designer

solarnut

Have two Grundfos UP26-99 pumps in series for a drainback hot water solar installation. One of the pumps died. Is there a fixed load ECM pump that would fit in its place and are the savings worthwhile?

hot_rod

wow

that's a lot of pump, what is the lift head from the water level in the tank to the top of the collector? How many collectors and how many GPM do you need to flow once the siphon is established? Two 26-99 in series gives you close to 60' of head!



It is possible to series two different circs also. I paired a 15-58 with an Alpha on my system. the 15-58 drops off after the fill, and it runs with about 20W of pumping power with the Alpha.



The 26-150 would give you near 45' of head and could be operated on a variable speed function, modulating it's speed based on detla t.



The 15-100 is another nice high head low flow circ from Grundfos, it covers a lot of residential DB systems.



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solarnut

Alexander

Thanks for your reply.

In answer to your question. Yes that is a lot of pumping power maybe too much.

One of the UP26-99s went down probably form a snapped shaft.

The tank is in the basement of a two story house. The pumps are piped well below the water line of a 900 gal storage tank and the return is piped into the tank with holes drilled into the 1 1/2 return. The 12 evacuated tube collectors are raised on kindorf about 4 ft of the roof surface. So the manifolds are about 10 ft above the roof. So we needed at least 40 ft of lift. There is a set of HVAC units mounted on I beams in the middle of the roof. So I split the array into two at the roof.

Thanks again for your interest.

hot_rod

remember also

the lift head is calculated from the water level in the tank,, not the pump location. This is why some installers install the drainback tank up high in the building. You just need to drainback the piping exposed to freezing not all the piping from the tank to the collector.



So it is possible your lift is less then 41'?



Regardless there are single pumps that can do the job.



I have heard of pumps snapping shafts when one is running and the second tries to start up??



With todays differential controllers the control could be sensing an increases delta T and trying to fire up the additional pump. Or maybe the control thinks the siphon head is lost due to a high delta T between the tank and collector and starts up the additional pump?



If you do in fact have a snapped shaft in a clean, debris free solar loop, I doubt it was a pump defect.



Here are a couple drainback schematics from Caleffi i-dronics 6



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solarnut

Alexander

Of course you are right about that the lift is calculated from the tank water level.

There are a lot of elbows etc. even so the job is over pumped. This is the reason for my post.

The original question remains. What pump can I replace the one 26-99 with and save the customer on the electric?

On the tank location; since this is a private house I couldn't very well put a drainback tank in one of the bedrooms upstairs. It's a flat roof so no attic.

Economic considerations are a very big factor. At the time and maybe this is still the case it was cheaper to install two pumps than one bigger pump. And the technical person hired by the owner specified these two.

hot_rod

so how many

tubes and what brand? 12 total tubes or 12 racks of 10 tubes? Using Apricus evac tubes for example they require .026 gpm per tube. So 12 tubes requires less than 1/2 gpm.



If you have 120 tubes (12 racks of 10 tube arrays) X .026 you need to move a bit over 3 gpm.



In a drainback that lift is only present until the system fills. then the pressure drop is just the resistence thru the pipe, fittings and headers, maybe 5- 8 feet of head?



I assume the control shuts down one pump after several minutes, using both pumps only during the fill phase?



So the pump required to flow the system after fill could be a small high efficiency circ like the Grundfos Alpha, if in fact you need around 3 gpm.



With 41 feet of lift you would need to pair the Alpha with at 26-99 to add the head of the two pumps. Run both pumps until the siphon is established, then drop to only the Alpha.



The pump is sized to the GPM required and the head. In a drainback the head changes from the fill phase to the run phase.



These slides show how the system curve shifts during the fill and run sequence.



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Kevin_in_Denver_2

Cast Iron pump?

One common problem would be the use of a rose colored pump in that open system.   The symptom is a snapped shaft.

solarnut

alexander

There are 11 24 tube collectors and 4 20 tube collectors.Sunpower from China is the brand. The manifold connections are 3/4 sweat.1 1/4 inch pipe brings the water to the roof then is manifolded  into two 1" branches and feeds 6 rows of collectors. 3 rows of 3x24 tube collectors and 3 rowns consisting of 1 2x24 and 2 of 2x20 a total of 344 tubes.

Two things I do not understand. If the system is drainback and the height from water tank level to the highest point on the system is what it is. In this case we figure about 35 ft. how is flow ever established?

The return is gravity and unless we over pump it to get the water back to the tank faster that gravity, there will never be "a closed loop"

Also how can one pump drop out? What controls that? My differential control is wired so that it turns on the pumps through one ice cube relay ( to protect the control). Both pumps turn on at the same time. Will I need to intall another control to drop out the 26-99?

solarnut

alexander

open system = oxygen = bronze pump

Fortunat

pumps

You are right, if you need 35 feet of lift, then the pump combination in Rod 's example is not sufficient. You need a pump which exceeds 35 ft of head at very low flow to start the system.



Once the collectors are full and the fluid starts back down the return pipe a siphon is established and the falling fluid effectively 'pulls' up the rising fluid on the supply side, so at this point the system is essentially a closed loop and at that point you can shut off the big pump and just use the low wattage circulator to keep the fluid circulating



~Fortunat

www.revisionenergy.com

hot_rod

DB systems

I could not find any info on that brans at the SRCC site. Perhaps listed under the manufacturers name from China? You need to know the flow they require of reccomend to properly size the pump(s).



Most of the newer style solar controllers have a drainback function. it gives you the choice of dual pumps, with one dropping off after the siphon is established. Or one pump sized to cover the lift which then runs on a variable speed function based on the collector to tank deltaT. It's possible a single pump like a Grundfos 15-100 could work for you with variable speed. the variable speed does save energy as the power drops as the speed decreases. Or an ECM pump if you can configure the control to vary the speed.



A DB system can be an open loop, closed loop, or closed loop pressurized. i prefer the pressurized as it eliminates pressure issues, flashing to steam, and related noise and loss of siphon. Putting pressure on the system assures you don't break up the water column at the hottest, and lowest pressure point in the system, the top of the collector.



Of course all the component need to be pressure rated in that type of system.



The pipe needs to be sized to assure at least 2 feet per second velocity to move the air bubble back to the tank. Over or undersizing the return piping will cause problems.



So to help with pump sizing you need to know how many GPM the collectors need, and the exact distance from the water level to the top of the highest header.



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