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Old 05-11-13, 09:31 PM   #411
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Originally Posted by JRMichler View Post
Ventilation will keep indoor humidity down if the outdoor dew point temperature is at least 20 deg F lower than the inside temperature. For example, the outdoor dewpoint right now is 58 deg F at the Oakland airport. If the indoor temperature is below 78 deg F, the house will be humid inside. If the house is poorly ventilated, it will be even more humid inside.

If the outdoor dewpoint reaches 70 deg F, the house will be a swamp inside. The solution is a dehumidifier. No need for ductwork, just put it anywhere in the house and set it for about 50% RH.
I know nothing about dew point and humidity, but I have read that HVRs can make a damp interior drier. Obviously I have to educate myself about this topic. So, why would my house be drier, while the cottage is damp, since they both experience the same climate? Doesn't infiltration have something to do with that? Right now, the dewpoint in Berkeley is 56F and it's 68F in my house, but I don't think it's really humid in here. This is all very interesting!

I think the problem with dehumidifiers is that they use a lot of energy, like hundreds of watts.

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Old 05-12-13, 03:44 AM   #412
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Dew points getting above about 55 degrees is where things get a little too moist inside. 55 degrees is the point where if my house is at that point and the air conditioner hasn't run in the past 24 hours, I will have it run for 2 hours. I kept my house hot last summer and stayed in the lower levels so the indoor dewpoint and outdoor dew point got a chance to get above 55 degrees. We had dew points well over 70 degrees outside so it was very necessary to run the A/C on a daily basis around the June, July, August timeframe especially. Running the A/C for two hours brought the house down to a dew point under 50 degrees and it was comfortable at about that point. I basically set my thermostat for 2 of the coolest night hours as long as they were going to be above 60 degrees outside and that is the most efficient temperature that my A/C would run without any risk of freezing the coil and lines back to the condenser.
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Old 05-12-13, 12:46 PM   #413
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I know nothing about dew point and humidity...

You might want to get one or two of these things.

I have one and it's been an amazing education. The thermometer is very accurate, as is the humidity sensor, and the instrument calculates the dew point, AND if your dew point coincides with temperatures that are conducive to mold growth, it will flash (and beep if you so desire).

-AC
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Old 05-13-13, 12:00 AM   #414
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We have a very different climate here in Berkeley, no need for air conditioning at all. I think that must mean that our dew point is usually below 60F, but I've only just started paying attention to that number. Right now, 10 PM, the dew point is 50F and the temperature is 59F. Most likely, the outdoor air will reach the dew point before the Sun comes up tomorrow. In my house, the temperature is 69F, but I don't know what the %RH is. Would be interesting to compare %RH in my house and in the cottage. Here's a question: are uninsulated concrete block structures damp because the cold walls and floor tend to condense moisture on their surfaces?
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Old 05-13-13, 07:19 PM   #415
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If a surface, any surface, has a temperature below the dew point of the air around that surface, there will be condensation on that surface. It does not matter if the surface is concrete, wood, or a cold bottle of beer.

Last edited by JRMichler; 05-13-13 at 07:22 PM..
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Old 05-15-13, 02:10 PM   #416
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Yes, that looks like a really great device. Thanks!
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Old 05-17-13, 06:36 PM   #417
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Found some interesting products from the U.K. The first two use the spiral/tubular heat exchanger that was mentioned in this thread at one point. The last one is really interesting - a passive HRV, apparently just going on the market in the U.K.

Retrovent by Envirovent

HeatSava by Envirovent

http://www.envirovent.com/downloads/...a_brochure.pdf

Ventive Passive HRV System
| Sustainability Workshop
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Old 05-17-13, 06:46 PM   #418
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I've decided to try building a heat exchanger core with the simple cross flow tube design. Below is a link to CAD model of the concept. I'm going to use polypropylene drinking straws for the tubes. They have a wall thickness of only .0055 inches (0.14mm). I'm still not sure how closely I will space the straws on my first attempt, but either way, it will be an experiment. I think I'm going to use this fan, which I found on Amazon Coolerguys 120x25mm Blower Fan 12v with 3pin Connector. It delivers 17 CFM and the specs say that it's very quiet (<20 dB)

Flickr: Drinking Straw HX
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Old 06-30-13, 01:19 PM   #419
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Hi All,

A while back I discovered that the HX-core in my HRV is not as efficient as I expected it to be and I found the cause. It comes down to the choice of material.

The plates are made of aluminium. Even worse, the plates are made of relatively thick aluminium, 1mm (1/25"). They conduct heat very well, which is a good thing when you want to transfer heat from one surface to the other surface. But heat will also spread out from any point on the plate throughout the plate in all directions.
That is a good thing if you are building a heatsink to cool a CPU and you want to transport the heat away.
In a heatexchanger you want to transport the heat on any given location only to the other side of the surface on that spot, you do not want to transfer the heat over the surface of the plate to another location.

Aluminium HX-cores are still a good idea if you can:
- make the material thin enough
- make the air move fast enough, say 5m/S
but now you need to:
- overcome pressure loss by using more powerfull fans
- higher airspeeds produce more noise, either live with that or deal with it in another way.

I have been playing with the calculator that can be downloaded from www.heatex.com and their HX-cores do the same, at lower airspeeds they are less efficient, reaching a maximum at a certain point and then declining again.

So what I would like is a heatexchanger build from some sort of plastic, but I find that hard to find in my neighbourhood, or else it is as expensive as aluminium.

And then I found the solution, see next post : )
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Old 06-30-13, 01:40 PM   #420
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...continued

Hi All,

I found a dutch company, Recair, that produces recuperators out of polystyrene.
Their heatexchangers are very efficient and of course they are counterflow. The internal design looks very good with triangular ducts, see this snippet taken from their productfolder.



Their website, www.recair.nl, is available in multiple languages. It also has a nice online calculator to play with.
They do not sell their products to individuals but at this website, warmtewisselaarwebshop.nl, I can order them in the Netherlands, no idea about shipping to other countries. These recuperators are very reasonably priced!

So this summer, after some other projects around the house, I will be rebuilding my HRV with one of these heatexchangers inside. I'll probably use the RS160 with a height of 250 or 300mm.

Fornax


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