Type of pump required for my drainback open solar panels

[mbaker]

Hello, My solar panels heat water which is dumped into the top of my hot water tank. Then water from the bottom of my hot water tank is pumped through the panels. Recently the pumped failed and I need a new one. I have tried several and the only one that doesn't require repriming everyday is a self priming pump. The old pump doesn't give me much information to say if the old pump was self priming or not. My installer doesn't want the work!!

The trouble is the pump I got requires too much power from my PV solar panel. However it works fine if powered by my power supply unit.

Question one: On my drain back system, do I actually need a self priming pump or should a non self priming pump work? Maybe air is getting into the system some how??

Secondly, I think I need a 12v DC self priming hot water pump 1M headish and 4-5 watts. MY panel only supplies 12v at 5W. Anyone any idea where I may buy pump like this?

[LucMan]

Hello, My solar panels heat water which is dumped into the top of my hot water tank. Then water from the bottom of my hot water tank is pumped through the panels. Recently the pumped failed and I need a new one. I have tried several and the only one that doesn't require repriming everyday is a self priming pump. The old pump doesn't give me much information to say if the old pump was self priming or not. My installer doesn't want the work!!

The trouble is the pump I got requires too much power from my PV solar panel. However it works fine if powered by my power supply unit.

Question one: On my drain back system, do I actually need a self priming pump or should a non self priming pump work? Maybe air is getting into the system some how??

Secondly, I think I need a 12v DC self priming hot water pump 1M headish and 4-5 watts. MY panel only supplies 12v at 5W. Anyone any idea where I may buy pump like this?

Relocate the pump to the bottom of the tank so it won't lose it's prime.

[mbaker]

How does a Drain back work anyway

Thanks for the reply, I've lowered the pump and it seems to be working ok.

The drain back system relies on water draining from the panels so they can't freeze. Why do they drain? How long would the panels take to drain? Does the water drain back through the pump? I guess it must do because where else will the water go to. Do all pumps allow the reverse flow of water? I've looked on the net for theses details but I've been unable to track anything down.

[inetdog]

Thanks for the reply, I've lowered the pump and it seems to be working ok.

The drain back system relies on water draining from the panels so they can't freeze. Why do they drain? How long would the panels take to drain? Does the water drain back through the pump? I guess it must do because where else will the water go to. Do all pumps allow the reverse flow of water? I've looked on the net for theses details but I've been unable to track anything down.

The system cannot drain back unless the tank is unpressurized and you have an air inlet valve at the high point of the system. If it is unpressurized, then when you turn off the pump the water will drain back through the pump or through a one-way valve into the normal return line at the top of the tank, with air coming in through the one-way valve at the top of the panels. When you turn the pump back on again, you will either force air into your storage tank or you will also need an air bleed valve at the top. That valve will allow air to pass out freely but not fluid.

If you are trying to circulate water from a pressurized system through the panels, you cannot drainback unless you have a bypass valve and way to dump the water out at zero pressure (e.g. on the ground or down the drain) when you switch off the pump and the outside is near freezing.

Drainback systems are most commonly used for pool heating where the system is not pressurized and the water just ends up in the pool. The flow rate will be high enough that the air will be forced out when the pump starts up again (and the input to the panel will be at the low point!) Bubbles into the pool will not be a problem.

Can you post a full set of photos of the system or a plumbing diagram so that we can help you figure out how it is supposed to work?

[mbaker]

Diagram

Hello,
I found this diagram on solarfriend.(http://www.solarfriend.co.uk/)

I just need to explain relatives heights as the diagram is not to scale. I live in a 2 floor house with a slanting roof.

The panels are on the slanting roof, the water comes into the panels at the bottom from my pump, and comes out at the top then directly into my hot water tank.
The hot water tank is on the second floor and the cold water tank in the attic above. Pipes are silicon tubing.

On the original system the failed pump was at the top of the water level of the cold water tank(more or less) in the attic. I've had to lower a new pump to the attic floor to get it to work consistantly.
I also power my new pump directly with main power as I 've not been able to find a suitable pump that can be powered by my 12V 5W PV. The old pump was a Charles Austen DL2. If I could find out the spec for this pump it would help.

My system may have an air vent at the top to allow air in.. I don't know(this is one of the things I am trying to find out). Remember my installer doesn't want to know and another 'expert' seems most reluctant to install my probe(he's says he has never seen one like mine)

I am also trying to get a temperature probe installed into the panels and connect this to a controller as at the moment I have no control other than turning the pump on and off manually. The system actually works now with me manually turning the pump on/off remotely.

[inetdog]

Hello,
I found this diagram on solarfriend.(http://www.solarfriend.co.uk/)

I just need to explain relatives heights as the diagram is not to scale. I live in a 2 floor house with a slanting roof.

The panels are on the slanting roof, the water comes into the panels at the bottom from my pump, and comes out at the top then directly into my hot water tank.
The hot water tank is on the second floor and the cold water tank in the attic above. Pipes are silicon tubing.

On the original system the failed pump was at the top of the water level of the cold water tank(more or less) in the attic. I've had to lower a new pump to the attic floor to get it to work consistantly.
I also power my new pump directly with main power as I 've not been able to find a suitable pump that can be powered by my 12V 5W PV. The old pump was a Charles Austen DL2. If I could find out the spec for this pump it would help.

My system may have an air vent at the top to allow air in.. I don't know(this is one of the things I am trying to find out). Remember my installer doesn't want to know and another 'expert' seems most reluctant to install my probe(he's says he has never seen one like mine)

I am also trying to get a temperature probe installed into the panels and connect this to a controller as at the moment I have no control other than turning the pump on and off manually. The system actually works now with me manually turning the pump on/off remotely.

That helps a lot. As long as the bottom of your panel is above the top of the gooseneck to the cold water tank, you automatically have a drainback system as provided that there is an air inlet somewhere at the top of the panels and your pump is the centrifugal impeller type which will allow water to flow backwards.
As long as the panels are high and the pump is run only when the panel temp is above the hot water temp, you will not be wasting heat into the panels.
But you must have a temperature sensor, and ideally the outlet of the panels should go to the middle of the tank rather than the top. Connecting it to the top will not waste overall heat, but may give you colder water to use than what the heating element has produced at the top of the tank. A separate pre-heat tank connected to the solar panels which then feeds as needed to the main tank would avoid that problem, but you might have a hard time installing it.

[mbaker]

The position / height of the pump seems to be critical, where should it be?

Thank you inetdog. Very useful.

Can you give some guidance on the height of the pump with respect to the attic cold water tank. Earlier it was suggested to move the pump below the attic water level (I did this and things improved).

What I dont understand is why the original pump was installed at the top of the attic water tank level.
The lower I put the pump, the higher it has to push water. Is it that some pumps can push water, but not pull it. If so that would make sense.

Are centrifugal impeller type pumps self priming? Must the pump I use be self priming. What are my options?

[inetdog]

Thank you inetdog. Very useful.

Can you give some guidance on the height of the pump with respect to the attic cold water tank. Earlier it was suggested to move the pump below the attic water level (I did this and things improved).

What I dont understand is why the original pump was installed at the top of the attic water tank level.
The lower I put the pump, the higher it has to push water. Is it that some pumps can push water, but not pull it. If so that would make sense.

Are centrifugal impeller type pumps self priming? Must the pump I use be self priming. What are my options?

There is a limit to how high a pump can suck, set by the air pressure. You cannot suck harder than a vacuum, so you are limited to an absolute maximum of 34 feet. Some types of pumps just do not suck even that well.
The issue of self-priming is that centrifugal or turbine pumps cannot move air very well, so if they end up above water they cannot suck the water up any distance at all.
Positive displacement pumps can move air, but the sucking to get the water in may not be very efficient and the pump may be damaged by running it dry. Especially if the water is expected to serve as a lubricant.
Some centrifugal impeller pumps are self-priming to some level, often less than 10 feet.

Any pump that can push water can pull water. The problem is entirely in making sure that the pump has water in it to start with.
If your pump is below the water level and there are no valves to prevent the water from filling the pump, it does not need to be self-priming.
I have no idea why the pump was originally installed at the top of the water tank.