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Need Heater Plumbing Advice

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Posted by Tom Kessler on March 18, 2013, 2:05 pm
 
I'd like an expert opinion on the best plumbing and control scheme for the
following solar hot water  system.  

1 - Two story home in a sub-tropical climate (20 N Lat) - never freezes, no
glycol, etc.  
2-  Evacuated tube collector with 25 gallon tank  on the roof.
3 - LP gas water heater on 1st floor.
4 - House is plumbed with hot water recirculation line to boiler from farthest
bathroom.  

Would like suggestions a completly automatic control and plumbing scheme between
solar only, boiler assisted, and boiler only modes.  I can retrofit the boiler
to electronic pilot ignition.

Has anyone tried to implement this and optimize number of valves, controller,
etc?
Or refer me to someone who has...  thanks

  




Posted by Ecnerwal on March 18, 2013, 2:55 pm
 


Many ways to approach this. Somebody did a sales job to get evacuated  
tubes in that climate - flat plate will do more bang for the buck, but  
what's bought is bought. My personal preference would be to put a PV  
panel and circulator pump on the system to move hot water down off the  
roof when the sun shines, and allow for more storage capacity by using  
the LP water heater tank (unless it's tankless - you switch between  
calling it a water heater and a boiler) as additional storage, or add a  
large tank for storage. If your roof can stand the load, a big tank on  
the roof allows things to be more passive, but that often is not an  
option.

Some folks don't want to booger things up with electrics. But you  
already have a pump. You can do it with line-powered pumps and sensors,  
but the PV panel powered pump simplifies the controls - if the sun  
shines, the pump runs, and the heater is heating. Cloud comes in, it all  
slows down. Night falls, it stops.

Does the recirculator run all the time? If so, fix that. You can put it  
on a switch, or even put it on a motion sensor so if someone moves  
_near_ the bathroom(s) it starts recirculating for 15 minutes. Things  
that run 24/7/365 when they are used a lot less than that can be  
massively wasteful. Also, insulate all the pipes on that loop as if you  
were in the arctic - they bleed heat from your hot water system into  
your air conditioning load, so you pay at least twice for the heat  
leakage.

Speaking of A/C, unless you are among the few that don't have it at that  
latitude, add a "desuperheater" to your A/C unit to get more efficient  
A/C and "free" hot water at the same time.

As for the "completely automatic control of modes" - the boiler/water  
heater draws water from the solar storage. If it's not hot, or just not  
hot enough, it kicks on; if it's hot enough, it does not. Cold water  
supply feeds into the cold end of the solar storage and/or through the  
collectors. Recirculation of the solar loop should feed the hottest  
water (from the collectors) to the hot end of the LP water heater, and  
the cold end of the LP water heater flows back to the hot end of the  
dedicated solar storage, and the cold end of that goes out to the  
collectors to be heated. Again, insulate the pipes in this loop well, or  
you'll lose a lot of heat you could be storing.

You will probably also need a tempering valve, since there is  
significant potential to have water stored that is MUCH hotter than you  
want coming out of the faucet (scalding/burning.) That will take your  
stored hot water and mix in enough cold water to limit the temperature  
if it is too hot.

--  
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.

Posted by Tom Kessler on March 18, 2013, 6:10 pm
 Dear Ecnerwal,

Thank you for the guidance I was looking for!  I can visualize a plan now.  A
few additional comments:

- While I'm at 20 N Lat, (103 W) I'm also at 5000 ft.  We still get chilly days
where convection losses are a factor.   I haven't bought the collector yet.
Both types are available.  How to make a more informed decision?  

- recirc pump will have a thermostat.  Have taco SS pump, not installed yet.  
- LP heater is conventional with 132L tank (purchased that big just in case of
-too many visitors and cloudy days)  Its also possible to buy a tube w/o tank
collector and leverage just the boiler tank.  

- tempering valve ok.  
- All hot water piping is well insulated.
- I left two additional insulated runs between boiler and roof , 3/4" and 1/2".  
- Have no AC
- Water is pressurized hydroneumatic.
- Tempering valve understood.  

Any additional thoughts?

thanks,

Tom




-  

Posted by Ecnerwal on March 19, 2013, 10:59 pm
 

http://www.solar-rating.org  

Basically, you look at the detailed rating information for the  
temperature rise fitting your application, and then you have to factor  
in the price. Before THAT you may need to see if anything you have  
locally available is even a rated item, if not, you are back to  
guesswork. A GLAZED flat plate will handle "chilly days" just fine,  
IMHO. The UNGLAZED ones are severely limited (basically only good for  
swimming pool heating.)

Evacuated tubes are "neat" but some of the Chinese ones are not that  
good (as with anything, they can make it look like the things that do  
well, but it may not perform as well if they only went for the look) and  
for DHW applications in less-extreme climates, flat plate nearly always  
wins on both BTUs per area (or kWh/m**2) and on $. Or Pesos ;-)

For instance, a Heliodyne Gobi HT rates at 4.01-4.05 kWh/m**2 (glazed  
flat plate) on the "clear C" condition. The worst flat plate is 0.9.

The best tubular is 3.67 The worst tubular is 1.62.

You will need to do enough reading to understand what the ratings mean -  
the main thing is the A B C D E class rating, having to do with the  
temperature rise over ambient - C happens to be the class for "warm  
climate domestic hot water." Good evacuated tubes thrash flat plates at  
class E performance, but that has little application to typical domestic  
hot water, and usually costs a LOT more for the collector. At 45 N I  
need to consider at least class D performance - you probably don't, or  
at least not much, in a climate that never freezes.


I'd suggest trying it with just the 132 L, or getting an extra tank NOT  
on the roof that you plumb in just ahead of the LP tank (unless you have  
a nice sturdy flat concrete hurricane-proof roof that can become a floor  
the next time you decide to build more walls and another roof. I've seen  
those houses in parts of the tropics.) Depending on how willing you are  
to rework it later, you can always add more storage capacity if you find  
that it runs out too soon at a given size of storage. I assume you get a  
bit more cloud cover than parts north of you with the sea nearby; the  
good news is that both flat plates and evacuated tubes collect some heat  
even on cloudy days - not that they can do miracles. If you get some sun  
nearly everyday (even if not all day) you may not need a huge storage  
capacity. We get long gray stretches, so much larger storage makes more  
sense here.

You'll also need (should already have with the water heater) a  
temperature/pressure relief valve, in case things get way too hot. You  
may find that at some times of the year or day you want to be able to  
cover (shade) part of the collector, if the T/P relief is blowing off  
too regularly (when you get much more sun than you need for your hot  
water use.) More storage can help with that problem as well. Make sure  
that the drain from that valve is directed somewhere that will not cause  
problems or burn anyone if or when it goes.

--  
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.

Posted by Tom Kessler on March 20, 2013, 10:39 am
 On Tuesday, March 19, 2013 4:59:28 PM UTC-6, Ecnerwal wrote:

Thanks very much!  This has been very educational.  I've two good options here
for certified flat plates, Kioto and Modulo Solar.   Not quite Heliodyne
performance, but seems very respectable.  It'll be interesting to quote and
compare.  

If you're into solar electric, check out this company, Heart Tranverter in Costa
Rica.  http://www.transverter.com  .  I bought two, their installation is my next
project!



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