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If you want to risk pump seals with a strong base, via con Dios. And if it did work without cooking anything where would all that loosened crud go?

The same place it goes when using the other additives that aren't 8-way, I guess. Don't shoot the messenger, but I doubt Rectorseal would advise the use of a product if it would harm boilers.

Regardless of the product, be wary of adding too much…the directions seem to be written by the sales department

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

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.

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.

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.