[Greenbuilding] vent waste heat recovery--in reverse?

[Chris Green]

Alan Abrams wrote:
>    still perserverating about improving energy efficency in my home--a 250
>    unit, 18 story condominium...
>    with maybe 18 soil stack vents, times +/- 160' per stack...or more than one
>    half mile of at least 4" id pipe...thermosiphoning air from the sewer,
>    through tempered space, and out through the roof.  Assuming winter time
>    ground temps (for a relatively shallow sewer) at 45-50d F, is there enough
>    flow, and great enough delta, to try and recover this heat?  if so, how
>    would one do it?  perhaps the outflowing air could assist a heat pump, which
>    could in turn pre-warm water bound for the water heater.
>
>    crazy?
>   

Crazy? No, but if I understand your idea as trying to use the municipal 
sewer system as a kind of annualized geo-solar heat source, it's 
certainly a novel idea.
 
What's the current average airflow through the vent stacks? (current 
airflow...sorry 'bout the accidental pun there)
Can you increase the volume of the airflow by ending the stacks on the 
roof with black metal pipes and focusing solar heat on them to increase 
the air flow, sort of like using a solar chimney set up?
http://en.wikipedia.org/wiki/Solar_chimney
How much can you depressurize the pipes before you start sucking water 
out of the toilets? ( this sometimes happens when it gets especially 
windy where I live. )

On the other end, in the sewer system, where will the make-up air be 
drawn from, and will this drop the ambient temperatures in the sewer 
pipes to a point where stuff could start freezing up in the depth of 
winter?

For that matter, is there anything in the system which could temporarily 
block the flow at some point? That might lead to bigger problems than 
the benefit is worth.

My gut instinct is that it would be worth the effort to model this on a 
computer just to see if, and how, it works. On the surface of it, I 
think the odds are it could work.
Don't have a clue if it's cost-effective, though, since you can only 
pull about 10 degrees max from 45 degree air. Any more than that and I 
think you'd face the risk of frost build-up in the chimney stack...

If it  works as a heating recovery system, it should also be capable of 
working as a cooling system as well.

As I said, a novel idea.

Cheers,

Chris Green.