Drainback vs. Closed Loop Circulators [RE-wrenches]

Tue Jun 6 22:04:38 PDT 2006

Hey Jeff and Conrad,

How neat is this?  We're actually talking about SWH on the wrenches list.

Sounds to me like the drain back systems Conrad is talking about weren't
installed properly.  The type of drain back I'm talking about does not
require a motorized spool valve or a vacuum breaker.  I think there were
several other variations on that same bad theme as well.

I can't imagine any potable water being so corrosive that it would ruin a
non-ferrous pump and copper collectors.  If that was the case wouldn't the
water be damaging the houses potable plumbing and appliances as well?

Something tells me they used cast iron pumps and didn't install the
collectors and plumbing properly (positive drain).

I thought about the two pump set up before but to me it adds unnecessary
complexity to the system.  Personally, I'll stick with the one pump set up
and use a line voltage controller.  I've used Tekmar controllers for radiant
heating and they are the cat's meow but I'd rather not use an expensive (2x)
controller that requires a power supply which plus a junction box for the
extra stuff.  I'm just a KISS guy.  Several hundred dollars of extra
equipment and labor plus a few more things to break add up to higher life
cycle cost.

The key benefits to a drain back system as I see them

-Fewest parts = All it needs is a pump, differential controller, and few
isolation and drain/fill valves. So that means the easiest install and the
fewest parts to break and require service.
A glycol closed loop requires and air bleeder, expansion tank, pressure
relief valve and check valve.

-No glycol to service.  Water never turns acidic, glycol does. I've heard
stories (fairy tales?) about glycol lasting 20 years but it's definitely not
the norm in my personal experience with the 100's of closed loop glycol
systems that I've laid my hands on over that last 15 years.  Even if it does
last 20 years you still need to be checking the PH regularly.  You can't
trust the home owner to do that so that means an extra expense for the
glycol system owner.  But then again service contracts are good for us poor
wrenches.

-Higher efficiency.  Water will collect and transfer more energy then
glycol.

-Power failures don't lead to problems with the type of drain back I'm
talking about.  A power failure in a closed loop glycol system can easily
stagnate the system.  If the pressure relief goes off a few times the
pressure often drop below the minimum head required to pump.  This is main
service call I've gotten for closed loop glycol system over the years.

-Lowest Life Cycle Cost.  Fewest parts initially and no need to ever change
the glycol out.   After a few stagnation events the closed loop glycol
system will need recharged which is service call you'll never have with a
drainback.  No chance of damaging the collectors assuming they are installed
correctly which is really easy to do.  If the pump fails or electricity goes
out the system drains out.

That's just my .02 and I'm very open to other ideas. 

Best,
Travis Creswell
Ozark Energy Services

-----Original Message-----
From: Jeff Clearwater [mailto:clrwater at earthlink.net] 
Sent: Tuesday, June 06, 2006 12:55 AM
To: RE-wrenches at topica.com
Subject: Re: Drainback vs. Closed Loop Circulators [RE-wrenches]

Hey Conrad,

Is the drainback of the '80s a fair comparison to the drainback of today?

It's the failures of '80s glycol that pushes us to drainback.  But 
then again is the glycol of the '80s a fair comparison to the glycol 
of today?

The debate goes on.

Travis, you said "all the benefits of drainback" - Want to spell that 
out for us?

Conrad, game to do the same for glycol?

Still Learning,

Jeff C.

>Jeff, Travis and other Drainheads,
>
>If you want to cut the power use more you can use two smaller 
>circulators, cutting the second one out after 5 minutes (once you 
>know you've filled the collector).
>Tek-Mar controller provides for this through a programmable second
output....
>.
>BTW, out of the several hundred 80's systems that we've serviced 
>here in the northeast, there is yet to be one surviving drainback.
>Some day I'll try to catalog this in the hopes of solutions.
>for starters:
>-corrosive drainback water?-circulators failing quickly and 
>collectors corroding
>-freeze ups
>
>Those glycol loops sure hold up....
>
>Conrad Geyser
>Cotuit Solar
>
>----- Original Message ----- From: "Travis Creswell, Ozark Solar" 
><ozsolar at ipa.net>
>To: <RE-wrenches at topica.com>
>Sent: Monday, June 05, 2006 3:52 PM
>Subject: RE: Drainback vs. Closed Loop Circulators [RE-wrenches]
>
>>
>>Hello Jeff,
>>
>>We just did a drain back system with six 4x10 flat plates.  Calculations
>>showed that we had to use Grundfos 26-96 to overcome the head and total
>>pressure drop.  I carefully added up every foot of pipe, valve, elbow etc
to
>>make sure I didn't oversize it.
>>
>>It's rated at 215 watts but we choked the flow down to ~9 gpm (1.5 per
>>panel) and dropped the wattage to 135 as measured with a Brand power
meter.
>>
>>The pump would do almost 20 gpm with out reducing flow which was higher
then
>>the charts said it would do so I assume there was a siphon effect adding
to
>>the flow.  But you can't factor that into the pump sizing as you have to
>>overcome the head initially.  And the next smaller pump left no room to
>>safely overcome the head plus it didn't use much less wattage.
>>
>>I'll gladly use the extra energy to get all the benefits of drain back. In
>>the morning I saw a 20f delta t at 9 gpm.  Of course the Delta T drops as
>>the days goes on but stays above 10.  That's 90,000 BTU's of heat for 460
>>BTU's worth of electricity.  So the average of the day that's 1% of the
>>energy collected is used to pump.  Share that with the "thermal engineer"
>>and see what he says.
>>
>>The common circulation pump is centrifugal which is not a positive
>>displacement type.  Very low wattage positive displacement pumps are
>>available that can handle the temp but their flow rate limits them to a
max
>>of several panels.  Ken Schaal of Commonwealth Solar can speak with direct
>>knowledge of them.
>>
>>Best,
>>
>>Travis Creswell
>>Ozark Energy Services
>>
>>
>>-----Original Message-----
>>From: Jeff Clearwater [mailto:clrwater at earthlink.net]
>>Sent: Friday, May 19, 2006 7:56 AM
>>To: RE-wrenches at topica.com
>>Subject: Drainback vs. Closed Loop Circulators [RE-wrenches]
>>
>>
>>Hi All,
>>
>>We have been getting back into offering Solar DHW - mostly due to two
>>factors that make it a doable business model in terms of
>>installation, service and liability.
>>
>>The first is switching to evacuated tube collectors.  They are light,
>>easy to ship and handle and easy to install - I don't have to worry
>>about cranes or major racking weight and it eases safety factors with
>>my installers.
>>
>>Second is going with drainback instead of closed loop glycol.   This
>>eliminates having to service the glycol every 2-3 years and actually
>>gives a chance that the system might live on when the folks sell the
>>house etc.
>>
>>However, I recently suggested drainback, closed loop to a low income
>>zero energy home development we are working with and the thermal
>>engineer came back with data saying that the drainback circulators
>>used up to 1/3 of the energy harvested by the system in terms of
>>electrical cost vs. gas saved!  I find that hard to believe. He
>>claimed the closed loop circulators use less but still was showing a
>>significant percentage.  I'm not sure whether he's just finding an
>>excuse to go with what he is used to or whether there is a
>>significant difference here.
>>
>>So the questions for my esteemed wrenches are:
>>
>>1)  Does anyone have KWH use figures for solar DHW circulators?
>>2)  Does anyone know of the most efficient circulators?  (are
>>circulators positive displacement pumps?) - any models to suggest for
>>drainback?
>>3)  Do folks recommend using PV direct pumping for drainback?  What
>>controller would one use then?
>>
>>Thanks!
>>
>>Jeff C.
>>
>>--

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