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Old 07-13-11, 03:59 PM   #1
iamgeo
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Default Solar Hot water

I need some advice.
This is my house. That part of the roof with the black lines on it gets full sun morning till about 7pm.
At the bottom of the roof from the front of the house to the back is about 42 ft.
I have an electric tankless water heater. I would like to heat the water before it gets to the tankless heater. Just trying to save electricity.
The black lines represent 1 inch diameter pvc pipe that I would paint black.
Where the pipes go under the house I would add a hose bib to facilitate having hot water outside and to drain the system in case of hard freeze
Is this idea sound? Is the pressure relief valve a good idea?
Too much weight for the roof?
Am I just spinning my wheels?


Last edited by iamgeo; 07-13-11 at 05:00 PM..
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Old 07-13-11, 10:35 PM   #2
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Why would you put a pressure relief valve up there?
One thing that I have seen up there, is an air bleeder.

On my old system (Hot water pre-heater using glycol & water),
I had a manual bleeder installed at the high point.
Once or twice a year, I would go up and open the bleeder to let any air escape.
IIRC, there never was much air in the system.
Some systems have an self-bleeder valve up there. (float type).

What kind of weather do you get at your location??
Around here, that type of open system wouldn't work real well in the winter.
Here, we need some good insulated flat panel collectors,
or some of those Newfangled vacuum tube units.
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Old 07-13-11, 11:10 PM   #3
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I'm guessing you live way down south, seeing as how your house is built on a platform.
First off, no, it's not to much weight, the weight of you standing on the roof is going to be more weight focused on a single point then the 20 or so gallons of water up on the roof (less then 200 pounds) so if your roof can handle the weight of a person it can easily handle the weight of this tubing.
something to think about of course is that you are going to have little or no storage and because of the large open, uncovered, uninsulated area of your collector your collector is going to cool off almost as fast as it heats up, it will loose heat from all sides while only collect heat from the side facing the sun, it will also take a bit of time to heat up, so unless you tend to use all of your hot water between say 11am and 6 pm then you are going to be disappointed.
But will it work? yes, it would work best for heating a swimming pool but not well for heating domestic hot water.
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Old 07-14-11, 12:51 AM   #4
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If I understand correctly, you want to plumb this before the hot water tank, not in parallel? If so, then this will work only if you are using water during the whole day, else the stagnant water in the pipes will overheat and boil. What you want is a circulation pump taking water from the tank and returning it after it has gone through the solar collector. Also, make sure that it does not circulate when the sun doesn't shine, else you'll be losing heat.

Other than that, it'll work, but as Ryland mentioned, it's not the most efficient way of doing it. DIYing a better collector (box with glass on top and insulation all around) wouldn't be much more expensive.
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Old 07-14-11, 04:51 PM   #5
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Quote:
Originally Posted by Xringer View Post
Why would you put a pressure relief valve up there?
One thing that I have seen up there, is an air bleeder.

On my old system (Hot water pre-heater using glycol & water),
I had a manual bleeder installed at the high point.
Once or twice a year, I would go up and open the bleeder to let any air escape.
IIRC, there never was much air in the system.
Some systems have an self-bleeder valve up there. (float type).

What kind of weather do you get at your location??
Around here, that type of open system wouldn't work real well in the winter.
Here, we need some good insulated flat panel collectors,
or some of those Newfangled vacuum tube units.
I was just worried about pressure in case the water got too hot.

I live at Medina Lake. That's just a few miles NW of San Antonio. So far last month and this month we have seen days in the triple digits and upper 90's.
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Old 07-14-11, 04:56 PM   #6
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Quote:
Originally Posted by Piwoslaw View Post
If I understand correctly, you want to plumb this before the hot water tank, not in parallel?
Yes, before the tankless to reduce and perhaps have it not turn on when I use hot water.

Quote:
Originally Posted by Piwoslaw View Post
If so, then this will work only if you are using water during the whole day, else the stagnant water in the pipes will overheat and boil.
That is why I thought I would need the pressure relief valve.
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Old 07-14-11, 07:56 PM   #7
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Quote:
Originally Posted by iamgeo View Post
I was just worried about pressure in case the water got too hot.

I live at Medina Lake. That's just a few miles NW of San Antonio. So far last month and this month we have seen days in the triple digits and upper 90's.
If it's not valved off from your tank, (Which should have a T&P relief valve on it),
then you aren't likely to need it.
But, if you have valves on it, that isolate it.. Yeah, it might start leaking from over-pressure.

I've been around the Medina Lake area a few times, I had an uncle that lived
in that area back in the late 1950s.

Since does get below 32F sometimes, I think you might need a drain down system.
That means you need to get about 98% of the water out of the outdoor
part of your system before it gets real cold.
And then be able to refill it, without too much air staying in the loops.

I remember jumping off the steps one really cold morning down in Del Rio,
I landed on the plastic water hose and it broke into about 50 pieces.
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Old 07-15-11, 01:24 AM   #8
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the pvc probably won't last long in the sun like that, even painted it'll breakdown pretty fast. It will also add a plastic smell/taste to the water, most people using PVC heat exchangers have switched to pex since it doesn't seem to change the water.

that system will be a nightmare if freezing is ever possible. you really need to do a draindown or antifreeze setup in those conditions.
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Old 07-15-11, 07:52 AM   #9
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Drain back system, that could use water in the loop.


Solar Heating - DHW | Colorado Solar Energy Industries Association




My Glycol system looked like this. The Expansion tank prevented any high pressure from building up.

My pre-heater tank was 84 gallons, and my standard water heater was my 76 gallon HS Tarm oil burning boiler..

My system worked great for years, but town water is very hard on metal.
First, the 84 gallon tank started leaking at the un-used heating elements,
not too hard to repair, then my heat exchanger started leaking at the clean-out gasket.
Sadly, the bolts wouldn't come out after the water did it's work..

Home heating oil was dirt cheap and my wife hated those panels on the roof..
The system had paid for itself.. So I took it down.. Wish I hadn't..
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Old 07-15-11, 08:40 AM   #10
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PEX tubing would be freeze resistant (it can stretch a little), but without it being in a frame it's still going to loose heat pretty fast and without a storage tank you are only going to have water when the sun is shining, that is going to be your biggest issue, roofs tend to be strong enough tho that if you really do live in an area that is warm all winter you could do a roof top storage tank and thermo-siphon so you don't need a pump, you would need a tank that could handle the pressure tho but then you just put it in a super insulated box on your roof and you have solar heated hot water 24 hours a day.
a system like this would not however work well in an area that had freezing temps as you could not be assured that the tank would never freeze, but if you got the rare frost at night the thermal mass of the tank in the super insulated box would even keep the water hot for a few days.
PEX, at least the stuff that NIBCO makes is UV resistant for short term exposure and with luck the black pant on it would keep it protected from UV longer.

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