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# Primary/Secondary serious clarification needed

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• Member Posts: 2
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Hydronic

Just finished Dan Holohan's book Primary Secondary Pumping made easy and read through it a second time and have some questions. Turn to page 39, you will see a diagram of a primary loop that is in the shape of a two pipe, direct return system, with the actual heat transfer loops being taken off of the "crossover bridge" piping. The "crossover bridges" are shown with full port balancing valves. Can that primary pipe loop, the one with no actual radiation load, be piped in with a reverse return? I dont' see why not, but. . . the time to ask questions is before the pipe goes in, not after. I also am curious about the GPM formula that is used in the book, BTUH/Dela T x 500 equals GPM. Designing each heat load for a 20 degree delta T makes the math easy, but is there any reason not to use a higher delta T, and therefore smaller pump? I mean, why couldn't you use a 75 degree delta T - I mean, the math isn't that hard!

One more thing, when I've bought pumps in the past I bought them based on pipe size and RPM, never GPM. When the author tells you to find the GPM for a circuit and then buy the pump, I'm confused. I feel inadequate, but I don't know how to buy a pump based on the required GPM. My penis is also small. There, I said it.

Thanks for the expected replies. You guys really seem to know your stuff.
• Member Posts: 54
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> Just finished Dan Holohan's book Primary

> second time and have some questions. Turn to

> page 39, you will see a diagram of a primary loop

> that is in the shape of a two pipe, direct return

> system, with the actual heat transfer loops being

> taken off of the "crossover bridge" piping. The

> "crossover bridges" are shown with full port

> balancing valves. Can that primary pipe loop,

> in with a reverse return? I dont' see why not,

> but. . . the time to ask questions is before the

> pipe goes in, not after. I also am curious about

> the GPM formula that is used in the book,

> BTUH/Dela T x 500 equals GPM. Designing each

> heat load for a 20 degree delta T makes the math

> easy, but is there any reason not to use a higher

> delta T, and therefore smaller pump? I mean, why

> couldn't you use a 75 degree delta T - I mean,

> the math isn't that hard!

>

> One more thing, when

> I've bought pumps in the past I bought them based

> on pipe size and RPM, never GPM. When the author

> tells you to find the GPM for a circuit and then

> but I don't know how to buy a pump based on the

> required GPM. My penis is also small. There, I

> said it.

>

> Thanks for the expected replies.

> You guys really seem to know your stuff.

• Member Posts: 54
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> BTUH/Dela T x 500 equals GPM. Designing each

> heat load for a 20 degree delta T makes the math

> easy, but is there any reason not to use a higher

> delta T, and therefore smaller pump? I mean, why

> couldn't you use a 75 degree delta T - I mean,

> the math isn't that hard!

If supply is 190 and return is 170, the radiation puts out heat based on a average water temperature of 180. If supply is 190 and return is 110, the average temperature is 150, and your radiation can only put out maybe 2/3 as much heat. So you'll have to increase your radiation by 50%, and if you have any of them daisy-chained, the ones at the end will not heat the room.

> One more thing, when

> I've bought pumps in the past I bought them based

> on pipe size and RPM, never GPM. When the author

> tells you to find the GPM for a circuit and then

> but I don't know how to buy a pump based on the

> required GPM.

The RPM is immaterial. Each pump has a curve relating the head pressure to the GPM. You have to work with that curve.

> My penis is also small. There, I said it.

Both the length and the ID (but not OD) are relevant to flow. Have you done a calculation? How much head do you get?
• Member Posts: 2,440
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Size has nothing to do with it

I will tackle (Most Of) your last paragraphs. Some things I just will not, ah, well, touch.

First, using different delta-T's besides 20 degrees: Done all the time. You are right, you move less water. But then other things happen to spoil the party. Your average temperature in the radiation drops. Heat output is therefore less or the radiation has to be over-sized to compensate. That is but one aspect to consider. We will not get into laminar flow here, we are on a roll!

GPM: The flow rate is the work you are doing, the amount of water (heat) you are transporting. Gallons per minute, obviously.

Head (AKA "feet of head"): Is the pressure or resistance which you must overcome or against which your flow must be delivered.

These conditions are plotted on a pump curve. Every pump has a curve, published or not. Any particular pump will move a combination of a lot of flow at a low head and less flow at a high head. Increase the flow you decrease the available head and vice versa (This is a REALLY basic primer here so bear with me).

So, you got your flow rate, you know the amount of water you need to move. You calculate your resistance of piping, fittings, devices on your most restrictive run, in terms of pressure loss, ultimately in units of feet of head. (One foot of head is equal to 0.433 PSI and 2.31 feet of head equals one psi by the way.)

You select a pump that meets the two conditions using the least amount of horespower (Watts).

There are other aspects such as how flat the curve is, how it responds to changes in system pressure but that is another class for another day.

RPM, (pump speed) is incidental to the above. Some pumps work at higher or lower speeds for similar duty. Lowest wattage that does the work wins, in simple terms. Generally higher pressures require higher RPM's, lower pressures, lower RPM's. Think of faster speeds to stuff water into a pipe, much as you pump a bicycle pump faster and harder in your 100 PSI 18 speed racing bike tires versus your kid's 40 PSI Schwinn tires.
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Last Line

LOL!
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"Head (AKA "feet of head"): Is the pressure or resistance which you must overcome or against which your flow must be delivered."

Ok, seems simple enough. . . when you say that, does that also imply that different static head pressures will affect the operation of the pump? For instance, if one circ/zone is operating the first floor, one the second, and one the third, the static head pressures in each will be different, correct? Or not correct? Now that I think of it, the pressure at the base of the system will be equal all over, determined by whatever water is highest in the system. Anyway, to determine the resistance, I should count the linear feet of pipe, fittings, and the type of radiation, and refer to a handy chart - oh, wait, I have no clue where to find such a chart! And if a lot of pipe is behind the walls, ( such as in a boiler redesign) I'd have a hard time knowing exactly which way all of it went. . . so I'm hoping there's some kind of standardized formula for computing this stuff.

Thanks a lot, fellas. You have no idea how helpful this is!
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Pressures

Yes, imposing different resistances (pressures) in a given hydronic circuit will affect flow. Throttle a valve and this is the perfect example. You decrease flow by increasing pressure resistance. (Or else why throttle? he asked whistfully..)

Multiple pumps working in parallel is another factor as you noted. Many things to consider.

E-mail me off line and I can set you up with a spreadsheet or other resources and if of general interest, I will post them of course.

If you really want a great book?? John Siegenthalers Modern Hydronic Heating (2nd Edition).

http://www.hydroncipros.com

Best \$100 bucks I ever spent in this business. There are others too right on this site. All at your fingertips!
• Member Posts: 54
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Head is not like regular pressure; it has nothing to do with 1st floor vs 2nd floor, etc. It is more like friction, it depends on pipe material, length and ID.

Yes, in practice it is not easy to estimate it in an existing system. If it is a converted gravity system with big fat pipes, the head is close to zero. For all other systems, it may be easier to measure it experimentally. Install a pump and measure the delta-T. Three-speed pumps (eg Grundfos 15-58) are great for testing purposes, or have a variety of pumps handy. Find the pump or speed that gives you deltaT around 20F. If you get 15F, no big deal. If you get 2F, that's wasteful, although it'll heat the house just fine (unless the flow is so high that you cause velocity noise in the pipes). Most pumps used in homes are oversized, some hugely, some slightly. Some people used B&G series 100s for damn near everything.
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