A different use for earth tubes or pipes
I have been intrigued for a while by the potential of Earth Tubes (most significantly discussed here at: https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/my-earth-tube-story’rel=”My Earth Tube Story”‘. I have a project coming up that will involve regrading, earth fill, and irrigation installation on a sloped half acre, so I may have the opportunity to bury some pipe (4-6ft deep, climate zone on the line of 5/6), if it seems worthwhile.
It seems the biggest negative for earth tubes is controlling air quality — which is assuming you want to pull that air into the house. What about NOT pulling it into the house?
1. Would there be any value to having cheap earth tubes open at an outdoor condenser unit, to provide a supply of more conditioned air for the heat pump to transfer heat to and from, mitigating hot and cold days?
2. Failing that, could sufficient air be supplied to provide a cool breeze to a patio or deck?
1 and 2 require calculating the amount of air flow that could be drawn through the pipe, and then what the blower could move (or the condenser fan on the heat pump–definitely would not want to stress that fan by choking it!), and of course how much of a difference the heat transfer would make. I’m not sure what variables I need to calculate all that. Volume of the pipe is easy, but heat transfer and how much air supply would be needed for fans to not be stressed, I don’t know.
3. What about using buried water pipes and a hydronic water-air heat exchanger that DOES go inside the house and have air blown across? Possibly matched to a spare water tank (from an old hot water heater) and a recirculating pump on a thermostat?
Could recirculating water through such a system condition enough air to be worth it?
What if the pipe already being buried for an irrigation system could also be used in a loop to recirculate this water (thus improving the economics)?
Yes, for that to work, there would have to be some way to charge the main supply line without activating the sprinkler heads…wondering if that could be done by some kind of pressure regulator valve on the risers, with pressure being low during circulation and high when the sprinkler needs to run.
Any thoughts?
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Replies
I'm dubious on the condenser-air angle. Outdoor units generally
flow a *lot* of air, which would dictate a fairly massive earth-tube
for the needed capacity. Even with that the flow would likely bring the
earth-tube down to ambient-air temp pretty quickly and keep it there.
_H*
Stuart,
Hobbit's analysis is correct about the outdoor condenser. For this type of tube to make a difference, it would have to be very large, very long, and buried very deeply. The value of the collected energy would be much too low to justify the investment.
Buried tubing (usually PEX) designed to convey fluid (usually a glycol solution) is a viable option. This type of buried ground loop is routinely connected to a copper coil in a ventilation system to condition incoming fresh air. Zehnder is one manufacturer of HRVs that offers this type of buried ground loop as an available option. However, many people have done the math and concluded that the cost of the loop is so high, and the value of the collected energy so low, that these buried ground loops have a very long payback.
Thanks Hobbit and Martin.
It's hard to let go of the idea -- ground temps of 55F in winter, 65F in summer seem a terrible thing to waste if you can easily install in disturbed earth. 65F sounds enough to potentially supply all cooling needs, for instance, without a heat pump.
So, rather than using an expensive commercial system like Zehnder, what about using Scenario 3 (pex loops to heat exchanger with blower) to condition air on a sunroom, or even to recirculate hot second story air over it in the summer to cool?
How would I calculate theoretical BTU per Hour?
I can figure volume of water in pipe, get a cfm rating for a fan/blower, and a BTU rating on a hydronic heat exchanger (though I don't know what that rating is based off of and how to modify it for the expected water temperature and how much heat exchange could actually occur at whatever CFM rate and air/water temperature differential).
Do I need anything else to calculate potential BTU/H?
Stuart,
As the season progresses, the temperature of the soil adjacent to these ground loops changes. If you are talking about winter performance, it will be better in November than March.
Similarly, if you are talking about summer performance, it will be better in June than September.
Changing soil temperatures makes these calculations tricky.
Zehnder provides some design guidance for their systems here:
http://zehnderamerica.com/wp-content/uploads/2014/11/ComfoFond-L-2015.03.16.pdf
If you are just interested in pre-heating ventilation air in winter (or pre-cooling ventilation air in summer), you could use the Zehnder guidelines. These guidelines call for PEX diameters of 3/4 inch to 1 1/4 inch, and loop lengths of 300 to 360 feet. Remember, though, that these loops can't possibly provide space heating or cooling -- all they can do is take the edge off a low volume of ventilation air (generally 50 cfm to 150 cfm of air -- not enough for space heating or cooling).
I'm not sure how humid your summers are, but in some cases a nice feature of these systems is their ability to dehumidify incoming ventilation air. 65 F won't dehumidify down to the level I'd want for indoor air. But at the boundary between zone 5 and 6, I'd think your ground would be cooler than that. And even if it is 65, depending on how humid it is, you might still get some dehumidification even if it is at 65 F, which you'd need to supplement with additional dehumidification via A/C or a dehumidifier.
The deeper you bury the pipes, the less seasonal variation you get in ground temperature.
Of course next logical step in taking better advantage of such a system is to connect the fluid-filled tube to a heat pump, and have a ground-coupled heat pump installation, which is generally disliked here because it's expensive. It might not be so bad if you can roll the installation of the tubing into the other earthmoving you are doing.
But back to the direct fluid system without a heat pump, there's another manufacturer who offers components for such a system, more of a roll-your-own than the Zehnder, and their data might also be helpful whether you buy their components or not. But my first attempt to post this trigger the spam filter so I'm trying again without their name. The initials are u and a.
Charlie,
Here is the link: UltimateAire.