AEE Dump Load Element for Water Heaters [RE-wrenches]

[Hugh Piggott]

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At 21:21 -0500 31/3/08, Travis Creswell wrote:
>  with DC I can PWM the dump load with the
>Morningstar controller and I feel the control will be better.  One option is
>to use the existing AC elements but run DC to them.  My wattage goes way
>down but its better then nothing at this point.

I buy Specialist DC elements to fit the 2 1/4 BSP fittings that are 
standard here.  TP Fay make the elements.  It does work better than 
relays for precise control of battery charge.

>
>One thing that I haven't figured out is what's going to happen to the
>existing T-stat when I run it on 48v (56v) and PWM that???  I've searched
>all over and find nothing rated for it.  Why don't the people that sell the
>elements have such a product?

In any case you do not really want to turn the load off.  I use the 
stat to energise a transfer relay that energises the water heater. I 
use AC for the coil in most cases.  Or 24 V dc.   The only problem is 
the rating of the relay contacts that switch the heating current. 
It's not a big deal to switch chopped DC off the Tristar, but when it 
turns the heater full on you can get a sustained arc that is pretty 
scary.   Another reason to over-size the dump loads.

>   I think a snap disc would be the most
>reliable way to do it but have yet to find one rated for it.  I've thought
>about all sorts of ways of doing it with thermistors, relays and contactors
>but they rapidly fail the KISS test.

it's nice to keep things simple, but when you are using dump loads 
for a useful purpose then things to get complex.
-- 
Hugh Piggott

Scoraig Wind Electric

http://www.scoraigwind.co.uk


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