Help with trouble shooting hot water

[LucMan]

Solar thermal was the best deal for the dollar until panels got well under a dollar a watt.I have a friend with solar thermal who has had several problems.Difficulties with overheated glycol loop when the pump jammed or the heat exchange tank reached its maximum safe temperature,pop off valves that stick open and of course with the possible exception of the more expensive vacuum panels they don't work well or at all when the temperature is below zero.Also in my experience plumbing is harder to do right than wiring.I had photovoltaic panels on my roof for 30 years with only occasional cleaning.I've never seen a system with a pump in it ;especially in hot water systems that held up anywhere near that long.I think solar thermal is great but I think the amazingly low panel prices now make for different economics.One thing I still need to research is the effect of less than optimum lighting.Of course lower than rated amperage will drastically lower the heat out put but would that somehow damage the panels?It would be easy enough to add a low voltage cutoff but would it be necessary?The load would be undamaged since unlike an electrical motor it is purely a resistive load.You would still need the usual diodes to prevent a brightly lit string from back feeding a dimly lit one of course.Over sizing the panels somewhat for the heating element would minimize the times when the voltage dropped off.

That's why I stick with drainback systems, glycol is a PIA especially when it's not replaced every few years, or especially when overheated. It gums up everything including the inside of the collectors & associated piping.

[Naptown]

Take a close look at the element locations in those standard tanks. the only place it makes sense to inject solar is into the bottom. The top element is very close to the top of the tank to provide a quick recovery to a very small volume of water.
Now take a look at a tank designed for solar. notice how the lower element is placed in about the upper 1/3 of the tank?
It's there for a reason. To allow the solar to heat the bottom cold water and allow it to stratify toward the top yet still allow a reasonable amount of hot water in the event of no solar available.
If you are going to use a separate preheat tank then the element at the top would never be connected. and the required tempering valve should be placed at the outlet of the last tank not between the tanks.

[Naptown]

That's why I stick with drainback systems, glycol is a PIA especially when it's not replaced every few years, or especially when overheated. It gums up everything including the inside of the collectors & associated piping.

Think continued revenue stream.

[muskrat]

preheat tank looks better

A thermal system benefits in two ways from drawing from the bottom and inserting halfway up (equivalent to being the lower element):
1. It lets the main or backup heater do the last bit of heating to get useful hot water if the solar is not able to keep up.
2. It gives the thermal panel first crack at the incoming cold water, since the panel will be more efficient the colder the incoming water to it.

For resistive heating, the advantage is that it can heat all of the water in the tank if the power is available, while the grid powered upper element will be called for only when the outlet water temp is too low. And it will benefit from the preheating done by the opportunity load heater.
If the solar electric heat is only available occasionally, the right thing go to would be to put it at the bottom of the tank and also have a grid powered element in that same position. A little harder to do with a conventional tank.
In either case, you need to allow it to raise the tank temperature higher than the normal elements, and that then requires a tempering valve on the tank output for safety.

I will probably be using a preheat tank in my first system as I am trying to supplement a natural gas heater.I may leave a top element on line power at first and put a usage meter on it to see how often it kicks on.This may require some real world experimentation to see what actually works.

[Naptown]

Think about this
To heat an 80 gallon tank from 60 - 120 degrees it takes about 40000 BTU's
A kilowatt of of electricity is 3413 BTU
So to heat that tank to 120 will take 11KWH.
So in theory to heat that tank on a perfectly clear day with 4 sun hours would take 2750W of PV. That's in theory not practice. With losses of say 10% which is a very rosy estimate add another 250W so lets say 3 KW of panels.
Now that is provided the panels are producing at full wattage the entire time which we all know is not true. Add in the fact that at the lower insolation portions of the day where a grid tie system can actually produce something the voltage will be collapsing on the panels with a fixed resistance element. This will drastically reduce voltage and BTU output to the element to the point of being almost nothing. So the alternatives are to increase the wattage of the PV to a more reasonable figure to account for the reduced voltage at the element which will reduce the output exponentially (The inverse of the square if I remember correctly.
So now you have 6 KW of PV on the roof to heat the water. Add in a tank and the additional controls and wiring and you are very close to the cost of an inverter.
Oh and by the way if you use a tankless water heater on the natural gas you say you have it will save you about $.50 a day on a good day.
Wouldn't that 6KW go a lot further if grid tied?

[solar_realist]

Thread has been hijacked

This was some interesting information but just wanted help with thermo siphoning issue, please

[Naptown]

Sorry about that.
OK EVERYONE BACK ON TOPIC