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Old 02-19-10, 08:45 AM   #1
Heynow999
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Default Solar hot water and electric tankless

HI

I install solar pool heaters, solar domestic hot water and PV

I don't have a lot of practical experience with domestic hot water, I have only installed 4 systems, but one of the systems is in my house, and I have taken factory training from several different manufacturers. I have seen the many different ways that companies deal with the heat exchange from gycol to domestic hot water.

I always want to try to make things work better. On my system at home I have a tank with an internal heat exchanger at the bottom of the tank and an electric element at the top. The problem I see with this is the solar has to heat the whole tank higher than the temperature that the top element is set at to stop that element from coming on. You also want to put the solar hot water at the top of the tank so it can be used first.

The conclusion I have come to is that the best system would be like the one "Enerworks" uses where they have an external heat exchanger that uses convection to draw the cool water from the bottom of the tank and put the then heated water into the top of the tank, where it can be used first.

(I just want to add a side note here. Enerworks has two systems, a US one and a Canadain one. The US one uses one tank and seems much more efficient. The Canadian one uses a second tank as a pre-heat tank, then goes into the existing water heater. I always thought that was dumb, your solar heated water has to work it's way through your existing water heater where it is going to cool. I asked the tech guy and he told me the Canadian government made them do it that way. So things are not always done with the highest efficiency in mind)

The second idea I have to improve efficiency if you are using electricity to heat water is to use an electric tankless water heater, for obvious reasons.

I have some other opinions as well, just didn't want to make too long a post


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Old 02-19-10, 11:11 AM   #2
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Hello and welcome to the site!

I think your current setup is superior to what you are describing going to. The point being that you always want your heat exchangers subject to the largest delta T (difference in temperature). By placing the heat exchanger at the bottom of the water tank, it is always subject to the coldest water. This means more heat will get transferred from the glycol to the domestic water. This cools the glycol down more and means when it passes through the solar collector it will also absorb more heat. The net effect is more heat transferred and a more efficient system.

With the natural convection external heat exchanger, by the time the water gets to the upper portion of the exchanger, its already decently warm and the heat transfer isn't as great. You can get the same heat exchange, but you'll need a larger exchanger for this style.

I would definitely look into putting a timer on your electric heating element though so it doesn't kick on until you actually need hot water (likely only in the early morning as the solar will keep it hot during most other times). A tankless heater is a great alternative.
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Old 02-19-10, 01:40 PM   #3
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Another option is, if you can add another heat exchanger to the top of the tank as well so the hottest fluid from the collectors heats the top first then goes to the bottom and gets the last bit of heat pulled out before going back to the panels.
A timer is a must as on a normal electric hot water tank it will cut the electrical usage by half.
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Old 02-19-10, 06:54 PM   #4
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I have compared the two different types of heat exchangers, external flat plate VS internal coil. I know that on my system they glycol will only drop about 4C after it goes through the heat exchanger. I have two thermometers on the pump station. A friend of mine has an external heat exchanger and he sees a huge drop in temperature when the glcoy goes through it. I don't remember any numbers and I know this is not scientific evidence but I remember being really surprised when he told me the numbers

The other thing I would do different next time is I would make a drainback system so I could use water as a heat transfer fluid. Water is better (and cheaper!) than glycol. I am tired of wiping glycol off the wall.

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Old 02-19-10, 07:51 PM   #5
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"I am tired of wiping glycol of the wall."

Ha ha! Just wait until you get old. They make you drink a gallon of glycol almost every year after you turn 60..
(I just finished my 4th gallon last month).

Anyways, when I had my solar hot water system (back in the 70s & 80s),
I never had any problems with the glycol loop.
But the domestic water loop in the exchanger and the 84 gallon tank started leaking after about 10 years.
The water in my town does not like copper!

I was using it to pre-heat the (cold) water input to my (oil)heating boiler,
http://i46.photobucket.com/albums/f1...HSTARMOT35.jpg
which also has a hotwater coil inside (hatch in the top).
So, I never hooked up the upper heating element that came installed on the unit.
I ended up removing it when it started to leak. Eventually, I took out the whole leaky system.

Now, I'm thinking of using PV and a resistance heater on the old boiler.
Which should help save a little fuel oil, provided I can find PV for $2 USDs a watt..
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Old 02-20-10, 08:28 AM   #6
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Quote:
Originally Posted by Heynow999 View Post
I have compared the two different types of heat exchangers, external flat plate VS internal coil. I know that on my system they glycol will only drop about 4C after it goes through the heat exchanger. I have two thermometers on the pump station. A friend of mine has an external heat exchanger and he sees a huge drop in temperature when the glcoy goes through it. I don't remember any numbers and I know this is not scientific evidence but I remember being really surprised when he told me the numbers
It sounds like his heat exchanger is just better sized for his application. I think I might try what Ryland suggested and run them in parallel. That way you don't need to buy a gigantic external exchanger.

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