Heating Help

Drainback booster pump question

I'm putting together a 384 square foot drainback system with approx 25 feet of vertical head, plus piping and collector loss. I'm planning to use a Tekmar 157 pump controller. The 157 can operate a booster pump to help fill the collectors at startup. In an effort to reduce energy consumption I would like to use a large pump as the booster and a smaller pump to continue circulation after the return siphon is established. Is there any problem with using two different size pumps in series for this application? I assume the larger pump should be pushing through the smaller pump to avoid cavitation?

it should work

I ran my system with a Grundfos 15-58 and an Alpha paired together. The 15-58 would drop out after 3 minutes and the Alpha would run the system with about 20W of energy.

I switched to a Grundfos 15-100 on variable speed drainback function, mainly to free up one of the relay outputs. Dual drainback pumps require both relay outputs in the control.

I use that second relay to shuffle to the hot water tank.

hr

15-100

HR,
Then what wattage does the 15-100 use for steady state pumping?

current draw

1.1 amp. For drainback it runs 100% speed until the siphon is established, then it modulates bases on delta T between the collector and tank.

pumping efficiency

I was going to use one large pump and just let the controller modulate the speed, but I had concerns about the efficiency of a large pump running at a very small percentage of it's rated capacity. I believe it's better to keep a pump operating as close to it's design curve as possible. A single large pump would only run in it's "sweet spot" for 3 minutes out of the day. My thought is that the pump should be sized to meet the whole day circulating requirement, not the 3 minute startup load, hence the need for two pumps in this type of system. A small variable speed pump that's closely
matched to the circulating requirements of the collectors, and a large
one on a timer to fill them at startup. I understand the pump curve changes as the motor speed changes, but the curves are not documented for anything but full speed (Grundfos Alpha excluded). The impeller and housing on a single speed pump are designed for maximum efficiency at full speed, who knows what happens to the mechanical efficiency when you slow them down.

I have Taco 005 and 0011 stainless pumps that I bought for this purpose. They looked good on paper, but seeing the two side by side and realizing how different they are in size made me wonder if it's a good idea to have them close coupled in series.

inefficiency is the name of the game

with wet rotor circs. At best they are mid to high 20%. I did test the pumps on a solar control with variable speed control. At 50% speed they do not consume 1/2 the energy. it's not a direct ratio.

If you have a flowmeter in the circuit and an amprobe you can calculate that efficiency under actual conditions. Here is a link to Siggy's article with the formulas

www.pmmag.com/Articles/Column/478f1350f4fc7010VgnVCM100000f932a8c0____


www.pmmag.com/Articles/Column/9b2042f2480d7010VgnVCM100000f932a8c0____


If uber efficiency is the goal, I'd look at one of the ECM's like the Alpha. Chose the fixed speed that matches the flow you need. I suspect you will use 40- 50% maybe less energy than a typical circ.

hr