Drainback vs. Closed Loop details [RE-wrenches]

Thu Jun 8 19:13:56 PDT 2006

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Ken Travis Geoff Smitty...
Trying to fill in the details a little as Ken was asking.
Most (not all) of my prejudice does come from small tank, vented SDHW 
drainbacks.
Sunhouse is the only packaged name that comes to mind.
These needed filled on a regular basis & had custom built in circulators, 
two unfortunate aspects.
Also a few site built collector/ big tankers have had to be de-commissioned.
Of these Acorn House systems are a branded name.  Freeze ups & overall 
system dissatisfaction have been the downfalls.
I work hard at trying to keep these on line, but haven't had success with 
the orphaned drainbacks.
I'm afraid I have to balk at naming contractors, sorry Ken.
- A retraction:  I recall two 80's drainbacks that are still going.  They 
both have small, unvented drainback tanks.
Mea Culpa, shooting from the hip.

There are many cases where a drainback is the preferred design.
Just in regularly freezing areas, where there aren't extensive 
overproduction issues, generally, I wouldn't support drainbacks.
Those narly glycol loops are just too hard to beat.  If something goes 
kaflooey and the glycol boils for a period of time, just change it, no 
biggie.
Those dime a dozen cast iron circulators go for twenty years in glycol.
The only time I've seen corrosion from bad glycol is when some psycho 
squirrel chewed a huge hole in the outlet pipe and the collectors sat and 
boiled for years afterwards.  Outwardly the collectors looked mint, but they 
were paper thin.
I think that it really comes down to the details.  Ken says he's got it 
worked out.  Ken, are your DB tanks vented?
The sol-reliant design sounds like it makes sense.  Do we know that the 
glycol residue in the collectors being boiled isn't an issue?  What about 
late day non controlled PV heat dumping?
details details
For instance, we've had trouble with Amtrol #15 expansion tanks.  And Vaughn 
HX tanks.  Now we don't use either.

Happy heating,
Conrad G.
Cotuit Solar

----- Original Message ----- 
From: "Ken Schaal" <ken at commonwealthsolar.com>
To: <RE-wrenches at topica.com>
Sent: Thursday, June 08, 2006 8:37 AM
Subject: Re: Drainback vs. Closed Loop Circulators [RE-wrenches]

>
> 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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>
>
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