improve the ventilation to start with.