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Community and Q&A

Fans for Earth Tubes

TruNorth | Posted in Green Building Techniques on

We are building a 24′ x 60′ barn to house a breeding flock of chickens.  We would like to keep the interior between 5 – 25C and have installed 6 x 100′ loops of 4″ tubing at a depth of 8′ to circulate air for winter warming and summer cooling.  The problem is finding the right fans to blow air through the tubes.  I’m assuming we will need input and output fans.  We would like this building to be off grid, so I would like fans that could be run with DC from solar panels… but I’m not finding any such thing.  How do people push/pull the air through earth tubes?  We are in climate zone 6a.

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  1. Expert Member
    Akos | | #1

    Each 4" tube (assuming 4" ID) is good for around 60CFM of air flow. You can push it to 100CFM but your blower power use goes up a fair bit.

    At 60CFM, each tube is about 0.25" w.c. plus say another 0.1" of a decent filter. This would put you into the realm of blowers only as most fans can't handle that much pressure. The simplest would be a pair of EC bathroom fans but these would run at 120V.

    DC options are limited to marine or automotive, you can get pretty decent blowers made for cabin air circulation (Spal) or marine bilge (Shurflow) that would be up to the job. You'll have to do a bit of digging to get specs, I would also try to find a BLDC version.

    You only need blowers in one location, either at the inlet or the exhaust works fine.

    If your soil temperature is around 50F (10C) at that depth, 240CFM would provide 2500BTU of heat into a 5C room, so not much heating. In zone 6 your soil temps are probably colder than that at 8'. The setup would fair better for cooling, just have to make sure you manage any condensation in the pipes.

    1. TruNorth | | #3

      Hi, Akos. Thanks for your reply and information. I have found some 4" fans (Spal 30103018) that are rated at 147 cfm; 12 v of course but I can't find out what wattage they draw, and I don't know if they actually perform at 147 cfm. The tubes (100' long at depth with two 90 degree corners) are recirculating air from inside the barn, not conditioning air from outside, so would the BTU gain be better in each trip through the Earth Tubes? Our temp at 8' is confirmed at 52F. My concern is to prevent freezing the chickens' water system so on a worst case winter's night of -22F outside, could the 6 tubes keep the inside temp above freezing? What size solar panel and battery would be needed to run this system? BTW I have been told that chickens generate as much heat as a 10w solar panel, and there would be 250 chickens inside the barn.

  2. Expert Member
    BILL WICHERS | | #2

    To move 60 CFM at ~0.35 inWC (Akos' specs for that 4" tube), you have to venture outside the building science world and into the technical equipment world. Such fans ARE available. They're not even all that expensive, and they come in multiple DC voltages too! The downside is they are LOUD. I call them "little blaster fans". I found one that meets your specs, and it's listed as around 59 dBA. That's loud for a fan, but if you're used to fans used with livestock it won't be a big deal.

    Here is a link to a suitable fan:

    It's fan curve shows about 60CFM at around 0.6 inWC or so. There is an "M" variant of that fan that should move about 60CFM at around 0.4 inWC too, and it's a little quieter and uses a little less power, but it is pretty much maxxed out in your application so no wiggle room if the backpressure is a bit higher than expected. Since I don't know what DC bus voltage you're using, I just picked 24v as a mid-range. The fans are available in 12, 24, and 48v options. I would recommend using the highest DC bus voltage you can for maximum efficiency in an off-grid setup. The "M" variant of the fan is about 14 watts, the "H" version at that link is about 20 watts.

    The letter in those part numbers just denotes the speed and airflow at pressure. The datasheets have all the info you need, and it's a standard 92mm square fan so the round part will be a bit under 4" allowing it to be easily adapted to fit the end of the pipe. I'd probably use a piece of rigid PVC to make an adapter plate, but some plywood would work too. The parametric search tool at that supplier (one of my favorite electronic component distributors, BTW), has lots of options to help you find the perfect fan for your application. Just remember that the specified pressures are a max, not the pressure the fan can deliver rated CFM at. You need to look at the fan curves in the data sheets to be sure what any particular fan can do in your specific application, but that site links to the data sheets for every fan they have so it's easy to do, and all in one place.

    BTW, that fan at the link is a little under $17 in quantity one, and they list 82 in stock. If you go with a different fan, I recommend you specify a ball bearing fan for long life since you're going to be running it pretty much continuously in an application that needs to be reliable. Those fans all have rated lifetimes specified too if you want to compare on that basis.


  3. TruNorth | | #4

    Thanks, Bill. Very useful! I can't find the fan curves on Mechatronics... actually, I'm not sure how to navigate the website. I suspect that 'parametric' means something different to engineers than it does to this retired statistician! Can you tell me what would be the difference in solar panel specs to run the fan you sent a link to vs. a Spal 30103018 (which is 12 v and 3x the price). You are right about the noise not being a problem - white noise is actually soothing to chickens.

  4. Expert Member
    BILL WICHERS | | #5

    Here is a link to the datasheet for the fan at that link:
    It's just a single page, and the fan curves are in the upper right corner.

    The higher power "H" version at the link uses 19.2 watts, 1.6 amps at 12v. The "M" version uses 13.92 watts, which is 1.16 amps at 12v. The SPAL fan you mentioned appears to use 24 watts, so more power. I was not able to find a proper datasheet for that SPAL fan, and I suspect it can't handle the backpressure in your earth tube application. I think you're better off with the cheaper fan here, which likely also has better specs. Sometimes things do work out in our favor :-)

    We engineers (I'm an EE :-) do sometimes steal the cherished terminology of other professions and twist it around to suit our own purposes... In this case, "parametric" means we can enter multiple, independent parameters to narrow the search down to more and more specific product offerings. The way I usually use that site is to type in a generic term in the "search" box, like "fan" or "capacitor". It tends to know engineering shorthand for a lot of things too. The search results will take to you a pretty broad component list by category, and from there you can get into the parametric search to narrow stuff down by whatever you're looking for.

    The reason I like that particular supplier is because their parametric search tool is very good. The other supplier like them is Mouser down in Texas, and I don't like their search tool as much so I don't use them as often. Both are good vendors though, and sometimes one or the other has slightly better pricing. They also have a few things that one carries and the other doesn't (Mouser has a much better selection of ferrite cores, for example).

    BTW, is there any particular reason you are using 12 volts here? Going up to 24v has a lot of advantages, and 48v is even better if you're planning on a large off-grid system. A lot of it comes down to how big a system (how much load) you're planning, but higher voltages generally have lower losses and better battery performance too.


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