pump selection and questions.

[warno]

I'm a newbie to all this and would like some help on selecting a pump.

I'm a little confused by the "head" number. I'm thinking that it's the pressure drop in the system but please correct me if I'm wrong. I have been trying to figure out the "head" number of my system.

My heat exchangers combined are just under 10 (based on the paper work for them) but I'm not sure how to figure the value for the pex lines and fittings.

This is the pump I have been looking at http://www.pexuniverse.com/grundfos-ups15-58fc-circulator-pump-59896341

I will have about 150 feet of 1" pex in the system. All sweeping bends maybe 2 or 4 90 degree elbows.

Could anyone help me out? Any help would be appreciated. If any more info is needed let me know.

 

[heaterman]

What is the required flow rate?

 

[warno]

On the heat exchangers? (Again sorry I'm a newbie) If so I believe it's 8.0 gpm.


Here's a pic of the paper work for the coil. I will have 2 of them in the system.

 

[warno]

I guess I forgot to ask this question in my OP. Is it smarter to run the pump 24/7 or just with demand for heat?

 

[ewdudley]

Long answer:

If I'm reading it right your WAHX coil is dropping 4.34 ft H20 at 8 gpm.

1 ft H2O pressure drop is 0.4335 psi, and 1 psi pressure drop is 2.306 ft H2O. (Just type "psi to ft h2o" in your web search bar if you forget these numbers.)

So your WAHX will see a pressure drop of 1.88 psi at 8 gpm (4.34 times 0.4335).

Now refer to a PEX pressure drop table like the one found here:

http://s3.supplyhouse.com/product_files/FostaPEX-Submittal.pdf

Now we're going to convert your WAHX pressure drop to an equivalent length of 1" PEX, since the rest of your problem is in 1" PEX.

The table labeled "Pressure Drop Table Expressed as PSI/ft. Pressure Drop" shows a pressure drop of 0.041 psi per foot for 1" PEX at 8 gpm.

If your coil drops 1.88 psi at 8 gpm and one foot of 1" pex drops 0.041 psi per foot at 8 gpm, then your coil is equivalent to 1.88 psi divided by 0.041 psi per foot, which is about 46 ft. [Edit: two coils in series would be about 92 feet equivalent, two coils in parallel would be about 13 feet equivalent.]

Assuming your 150 ft number is round-trip and not one-way -- just re-do the arithmetic if this is not the case -- then total ft of 1" PEX equivalent is 150 PEX plus 46 WAHX plus three or four feet per elbow, which comes out about 225 ft of 1" PEX equivalent.

Now you can go back to the PEX pressure drop table and compute total pressure drop for various gpm: 225 times 0.011 psi at 4 gpm; 225 times 0.017 psi at 5 gpm; and so forth. This will give you a system pressure drop, or head, at each gpm. (In psi. See conversion factor above to convert head in psi to head in ft H2O.) The point is there isn't a single head number for the loop, there's a different head for any given gpm.

Then find a pump curve for each candidate pump. Here is the 15-58, for example:

(broken link removed to http://www.pexuniverse.com/docs/pdf/gr-59896341.pdf)

Print out the pump curve and plot out the numbers you got for each gpm above. For instance at the 4 gpm line you would pencil in a point at 5.7 ft of head. (You remembered to multiply the 225 times 0.011 psi by 2.306 to get ft H2O, right?)

Now sketch a line through the points you've plotted, and where that line crosses the pump curve is the gpm the pump will deliver through your WAHX and PEX circuit.

Now if the gpm the pump will deliver about the same as the gpm you need then you're all set. Else find a bigger or smaller pump that will match up better.

Be aware if the gpm you need is too large there may be no such thing as a pump that will supply the flow you're looking for.

Short answer:

ECM: Bell&Gossett 19-14 Vario, Wilo Stratos, Grundfos Alpha
Conventional: Taco 008, Grundfos 15-58, Bell&Gossett NRF-22

 

[ewdudley]

I guess I forgot to ask this question in my OP. Is it smarter to run the pump 24/7 or just with demand for heat?

If you run the pump only when there is a call for heat then sometimes you'll be blowing cooler air for some period of time while you're waiting for the really hot water to arrive.

Some guys use a 'clixon' temperature sensor or some other thermostatic device that disables the fan until the water leaving the coil proves, which gives you the best of both worlds: pump off if no call for heat; and fan off if water not hot enough yet.

 

[warno]

If I'm reading it right your WAHX coil is dropping 4.34 ft H20 at 8 gpm.

1 ft H2O pressure drop is 0.4335 psi, and 1 psi pressure drop is 2.306 ft H2O. (Just type "psi to ft h2o" in your web search bar if you forget these numbers.)

So your WAHX will see a pressure drop of 1.88 psi at 8 gpm (4.34 times 0.4335).

Now refer to a PEX pressure drop table like the one found here:

http://s3.supplyhouse.com/product_files/FostaPEX-Submittal.pdf

Now we're going to convert your WAHX pressure drop to an equivalent length of 1" PEX, since the rest of your problem is in 1" PEX.

The table labeled "Pressure Drop Table Expressed as PSI/ft. Pressure Drop" shows a pressure drop of 0.041 psi per foot for 1" PEX at 8 gpm.

If your coil drops 1.88 psi at 8 gpm and one foot of 1" pex drops 0.041 psi per foot at 8 gpm, then your coils is equivalent to 1.88 psi divided by 0.041 psi per foot, which is about 46 ft.

Assuming your 150 ft number is round-trip and not one-way -- just re-do the arithmetic if this is not the case -- then total ft of 1" PEX equivalent is 150 PEX plus 46 WAHX plus three or four feet per elbow, which comes out about 225 ft of 1" PEX equivalent.

Now you can go back to the PEX pressure drop table and compute total pressure drop for various gpm: 225 times 0.011 psi at 4 gpm; 225 times 0.017 psi at 5 gpm; and so forth. This will give you a system pressure drop, or head, at each gpm. (In psi. See conversion factor above to convert head in psi to head in ft H2O.) The point is there isn't a single head number for the loop, there's a different head for any given gpm.

Then find a pump curve for each candidate pump. Here is the 15-58, for example:

(broken link removed to http://www.pexuniverse.com/docs/pdf/gr-59896341.pdf)

Print out the pump curve and plot out the numbers you got for each gpm above. For instance at the 4 gpm line you would pencil in a point at 5.7 ft of head. (You remembered to multiply the 225 times 0.011 psi by 2.306 to get ft H2O, right?)

Now sketch a line through the points you've plotted, and where that line crosses the pump curve is the gpm the pump will deliver through your WAHX and PEX circuit.

Now if the gpm the pump will deliver about the same as the gpm you need then you're all set. Else find a bigger or smaller pump that will match up better.

Be aware if the gpm you need is too large there may be no such thing as a pump that will supply the flow you're looking for.

Wow, that's alot of info for a newbie to digest. Lol


Yes the 150 feet of 1" was round trip.


Thanks for the replies everyone. Anyone else have any info I could use?

 

[Bob Rohr]

E dub pretty much has it. Is you really want more math, this issue is rolling off then press today, here is a pdf link.

http://www.caleffi.com/sites/default/files/caleffi/uploads/files/idronics_16_na.pdf

 

[warno]

I'm in the process of plumbing my system up right now and I have a question on my circulater. I'm running a thermomix mixing valve in my system and it recommends that the water be pulled through it instead of pumped through it. This puts my pump at the very end of my system before returning to my boiler. I was wondering if that will affect my pump at all pulling the water through the whole system as opposed to pushing it?

The total head number in my system is around 12 ft plus about 60 ft of 1" pex round trip. My pump is a Bell and Gossett nrf 36.

My mixing valve is this one
(broken link removed to http://www.caleffi.com/usa/en-us/catalogue/thermomixtm-thermostatic-mixing-valve-high-flow-npt-280165a)

Could anyone help me out here?

 

[velvetfoot]

I'll let the experts reply, but having a Vario in front of the boiler pulling water through a thermomix seems to work fine for me.