[RE-wrenches] On Demand Heaters for SDHW

jay peltz jay at asis.com
Thu Sep 16 14:40:43 PDT 2010 

HI Darryl,

Having worked with ODH a lot, the usual cause of the problem you mention is the wrong type of fixtures or not understanding how to use them as there are differences.

One of the issues you mention below on/off/on/off has to do with low flow rates being caused by a number of factors.  Often the main one has to do with single handle fixtures which if not full port and balanced  you get to that spot that can reduce the flow of hot water to the point that it turns off and hence the temp wildly changing.

ODH are not for everyone and they can work extremely well and be very efficient and be very cost effective. However retrofits can be a nightmare unless great care and understanding of fixtures, loads etc are understood, sounds a lot like offgrid solar.

jay

peltz power


On Sep 16, 2010, at 8:40 AM, Darryl Thayer wrote:

> Hi all
> I have done several On Demand Heaters with SDHW, typically the SDHW provides 100% all summer (7-8 months) and then falls short in the winter.  I have never done a On Demand Heater that has made the customer happy.  They all have complained about 'chase the temperature' because there is a range in which the heater will want to fire or not fire, and at some time they will be in that temperature range.  then the unit will fire and not fire and the temperature changes at the fixture.  They complain.  I get complaints even when the On Demand Heater is installed by others.  I tell customers, that the system will work with the standard design, but if they want to install a On Demand, I am out of the loop. 
> Darry;   
> 
> --- On Thu, 9/16/10, Todd Cory <toddcory at finestplanet.com> wrote:
> 
> From: Todd Cory <toddcory at finestplanet.com>
> Subject: Re: [RE-wrenches] On Demand Heaters for SDHW
> To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> Date: Thursday, September 16, 2010, 10:16 AM
> 
> I know this was not the question, but this whole thread is predicated on the use of a demand hot water heater. Unless there is a space limitation, why would this be done over a traditional tank unit?
> 
> Todd
> 
> 
> 
> Bill Loesch wrote:
>> 
>> 
>> Kris, 
>> 
>> Boiler manufacturers today get starry-eyed when their machines are able to provide a 10:1 turndown ratio. Even the venerable natural draft Bosch 125BS * provided a better than 4:1 turndown ratio and all the condensing powervent units compared below regularly offer 10:1 and some better than 20:1. With each benefit comes a disadvantage, like with current production cars, serious repair/maintenance takes place primarily at the dealer since they are usually the only ones to pony up for the multitude of specialized test equipment. I find it a bit of backward progress to have to tell someone that the reason they don't have reliable hot water is because they don't have the current software update (due in no small part to the marketing gurus who have no qualms about using the customer as the proving grounds for their product). Many plumbers have no clue to the workings of a combustion analyzer, a necessary tool for  negative pressure gas valve tuning and replacement (and a multitude of other tasks where the blue flame is no longer sufficient to be able to deliver optimum performance). 
>> 
>> Your Takagi needs AC power to (in order of decreasing current requirements for a generic power vented tankless) 
>>    power the freeze protection heating elements 
>>    run the primary combustion fan 
>>    run the secondary combustion fan 
>>    provide control to 
>>        gas valve 
>>        water valve 
>>        control board (oftentimes with digital display) which provides the logic and timing circuits for the multitude of sensors and limit switches and remote controls and wireless remote controls etc., etc. 
>> 
>> Long live the thermocouple. 
>> 
>> My compliments on your simplistic approach to coordinate the solar storage tank and tankless. Simplicity is the ultimate sophistication. 
>> 
>> Bill Loesch 
>> Solar 1 - Saint Louis Solar 
>>> 
>>> 
>>> 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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> 
> -- 
> Todd Cory
> KE6SXS
> toddcory at finestplanet.com
> 
> Mt. Shasta Energy Services
> License C-10  #811428
> P.O. Box 689
> Mt. Shasta, CA. 96067
> (530) 926-1079
> 
>  
> 
> “I'd put my money on solar energy...I hope we don't have to wait till oil and coal run out before we tackle that." 
> ∞ Thomas Edison, in conversation with Henry Ford and Harvey Firestone, March 1931
> 
>  
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