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Posted by Mark 
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05 February, 2018 23:30
Hi folks
I need some advice about dehumidifier
My one freezing on in cold weather around +5 degrees celsius
a bow 5 degrees works perfect
Machine is brand new and everything works how should works
Just wondering maybe i need yous some kind of heater or bulb
or maybe it is som different trick
Thanks Mark
Re: Dehumidifier
06 February, 2018 10:35
Hi Mark,

Compressor de-humidifiers will not work below certain temperatures - this value varies but will be in your documentation - but typically 5 to 8 degrees centigrade is the minimum.
If you do not provide heating as well you need a desiccant type de-humidifier which work at lower temperatures.

Re: Dehumidifier
07 February, 2018 11:25
Hi Mark
I completely agree with Brian, you need to supplement the temperature up a few degrees to get the dehumidifier to work correctly, these machines are designed to work at this slightly higher temp range 5/8 Degrees and up, they are not effective at removing moisture from the air at colder temperatures, as you probably have a domestic style machine and are set to work in that environment, I have 2 machines running on my boat and a heater set on low temp, and it is removing moisture continuously. Your machine frosts up and stays that way, probably as it cant defrost correctly !!!!!!!.

On the safer side of things, perhaps bring it home and check that it functions correctly at a higher room temp, as you dont want it over heating and causing a fire on your boat
Safety First......
Re: Dehumidifier
08 February, 2018 15:15
I got a Celect plug in heating/frost protection thermostat.

Covers -5c to +25C

They are for sale on Amazon, the supplier being Coronado Controls Ltd, about £25.


Plug the thermostat into the power socket, set the temp for 5 or 6 deg, and then into the thermostat I plugged a timer switch to the dehumidifier.

If the temp is below your setting the power does not flow, irrespective of the timer.

If the temp is above your setting the power flows and when the timer trips....Voila, your dehumidifier starts.

Of course you don't have to have a timer switch and can plug the dehumidifier straight into the thermostat.

It has worked like a dream for me for 5 years, all through the harshest of winters. Never had a frozen dehumidifier.

I do not heat my boat in order to lift the internal temperature. That just makes the boat sweat and guarantees condensation.

Re: Dehumidifier
08 February, 2018 18:12
Am I missing something here? Surely these are frost protection thermostats designed to switch on a heater if the temperature falls below a preset value. Therefore power will flow if the temperature is lower than your setting and will switch off if the temperature is higher than that setting...the opposite of what you say.

Re: Dehumidifier
08 February, 2018 19:20
Hi Peter, I have exactly the same setup as David, works a treat.
And yes, you can get the units that either switch on above a temp setting or below a temperature setting. I use the ones that switch on below a setting for the heaters and the ones that switch on above a setting for the dehumidifiers. They don’t work below about 8 degrees anyway.
Les 41M
Re: Dehumidifier
08 February, 2018 22:45
Hi Les/David,

Thanks for that. It's just that, to me, the ones David linked to seemed to be the low/on high /off variety for frost protection. Can't imagine what you would normally use the other type for...unless to control a cooler or air conditioner that would need to be high/on...or a Heath Robinson controller for a dehumidifier. grinning smiley
I don't need one but just out of curiosity where can you get them?

As others have said a desiccant wheel dehumidifier is the way to go. With a built in humidistat it only dehumidifies when it needs to. It also has the advantage of expelling air about 10 deg warmer than ambient which goes a little way towards keeping the unit and the boat frost free. I keep an oil filled radiator on frost setting near the unit just in case. Another good investment is a set of window covers. These prevent radiative cooling on clear nights and lessen the likelihood of condensation forming on the inside of the windows. That and plenty of ventilation has kept our boat damp and mold free for years. Visiting the boat during the winter I always head straight for the loo and the toilet paper test. If it's crisp and dry everything's OK...if it's limp and clammy something has gone wrong and needs investigating.

By the way I've never used a thermostat since one went on fire on me some years ago. Luckily I had just set it up and was still on board at the time. It was the same type as in this YBW post.


The instruction manuals for some appliances, my radiator for example, warn against using external timers and thermostats. The reason given is that a fault in the appliance may activate a safety cut off device which would be reset in the event of a power interruption by the timer. Power could then be restored by the timer allowing the original fault to reoccur...repeating this a few times could be a bad idea according to the manual.


Edited 1 time(s). Last edit at 08/02/2018 22:46 by Peter.
Re: Dehumidifier
09 February, 2018 10:43
Good old Amazon link has changed.

The Cooling thermostat (vs the T30 heating one I linked) comes from the same supplier.

It is available on eBay....seems to be sold out on Amazon.


It is a Celect T38.

Re: Dehumidifier
09 February, 2018 21:08
Ahh...that's cleared the confusion...couldn't figure out how your system would work...thanks for the new link.
Can't imagine there would be much demand for cooling thermostats in our clime.

Re: Dehumidifier
11 February, 2018 19:52
Thats some good information on the Dehumidifiers.
When I changed my boat insurance to Allianz they wanted to know if I ran a dehumidifier on the insured craft. Apparently they are a fire risk and will increase your premium. There is also the possibility that if you dehumidifier sets fire to your boat, and you have not declared it to you insurance company, they may try to wiggle out of paying for your loss.

Robert M
Re: Dehumidifier
12 February, 2018 22:21
I've found these pretty good in the past. Didn't used to be available in ROI but now imported by CH Marine, prob through NI store.


Re: Dehumidifier
15 February, 2018 23:43
Thanks everyone for reply
its very much appreciated

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