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Too cold for dehumidifier


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My dehumidifier has run for years on a low setting keeping the obsy at 60% humidity.  It's so reliable I forget about it most of the time.  Today I got an alert that the humidity was over 70% so went for a look.   Hmmm, a few hours at -6c has proved too much and it's stopped.  That's -6c inside the obsy 🥶 and -8c outside for a while overnight.

I first thought that the outlet hose might have frozen causing it to shutdown but it wasn't that so I've unplugged it for now.  I've popped a heater on and will investigate further once the obsy has warmed up a bit.

 

2021-01-09 Dehumid.JPG

Jan 9 Temp.jpg

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I've got a small tubular greenhouse heater in my obs'y controlled by a thermostat to come on when temperature drops below 5 degrees.

Dave

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Good idea. I can see another trip up that long river place coming on. I was going to ask if anyone had a greenhouse heater in their obsy.

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Is there any point in a dehumidifier if one's obsy isn't hermetically sealed? My RoR has so many gaps I think I would be trying to dehumidify Dorset, and I'm only a mile from the sea.

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I also use a tubular greenhouse heater in my fibre glass observatory in conjunction with a 12 volt 6 " diameter fan. This seems to work as well as the mains operated dehumidifier which I run as well if the conditions are really acute.   Dave

 

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I have a greenhouse tubular heater and dehumidifier in my shed where I keep all my astro and modeling tools and equipment.

The heater is on permanently at the moment and the dehumidifier is on a timer 2hrs on 4 hrs off. The RH is kept in the region of 50-60% the heater keep the inside roughly 8deg above outside. Its also insulated with the foil backed bubble wrap stuff from screwfix 

It's also ventilated.  4 soffit vents in each corner of the floor and 2 small vents in adjacent walls. Always feels fairly warm in there and never feels damp.

I also keep 35mm film cannisters with reusable sachets of dessicant in the focus tubes of my refractors which I change weekly.

Steve

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There was nothing obviously wrong with the dehumidifier so I switched it on about an hour ago. It sounded normal so I was hopeful.  By now the RH should be 60% but its not moved at all. I'll have another look tomorrow in daylight.

I used to let natural ventilation keep the obsy damp free but drawing in fresh air at 90%+ Rh wasn't really achieving anything so I closed as many gaps as possible with brush strip and run the dehumidifier 24/7.  The only time its off is when I'm inside with the roof open.

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I’m not sure that RH actually means too much below 0’C as the water left in the air is from the partial pressure of ice rather than liquid water.  I’m sure the physics graduates will put me straight but I think there is very little water in sub zero air.

I used to run a heater in my obbo because I was worried sub zero temps would damage my dessicant dehumidifier but touch wood things have been fine without it these past couple of winters.

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2 hours ago, DaveS said:

Is there any point in a dehumidifier if one's obsy isn't hermetically sealed? My RoR has so many gaps I think I would be trying to dehumidify Dorset, and I'm only a mile from the sea.

I run one in my glass fibre dome because natural ventilation Does Not Work - the condensation was horrendous.  I did put some P section tape around the flange of the dome and the opening hatch runners to cut down the air ingress.  I have the dehumidifier wired through a humidistat and it holds the obbo nicely between 55 and 65% Rh.  If things are really soggy it might come on three times an hour for around 3-4 mins a time, usually much less often.

The RH readings on the datalogger go a bit haywire when things get below freezing but there is so little physical water in the air that it doesn’t mean anything in practice.

Edited by Hallingskies
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  • 2 weeks later...

I forgot to post the end of this little saga.   After letting the dehumidifier run for several hours it slowly began to bring the RH down but stuck at 80%.  I watched a few youtube videos showing dehumidifier repairs and discovered that they have two drains.  The primary drain for the water from the condenser and another smaller drain from the heating element chamber which lead to the thought that maybe this was blocked.

Having switched off and unplugged it I turned the dehumidifier on its side.   About half a cupfull of water came out so I returned it the right way up.   Shortly after switching on warmer air was being vented and water was appearing in the main drain.   The RH dropped to 65% quickly and then down to the normal 60%.  Success!

I can only assume that when the dehumidifier thawed out a certain amount of water became trapped inside the heating chamber preventing the heater from working.  This created the fault condition.   It's all working now so I'm happy not to have to pay for a repair or get a new one however I might take the back off and see what prevented the water from escaping properly.

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