Bill and Peter, I stand corrected. Tankless units do not modulate to zero. What a shame.
My Takagi needs AC power to the exhaust fan in order to operate. I will be installing an aquastat relay on my solar storage tank that removes power to the Takagi when the solar tank is at or above 110 degrees. Kris Legacy Solar 864 Clam Falls Trail Frederic, WI 54837 715-653-4295 sol...@legacysolar.com www.legacysolar.com -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Bill Loesch Sent: Tuesday, September 14, 2010 7:29 PM To: RE-wrenches Subject: Re: [RE-wrenches] On Demand Heaters for SDHW 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.parr...@calsolareng.com> To: "'RE-wrenches'" <re-wrenches@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.parr...@calsolareng.com > Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 > > > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > > _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org