I’m not sure of the benefits over just reading the external temperature and humidity locally – assuming there is some lag between inside and outside levels?

My larger machines are in a very damp, cold outside garage currently but I’m looking to improve this very unsatisfactory situation with a new (insulated) shed, as soon as I can level it’s intended site – which will involve hiring a digger & skip etc.

I suspect a custom system could be more finely tuned (and therefore cheaper to run) than the in-built humidity control though. I could do this very simply (taking both internal & external measurements) using one of my PicoMites. It has an inbuilt ‘Humid’ command that supports the DS18B20 temperature sensor and DHT22 (or RHT03 or AM2302) temperature/humidity sensor directly. A few lines code (and a relay switch) sounds simple enough.

Your scheme does sound technically impressive but a bit complicated. Do you have any data on what your new scheme is saving you over just setting the dehumidifier?

Regards,

IanT

Like Graham, I have used dehumidifiers (they don’t last forever) for many years now.  They’re not a panacea for rust prevention, but certainly go a long way towards helping.  The first couple were refrigerant ones, and they worked well except when the temperature got too low.  The advent of desiccant dehumidifiers has been great, as they work at lower temperatures.

My desiccant dehumidifier is permanently connected to a drain (protected from frost), and works all year round.  The drain can occasionally get blocked (I don’t know what by), but is easily blown clear with an airline.  In the event of a blockage, the dehumidifier just reverts to the built in reservoir and throws up a full indicator lamp.

One lesson I have learnt the hard way however, is to regularly clean the air inlet filter screen.

Unless the machines are covered you need to measure the temperature of the main mass of the machine(s) and the dewpoint of the air in the garage to ensure no condensation on the machines.  Probably more cost effective to measure relative humidity and air temperature and calculate dew point.
Using outside dewpoint will bear no relation to what is happening in the garage.

Robert.

If you want a rust free workshop then you have pay up. 28 or more years ago I bought a preowned dehumidifier and the only time is OFF is in the height of summer. In my home made shed/workshop, the tools and machinery is rust free, and its comfortable to work in. Before buying a dehumidifier I made one, following an article in Model Engineer, using the ice box from a fridge. The home made dehumidifier worked well but was bulky, unlike the compact manufactured one. By the way the water from a dehumidifier is good for use in the smoothing iron and cleaning windows.   John

Elec rates vary between rural and town and cheap overnight rates.
One thing to avoid is the ‘solid state’ versions. Best I can determine from online sources is that they have a COP of only a few % nowhere near efficiency of a refrigerant one and tend to quickly corrode themselves to death with the condensate.
I have considered that if I had a solar panel on the shed it might be worth using a peltier as a lathe  heater not as a dehumidifier just because it would add a little bit extra to meagre sun production.

On 15 March 2026 at 12:51 Robert Atkinson 2 Said:

Unless the machines are covered you need to measure the temperature of the main mass of the machine(s) and the dewpoint of the air in the garage to ensure no condensation on the machines.  Probably more cost effective to measure relative humidity and air temperature and calculate dew point.
Using outside dewpoint will bear no relation to what is happening in the garage.

Robert.

A useful reference chart relating temperature to relative humidity and consequent dew point.

https://www.typecalendar.com/dew-point-chart.html?_gallery=gg-890-39996

On 15 March 2026 at 20:57 Bazyle Said:

Elec rates vary between rural and town and cheap overnight rates.
One thing to avoid is the ‘solid state’ versions. Best I can determine from online sources is that they have a COP of only a few % nowhere near efficiency of a refrigerant one and tend to quickly corrode themselves to death with the condensate.
I have considered that if I had a solar panel on the shed it might be worth using a peltier as a lathe  heater not as a dehumidifier just because it would add a little bit extra to meagre sun production.

The solid state de-humidifiers are Peltier based. Their poor efficency does mean they add heat to the environment. Using as a machine warmer and de-humidifier is valid. You do need a sealed type to prevent failure through condensation driven corrosion. Thermal cycling also shortens their life. Running with proportional control (varying the voltage to control the temperature) gives a longer life than on-off control.

Robert.

On 15 March 2026 at 20:20 beeza650 Said:

On 15 March 2026 at 19:48 not done it yet Said:

My dehumidifier both reduces the humidity and raises the temperature a tad.  It is costing me just over 3p per night, currently.

It has not cost me more than the price of 1.5 units of leccy per night (in the depths of a cold spell) for the last several years.

The aggravation of automating it further, is just not worth my effort.  I can afford the 12p/night, to protect my machines, tools and metals from condensation over the worst few days of winter – and, as I wrote above, only about 20-25p/week at this time of the year.🙂

What wattage is that then? Let’s say it’s 600W and the tariff is 27.5p per KWH (mine is) that’s 11mins run time for 3p. What humidity do you have it set to and how airtight is your space?

“aggravation” hang on sec – the whole reason you are on this forum is because you enjoy doing things that don’t HAVE to be done. I’m personally super impressed at what you can do with Gen AI and free APIs/hosting etc. It was a really interesting journey getting there.

Wattage:  ~375W at lower setting, ~750W on the highest setting.  So let’s neither assume 600W, nor 27.5p/kWh.  My current night-time rate ‘(between 00:30h and 05:30h) is 8.5p/kWh.  I don’t set it to a humidity level, although they can be.  My space is draught proofed.

It would be aggravation for me – maybe not for you – so I have no intention of running with more than my simple timers.

It may be the only reason why you are on this forum.  That is not the case for me!  I can actually get by without the help and assistance of AI, thanks.  I have managed without AI for the past 7 decades!

On 15 March 2026 at 21:52 Macolm Said:

A “refrigerator” type dehumidifier works by producing a temperature difference between evaporator and condenser, roughly balanced around ambient temperature. As the air then moves right through the process, the only change in its temperature is a small increase due to the electrical power consumed.

A Peltier device similarly separates into hot and cold, but to heat up something it would need a second thermal mass from which to extract the heat, and which would become cold in the process. This could certainly be configured to heat the object to be protected, and also to extract water at the cold surface, but would need a somewhat complex design including a closed space for it to be effective. It might be well worth doing to exploit free electricity.

A) All dehumidifers extract the ‘heat of evaporation’ from the water they drip into a bucket so produce a bit more heat energy than they consume, but nothing like the ‘extra’ you get from a heat pump which is moving heat rather than extracting it.

B) I don’t think you can just plug in a peltier chip alone and expect magic heating results. It would require a fan circulating outside air or perhaps water from the shed waterbut against one side as a heat source and another system on the other side to extract it, all speed controlled depending on temperatures and power in the device. Not a trivial project but perhaps a fun one.