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

[Bill Loesch]

----- 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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