The garage is unheated and damp, and I've noticed mildew on things.
I have left the doors and windows open for a couple of days and that seems to have helped, but I'd like to do a bit better.
There's a chest freezer in there, that collects a lot of condensation on the sides and lid.
I don't want to buy or run a dehumidifier unless absolutely necessary, but perhaps the warmer (therefore damper) air around the freezer could be ducted outside with the help of an extractor fan - it seems a shame to waste the heating and damp-collecting properties of the freezer.
If it's anything like my garage, with gaps around the door, then trying to keep the humidity lower than outside ambient is at best going to be expensive (using heating or a dehumidifier, for example), at worst impossible. Given the appalling weather we've had these last few weeks, with heavy rain almost everywhere, the ambient humidity outside will be close to 100% most of the time. I can't see that opening the doors and windows would have made much difference.
If the freezer is collecting a lot of condensation on the outside, the insulation is shot and it's wasting electricity in its efforts to keep the contents frozen. When was it last de-frosted, not that that is a sure cure, but it's a start. Nor will it help with the overall dampness in the garage.
If the air around the freezer is warmer (due to it's flogging hard to keep the contents cold), the air will be marginally drier there, not damper as you suggest. The actual moisture content in the air (in terms of grams of water per cubic metre of air, or whatever) will be pretty uniform throughout the garage, it's just that warmer air is capable of holding more moisture, so the relative humidity will be lower near the freezer.
An extractor fan will merely replace the inside air with cold damp air from outside, drawn in around the door as the fan exhausts through the window or wherever.
If it were mine, I'd do my best to seal gaps around the garage doors; I'd defrost and even replace the freezer (purely to save electricity), and perhaps install some low-wattage tubular heaters or even a dehumidifier as you suggest. But the latter will need to be emptied frequently, probably daily.
What is it in the garage that's going to suffer from the dampness anyway? If it's tools or whatever that may rust, consider moving them into the house, into a spare room perhaps, or spray them with a light oil such as 3-in-1 or the dreaded WD40.
Car wax the outside of the freezer so it doesn't rust. Collect the drips from it in a bucket and dispose of it outside. You have a dehumidifier (just not a very good one).
There have been a lot of days in December where the outside air has been saturated. You can't get rid of damp by ventilation alone when this is the case. You need some heat from somewhere. A sunny window, the freezer motor, a fan or a small heater/dehumidifier.
I have been toying with the idea of using an air source heat pump to control the air temperature, target 10deg.C, in an insulated garage to allow use as a utility area. So far I have not progressed beyond a few lazy brain cells:-)
I run a dehumidifier in my shed for a couple of days every fortnight or so in winter - pulls out about 14l of water and seems to keep the condensation at bay (theres a lot of exposed wood so I aim to dry that and let the buffer new damp).
If the freezer has an anti-condensation case heater, you should switch it on. However, those often operate only around the door seal, and that's to prevent condensation there freezing and sticking the seal to the case.
They don't work at outdoor winter temperatures anyway.
That's a misunderstanding of the problem. The condensation forms on the freezer where the cold is leaking out from the inside (at least, that's the easiest way to think of it;-). The heat from the freezer is actually very welcome, and the heated air will have a lower relative humidity, not higher (unless the freezer is frost-free).
A friend has a large garden shed which they use for storage. Problem was that many of the items in there would be damaged by damp, so I designed a system in there to maintain the humidity at no more than
80%. This has been running for 15 months now, and it has worked perfectly - there's been no damp in the shed over this period - no new metalwork in there has gone rusty (which it used to before), and there's no smell of damp timber or fabric. Whilst the outside humidity often reaches 100%, the humidity in the shed has never exceeded 81%.
The system uses a Raspberry Pi to continually monitor the humidity, temperature and dewpoint inside the shed (and also outdoors for comparison). It switches on a heater in the shed if the RH exceeds
80%. This has completely prevented any condensation forming over the 15 month period. It has cost £55 in electricity over the 15 months (mostly in the winter), which is much less than the £200/month they used to pay in storage rental space.
At some point when I have time, I will write up the system in a blog, but basically, you need a low power heater (depending on thermal properties of the garage) to kick in when relative humidity exceeds 80%. One slight complication here is that most humidity sensors become wildly inaccurate outside the 30-70%RH range. I use SHT75 sensors driven by the Raspberry Pi directly, which are accurate from 0-100%RH.
If the air circulation and temperature uniformity are good inside your garage, you might even get away with a 90%RH setpoint, but I didn't have time to try that before the items arrived for storage. (I did start with a 70%RH setpoint initially, and bumped it up.)
Its science though is it not? Warm air holds more water vapour than cold air, so one assumes the hot air around the exchanger in the freezer can mop up the damp and then when it moves onto cold surfaces, it gives up its water. there is a reason why air conditioning units heat exchangers are normally outside!
There's a number of systems that reduce humidity by monitoring the absolute humidity inside and outside, and turn on a ventilator when it's drier outside than in... Less useful in a shed than in a cellar, where the temperature can differ greatly from the outside temperature.
Most of the systems are homebrew, Raspi, Arduino, or such; the commercial systems tend to be expensive and limited.
There are a few occasions when the absolute humidity (dew point) in the shed is higher than outside. This tends to be in spring and autumn around midday, and I have thought about adding a ventilator to kick in when this happens. But it's not often enough to be worth rushing to do it, so I haven't done it yet.
However, there are days on end when the RH outside is 100%, and even periods when it's super-saturated (over 100% and moisture condensing out on everything, dripping from trees, etc). Ventilation won't help at all in these cases, and this is what you really need to protect the shed contents from.
Yes -- much different if there are thick walls, buried. Apparently one can dry a cellar well just with ventilation.
And I saw a website with a diy heat exchanger that ventilated and recouped the heat from the exhaust. The heat exchanger was either very many soft drink straws, or polycarbonate plates. And the heat exchanger tan into problems with condensate freezing in sub-zero weather.
I have bag of (I think) zeolith, absorbs moisture and releases it again when heated in an oven. It keeps the contents of one plastic box dry, if reasonably well sealed. The bag absorbs about 200 grams of water.
I read somewhere that you need to keep the RH below 60% to stop rust on steel. Many new box girder bridges use dehumidifiers inside the boxes rather than paint as its cheaper and lighter.
At the start of this project, I didn't know what the value was, or if it was simply that the atmosphere must be non-condensing (i.e. less than 100% at metal surface). Also, I was suspecting a different (lower humidity) requirement for timber and furnishings.
Based on what I've learned in doing this, I suspect the requirement for steel is that the atmosphere must be non-condensing. I have not measured the moisture content of the timber in there - I keep forgetting to do this when I'm visiting. However, there is no musty/damp smell which you often get in damp sheds/cellars/garages, so I think the 80%RH limit seems to be working for the timber and furnishings. What I don't know is if 90%RH would also work (it would reduce the heating bill, but I can't experiment with the furnishings which are now stored in there).
I have very accurate control on the humidity limit and the shed is airy inside, with nothing stored up hard against the walls or floor. If you don't have both of these, then the temperature (and consequently the RH) will change in different places in the shed. Then it would be necessary to lower the limit on the measured RH to try to ensure that pockets where it is higher are still low enough to prevent dew point condensation.
There are cheaper humidity sensors, but as I said, I needed some that were accurate up to 100%RH. There is a cheaper version - SHT72, which I think also works up to 100%RH but is not so accurate across the whole range.
BTW, I think there's a Pi driver for these now, although there wasn't when I did this so I had to write my own. They are similar to I2C, but without any device addressing, so they need a dedicated bus for each one. Timing is not critical (all I/O is clocked), so you can bit-bang it from user-space, which is what I do. Connections can be made directly to the Pi's GPIO pins without any interface buffers/logic, although you can't drive more than a few feet of cable.
Warmer air can hold more moisture than colder air. Passing it over the condensed moisture on the surface of the freezer, and then out of the garage, would seem like a neat trick if it could be achieved.
I don't mind if that makes the garage colder... unfortunately, the thing that mostly warms it up is people running around playing ping-pong in it, which visibly contributes to the humidity.
I'd definitely be interested in giving that a go if you can provide mor information. I have a Pi Zero begging to be used for something like that.
Yes, so heating the air with a fixed absolute humidity results in a reduction of relative humidity.
The moisture on the surface is condensing out because the freezer case is colder than the air, and below the dew point. If you collected the condensate and piped it out of the garage, that's actually a mini dehumidifier. Although parts of the freezer case are colder than the air, the freezer outputs more heat into the garage than the cold parts of it remove. Its net heat output to the garage is its power consumption plus the latent heat of condensation that forms on it and is removed from the garage in liquid form without evaporating.
The heat from the freezer is a bonus because it will be raising the temperature of the garage (slightly), and when air comes in from outside, its relative humidy will be reduced when it's heated. If the air and garage were uniformly heated, it would make condensation in the garage impossible. Unfortunately, the heat output from the freezer is unlikely to be enough and the temperature rise won't be uniform.
Unfortunately, it's a piece of software I wrote 17 years ago and have hacked about ever since, and it's not in any state to be opensourced. Maybe when I retire, I'll rewrite it designed for Raspberry Pi, rather than as the current set of emulators of other products I used to use years ago (and still do in a couple of installations which well predate the Pi). I am surprised something like this isn't readily available already.
However, for just this application, a shell script loop reading the humidity and deciding to switch a heater on or off is all that's required (providing there is now a Pi driver for a suitable humidity sensor).
You might be right. However, these use more power (for the same level of dehumidifying), and I suspect a heater of the same power rating would be just as effective.
The heater I use in the shed is a 1kW oil-filled radiator, but with a series diode to limit it to 500W. When the system is running, the heating duty-cycle is so low that when you feel the radiator, you would not even think it has been on. The power output required to drop the RH from 100% to 80% at low temperatures simply by raising the temperature is remarkably low.
I had not expected this initially and using a heater was going to be a stop-gap before trying a dehumidifier, but when I found how little heat was required, I dismissed using a dehumidifer in the shed.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.