Please check my HEAD calculations

[MNBobcat]

Hi guys,

I've been all over the Grundfos web site trying to find a pump curve chart and I've also Googled the Internet and not found what I was looking for. You guys have been so much help (thank you!) that I thought I would ask here.

I have an outdoor wood boiler located about 275 feet from the furnace in my house. 1 1/4 Thermopex.

Central Boiler provided a really nice pump selection chart that gives pump model along with GPM through 1 1/4 pex at various distances.

What I was looking to do was to compare Grundfos pumps to the Taco. My chart shows on a 600 foot round trip loop that an 014 taco will produce 8.26 gallons per minute. Ideally, I'd like to achieve a GPM over 10 if its possible to do so with my 1 1/4 pex at that distance. I have one 90 degree 1 1/4 fitting on the supply and one on the return. The only Grundfos charts I've found list the distance column as "H" which I assume means head. But they do not show GPM by pex size.

I'd also like a Grundfos pump that has multiple speed options so that I can fine tune the system.

Anyone have a recommendation on a Grundfos pump model would meet my needs? Anyone have a chart or computer program that can calculate the GPM of the pump?

Thanks!

 

[dogwood]

Try http://www.grundfos.com/Web/HomeUs.nsf/Webopslag/PAVA-53CT8Y. I've got the pump curves for the Superbrute Series of 3- speed pumps but get an error message when trying to attach it. I'll try to locate the web address it came from. You might find this pump comparison chart useful, http://www.us.grundfos.com/web/download.nsf/14f6f0b549ed2aeb082564f9005a292c/d0ec3df6e3305a3e0825691a00637af3/$FILE/L-UP-TL-008.pdf

Found it, http://www.grundfos.com/web/HomeUs.nsf/GrafikOpslag/superbrute/$File/UPSpecpages.pdf

Mike

 

[dogwood]

Try this too, (broken link removed)

Mike

 

[MNBobcat]

Hi Mike,

Thanks for the info. Those are helpful.

Where I'm struggling a bit is trying to understand what kind of GPM rate I could expect over 1 1/4 pex on a 600 foot round trip loop with a given pump. Seems like there should be a chart around that takes pipe size and distance into account.

 

[DaveBP]

One of the things you get when you buy Siegenthaler's "Modern Hydronic Heating" is a simplified version of some of his software for hydronic design. One of those modules will calculate just what you're asking for. The problem is there are about 100 circulators on his list to pick from all Armstrong, B & G, Grundfos, and Taco models.

Examples with pure water at 180F:

Taco 007 5.71GPM @ 9.53ft. of head
Taco 0013 10.28GPM @ 26.64ft.
Grundfos 15-58 spd 3 6.87GPM @13.16 ft

That's with only 600ft. of 1.25" PEX, no fittings.

So which makes and models are you considering?

 

[heppm01]

One of the things you get when you buy Siegenthaler's "Modern Hydronic Heating" is a simplified version of some of his software for hydronic design.

I was able to check out both his book and the accompanying CD via interlibrary loan. Worth looking into!

 

[dogwood]

MNBobcat, I did the interlibrary loan just like DaveBP, prior to purchasing the Siegenthaler book, and the CD was in the library book as Dave and Heppm01 indicated. I did find a pipe fluid flow calculator online a while back that may or may not address your needs. Try http://www.gearhob.com/eng/design/calculators/calculator-Fluid_Flow.htm and see if this is of any use. A pro like Heaterman might have more of a clue on how to figure this out than I ever will.

Another calculator is at http://www.freecalc.com/frdiresl.htm

Mike

 

[Gooserider]

I think the reason you are having a problem is that you are trying to solve two questions at the same time...

Separate the questions and it will be much easier...

1. Solve for the system head value at various flow rates - For a starter on that, look at the Taco circ sizing paper I mention in the "tidbits" sticky... When figuring don't forget to include the resistance of whatever HX's or other emitter hardware you go through (I really can't believe you just pump water to the house and back, w/o doing anything to get the heat out of it :coolgrin: ) At the very least solve for your desired 10gpm, If you want, go for other flow rates as well, and you can then generate a "head resistance curve" for your system... Note that when figuring this, you do NOT make any reference to what pump is in use - you assume a "magic pump" that simply gives the desired flow rate, but doesn't have a label to tell you what it is...

2. As the paper suggests, look up the curves of different pumps and pick one with a curve that will supply your desired head pressure, preferably near the center... Note that if you have a head resistance curve generated, it will be curved in the opposite direction from the pump curves. If you plot the head curve on the same chart as the pump curve, where the two curves intersect will give you the flow rate and head pressure values for that pump (to the limits of the calculation matching the real world...)

Gooserider

 

[heaterman]

A 26-99 Grunt will do 9.96gpm through 600' of 1.25" pex with no other fittings accounted for. Add 10 90* ells and a couple ball valves and you get 9.64 according to the software I have.

A 0013 Taco will do 9.78 with the fittings noted and a 0014 will do 7.89.

The issue you may have is that to produce those numbers you are developing around 23-25 ft of head. This means you are going to have to somehow maintain about 5-7PSI or pressure which may be difficult to do with an open system.

 

[brad068]

A 26-99 Grunt will do 9.96gpm through 600' of 1.25" pex with no other fittings accounted for. Add 10 90* ells and a couple ball valves and you get 9.64 according to the software I have.

A 0013 Taco will do 9.78 with the fittings noted and a 0014 will do 7.89.

The issue you may have is that to produce those numbers you are developing around 23-25 ft of head. This means you are going to have to somehow maintain about 5-7PSI or pressure which may be difficult to do with an open system.

Heaterman,

The psi you are talking about is if you run up to 210*F isn't it? Is that the intake press. needed to prevent cavitation at higher gpms or is that the same throughout the pump curve? If you can't move out the water as fast do you need it to come in the suction as fast either. Either way its probably no good for the impeller. I don't no... see what x-mas shopping does to the brain!

 

[heaterman]

A 26-99 Grunt will do 9.96gpm through 600' of 1.25" pex with no other fittings accounted for. Add 10 90* ells and a couple ball valves and you get 9.64 according to the software I have.

A 0013 Taco will do 9.78 with the fittings noted and a 0014 will do 7.89.

The issue you may have is that to produce those numbers you are developing around 23-25 ft of head. This means you are going to have to somehow maintain about 5-7PSI or pressure which may be difficult to do with an open system.

Heaterman,

The psi you are talking about is if you run up to 210*F isn't it? Is that the intake press. needed to prevent cavitation at higher gpms or is that the same throughout the pump curve? If you can't move out the water as fast do you need it to come in the suction as fast either. Either way its probably no good for the impeller. I don't no... see what x-mas shopping does to the brain!

That psi is at a temp of 180 and the pressure is based on the required differential created by the impellor in order to move a given gpm.
If you get past 190-200 all bets are off. You would likely need 12psi + to prevent cavitation/suction boiling. You are correct in that a lower gpm would reduce the head but any way you slice it, 600 feet is a pretty good stretch for 1-1/4 tube. As an example of what I'm talking about with it being "a stretch", you can connect a 15-58 and still get 6.42 gpm with only 12.7 ft of head. When you reach the tipping point combination of tube length/diameter pretty much all that happens is that you create a lot more head and not much more flow. Any time we get in the 500+ feet of tube I start looking real close at the flow rate and head and usually wind up with 1-1/2" or even 2".

 

[MNBobcat]

Heaterman,

Thanks so much for taking the time to calculate those numbers for me. I really appreciate your generosity.

I'm doing my best to learn as much as I can but still have a ways to go. When you said, "somehow maintain about 5-7PSI or pressure which may be difficult to do with an open system." what do you mean? Doesn't the pump maintain a constant pressure?

My pump will be located in the house and about 5 feet lower than the boiler. I've been told that anything over 6 gpm and I should be in good shape. Has that been your experience with these OWB?

 

[heaterman]

Heaterman,

Thanks so much for taking the time to calculate those numbers for me. I really appreciate your generosity.

I'm doing my best to learn as much as I can but still have a ways to go. When you said, "somehow maintain about 5-7PSI or pressure which may be difficult to do with an open system." what do you mean? Doesn't the pump maintain a constant pressure?

My pump will be located in the house and about 5 feet lower than the boiler. I've been told that anything over 6 gpm and I should be in good shape. Has that been your experience with these OWB?

The issue with any open system is that you can only supply the required pressure at the inlet of the pump/impellor via the height difference between the tapping on the boiler and the circ inlet. The thing to remember is that you are working with actual feet of head in this scenario as there is no static fill pressure in an open system..... So, for every 1psi of pressure you need, there should be 2.31 feet of elevation above the circ inlet.

 

[MNBobcat]

496 feet (248 each way) of 1 1/4 pex

6 - 1 1/4 90 degree elbows (3.45 feet each or 21 feet)

2 - Webstone 1 1/4 x 1 purge tees (est: 7.00 feet each or 14 feet)

1 - Webstone 1 1/4 x 1 1/4 purge tee (est: 7.00 feet each or 7 feet)

1 - Webstone 1 1/4 Y-strainer (est: 3.0 feet each or 3 feet)

2 - Webstone 1 1/4 isolator valves (est: 6.80 feet each or 14 feet)

1 - Primary pump

I called Webstone and they don't have any numbers on how much "equivalent length" their components add to the calc. Go figure. I estimated the numbers above. I come up with:

496 + 21 + 14 +7 +3 +14 = 555 total feet

I have the following values taken from the Taco head loss calculations

k = .000794 (1.25 pex)
c = 0.933 (180F water)
L = 555 (total feet)
f = 56.234 (10 GPM)


.000794 x .933 x 555 x 56.234 = 23.12 head


If my calculations are correct, then a grundfos up-96F or a UPS 26-99FC running on high would be a good pump choice (see attached curves)

All comments, corrections and advice would be appreciated! Please let me know what you think. I want to make sure I get the right pump.

By the way, if a pump is rated at 197 watts how do I calculate the killowatts used per month if it runs continuously? Is that 197 watts for 1 minute of usage?

Thanks!

 

[MNBobcat]

Hi Guys,

I've made some progress on the calculations and search for the correct pump. I've posted the info above...
If I pull this off and everything works well, I'm going to owe a lot of you a beer!

Moderator note - Rather than having to make people chase threads around, I've merged the calcs that were mentioned into this thread... Simpler to keep the stuff related to one project (or big chunk of a project) all in one thread...
Gooserider

 

[MNBobcat]

Hi Guys,

I've made some progress on the calculations and search for the correct pump. I've posted the info above...
If I pull this off and everything works well, I'm going to owe a lot of you a beer!

Moderator note - Rather than having to make people chase threads around, I've merged the calcs that were mentioned into this thread... Simpler to keep the stuff related to one project (or big chunk of a project) all in one thread...
Gooserider

I understand. I've been a forum owner/moderator for 14 years myself.

Could you please rename this thread to "Please check my HEAD calculations" ? I'd like to make sure the focus now is on the calcs and less-so on a pump (which was the original posts' question.) Two different though somewhat related topics.

 

[Gooserider]

I haven't checked your math in detail, but it looks about right, far as it goes. However I see some potential issues...

1. I assume the water is getting circulated through some sort of HX or other hardware to get the heat out of it - this needs to be figured into the calculations as well, but I don't see it! (And a heat exchanger can be a MAJOR flow restriction...)

2. Looking at the pump curves, I see that 23-24 feet is only going to get about 6-8 GPM, not the 10 GPM that you were using in your equation... A lower GPM will slightly lower the head pressure, but not by much...

3. The ideal pump should be working in about the middle third of the pump curve, however both of these are way high up on the curve, I would say top 1/4... The pumps can do it, but it suggests that they won't be happy about it.

4. What heaterman was referring to earlier - high power pumps like this MUST have a certain minimum amount of pressure, known as "NPSH" or Net Positive Suction Head at the inlet in order to avoid getting vacuum cavitation at the impeller - I didn't see the exact number on the attached spec sheets, but HM was saying about 5-7psi - which is going to be a problem with an open system - figure you get about 1/2psi per foot of elevation between the waterline and the pump, so to get 5-7psi, you'll need 10-15' of elevation...

That high head loss number is what really causes you problems, and unfortunately there is no easy way around it, as it looks like you can't do much better with your installed PEX lines... Only way you can significantly lower the head would be to bury more / bigger PEX.

By the way, if a pump is rated at 197 watts how do I calculate the killowatts used per month if it runs continuously? Is that 197 watts for 1 minute of usage?

Watts don't have time as part of their value - your electricity is measured in Kilowatt / Hours, which is the equivalent of drawing 1,000 watts for one hour. Thus to determine your KWh usage, you take 1,000 over the watt rating, or in this case, 1000 / 197 = 5.076 hours of operation per KWh, divided into the number of hours of operation (24 x 30) / 5.076 = 720 / 5.076 = 141.8 KWh of operation per month, times your per KWh cost off your bill - note that big pumps cost a lot to run!

Gooserider

 

[Gooserider]

Hi Guys,

I've made some progress on the calculations and search for the correct pump. I've posted the info above...
If I pull this off and everything works well, I'm going to owe a lot of you a beer!

Moderator note - Rather than having to make people chase threads around, I've merged the calcs that were mentioned into this thread... Simpler to keep the stuff related to one project (or big chunk of a project) all in one thread...
Gooserider

I understand. I've been a forum owner/moderator for 14 years myself.

Could you please rename this thread to "Please check my HEAD calculations" ? I'd like to make sure the focus now is on the calcs and less-so on a pump (which was the original posts' question.) Two different though somewhat related topics.

Done... :coolsmile:

Gooserider

 

[MNBobcat]

GooseRider,


Per another discussion you and I had where you gave me some good advice (as always), I'm installing this as a primary/secondary configuration. The secondaries shouldn't change the head on the primary, other than what the closely spaced tees add, should it?

The UPS 26-99 looked to me like it would be exactly 10 GPM at 23 feet of head. I'd fall short of 10 GPM with the other pump.

I would have gone larger pex but thermopex is only available in 1 inch and 1 1/4 inch. My goofy dealer tried to tell me all I needed was 1 inch. It pays to try and educate yourself.

Central Boiler claims anything over 6 GPM I'm golden but if I could get 10 GPM I'd be a happy camper.

 

[jebatty]

You know well that 6 gpm at Δ T20 is 60,000 btu's. I didn't notice that you posted a heat loss calc, but this is pretty low for most wood boilers.

 

[Gooserider]

GooseRider,
Per another discussion you and I had where you gave me some good advice (as always), I'm installing this as a primary/secondary configuration. The secondaries shouldn't change the head on the primary, other than what the closely spaced tees add, should it?

You are quite correct... I'd forgotten about that (Easy enough to do when you have as many threads going as I do..) Primary/Secondary should give you hydraulic isolation, and thus no added head other than from the tees themselves...

The UPS 26-99 looked to me like it would be exactly 10 GPM at 23 feet of head. I'd fall short of 10 GPM with the other pump.

Zooming in on the diagram some more, you may be right, especially if you leave out the flowcheck... It is still on the upper end of the curve, and I still don't know about the NPSH issue, but I don't know if there is anything that would really do any better either...

I would have gone larger pex but thermopex is only available in 1 inch and 1 1/4 inch. My goofy dealer tried to tell me all I needed was 1 inch. It pays to try and educate yourself.

I know about the size limit on thermopex - it's one reason I like the approach several have suggested of doing plain PEX and foam instead - allows for either bigger sizes, or multiple runs, but it's to late now to easily change that part...

Central Boiler claims anything over 6 GPM I'm golden but if I could get 10 GPM I'd be a happy camper.

Seems to me like CB claims all sorts of things that aren't necessarily in line with reality, IMHO it seems like their marketing department is "credibility challenged"

That said, they may be right - essentially you have the bottom-line question of how many BTU's per hour can you transport - you really don't want / need to transport more than you actually need to meet the house demand.

Having a good handle on the demand should allow you to calculate the BTU's/hr you need to transport, and allow you to figure out what the lowest GPM rate is that you can get by with - If you can drop the flow rate down to the point where you can use a smaller pump or lower pump speed, it will pay off big-time on your electric bill... It might even be worth trying lower pump speeds when you have warmer-than-design temps. If you look at the specs, you notice that you drop your power needs by 25% going from speed three to speed one... You also reduce all the other issues with trying to drive an excessively high head load.

One of the things I'd be looking at is to see if you can increase the amount of heat you are pulling out of the water on the house end - If you can double the ΔT between the boiler supply and return, then you get twice as many BTU's per gallon of flow, which means you drop the flow rate needed by half, and life suddenly gets a lot better... Without running the numbers, I'd bet your head would drop down to the 10-12 foot range, if not less, and let you get back down to the 100W range of pumps...

Gooserider

 

[MNBobcat]

I have a 100,000 btu furnace in the house. I installed a 140,000 btu heat exchanger in the furnace tonight.

My furnace flows 1570 CFM. The chart on the HX only goes up to 1400 CFM.

With 185 degree water temp with 1400 CFM air flow it shows the following.

7 GPM = 94,000 BTU
10 GPM = 110,500 BTU

I'm only using 180 degree water. Sounds like I may be okay if I can't get 10 GPM.

I plan to plumb this thing so that the first secondary in the loop, receiving the hottest water, is the HX for the furnace. The next secondary will be the modine style of hanging heater in my garage. The last secondary, getting the coolest water, will be the plate heat exchanger for the DHW. Since DHW only needs to be 120 - 140 degrees, I'll send it the coolest water. Seemed to be smarter than sending it hotter water and then cooling it with a mixing valve when it goes out to the house. I'll still use a mixing valve, but that's the logic I'm using putting it last on the loop. Also, I have two 80 gallon hot water heaters. Once they come up to temp, shouldn't need to call for heat very often.

I'm hoping a few people will weigh in on my calculations and let me know if the head I came up with is right.

 

[Gooserider]

Sounds like you should be OK with a lower than 10gpm flow. If not, I'd consider trying to add more radiation, like retrofitting some of your rooms with infloor or flat panel rads... This would take some of the load off the HX, and since the added radiation could use lower water temps, then you'd be able to get more heat out of every gallon pumped...

Gooserider

 

[MNBobcat]

I just wanted to report that my boiler is up and running and the Grundfos 26-99 pump is working awesome with the 1 1/4 pex. I'm going to post (on a new thread) pictures of my wood shed, boiler and the primary/secondary plumbing, when I get some time in the next few days. Thanks for all of the help!