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GBlist: Drainwater heat recovery

Domestic water heating is a significant energy end use in almost all 
housing, but has, I believe, been neglected compared to HVAC and 
envelopes.  Drainwater heat recovery looks like one technology that 
can improve system efficiency.

As mentioned by Craig DeWitt, the GFX (from Vaughan Manufacturing in 
MA), is a very simple system, basically a copper drain line 
(typically 5 feet long) with copper supply tube wrapped around it.  
The supply tube carries cold water on its way to the water heater 
and/or directly to showers.  When there is simultaneous flow of warm 
drain water and supply water (when showering or otherwise by 
luck), the drainwater preheats the cold supply.  

The GFX has proven performance in a lab setting, with an 
effectiveness of about .60 (i.e. captures about 60% of theoretically 
available energy) during simultaneous flow.  It has not been subject 
to any rigorous field trial yet, but I have a proposal out to a 
number of utilities to install and monitor systems in occupied homes 
to obtain this information.  

We at NAHB Research Center, working on behalf of Vaughan, have 
also just completed the evaluation of the GFX required to obtain 
an IAPMO listing, which will help with code acceptance.

The concept of a heat exchanger combined with storage capacity, to 
allow heat recovery when supply flow and drain flow are not 
simultaneous, makes sense from a pure energy perspective.  
However, adding storage raises practical issues such as added cost 
and complexity (added storage volume on either drain side or supply 
side, and controls and pump if storage is on the supply side), and 
code acceptance issues, especially if drain side storage is added.  
I'll watch for more ideas on this.  
Dan Cautley
Phone 800-638-8556 x706
Fax   301-249-3265

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