Demand heater for SHW backup [RE-wrenches]

[Ian Woofenden, Home Power]

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Hi Todd,

>While everyone thinks demand heaters are oh-so-efficient, in all 
>honesty they really do not save much energy.

I tend to agree that the advantages are exaggerated, and they also 
have the dark side of allowing endless hot water use.

>If you have a SDHW system, you already have a tank... so super 
>insulate that and use the top element as your (electric) back up.

I have heard other SHW people question this approach, saying that it 
doesn't maximize solar production. In other words, there will be 
times when you heat water with electricity and then when the sun 
comes out, you can't make the most of it, because your tank is 
already hot. How do you answer that?

>Propane demand units for backup are prone to freezing, scaling and 
>not 100% efficient like an electric unit is.

All good points.

>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?

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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