Tankless misconceptions WAS: Demand heater for SHW backup [RE-wrenches]

Peter Parrish peter.parrish at calsolareng.com
Wed Aug 22 16:08:35 PDT 2007


The mantra that I repeat when I talk to people that want solar DHW and
tankless HW is 

"the tankless water heater must modulate on input water temperature"

Modulate on _output_ water temperature (the mode of operation of almost all
tankless water heaters) won't cut it, as at least one person mentioned
previously. 

There is only one tankless heater that I am aware of that meets this
criterion: the Bosch 125BS

The problem with it is that it only puts out 117 kBTUs and has 1/2" copper
lines which limits its water flow.

Since we don't do many SDHW systems any more, I haven't been keeping up on
tankless heater specs, there may now be some big dogs with modulation on
input water temperature.

Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com 

-----Original Message-----
From: Bill Loesch [mailto:solar1online at charter.net] 
Sent: Wednesday, August 22, 2007 5:22 AM
To: RE-wrenches at topica.com
Subject: Tankless misconceptions WAS: Demand heater for SHW backup
[RE-wrenches]



----- Original Message ----- 
From: "Ian Woofenden, Home Power" <ian.woofenden at homepower.com>
To: <RE-wrenches at topica.com>
Sent: Tuesday, August 21, 2007 9:53 AM
Subject: Re: Demand heater for SHW backup [RE-wrenches]


>
> I tend to agree that the advantages are exaggerated, and they also
> have the dark side of allowing endless hot water use.
>
Well, Comrade Woofenden, don't forget about the dark side of an entire
(perhaps extended) family _all_ being able to take hot showers in the
morning even after a day (or week) of cloudy days.
>
> >Propane demand units for backup are prone to freezing, scaling and
> >not 100% efficient like an electric unit is.
>
> All good points.
>
Perhaps I am missing something here, a properly installed propane tankless
water heater is more suseptible to freezing than an electric tankless? I
don't think so.

Scaling is an issue for tankless (as well as tanks or solar tanks or clothes
washer or dishwashers or every valve and pipe) in any location that has hard
water. Installing any of this equipment in a hard water location without
water treatment is not doing the user (or your solar bedfellows) any favors.

> >If you must use one of these things, electric is the way to go but
> >be warned the current draw is huge.
>
> Have any specific models that sense incoming temperature and modulate?
>

Ian, I think what you are asking for is a machine that modulates in response
to temperature. All tankless that I am aware of modulate. Most modulate in
response to flow. For the solar backup application to work well (and
simply), the backup machine would have to modulate _down to zero_ in
response to temperature. To the best of my knowledge there is no gas
tankless that has a minimum input below 11,000 BTU/hour.

High ground water temperatures (in the summertime) and low ground water
temperatures (in the wintertime) pose some of the most challenging
conditions for the tankless. What ever marketing idiot came up with the
"average ground water temperature" as a basis for sizing a tankless should
be exposed to his handiwork on an hourly basis during winter in a low ground
water temperature area. If he survives the hypothermia, he might become an
advocate of proper sizing criteria.

Respectfully submitted,

Bill Loesch
Solar 1 - Saint Louis Solar
314 631 1094

> Thanks,
>
> Ian
>
>
>
> >Ian Woofenden, Home Power wrote:
> >>What do you recommend in on-demand water heaters for solar domestic
> >>hot water systems? I'm looking for specific brands and models that
> >>you have used and are happy with.
> >>
> >>Propane and electricity are both available. Which is better/more
> >>economical? I'm partial to the idea of electric because it could at
> >>least potentially be powered renewably, either on-site or via
> >>"green power".
> >>
> >>A modest or non-existent idle draw would be nice, since there will
> >>be a PV system, and it would be preferable not to use a significant
> >>amount of its production maintaining the heater's electronics.
>
> -- 
>
> Ian Woofenden <ian.woofenden at homepower.com>, Senior Editor
> Home Power magazine; Subscriptions: $24.95 per year, PO Box 520,
> Ashland, OR 97520 USA
> 800-707-6585 (US), 541-512-0220, or download free sample issue at
> <http://www.homepower.com>
>


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