How to size my Circulator pump ?

joseluisheating

Hey everyone! All the baseboards have been installed, and we’re close to finishing the connections to the manifold. For both supply and return lines from the manifold to the utility room are approximately 100 feet of 1" piping.

Each loop will operate between 1 GPM and 2 GPM (I’m still deciding on the exact flow rate). To help finalize things, I’d appreciate your input on determining the proper circulator pump size and the required head.

At the moment, I have 300 feet of 1/2" PEX already installed for the loops. I estimate that I’ll need an additional 150 feet to fully connect all the loops to the manifold.

I’ve been considering the Alpha 15-58F Smart ECM pump, but I’m unsure if it will provide sufficient performance for this setup. Could you help me confirm whether this pump will meet the system’s requirements?

Attached is the full diagram of the baseboard layout, house dimensions, and the utility boiler setup for reference.

Thanks for your help!

EdTheHeaterMan

On a residential system we usually select the Pump Head by the longest run method. That is the farthest the pump has to overcome the friction of the piping. So use the length of the loop from the outlet side of the pump, to the manifold, then from the manifold to the radiator that has the longest travel to get there, then follow that back to the manifold and then back to the circulator.

You mentioned 300 feet of 1/2" PEX. Is that all the PEX or are you actually running 300 feet of PEX to one of the radiators?  I’m assuming that you are cutting up that PEX into shorter lengths to get to each radiator.  

Let’s say that you have about 60 feet of 1” PEX from the boiler to the manifold.  Then you have another 50 geet of ½” PEX to the farthest radiator. (room #11)  That would be a total of 110 feet from the boiler to the radiator. The return path would also be 110 feet.   So, you have a total of 220 feet of pipe friction to consider. Now add to that a factor of 50% for the equivalent length for all the fittings and the manifolds.  That comes to 330 feet total equivalent length for the longest path.   (if the pump can do the longest path, all the shorter ones will be easier to do.)

To keep the system from being noisy, we design to a 4 feet of head for every 100 feet of equivalent length.  So the equation will look like this:    

1. Total length of pipe for the longest run
2. Add 50% to that for the total equivalent length (220 x 1.5)
3. Multiply that by .04
4. This is your pump head.  

In this example given above you need a pump that will deliver the GPM required at a pump head of 13.2

If you could locate the manifold closer to the boiler , or perhaps central to the home so that room #11 is not so far away, then you could cut down on the amount of Pump Head required.

1. Since your shared piping is 1", then I'm assuming the you do not need any more than 8 gallons per minute. (ask me why)
2. That means that your pump needs to develop a flow of 8 GPM at 13.2 ft of head
3. The maximum Head for the Alpha 15 pump is 19 feet head and the maximum GPM is a little over 13 GPM that pump looks fine, but a closer at the performance curve shown that at 8 GPM the maximum pump heat available is 12. So you will want to use less tubing in some fashion on that longest run, or use less GPM which means less BTUh. BTW That problem will only show up on the coldest days because you only need maximum performance when it is at design temperature. At higher outdoor temperatures the system will be able to keep up just fine

By the way… Room # 9 has no return pipe to the manifold, that room's gonna be cold this winter.

Ironman

I’m not sure from your description if you’re saying the combined length of all your loops is 300’ or that’s what each one is.

You’re not gonna get 2 gpm from a 1/2 loop, particularly if it’s 300’, more like 1/2 gpm.

A pump is selected by the required gpm and the resistance (head) it must overcome.

The gpm is determined by the btus required at a specific delta T (usually 20* for baseboard).

The head is determined by the size, type and length of piping at the required gpm. You only use the longest run of piping in a parallel system to determine head plus the fittings, valves, etc.

Once the required gpm and the head are determined, that is compared to the pump manufacturer’s pump curve for the circulator being considered to see if it meets the requirements. You wanna stay within the middle 1/3rd of the performance chart when selecting a circulator.

joseluisheating

@EdTheHeaterMan @Ironman Thanks for your comment. And thanks again Ed for that specific example.

I now know how to size a pump. 300' is for all the manifold loops. The longest loop is the room#11 it is something like (1/2" pex) 75"X2 = 150 feets. Plus (1" pex) 50"X2 = 100 feets from the manifold to the boiler. Then I use the longest run math to determine the head. In my case, (50,000btu's / (500 X 20DT)) = 5 GPM / 7Loops = 0.7 so each loop roughly is 1gpm. Then I use the 5gpm and the head of the longest run to choose a pump that can deliberate that gpm and head at the curve. Very good explanation.

This is my first time installing hydronic system, you are the best guys!

joseluisheating

@EdTheHeaterMan @Ironman

Correct me If I type any wrong. How about the boiler set up. It is looking good or it is missing something.

EdTheHeaterMan

That looks fine. but if you could make the 1" loop shorter. that would be great.

From the looksa of you room floor plan it appears the the red and blue 1" PEX is over 120 feet round trip.

If the red and blue 1" PEX can be under 60 feet round trip by moving the manifold closer to the boiler room, you can use the Alpha 15 pump.

So add up all the total length of the red and Blue 1" PEX and try to get it to be as short as possible.

EdTheHeaterMan

I just saw your actual 1" PEX numbers and your longest run of 75 feet (x 2) of 1/2" PEX.

The numbers would look like this 100 feet round trip 1" and 150 feet round trip 1/2" then add 50% for total equivalent length.

250 x 1.5 = 375 total equivalent length divided by .04  375 x .04 = 15 head.  

Now with a need of only 5 GPM total, you may be able to use the Alpha 15,  lets look at the performance curve.

It looks like the GPM and the Head intersection falls below the maximums on the performance curve. So Go for the Alpha 15-58e

However you cant add another zone to the out building 40 feet away, you are already near the MAX of that pump. and did you add that return to room #9 yet. LOL

joseluisheating

@EdTheHeaterMan Thanks you so much ed. I really appreciate your help, you took ton of pressure from my brain.

Because it is a combi boiler, it would be compact so I will put it on the room#2 which is the electical room it has enough space (7"X6") to install the set up and therefore use the Alpha 15. From the room#2 to the manifold there are around 10-15 feets supply and return together, it would drastically reduce the head require. The manifold and boiler are in the basement, but the diagram is on the second floor. Does it change anything ?

EdTheHeaterMan

The location of the manifold or the boiler or the radiators do not affect the head requirements. the only thing that pump is looking at is the friction of the piping walls. that is what the pump needs to overcome.

I thing you will get a good idea of how this works if you read pages 7 and 8 of this booklet.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/108119-Reference%20Guide.pdf

joseluisheating

@EdTheHeaterMan Thank you so much ED. It would be so hard without you.

This was draining my energy. It is now way easier.