Drainback vs. Closed Loop Circulators [RE-wrenches]

[Ken Schaal]

 Ahh-- how the past design failures affect form our thought patterns  [
prejudices?] !

1- If we are talking about side arm, small dB tank w/ HE, I agree that
control failure is a concern .  I've very rarely encountered a broken tank
sensor wire to cause constant on. More likely on the collector sensor,
corroded connection causing delayed on, or broken, typically from ice or
squirrels-- buzzards once- causing no pump on.

2-- Large tank DB makes prolonged 'pump on ' much less critical.

3-- The ultimate issue is which system, overall, is more effective at
producing useful energy for extended years. For all our efforts, weather PV
or SHW or Passive Design or Wind or Hydro, the devil is in the details. The
best use of this list is to get those details RIGHT.

4--We never had many side arm DB's in Richmond, mostly Reynolds anti-freeze
systems, my large tank DB, various other systems both AF and side arm DB. We
didn't have too many shysters to give a really bad taste ! Our climate is
quite variable 0-100, cloudy to blazing sun, ice and snow . A good sampling
of weather. We need to be efficient for a SWH to be cost effective. [ Large
tank DB tests out at 600-800 BTU's per sq ft per day annual ave. depending
on the details.]

I have many more service calls for AF than large tank DB. The only frozen
collectors were when they were immediately above a very hot tank-- one tied
into a wood fired boiler, another when the house was vacant-- which took a
simple shallow P trap in the return line to keep solar tank vapor from
migrating into the collector.

5--On residential size systems filling the collectors under stagnation
conditions is always impressive and should not to be typical, but is hardly
catastrophic. With a typical high head circulator, the flow is self limiting
as soon as steam forms. At least with large tank systems with the return
carried below water level and appropriate vacuum break to allow draining,
steam dissipates quickly. Larger systems deserve more care, but again have
not been a problem in real world conditions.
Fortunately we now have a great control from Thermo Technologies,-- the
evacuated tube guys-- that gives digital control, temps, high limits for
bottom and top of tank , as well as collector. Also provides BTU calc. all
for a $100.  Ahh, progress!  [ No. I have no financial links . Mahjouri
hasn't even been able to get me to try evac tubes ! I still have to many
flat plates from a poorly designed AF commercial system that was taken off
the roof after leaking ethylene glycol onto a built up tar and gravel roof.
100 --4x12 's ]

6-- Todd's comment were right on-- very little risk, lot's of performance
advantage, details matter, as well as basic system design. Remember, if
there is a failure mode, the water is moving and it's the whole tank, which
has probably been at least partially heated the day before, that has to be
cooled to below 32.

OK Smitty-- let's here the bloody details. We don't want PV to get all the
gory attention !

It's all for the good of SOLAR

Be careful out there all  -- Ken



----- Original Message -----
From: "Bill Loesch, Saint Louis Solar" <bill_loesch at compuserve.com>
To: <RE-wrenches at topica.com>
Sent: Thursday, June 08, 2006 3:25 AM
Subject: Re: Drainback vs. Closed Loop Circulators [RE-wrenches]


>
>
> I cannot agree more with Todd that using water in a freezing climate is
> _asking_ for a catastrophic failure at some point. Those KISS drainback
> folks need to keep their brazing skills in good practice in anticipation
of
> the control failure mode.
>
> Another rarely discussed aspect of drain back is the thermal shock of
> pumping "cold" fluid into "hot" collectors.
>
> Bill Loesch, Solar 1- Saint Louis Solar
>
>
> Message text written by INTERNET:RE-wrenches at topica.com
> >
> The only caveat with drain backs is the possibility (albeit very small)
> of freezing. I agree with all the advantages mentioned about drainback
> systems versus glycol... but I do not think it wise to trust a snap
> switch and/ differential controller or its temp sensor to not accidently
> pump water through the system during times of freezing weather. Brazing
> leaks on a burst absorber panel is no fun.
>
> Todd
>
> <
>
>
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