|This is an interesting new idea for heating domestic water.
Water with antifreeze is circulated through PEX pipes that are suspended in a special roof ridge vent. The fluid in the pipes picks up heat from the hot air exiting from the ridge vent and transfers this heat via a heat exchanger to a preheat tank that is plumbed in front of the regular hot water tank. It is said that under favorable conditions water can be heated up to 120F.
Advantages include a relatively low cost (compared to full commercial solar heating systems), and the aesthetic advantage that the look of the roof is unchanged.
The Greenward website:
Article with a little more detail:
The article above says that a 40 ft roll of the ridge vent with PEX pipe embedded costs about $300, and that a kit for a full system (including the preheat tank) is about $3100.
Some pictures of the system from their website.
Roof vent with suspended PEX pipes being
installed. This is then covered with ridge
roof shingles -- it looks like a regular
roof ridge vent.
The antifreeze fluid circulates through the 4 PEX pipes that run the length of the ridge and pick up heat from the hot air exiting the attic ridge vent.
They do not give any actual performance data, stating only that the payback will vary with climate.
I would guess that in most climates, and certainly climates with cold winters that this is a three season water heater.
The $3100 price of the full system is still (to me) depressingly high.
Here is a thought on a less expensive DIY alternative.
Either buy the ridge vent part from Greenward, or or make it yourself. They apparently get about $300 for 40 ft of the roof vent, so its not really the expensive part of the system.
Then use the storage tank and heat exchanger scheme used in this $1K solar water heater. This scheme uses an non pressurized tank made from plywood lined with EPDM rubber. The domestic water to be heated passes through a large PEX pipe coil that is immersed in the tank. Its a very efficient system that is cheap to build and has a very good track record for a long and low maintenance life. Ideally, water from the storage tank could be pumped through the ridge vent PEX, and then be allowed to drain back to the tank when the ridge vent goes below a set temperature to provide freeze protection. This would be like a solar collector using a drain back system. Hopefully, the ridge PEX would drain sufficiently to provide freeze protection, but that is something that would have to be tested. If drainback could not be relied on to protect the ridge PEX, then antifreeze would have to be run in the PEX ridge pipe, and a heat exchanger used to transfer heat from the ridge PEX loop to the tank -- this could be a coil of copper pipe.
The cost of the tank described above with insulation, liner, and large domestic water preheat coil is about $300.
It is possible that the control that turns the pump off and on could simple be a $10 thermal snap switch located in the ridge vent area -- when the ridge goes above the snap switch setting (maybe 120F?) the pump is turned on.
Such a system could be under $1000.
It would be an interesting alternative to try and measure the performance of.
Thanks very much to Chris for finding this!
Gary November 13, 2009