mixing valves on SHW systems with gas backup [RE-wrenches]

Sat May 10 09:44:18 PDT 2008

Hello:

Here's a bit of history, to the best of my recollection.  Tempering  valves 
were initially 'required' because there were no UL listed controllers  
available - and therefore no UL listed over-heat protection.   From  that time 
forward, every manu who had a panel provided a schematic showing the  tempering 
valve.  When the controls became UL listed, the requirement for  the valve was 
never modified and in light of the deluge of passive systems  entering the US in 
the early 80's, it was probably a good idea. Tripping the  fusible link in a 
gas water heater valve is expensive, running tank temps above  160F is bad 
practice and will void the warranty of the storage  vessel.  The real problem with 
these hot water extenders (as we now call  the cheaper mixing valves) is that 
they will fail, and when they DO, they  over temper the solar hot water 
entering the conventional heater which  significantly reduces savings, completely to 
the ignorance of the user.  We  take advantage of the hot water extending 
capabilities of the valve by placing  it AFTER the conventional heat source, and 
set the upper limit in the controller  to 150F.  WHEN the valve fails, the 
customer will call you.  You can  set the gas/electric heater thermostat lower 
and drive 150F water into it, which  will allow it to have more overnight heat 
loss reserve.

Seated at the rear emergency exit,  
Patrick A.  Redgate
AMECO

(888)  595-9570

In a message dated 5/10/2008 3:55:23 A.M. Pacific Daylight Time,  
solar1online at charter.net writes:
Hi Fellow Thermal Wrenches (seated in the  back of this Solar PV Bus),

It doesn't even have to be "Energy Smart".  Once the limit switch (on every
electric water heater that I have ever seen  or serviced) trips, the
corresponding element is deenergized until _manually_  reset. Not a big deal.
You tell the customer about the situation when you  install. Unfortunately,
they forget. There is no hot water one day in the  early fall. They call. You
explain again. They remember this time, especially  when you tell them they
can save the cost of a service call if they are  willing to turn off the 240
volt power to the elements, remove the cover  plate, push the red button
until it clicks, reassemble, and remember to do  the same thing again next
fall when there is no hot water.

This user  intervention eliminates the second mixing valve (and any
associated  maintenance issues).

The ultimate in control simplicity is Bob-O's  technique of a caution label
at the faucet of "Solar Heated Water". This  technique alone; however, does
cross more than one Code line.

Bill  Loesch
Solar 1 - Saint Louis Solar

----- Original Message  ----- 
From: "Travis Creswell"  <tcreswell at ozarkenergyservices.com>
To:  <RE-wrenches at topica.com>
Sent: Saturday, May 10, 2008 5:09  AM
Subject: RE: mixing valves on SHW systems with gas backup  [RE-wrenches]

>
> The same thing can happen on electric  water heaters with "Energy Smart"
> controls.
>
> Yes, thermal  questions are allowed and I would like to see more.
>
> Travis  Creswell
> Ozark Energy Services
>
>
> -----Original  Message-----
> From: Karl Schwingel  [mailto:karl at northwindrenewable.com]
> Sent: Friday, May 09, 2008 10:00  PM
> To: RE-wrenches at topica.com
> Subject: RE: mixing valves on SHW  systems with gas backup [RE-wrenches]
>
>
>  Davis,
>
> What I've heard is that you run the risk of tripping the  overthermal
> protection in the gas water heater if you should on a hot  summer day run
190
> deg water into the bottom of the tank.  Of  course you don't realize it
till
> months or weeks later when there's  no solar preheated water and you get
the
> call that "my water heater  doesn't work"
>
> I've never put a mixing valve on the preheat tank,  and have only seen it
> done when you want to keep the tank at a high temp  (such as when you're
> pulling water off for a space heat loop.   However, I'm working mostly
around
> electric water  heaters.
>
> Is this forum open to thermal questions too? All I've  seen so far has been
> PV related.
>
> Karl Schwingel
>  Nabcep Certified Thermal Installer
>
> NorthWind Renewable  Energy
> Karl at northwindrenewable.com
> PO box 723,
> Stevens  Point Wi.
> 54481
> cell: 715 209  0446
>
>
>
>
>
> -----Original  Message-----
> From: Davis Terrell  [mailto:davis at brightearthsolar.com]
> Sent: Saturday, May 10, 2008 12:28  AM
> To: RE-wrenches
> Subject: mixing valves on SHW systems with  gas backup [RE-wrenches]
>
>
> Wrenches,
>
> I  have seen some literature call for a mixing valve in between the solar
>  storage tank and the gas hot water tank.  Can anyone give some ideas  as
> to why this second mixing valve may be necessary?  Does anyone  else use
> one?  Thanks.
>
> -- 
> Davis  Terrell
> Bright Earth Solar
> (802) 492-2273 / brightearthsolar.com   

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