Finding a good pump for this system is still a bit of an open issue. Given that the pumping requirements are modest and the maximum temperatures are reasonable (140F), it seems that there ought to be a good, long life, pump out there for a reasonable price. But, it has not been easy to find one. Some alternatives are listed below -- if you know of any others, please let me know.
In a nutshell, the pumping requirements are: around 2 gpm flow rate, sufficient starting head to pump water to the top of the collector, OK for 140F water, low power consumption, long life.
Some PC owners who want the best performance have started to cool the PC chipset with water. The pumps they use for this look like they might work for this system. Its an application where long life, low power, and moderate temperatures are important -- just like a solar water heater pump.
Laing/Shurtech MCP355, MCP350 and a couple other models from the same outfit:
These are 12 volt DC pumps, 50,000 MTBF, 5 year life, 18 watt, max head might be 13 ft or 20 ft (I've seen both listed), 140F max operating, about 2 gpm -- about $65.
It will need a 12VDC power supply -- a wall-wart type should do.
I believe that these are non-submersible, and are not self priming, so a U-Tube style mounting outside the tank is probably in order.
The MCP655 is a similar design with higher output.
Given that the MTBF is 50000 hours, it seems like it should do better than a 5 year life with 5 or 6 hours of operation per day?
I have an MCP350 on order, and will report on how it works out.
There are other brands of this type of PC cooling pump that might also be worth a look.
Taco and Grundfos both make a line of HVAC circulators that are commonly used in radiant heating and solar systems. They are rugged, will withstand high temperatures, and typically have a long life. For this kind of vented tank and plain water, one should use the bronze or stainless steel case pumps -- not iron.
These tend to be in the $200+ area, but do come up on ebay for less at times.
The power consumption of these pumps is fairly high -- in the 70 watt area depending on model.
For example: a Taco 007 has a startup head capability of 12 ft. The flow rate is quite high for moderate resistance. Good up to 230F. Cost is about $200 for a bronze case pump.
These pumps have to be installed outside the tank, but below the tank water level to retain prime -- probably using a U tube arrangement.
Pump curves for these pump...
March also makes pumps in this category.
In all, they seem like overkill on flow rate, fairly expensive, and kind of high in power consumption -- but sturdy and long lived.
Notes on installing these pumps:
They must be installed below the storage tank waterline in order to retain their prime when the collectors drain back -- that is, the pump housing must remain full of water after the drain back so that the pump will be able to start up the next time.
Also, Matt reports that mounting these pumps in the horizontal position can result in an air pocket in the pump housing that prevents the pump from starting after drain back. These pumps should be mounted vertically (i.e. with inlet down and outlet up to prevent this problem).
Laing makes a PV powered pump intended for use in solar systems. It gets a lot of good reviews from people who have used them. They are optimized for use with PV panels. The unique design is said to result in a very long life.
Search for Laing D5.
Its about $200, and would be a good choice for a PV powered system. But, see notes on PV powering a drainback system below.
This might be a good, long life pump to just use with a DC power supply?
Garden center type places sell submersible pumps for fountains that will work (at least for a while). They need to have a maximum head rating that will reach the top of your collector.
These pumps vary a lot in quality. I've noticed that some of them are brushless, seal-less, magnetically driven, and one that I took apart had a ceramic shaft for the impeller, so they are not all junk. I would guess that they all would say that 140F is outside the range they are designed for, but they may still do pretty well at this temperature.
They do not tend to be very energy efficient, but some of the recent ones have "energy efficient" printed on the box (whatever that means).
They often go on sale toward the end of summer.
I have a whole box of these things, and use them for tests and what not -- I've never actually had one stop working, but then again, I've not put many hours on them.
Some of these pumps could be powered with a PV (solar electric) panel. PV power works well in closed loop systems where the pump only has to overcome the friction losses in the pipe (there is no high startup head requirement). In a closed loop system, when there is some sun, the pump will run slowly and produce some flow, and as the sun gets stronger, more flow is produced.
PV is not often used in drainback systems -- I guess this is because the pump must get up to full RPM to get the the flow initially started, so if you have weak sun the the pump would not be able to get the fluid up to the top of the collector to start the flow. That said, I did run the PEX prototype collector on the March 893 pump using a PV panel, and it seemed to work fine. So, it may just be a mater of choosing a bit larger a PV panel, and (perhaps) using one of the small linear current boosters that are made for PV pumping applications.
There is a whole debate that goes on (and on) about whether its better to use PV or grid powered pumps -- with a whole raft of pros and cons for each. I'm inclined to think that either works just fine.
Gary October 1, 2008