I'm a rookie at most of the topics on this site, but its helping come up to speed quickly and I thank you all for that! I recently saw a design for an ERV system where they recommend a long (I know, what does long mean)intake buried about 10' below surface, and that the fan be running slowly. The idea was that in the winter, the cold air would have a chance to warm up before reaching the ERV. Also, in the summer the warm air would have a chance to cool down. This seems logical. Is there something I'm mission here or is this a good idea? Has anyone installed a system like this?

How do you keep the crawlers out it?
Does it stick up out of the ground for water and snow?
What do you use to pipe it?

If it is not completely air tight, I wonder if radon can be an issue?

If you try it let me know. It sounds very interesting.

Take a look-
http://na.rehau.com/construction/renewable.energy/ground-air.heat.exchange/ground-air.heat.exchange.shtml

You'll get not only tempered fresh air in the winter but dehumidified fresh air in the summer.

I wonder how much their special anti-microbial silver particle impregnated pipe costs?

I once inspected an older home (but contemporary style) that had a system installed like you are talking about. I referred to them as 'earth tubes'. These were very large (2 ft dia) metal (galvanized?) tubes. However, they only ran a short distance (maybe 20 ft) outside the home just below grade (maybe 3 ft down below the frost line). The tubes terminated to daylight (sloped property) with screens on the ends. Personally, I don't thnik they worked very well, however the concept back in the day probably seemed sound.

I had a neighbor put in some corrugated tubes to try to cool his garage. He had to abandon them a few weeks in to the summer because of the moldy air that was coming out of them. He drilled drain holes for condensate but mold grew on the surface anyway and he was worried about breathing it for hours.

Its one of those ideas that sounds good on first inspection but fails various reality checks.

G.O. Joe

Great link! Thanks for posting. I have an HRV so I went to the link and called the company about their system. They were very helpful and nice. Here are the bullet points I took away from chatting with them:

- pipe length (and cost) varies - the longer the pipe the more heat you exchange with the ground and the greater the cost.
- for my application (3500 sq foot - ICF with an HRV) and requesting high performance - the cost would be about 4000 dollars for the hardware - then you have to add on the install.
- They say they have data showing that payback is similar to that for a solar or wind system
- It is a little vague as to whether the cost gets the tax writeoff. Engineer - what reality checks does it not pass?

I just got back from a business trip. I appreciate all the replies. Air quality is of prime importance to me and the advice about the mold is well taken. I noticed on the link they claim "antimicrobial-protected." I would really have to look into this closely to be convinced.

Would a product like this help with the mold?

http://www.honeywellstore.com/air-quality/uv-air-treatment/RUVLAMP1-C

Why are we concerned about the mold - because some guy buried a home made pipe in his yard and it got moldy?

As long as the pipe stays dry it should be fine - and the REHAU product has two different condensation management systems that go along with it (choose whichever you prefer.)

If the pipe drains well it should stay dry and be fine - I think the cost would be the bigger obstacle.

Please let us know how you can keep a cold pipe dry when pumping warm, moist air through it. You can drain the condensate but the sides will stay wet as long as the pipe temp. is below the dew point.

Before I would consider purchasing this product, I would want to know: How long is the anti-mold coating guaranteed for? Will they pay for labor should it fail or delaminate, or just replacement product? If one piece fails, and they will only replace that one piece, would you want to continue using the system knowing that the rest of the pipe hasn't been replaced?

Reality checks:

1) condensation / mold / anything else that likes a damp dark environment would flourish. The intake is sure to bring in dust with organic / cellulosic content and I can't imagine an anti-mold coating working forever, or even after a good coat of crud has built up on it

2) How much energy exchange does it really buy you?

3) Isn't an HRV / ERV already designed to transfer most of the desired enthalpy from the leaving air to the entering air anyway? If a long intake pipe buried deep adds a bit more to the HRV / ERV efficiency, what will the payback realistically be?

4) Is there data to support a given claim of heat exchange over time? Does it account for soil type and moisture.

I have been thinking about doing something like this when I actually start building my home. I am with engineer 100% on point #1. But not 2 or 3. My work around for moldy/dirty air would be to instead bury HDPE tubing like for a geothermal system, and circulate water or some other fluid through that piping loop. Then have the ground temp fluid exchange it's heat with the freash air supply to the HRV/ERV (either infront of or behind the HRV, ahead of the unit is better in the winter, after the unit is better in the summer). In this way you avoid any possible moisture issues in the air based earth tubes.It come at the cost of an additional electrcal motor running many hrs/day. Adressing points #2 and 3: Depending on what your home setup is, this would either makes sence, or be a waste of time/effort. In my case, I would be building an off-grid solar home in southern Ohio. So i would not/ could not include air conditioning in my appliance list :). Given this, it would make sence that I try to cool the incomming outside air (and de-fumidify it) before it enters the house. This gives me some relativly effective A/C for a very modest ammount of electrical usage. It would also lower the required run time of any heating system in the winter, which would be much more important for a solar home. Now if you already have A/C and are connected to the grid, I could see this being too much effort and $$ for the energy savings. But for an off grid solar home, every watt-hour counts and is worth 10 times that of any grid-tied home.

I am in agreement with a few points brought up:

1) The pipe has to be opened to the air (otherwise you would not be exchanging inside air for outside air) and as such, dirt, dust, bugs, water, etc, will be able to get inside. And since it doesn't get as hot as the furnace flue, I doubt any moisture will be completely dried and I doubt any bugs would be killed. I suppose most of this could be fixed (for the incoming pipe) by changing the look of the pipe above ground...instead of having the pipe stick straight up, put a 90 or 180 degree bend on it with some mesh wire to cover the opening (probably a good idea to use a plastic plumbing elbow with screw type connectors).

2) The theory is sound, it is the same theory as heating/cooling with geothermal, in the sense that you are using the soil as the heat exchanger.

3) mold and mildew also thrive on air, so unlike in a closed loop (read as sealed and air tight) ground loop for a geothermal unit, mold will eventually grow. The anti-microbial coatings only last so long, and don't cover the stuff that falls into the pipes. Think of the coating as teflon in a frying pan...eventually, it will wear off.

But I am kind of curious about the whole HRV/ERV set up.

1) Isn't the whole idea of an HRV so that the outgoing stale air would help warm (or cool, depending on the season) the incoming fresh air? If that's the case, Why wouldn't you set it up so the air return duct of whatever furnace you are using is linked to the HRV/ERV duct going outside (that is before it gets to the furnace), and the HRV/ERV duct incoming from outside would be linked to the air return on the furnace itself? This would allow the fresh air to be heated or cooled in accordance with your thermostat. The system would also not run during those days when you would have the windows open and the furnace/ac off. And when you have the furnace/AC on, you would always have fresh air.

What SolarOH proposes is essentially passive geo

I agree that the mold and humidity issue would be eliminated since air is eliminated from direct contact with dirt and a cool pipe wall.

Looking at the rest of is on the back of an envelope:

 Suppose in southern OH deep ground temp is upper 50s, maybe 60. At 6-8' depth there is seasonal ground temp variation, perhaps of 5-10 degrees. Add to that the differential needed to make a heat exchanger work and the outcome is loop fluid of maybe 65-70 in summer and 40-45 in winter. The transfer to air adds another 5-10 degrees in the unwanted direction.

While some heating and cooling of outside air is possible with these temperatures, the incremental benefit over a conventional ERV is likely small. There is considerable first cost (buying and burying tube, buying pump and airside heat exchanger, controls, circuits, etc)

Then there is operating cost of the pump and some blower power by the ERV to overcome the added arcoil. As has been noted, power in an off-grid house is particularly dear.

It is difficult for me to envision this being usefully effective, but I welcome thoughts to the contrary by anyone with a bigger or better back-of-envelope

I decided to do a little research into this as engineer brought up some very valid arguments. The following is what i determined after some very brief work.

Attached is a psychrometric chart, commonly used in air conditioning design. By useing this graphical approach one can estimate energy needs and performance to within 10% with little effort.

In the upper right I have shown what I believe would be the case if this "passive geo" system were implemented in a typical ohio summer day. Assuming I'm off grid with no other means of air conditioning with an indoor air temp of 75 F. The blue line shows what the ERV might be capable of doing to the incomming summer air, notice that it is unable to de-humidify the air, but it does cool it do within 2ish degrees of the ambient air it would replace. After the ERV it encounters the passive geo cooler (red lines), which would be a duct mounted water to air heat exchanger. I made some general assumtions as to the delta T's we would see.

I calculated a 6.5 BTU cooling load for every pound of air that passes through the exchanger. After several rounds of conversion factors, and running with the assumtion I am circulating around 60CFM with my ERV (assumtions abound, I know) I calculated a 1800 BTU/hr cooling ability. It may not seam like much but in an off grid home, it's better than nothing, at least it lowers the indoor relative humidity which helps a great deal with comfort.

I also looked into how much power i would need to run this passive geo system to attain that type of cooling. I, VERY roughly, determined that I could use a Taco circulator that uses approx 80 watts to circulate enough fluid with the right Delta T's to achieve my goal, that added to an assumed 100 watts tio run the ERV equals 180 watts. 1800 BTU/hr equates to roughly 500-550 watts of cooling, so this means I might get a C.O.P. of 3 for this setup. I don't think that's too bad at all.

An interesting though too is that the ERV is a part of the home no matter wahat. So if we ignor the ERV power needs, we see that the only power input for cooling is 80 watts for a 550 watt cooling ability, that's a C.O.P. of up to 6.8! It all depends on what you consider to be the input.

The lower left of the chart shows two options, the upper line is 100% ERV performanc, the lower is the passive geo pre-heating the ERV input, it shows very little effectiveness, so I would imagine this concept is only energy saving in the cooling season.

Like I said, I did this pretty quickly and haven't triple checked my numbers. I'm open to opinions on my approach.

A couple of years later, to revisit this topic: I've been thinking about this again. Concerning ERV/HRV's, I don't know what their incoming temp would be AFTER the recovery from the outgoing air, but I can only assume that the colder the outside air, the lower the air temperature after the energy recovery. The system as I imagine it would be the following: An HRV/ERV or just a mechanical ventilation would bring outside fresh air into the house. The incoming air would go directly to a ground loop (back outside) of air sealed PEX piping. The air would then be warmed by the earth, same as a geothermal heating unit. After the ground loop, the air would then go to the air return on the furnace. So, all the air that is heated or cooled and taken to the various rooms of the house via hvac systems will be fresh air. For a very tight building envelope, this would allow the air changes per hour the house requires without a separate system that runs independently of the hvac system. It also would minimize the energy needed from the hvac system to heat/cool the air to the comfort setting. I imagine it would be minimal increase in installation cost if installing at the same time as the geothermal system. Any thoughts?

The incoming air would go directly to a ground loop (back outside) of air sealed PEX piping.

I'm thinking that the fans needed to move all this air through the "air sealed" tubes would take away from the incremental energy saved. Even though the in ground tubing was perfectly air sealed we are still talking about air being passed through it from the outside. I believe eventually mold will grow inside that tubing.

When I first began this thread, wow almost three years ago, I had not yet purchased a lot for my planned retirement home. Well, I now have a lot in Georgia, where it is nice and humid. :-) The mold issues concerning this type system have caused me to drop the idea. Like someone mentioned earlier, you really need to give these types of ideas a good reality check.