Drainback vs. Closed Loop WAS: mixing valves on SHW systems with gas backup

Ken Schaal ken at commonwealthsolar.com
Sat May 10 11:24:21 PDT 2008


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Hi Bill

Not to worry--' your Dad' is just fine ! Actually, I have an anti-freeze 
system on a rental house next to my office, and for my office radiant slab. 
Circumstances dictated.
---practically free, used collectors  ( Reynolds , all aluminum absorber, 
not compatible with copper HE, and needing anti-freeze for corrosion 
protection)
---ground mounted array making drain back impossible at the time.
    note; I now make my own tanks ,which can be in ground if needed, and we 
use a positive displacement pump, with bypass to allow drain back, that can 
be mounted at water level, ie. don't need positive inlet pressure.

Of coarse I've had problems, since I've learned from experience over the 
last 30 years. My own experience , plus what I've seen others do, is briefly 
reviewed below.
For drainback reliability issues, we should first distinguish as to type of 
drainback.
The typical on the market has a small drainback tank, with integral HE, or 
uses a storage tank with integral HE. The smaller volume of water makes it 
more vulnerable to control or, more likely,  sensor wiring failure resulting 
in continuous pump operation, which in freezing weather may eventually 
result in a frozen absorber. If this type of system is used, a freeze sensor 
to prevent operation with collector temp below 40 + would be best, and 
sensor wiring should have particular attention paid to critter damage like 
squirrels, vultures, or ice, hanging or sliding. Piping is also vulnerable 
to the ice issue as well. The tank sensor needs to be cared for well also, 
since a break in that wire will cause the control to keep the pump on.

One important consideration for piping, including horizontally  mounted 
collectors, is the relationship between slope and pipe diameter. The smaller 
the pipe the greater the slope needed to overcome the  surface tension of 
the water  within the pipe. At 1" dia. the pipe can be minimally 
sloped--1/16 to 1/8"/ ft of run, even dead level if you are very trusting of 
your level and the run is only one or two internal header collectors.Even if 
1" pipe is 1/2 full of water, the freezing water will not expand enough to 
split the pipe. Too much slope on the collector keeps the upper capped off 
header full of water, which, if the solder has not flowed well around the 
cap, can eventually lead to failure of the solder bond, from freeze/thaw 
cycles, with consequent complete failure of the bond, and the cap popping 
off, usually when the collector fills at stagnation temps after a sunny day 
power outage. Some of the German controls are nice as they have a collector 
hi limit in addition to the storage hi limit. Goldline, since Hayward bought 
them, doesn't seem very interested in advanced features, last we checked.
But they are quite reliable and have more robust relays.

The other drainback type uses a large volume tank-- our are 2gals/sq ft of 
collector, usually-- with the HE submerged in the storage water, either 
copper coils or SS tanks ( my preference) for better performance.If the pump 
runs continuously, it is much less likely to freeze since there is enough 
heat in storage to go thru the night and then be reheated the next day.

Generally this type of drainback is used for larger commercial systems, 
where Btu's/ $ are looked at closer, and multiple 80 or 120 gal tanks get 
expensive, take up too much space, and are a pain in the ass. Since we make 
our own tanks, we use them for residential as well. They can also easily be 
made from poly tanks. We used to get our from Chem-Tainer. There is still a 
picture on my old website. www.commonwealthsolar.com

Gravity has been pretty reliable, but air venting to allow the collector to 
drain needs to be virtually foolproof.!

Others can probably add to this list of caveats.

As for why the European imports are all anti-freeze,  maybe it's because 
their primary market is newcomers who don't have experience , and so long as 
the connections are tight, the temp gauge will get hot, and most homeowners 
think hot is good, not understanding that energy saved is more than 
temperature ??
If I was exporting I wouldn't want installer problems either, and how many 
residential systems are performance tested ?
A typical large tank drainback in Virginia, with typical residential load 
patterns, gets 650-750 Btu's/ sq ft/ d annual average.

What do others think of the imports ??
They do look slick though !

And there's room for all !

Typical for us, we had a call for service last week, from a customer that 
got a large tank drainback in '84. Control got zapped by lightning. Also 
switched out the 009 pump (160W) for our new progressive rotor w/ inverter 
based electronics that uses about 30 watts for 30 ft head applications.
Good for another 20 years?


Take Care
Ken


In
----- Original Message ----- 
From: "Bill Loesch" <solar1online at charter.net>
To: <RE-wrenches at topica.com>
Sent: Saturday, May 10, 2008 8:34 AM
Subject: Drainback vs. Closed Loop WAS: mixing valves on SHW systems with 
gas backup [RE-


>
> Hi Ken,
>
> I'm changing the subject as this has almost nothing to do with the initial
> subject and the potential to deteriorate into a "my Dad can whip your Dad
> scenario".
>
> I am very pleased that you have had no(?) problems with drainback systems 
> or
> electronics or plumbing as well as gravity.
> I wish you continued success.
>
> There have been a plethora of SDHW system manufacturers entering the North
> American market, primarily European, and I have not seen one of their
> systems utilize anything but closed loop principles. Food for thought?
>
> Bill Loesch
> Solar 1 - Saint Louis Solar
>
>
> ----- Original Message ----- 
> From: "Ken Schaal" <ken at commonwealthsolar.com>
> To: <RE-wrenches at topica.com>
> Sent: Saturday, May 10, 2008 5:35 AM
> Subject: Re: mixing valves on SHW systems with gas backup [RE-wrenches]
>
>
>>
>> As we approach the summer season, and overheating of anti-freeze systems,
>> it's a good time to remember that drainback designs require nothing but
>> gravity to prevent dangerous temperatures that lead to increased 
>> callbacks
>> and maintenance problems. Not to mention the liability issues of scalding
>> water, and the increased corrosion activity in the tank.
>>
>> Ken Schaal
>> CommonWealth Solar
>>
>>
>> ----- Original Message ----- 
>> From: "Travis Creswell" <tcreswell at ozarkenergyservices.com>
>> To: <RE-wrenches at topica.com>
>> Sent: Saturday, May 10, 2008 6:09 AM
>> Subject: RE: mixing valves on SHW systems with gas backup [RE-wrenches]
>>
>>
>> >
>> > The same thing can happen on electric water heaters with "Energy Smart"
>> > controls.
>> >
>> > Yes, thermal questions are allowed and I would like to see more.
>> >
>> > Travis Creswell
>> > Ozark Energy Services
>> >
>> >
>> > -----Original Message-----
>> > From: Karl Schwingel [mailto:karl at northwindrenewable.com]
>> > Sent: Friday, May 09, 2008 10:00 PM
>> > To: RE-wrenches at topica.com
>> > Subject: RE: mixing valves on SHW systems with gas backup [RE-wrenches]
>> >
>> >
>> > Davis,
>> >
>> > What I've heard is that you run the risk of tripping the overthermal
>> > protection in the gas water heater if you should on a hot summer day 
>> > run
>> > 190
>> > deg water into the bottom of the tank.  Of course you don't realize it
>> > till
>> > months or weeks later when there's no solar preheated water and you get
>> > the
>> > call that "my water heater doesn't work"
>> >
>> > I've never put a mixing valve on the preheat tank, and have only seen 
>> > it
>> > done when you want to keep the tank at a high temp (such as when you're
>> > pulling water off for a space heat loop.  However, I'm working mostly
>> > around
>> > electric water heaters.
>> >
>> > Is this forum open to thermal questions too? All I've seen so far has
> been
>> > PV related.
>> >
>> > Karl Schwingel
>> > Nabcep Certified Thermal Installer
>> >
>> > NorthWind Renewable Energy
>> > Karl at northwindrenewable.com
>> > PO box 723,
>> > Stevens Point Wi.
>> > 54481
>> > cell: 715 209 0446
>> >
>> >
>> >
>> >
>> >
>> > -----Original Message-----
>> > From: Davis Terrell [mailto:davis at brightearthsolar.com]
>> > Sent: Saturday, May 10, 2008 12:28 AM
>> > To: RE-wrenches
>> > Subject: mixing valves on SHW systems with gas backup [RE-wrenches]
>> >
>> >
>> > Wrenches,
>> >
>> > I have seen some literature call for a mixing valve in between the 
>> > solar
>> > storage tank and the gas hot water tank.  Can anyone give some ideas as
>> > to why this second mixing valve may be necessary?  Does anyone else use
>> > one?  Thanks.
>> >
>> > -- 
>> > Davis Terrell
>> > Bright Earth Solar
>> > (802) 492-2273 / brightearthsolar.com
>> >
>
>
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