[RE-wrenches] On Demand Heaters for SDHW

[Bill Loesch]

Peter,

With all due respect, if you are only doing one SDHW system a year you might
want to consider hiring (subcontracting) someone who does keep up with at
least current technology and preferably state of the art.

As you are hopefully aware there are a number of ways to construct a SDHW
system. I personally prefer the single (larger than conventional) tank with
integrated HX for simplicity and efficiency. That is the system type which
will be addressed here. Since the tank in all but the latest (uninformed)
solar johnny come lately scenarios, use an electric tank, the _need_ for gas
fired backup is an expensive backup solution.

Provided that the application does call for gas backup then the _ideal_
machine would in fact be as you described. Such a machine has not existed
and does not exist today. All tankless, including solar compatible (heaters
which modulate fire rate based on _inlet temperature_) have a minimum
fire rate which is well above zero. The current iteration (Bosch 1600PS) of 
the
Bosch 117,000 BTU/h natural draft tankless water heater you cited uses
23,400 BTU/h as its minimum fire rate. Power vented tankless water
heaters from Bosch that provide up to 225,000 BTU/h have min fire rates of ~
25,000 BTU/h. You need to be exceptionally clear which flavor of
heater you are working with as a natural draft heater has totally different
installation and vent materials requirements compared to a power vented
model. The vent materials differ within the power vented category depending
upon condensing or non-condensing..

While you can electrically gang/cascade/group multiple power vented heaters
to provide increased flow (and you should be plumbing them in a reverse
return configuration). The natural draft heater you previously mentioned has
no inherent capability to provide such multiple unit compatibility (even 
with
proper reverse return plumbing configuration).

The biggest issue with the separate gas fired solar compatible tankless
backup SDHW system is the incremental heating of the almost warm enough
solar heated hot water.

The biggest issue with any tankless water heater is the potential for scale
and the resultant efficiency and functionality loss and descaling
requirement.

Best wishes to you and your solar hot water enthusiast client,

Bill Loesch
Solar 1 - Saint Louis Solar
314 631 1094

FYI - The Bosch 125BS has had essentially only a name change to Bosch 
1600PS.

PS I reiterate, despite other RE-Wrenches list comments to the contrary 
about
"modulate to
zero", NONE of ANY of the Big Five tankless manufacturers that market to
North America have a low fire rate which modulates to zero or anything near
it for the very same reason that you will never get a trickle of hot water
out of any tankless water heater. (~0.5 gpm minimum activation flow
requirement)

Comparing condensing heaters with ~200,000 max BTU/h input
Bosch GWH C 800 ES     19,900 BTU/h
Noritz NRC 111               11,000
Paloma   no condensing heaters
Rinnai RC98HPe                 9,500
Takagi T-H2                     13,000

Thanks for your patience, I hope it was not too technical.

----- Original Message ----- 
From: "Peter Parrish" <peter.parrish at calsolareng.com>
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Sent: Tuesday, September 14, 2010 10:37 AM
Subject: [RE-wrenches] On Demand Heaters for SDHW


> We have a SDHW client that wants to replace his old water heater with a
> new
> tankless water heater. We do a SDHW system about once a year, and only for
> clients that are getting a PV system from us to begin with. So this is not
> a
> big business line for us and we are behind the curve in terms of
> understanding the latest technologies.
>
> As I remember from an excellent workshop that I took about four years ago:
> when used in conjunction with a SDWH system, the tankless heater should
>
> (1) Modulate heat input based on INPUT water temperature
> (2) Be able to modulate down to ZERO BTU/hr
>
> The only unit I knew of back then was the Bosch 125BS (I believe). Today I
> can't find any bigger units that fit the above requirements.
>
> Short of using two Bosch 125BS units in parallel, does anyone have a
> solution?
>
> - Peter
>
> Peter T. Parrish, Ph.D., President
> California Solar Engineering, Inc.
> 820 Cynthia Ave., Los Angeles, CA 90065
> CA Lic. 854779, NABCEP Cert. 031806-26
> peter.parrish at calsolareng.com
> Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
>
>
>
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