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Old 05-31-12, 09:52 PM   #30
GaryGary
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Join Date: Nov 2008
Location: SW Montana
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Hi,
It looks like your collector area is 166 sf and your vertical distance from tank water level to top of collector is 16 ft?

If so, I'll make the case for something like a Grundfos 15-58, and you can compare it to the Laing pumps.

The Grundfos 15-58 looks like a good match for your startup head requirement of (16 ft)(1.2) = 19 ft.
And if you go with about 0.04 gpm per sf of collector, you need 6.6 gpm. At 6.6 gpm, the 15-58 delivers about 13 ft of head, which is probably enough to overcome pipe friction (which is all you need to do after startup).
You can work through this sizing procedure to make sure: Pump and Pipe Sizing for a Solar Water or Space Heating System
This is something you want to do for any pump you are considering.
The pump curve for the 15-58 is on this page:
New Page 1

The 15-58 may actually do more than the 0.04 gpm/sf, which will give you somewhat greater efficiency -- all good.

The 15-58 is $75 depending on where you buy it.

On high speed power consumption is 85 watts.

The pump is very quiet -- I have to touch mine to see if its running.

Its good up to 230F.

These cast iron circulators are built like tanks and have the reputation for very long lives.

I don't mean to sound like a salesman for Grundfos -- Taco and others make similar pumps that would likely work as well.

I have trouble with the argument for varying pump speed to save pump power under lower sun conditions for these reasons:

When your 166 sf of collector is in full sun and assuming it achieves a standard sort of efficiency, it will be producing about 24000 BTU/hr or about 7035 watts.
This is from: Solar Collector Efficiency Calculator

If you look at this page:
Determining Solar Water Heating Collector Flow Rate
The drop in collector output in going from 0.04 gpm/sf down to (say) 0.02 gpm/sf is about 3%, or 210 watts. So, to me, one message is that you want to be very careful in dropping pump flow rate on the idea that you will end up saving pumping power -- the drop in collector output for such a change is likely to be a lot more than the saving in pump power.

Granted, the example above is done with full sun, and the drop in collector output would be less than 210 watts in part sun, but is it really going to be less than the 20 watts or so that you save by ramping the pump speed back?

To me it, the variable speed pumps seem like a lot of complication and expense to maybe end up not saving anything? I'd be glad to be wrong about this if someone has good numbers to show a real saving under real operating conditions.

The one advantage I do see to variable speed pumps for drain backs is that they can prevent the kind of short cycling that sometimes occurs under low sun conditions when the collector can get hot enough to trigger the controller to start the pump, but cannot collect enough heat to keep the controller from turning the pump off as the collector cools when the flow starts. But, this is not normally a problem worth worrying about much, and a simple 2 speed pump would be plenty to handle this.

I'm not saying this is as slam dunk decision for either Laing or the Grundfos (or similar) pumps -- I can see pluses and minuses to both.
To me it comes down to is the power saving worth the extra cost and the risk associated with the low 140F operating temperature limit?

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Very nice job on the tank!


Gary

Last edited by GaryGary; 05-31-12 at 09:55 PM..
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