Simple Hydronics Question with a Lot to Unpack: Why Two Pumps?

I get a lot of text messages, that new kind of way to exchange information. I like it when it works and hate it when it doesn’t. The part I like the best is the speed I can sometimes get an answer if I have a question. It sure beats the old way of pay phones, pagers, and phone tag with messages left with the office.

This photo is from a younger guy who grew up with his dad being a heating contractor, like so many guys in the business. He asks a lot of questions, which I think is great. He has a natural curiosity, which he might have gotten from his father. He also isn’t afraid to ask questions, which I also think is great.

When I was new in the business, I wouldn’t always ask questions. I think the reason was to not look like I didn’t know the answer already. I think I have gotten over that bad behavior over the years. I encourage you to ask questions to make sure you understand that concept or suggested piping diagram. A better understanding of what you’re working on results in better work.

Two Pump Problem

He wanted to know if “you get twice the flow out of these pumps or same flow at lower head?” What you see are two pumps flanged together so one pump discharges directly into the other. In the trade, it is referred to as pumped in series.

To answer the first part of his question, No. The gallons per minute flow rate stays essentially the same. If the pumps are the same size, the flow stays the same if one pump or both pumps are running when piped in series.

If the pumps are piped in parallel, side by side, then the flow rate increases to essentially the total of how many pumps are running. If the same size, you can assume that the flow rate doubles (although the engineers of the world will probably say something else, but very close to double).

To answer the second part of his question, No. Actually it is essentially the same flow rate but at close to double the head rather than at a lower head. Again, the engineers of the world will be able to calculate that more accurately. The rule of thumb is, pumps in series double the head with flow remaining the same, while pumps in parallel double the flow with head remaining the same.

Max Ratings

I texted back, “same flow and almost double the head of one pump”. He then stated that one pump is rated for 17.6 gallons per minute at 15.4 feet of head, so these pumps in series would produce 17.6 gallons per minute at 30.8 feet of head. Knowing that this mini-pump wasn’t capable of that kind of performance, I told him that he might be reading the curve incorrectly.

He replied “that’s what that pump was rated for at the max.” Now I knew where he had gotten off track. The max ratings for the pump were correct, but pumps don’t operate at their max ratings. They operate at somewhere in between the max ratings, where the flow rate is limited by the head of the system.

You might get close to 17.6 gallons per minute if the head of the piping is about zero, or you might get 15.4 feet of head if the gallons per minute is about zero. Zero doesn’t happen in closed loop hydronic systems. Once the pump is energized, water starts moving and the system head is quickly created by the friction of the water moving through the pipes.

The more gallons per minute in the same size pipe creates more head. The other part of the pump head question is the length of the pipe. Again, the longer the length, the more head it creates. The reference I use for these calculations is the B&G System Sizer. Some systems have very little head loss because of over-sized pipes, like 100-plus-years-old gravity hot water systems. Some systems have very high head loss because of long piping runs, like modern geothermal loop fields.

The Water Doesn't Care

His next reply was “never knew why that was done.” I mostly see pumps in series on geothermal jobs. But the picture looks like a radiant floor job, which can also have higher head because of under-sized PEX tubing or unusually long loop lengths. The reason is higher head at lower flow rate, which can be dictated by design.

My reply was “two pumps are cheaper than one pump at the same duty point.” That’s what I was always told and the cost of goods is a powerful driver in this industry. If it’s cheaper to bolt two pumps together than the price of one pump, then somebody’s going to do it. The water moving around doesn’t care.

We then moved on to sizing a pump, “have to feed a hot water coil with little head loss, around 50k btu.” I know from experience that a typical mini-pump—choose your favorite flavor—will do just fine for that type application. You can assume about 5 gpm for the flow from the btus and maybe 5 feet of head from his description. That hits the curve perfectly on the low speed of the mini-pump that we stock.

Update

For those of you tracking my health, I’ll report that chemo is a challenge and I can’t wait until it is over. I now have a better perspective of people suffering through medical issues.