[Greenbuilding] Low flow with tankless heaters

[David Bergman]

This report is disturbing because I've been recommending tankless 
(electric) heaters for current clients - and I wouldn't to have them 
dissatisfied (and complaining to me about it).

Have others experienced these problems? Am I asking for trouble by 
specifying tankless for my clients?

David
DAVID BERGMAN ARCHITECT / FIRE & WATER LIGHTING + FURNITURE
architecture . interiors . ecodesign . lighting . furniture
bergman at cyberg.com    www.cyberg.com
241 Eldridge Street #3R, New York, NY 10002
t 212 475 3106    f 212 677 7291

At 08:08 AM 11/17/2006, Lawrence Lile wrote:
>I've noticed that both of the tankless water heaters I've owned have 
>problems regulating temperature at low flow.  (Bosch and 
>Tagaki).  Some friends were sitting around complaining about this, 
>(that's how popular the tankelss heaters are getting!) and one of 
>them says he always suggests that Tankless owners will be happier 
>with thier showers if they take out the flow restrictor in the 
>shower head.  A little work with a pocket knife opened up the 
>restriction in mine, and suddenly the shower quit wandering in 
>temperature throughout the experience.
>
>I had hoped the new Tagaki was better about this, so I did a 
>test.  At 1 GPM flow, the heater cycles on and off, and the water 
>temperature wanders from cold to about 80F and back again every two 
>minutes.  Pretty sad, actually.
>
>Of course coring out the flow restrictor uses more hot water, which 
>uses more energy, and the point of the tankless water heater is to 
>save energy.  The shower is just one component of the water system, 
>this modification doesn't affect the dishwasher, kitchen sink, 
>handwashing or clothes washing.  So maybe in the grand scheme of 
>things it isn't that much of a sacrifice.
>
>The other situation that the low flow failings are annoying is hand 
>washing dishes.  I'll often run a little hot water to wash a pot, 
>shut it off, run a little more, and these short bursts of hot water 
>use don't get the unit up to temp, they are just bursts of cold 
>water.  In frustration I end up leaving the hot water running, at a 
>fairly heavy flow, so it stays hot while I am handwashing.  Once 
>again, using more hot water to compensate for the tankless unit's failings.
>
>I started looking into the idea of a very small electric tank unit 
>in series.  Fed by the tankless, which does most of the heavy 
>lifting, the tank would hold 5 gallons of hot water, and deal with 
>short bursts of hot water use.  Presumably if you are using a lot of 
>hot water, the tankless unit would catch up since it works well at 
>higher flows.  So the small tank would mostly just have to 
>compensate for it's own heat loss, would not have to heat much water 
>from 52F.  I calculated the energy loss for such a tank as costing 
>about $5 a year, if it doesn't have to heat any water, and that's 
>before I wrap it.
>
>All of these modifications are taking me in the wrong direction, 
>toward more energy use.  Has anyone else dealt with the low flow 
>issue with a tankless heater?  How did you deal with it?  Or did you 
>just tolerate it?
>
>--Lawrence Lile
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