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Discussion Starter · #1 · (Edited)
I realize this isn't a conventional HVAC question, but couldn't think of a better section to put it in. Our manufactured home has a small pseudo-attic space with four squat vents thru the roof and no access from the inside. Despite installation of reflective roof panels (as well as the aforementioned vents), this space still traps a lot of heat in the east Texas summer, much of which seeps into the living space below.

Replacing at least one or two of the short and ineffective vents with solar chimneys appeals to me as a relatively simple fix. However, I have no experience with them, merely an understanding of the general principles involved. Would wedging a screened and capped-off length of black stove-pipe thru the vent hole(s) work well enough? If so, how tall should they be for a reasonable amount of draw? Any other design considerations I should be aware of? Also, any "RTFM" replies with relevant resource links/references (books, websites, etc) would be welcome.

Thanks in advance,
Rob
 

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Their effect will be highly dependent on the final temp difference between inside and outside the pipe, as well as the height and width. Wider means higher potential volumes but you'll also have to go higher to get a similar velocity. Generally, the taller, the better. My guess is that you won't need much to have a decent effect. I'd probably pick something that is easy to fit. You'll probably also want a rain cap, to keep inside dry

There's formulas to help you. You can start with the classic chimney formulas that include temperature. (some formulas were simplified by assuming some temperatures depending on application.) I don't have any handy at the moment.

You'll want to paint the pipe with mat black to absorb as much heat as possible. Use single wall pipe, as you want heat transfer into the pipe.

Cheers!
 

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Four roof fence and the single wide it’s not enough vent Consider adding a couple more or turbine vents help to evacuate the space, also is this a new home or old home maybe there’s not much insulation in the attic


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Chimneys, stove pipes and other high vents do not "draw" to create air movement. They are merely a path of less resistance but for the air to move it needs a push, typically the inflow of cooler (heavier) air flowing underneath (called displacement). So, yes that chimney(s) can help exhaust some of that hot air, but you need to provide an intake source to compliment the process.

I suspect your remaining squat vents will become intake vents if there are no other low vents.

What is the current roofing material?

Bud
 

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Discussion Starter · #5 ·
Four roof fence and the single wide it’s not enough vent Consider adding a couple more or turbine vents help to evacuate the space, also is this a new home or old home maybe there’s not much insulation in the attic


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Switching to turbine vents seems like a good idea, too. In fact, 4-5 feet of black stove pipe to make a solar chimney with a black turbine vent on top seems like an even better idea for maximum draft without a blower. What do you guys think?

It's a new home, by the way, and I saw lots of insulation when I popped the vents off to install the new roofing.
 

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Discussion Starter · #6 · (Edited)
Chimneys, stove pipes and other high vents do not "draw" to create air movement. They are merely a path of less resistance but for the air to move it needs a push, typically the inflow of cooler (heavier) air flowing underneath (called displacement). So, yes that chimney(s) can help exhaust some of that hot air, but you need to provide an intake source to compliment the process.

I suspect your remaining squat vents will become intake vents if there are no other low vents.

What is the current roofing material?

Bud
There are small soffit vents spaced at roughly 10 ft intervals below both eaves, so hopefully the intake side of the attic space is adequate.

The roofing is galvanized metal.
 

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Switching to turbine vents seems like a good idea, too. .
They do work great. I just thought you were going for the unconventional, and quiet solution. Turbines can make noise, even expensive ones after a while. Some people don't like it.

Cheers!
 

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There are solar reflective paints that can be used to cover that metal roof, effectively rejecting the solar energy before it enters your attic. You may already have this but wanted to mention.

When a fan is used to exhaust a hot attic they want something like 700 cfm for 1,000 sq ft of attic. I have a blower door fan that can move that much air and I assure you the chimney you are considering will not come close. Wild guess would be less than 50 cfm on the hottest day. In other words it will provide very little benefit if any.

Bud
 

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Discussion Starter · #9 ·
They do work great. I just thought you were going for the unconventional, and quiet solution. Turbines can make noise, even expensive ones after a while. Some people don't like it.

Cheers!
Good point about the potential noise; but I reckon I could replace them with a simple rain cap if they make too much racket. My main short-term goal is to maximize passive airflow (i.e. avoid powered blowers). Depending on how well it works for the pseudo-attic space, my longer-term goals would be to explore a combination of solar chimneys and earth tubes for whole-house passive cooling.
 

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Good point about the potential noise; but I reckon I could replace them with a simple rain cap if they make too much racket. My main short-term goal is to maximize passive airflow (i.e. avoid powered blowers). Depending on how well it works for the pseudo-attic space, my longer-term goals would be to explore a combination of solar chimneys and earth tubes for whole-house passive cooling.
Both passive options work great. Length is your friend. Your neighbours (if any are even close enough to care) might dislike the look. Other then that, the longer the better. For the earth tubes, getting deep enough to avoid the solar heating effects is a must. Longer tubes, will create more resistance. There's formulas for that too.

Cheers!
 

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Discussion Starter · #11 ·
There are solar reflective paints that can be used to cover that metal roof, effectively rejecting the solar energy before it enters your attic. You may already have this but wanted to mention.

When a fan is used to exhaust a hot attic they want something like 700 cfm for 1,000 sq ft of attic. I have a blower door fan that can move that much air and I assure you the chimney you are considering will not come close. Wild guess would be less than 50 cfm on the hottest day. In other words it will provide very little benefit if any.

Bud
I wouldn't mind painting the roof to become more reflective, if it weren't for the fact that we're using the roof for rain catchment (I'd be concerned about components of the paint leeching into the water). But just out of curiosity, would solar reflective paints offer a substantial reflectivity boost over the semi-shiny galvanized metal surface I've already got?
 

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Your galvanized roof should already be reflecting much of the solar energy although that decreases with age. You might also want to test for zinc and if above desirable levels they recommend coating the roof. I'm going by just what I read and have no prior experience other than considering reflectivity.

But if a coating is needed you might benefit in both directions. Here is a reference link.
Roofing Materials for Rainwater Harvesting

If zinc levels are ok I doubt you could justify a coating just to improve reflectivity.

As for the reflectivity of galvanized steel there were a variety of links, just Google: "reflectivity of galvanized steel". They show a new roof being 70% which is rather good.

Bud
 
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