[RE-wrenches] SWH - mixing valves

[Bill Loesch]

Hi again, Kelly,

You mentioned backflow issues in your post, which I should have addressed
with my previous reply.

If the Honeywell valve you are using is a Honeywell AM-1 Thermostatic Mixing
Valve it incorporates dual check valves on the inlets. Those check valves
are removable and those check valves are also susceptible to scaling if the
water quality is deficient. (See previous post.) The AM-1 has a Teflon
coating which would delay the effects of scaling, but would ultimately
succumb. The integrated check valves as well as the mixing valve itself,
will also produce a pressure reduction in the system.

A phenomena known in the plumbing trade as "plumbing crossover" and perhaps
more descriptively labeled as plumbing bypass, is the situation where
(typically) the cold water flows into the hot water side. This can be
illustrated with the homebrew shower where the throttling valves on the hot
and cold lines are supplemented with a third valve to allow the mixing
(throttling) valves to remain preset and the shutoff is downstream of the
two mix valves. This is an excellent written illustration and relatively
obvious. Unfortunately, what is much more common is the not so obvious,
widespread use of  single handle faucets. Once the seals on the cartridge,
module, insert (terminology dependent on manufacturer) become worn, the same
situation exists with the single handle faucet as described with the
homebrew three valve shower.

The easy test for this condition is to stop the flow of hot water at the
source, then open all hot water faucets. _Any_ flow, at any faucet, after a
delay to allow gravity drainage to occur, would indicate a plumbing
crossover/bypass _somewhere_ in the system.

Some single handle faucets also incorporate dual check valves into their
construction. Again, poor water quality can render those inop in either or
both directions.

Thanks for your interest in getting a solution to this pesky problem.

Bill Loesch
Solar 1 - Saint Louis Solar



----- Original Message ----- 
From: "Kelly Keilwitz, Whidbey Sun & Wind" <kelly at whidbeysunwind.com>
To: "RE Wrenches listserve" <re-wrenches at lists.re-wrenches.org>
Sent: Wednesday, May 19, 2010 1:14 AM
Subject: [RE-wrenches] SWH - mixing valves


> Thermal wrenches,
> I'm having an ongoing issue with mixing valves in my own system not
> keeping a steady mix domestic HW temperature as the tank temperature
> varies with solar heating. If I set it for (say) 122˚F mix when the
> tank is 135˚, it seems to hold that differential at all tank temps -
> i.e. when the tank is hotter the mix is too hot (sometimes dangerously
> so), and when colder the mix is too cold for a comfortable shower.
> Thus, I need to change the setting all the time.
>
> I'm now using a Honeywell valve with a 90˚ - 145˚ range. This is a
> replacement for a valve with a wider range that acted the same way.
> Any ideas,  suggestions for better valves, or....?
>
> This has me wondering how many of our SWH customers are experiencing a
> similar thing, but just aren't bothered enough to say anything. We
> seem to have encountered many plumbing situations where the mixing
> valve allowed weird backflow or siphoning situations to mess with
> system operation, especially with HW recirculation. I thought I had
> all such causes eliminated on my system, but maybe not.
>
> Thanks for sharing your experience with such matters.
> -Kelly
>
> Kelly Keilwitz, P.E.
> Whidbey Sun & Wind
> Renewable Energy Systems
> kelly at whidbeysunwind.com
> 360-678-7131
>
>
>
>
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