Help sizing a pump

[Tim S.]

With an outside wood boiler when sizing a pump do you add in elevation head with your pressure drop of an open system using 1/2 inch pex in concrete floor.

Thanks, Tim (Nubie to the forum)

 

[Tim S.]

Am I on the wrong type of forum to ask this question

 

[Tennman]

You're in the right place Tim. I'm not a pro installer but don't want you to feel ignored. Pretty busy this time of year for the pros, but hopefully they'll chime in so I don't tell you something wrong. To size the pumps in our system I used the Taco pump sizing datasheet (TD10). Those calcs do not consider elevation change for our types of systems probably because length of run (friction) and turns (ells) dominate head loss. Somewhere in your circuit water is going downhill because once it went uphill. Does make me wonder if its a factor in skyscrapers, but I doubt you care about that.

Oh.... and welcome. Come and hang out.

 

[ewdudley]

Welcome. Definitely the right type of forum.

Perhaps we didn't understand the question. As long as the pump is in a circuit then the elevation at the highest point in the circuit doesn't affect the head that the pump sees.

 

[Tim S.]

Thanks for the reply's, okay I take it elevation head balances itself out. I looked at the TD10, Q=btu/hr and delta T=20 degree difference on supply and return. Is Q what the outside wood boiler is capable of putting out in btu/hr or how is Q arrived at.
Thanks again, Tim

 

[ewdudley]

Thanks for the reply's, okay I take it elevation head balances itself out. I looked at the TD10, Q=btu/hr and delta T=20 degree difference on supply and return. Is Q what the outside wood boiler is capable of putting out in btu/hr or how is Q arrived at.
Thanks again, Tim

Q is the rate of heat delivery, so sizing of lines is according to load. As the boiler cycles it will be putting out more heat than needs to be delivered.

deltaT=20 is an industry rule of thumb for baseboards, WAHX, and whatnot. If your are supplying hot water to a mixing device then the deltaT should be much higher, 70 degF or even 100 degF in some cases.

 

[Tennman]

My pump/HX calcs were based on 80% of the advertised 205 output (which is under perfect conditions) or 180kbtu/hr. That is particularly important for sizing the HX if you have one, because you want the HX to keep your home warm under normal conditions and burning good, but not "perfect" wood. In other words, don't expect to get the advertised rating out of the boiler very often if ever. That was the advice given me ~6 years ago which as turned out to be good.

Pay attention to flow velocity, not just the gpm on the main line. Undersizing the pex for your required gpm can drive you to a larger pump than necessary. As I recall Taco recommends flow velocities less than ~4 feet/second. I wouldn't have had to use a Taco 0013 between my boiler and house if I'd have used the next size up pex. But my run is also longer than most. The TD10 will hold your hand if you're designing this stuff on your own like I had to.

Is the supplier of your boiler helping you any? I'd sure want to understand all the radiant lines/zones in the concrete before I poured the slab. I can't help you on anything about radiant, but that's a big deal and has to be right. Ask lots of questions!

 

[Clarkbug]

Thanks for the reply's, okay I take it elevation head balances itself out. I looked at the TD10, Q=btu/hr and delta T=20 degree difference on supply and return. Is Q what the outside wood boiler is capable of putting out in btu/hr or how is Q arrived at.
Thanks again, Tim

Tim,

You mentioned this is an open system? Where is the system is the pump, the boiler, and the load elevation wise?

 

[Tim S.]

Here's what I got Central boiler 6048 Brand new,house new, 3-300 foot runs 1/2 pex in garage, 3-300 foot runs 1/2 inch pex in basement slab which is about 6 feet below ground level. Electric furnace going to install 120,000 btu coil, nothing is installed in house yet such as pump or pumps or manifolds. This is son-in-laws home .Next year going to install pex in 60 by 40 garage. The boiler is 170 feet away from house with 1 inch thermopex 30 inches in ground, probably should have been at least 1 1/4" but it's in now. Should I separate the 2 slabs in house from the furnace and I forgot going to install side arm on DHW. Pump for both slabs and pump for furnace and water heater. Boiler sets about 4 feet above house level of 1st floor in about a 170 foot run. Furnace is forced air. any help appreciated. Didn't know how to edit above, boiler is 350,000 btu I believe.

Thanks again, Tim

 

[Tennman]

Per Taco TD10 to move 280kbtu/hr (80% of the 350kbtu/hr) with a 20deg F deltaT you need to move 28 gpm. For a 30deg F deltaT you must move ~19 gpm.

Per Pexuniverse 1" pex is .863" ID.

28gpm = 15.4 feet/sec in 1" pex
19gpm = 10.2 feet/sec in 1" pex

1 1/4" pex is 1.054" ID

28gpm = 10.3 feet/sec in 1 1/4" pex
19 gpm = 7 feet/sec in 1 1/4" pex

Recommended flow velocity is 2-4 fps. 1 1/4" pex would have been too small to move the capacity of that boiler. I'm sure your Central salesman said, "Use 1", you'll be fine. Do it all the time." The maximum flow rate for 1" pex per TD10 is 7.5 gpm.

At 20 gpm for ~170' run and the usual count of ells and tees your head loss is over 66 ft because of the velocity. For 28 gpm like you really need the head loss is over 130 ft.

Note, this is just moving 80% of the capacity of that boiler. Bottomline, leaving the underground alone, you will need a monster pump to operate anywhere near that boiler's capacity. Or, use like a Taco 0013 and operate it as a boiler at less than half it's capacity.

Yeah, I know.....

 

[Tim S.]

Thanks for the figures Tennman, the other part of the boilers capacity will be used next year when the 60 by 40 garage is complete. I shouldn't be trying to use all the boilers capacity in the house alone correct, so if I can get to delta T 30 or 40 with a little smaller pump should still be okay right.

 

[Tennman]

Tim, It starts with how much energy the house needs. Based on the home's demand the pump's sized to feed that energy thru the 1" pex. If the house only needs 80-120kbtu/hr or 1/3 of the boiler's capacity you won't need a monster pump, but your boiler will be doing a bunch of idling or you burn a bunch of small fires... I guess. Not sure how the Central will manage all that excess capacity.

I would think someone helped determine the expected energy demand of the home.

 

[Tim S.]

Okay, I understand now thanks Tennman

 

[Tim S.]

Anyone familiar with the Taco 0013-vdtfe variable speed that you set your delta T and it automatically adjusts its performance.

 

[Tim S.]

Okay next question, where should the pump be located, on supply or return side and bypass location. Can I pump all the zones with one big pump.

 

[Clarkbug]

A diagram of your intended layout (and current setup before other buildings) might be helpful for us to try and help you out. Pump location will depend on elevation and if you want it at the boiler or in the building.

 

[Tim S.]

Here is a very rough set up central boiler dealer said taco 009 at boiler supply would work for house/garage layout. I imagine the pump should be where pex comes into house 30 inches below grade, that would be roughly 7-8 feet below boiler from 170 feet away. Will one 009 work or 013 or do I need two pumps. One inch thermopex is already in the ground, sidearm water heater is installed, no lines are run inside house yet. intended to run 1 inch copper. Do I need an air scoop Where should it be located. Total feet with fitting factors added in the longest loop will be 750 feet. I come up with 47.8 feet of head, does this seem right. I doubled the 170 foot to 340 to make a complete loop from supply to return at the boiler or do you just count the 170 feet one time with the 300 foot run of the pex for the longest run. I figured this at 200,000 btu with a delta T at 40 degree. Do I need two pumps in series to maintain the head.

Thanks, Tim

 

[Clarkbug]

Tim, thanks for posting the diagram.

Quick question for you... You show the supply going through a thermo valve, then the sidearm, then a furnace, and then into some other boxes. What are those other boxes? And what is the thermo valve?

I havent run the math on the flow/BTU delivery, but I will say that I would advise separate pumps for the main loop vs. the radiant loops. With an OWB you want to pump that main loop all the time to keep from freezing, but also so the the water in the boiler is constantly getting turned over. That way when the temp drops, the boiler knows to open the draft door to heat up. You dont want to circulate through your radiant piping all the time, or it will heat you right out of the house.

You shouldnt need an air scoop, because your CB is an open system, so there will be air always getting into your water. You DO need to install vents at the high points in the system so that you can vent any big air bubbles when you first get going, or if you ever have to drain down the system.

 

[Tim S.]

Those boxes are manifolds for the pex in the concrete slabs for basement and garage. I planed on pump or pumps running 24-7, slabs would have zone valves or balancing valves. From taco site pumps in series would increase head but keep flow same. I kinda know about high point vents and low point drains just not sure of pump arrangement and sizing and if I did the TD10 formula correct. If air is always getting into the system isn't that the reason for air scoops or air separaters to keep the system from getting air locked. The thermostatic mixing valve would mix the supply and return to make sure return water doesn't go below I believe it is 140 degrees going back to boiler, Central boiler says to use so not to void warranty. I doubled the 170 foot to 340 to make a complete loop from supply to return at the boiler or do you just count the 170 feet one time with the 300 foot run of the pex for the longest run.
Thanks again Tim

 

[Tim S.]

Can anyone help me out. Does the math seem right. Will I need 2 pumps in series to keep the head.

 

[ewdudley]

Here's a pressure drop and feet-per-second table for 400 ft of nominal 1" PEX. (170 ft each way plus whatever.)

l=400 ft d=0.863 in
  gpm    head    fps
3.00   6.15    1.65
3.25   7.13    1.78
3.50   8.18    1.92
3.75   9.30    2.06
4.00  10.47    2.19
4.25  11.72    2.33
4.50  13.02    2.47
4.75  14.39    2.61
5.00  15.83    2.74
5.25  17.32    2.88
5.50  18.88    3.02
5.75  20.50    3.15
6.00  22.18    3.29
6.25  23.92    3.43
6.50  25.72    3.57
6.75  27.57    3.70
7.00  29.49    3.84
7.25  31.47    3.98
7.50  33.51    4.11
7.75  35.60    4.25

You can't push much more than 6 gpm with less than 200 watts of pump.

To maximize deltaT you need to move the thermostatic mixing valve close to the boiler so that the return temperature from the house is minimized. With your undersized pipe from the boiler to the house you need all the deltaT you can get.

The slab loops need their own mixing valve and pump. One pump with zone valves should be enough but if the layout doesn't allow it then two mixing valves and two pumps.

Below are two near-boiler layouts and one house layout. Using two pumps at the boiler would would remove some pressure drop from the boiler-to-house loop, plus it would avoid having such a huge difference in resistance between the bypass leg and the load leg.

 

[Tim S.]

ewdudley thank you, 009 and 007 would leave me about 5 feet of head short of my figures, should i go up to 008, will it hurt the pump or flow or is that close enough.

Thanks again, Tim

 

[ewdudley]

009 and 007 would leave me about 5 feet of head short of my figures, should i go up to 008, will it hurt the pump or flow or is that close enough.

Not sure if I'm following what you're saying.

First of all the in-floor loops need one or more mixing valves and pumps that draw water downstream of the DHWHX and the WAHX as shown in the house side of the diagrams above. By drawing from the circuit downstream from the DHWX and WAHX you will be able to increase the deltaT of the boiler-to-house circuit, which increases its btu per hour heat carrying capacity. These circuits do not help or hinder the pump that moves the water through the DHWHX and WAHX.

On the main 340 ft round-trip loop from the boiler to the house; as you can see from the flow vs. head chart above a 1.4 ampere 009 will get you a little less that 6 gpm, a 2.0 ampere 013 will only get you about 7 gpm. You could throw two 013 pumps in series at it and get maybe 10.5 gpm, but now you're using 500 watts, yikes.

So I'm saying if you can't do the job with 6 gpm then need to increase deltaT on the 340 ft round-trip loop or start planning on laying another line from the boiler to the house.

If you use a single 009 or 013 to draw flow through the mixing valve then you would be pulling 25 or 30 feet of head through the 340 ft loop, which is a recipe for cavitation and destroying the pump. I'm now thinking that my drawing that shows this setup above in the first drawing is a bad idea.

So now I'm saying you need a near-boiler loop, as shown in the second near-boiler diagram above, with its own small circulator to move water through the mixing valve and the boiler, with another pump to push water to the house, like a 009 or 013. I'm showing a 008 above but it would only get you a couple gpm so that is looking like a bad idea.

So:

• A separate pump (or two) to draw water through a mixing valve (or two) for the in-slab circuits. A single small pump (like a 008, 15-58, Bell&Gossett Vario, Wilo Sratos, or Alpha) would easily supply your six loops if you add zone valves to control flow to either floor.
• A separate near-boiler pump circuit with a thermostatic valve to maintain minimum return water temperature.
• A separate high-head pump (for instance a 009 or a 013) to push water from the near boiler loop to the house and through the DHWHX and WAHX.

 

[Tim S.]

On the main 340 ft round-trip loop from the boiler to the house; as you can see from the flow vs. head chart above a 1.4 ampere 009 will get you a little less that 6 gpm, a 2.0 ampere 013 will only get you about 7 gpm. You could throw two 013 pumps in series at it and get maybe 10.5 gpm, but now you're using 500 watts, yikes.

I'm not sure i understand now, from Taco FAQ pumps in series only increase head, pumps parallel increase flow. I had a total of 47.8 feet of head in the system, now are you treating the 340 ft.and 300ft. pex loops as separate loops, not counting the head loss with fittings, valves. Are you saying an 009 or 013 will take care of the run from the boiler to house loop then an 008 will take care of the slab circuits. Is it normal to put a bypass loop on an outdoor wood boiler 170' from the house.

Thanks for your patience, Tim

 

[ewdudley]

I'm not sure i understand now, from Taco FAQ pumps in series only increase head, pumps parallel increase flow. I had a total of 47.8 feet of head in the system, now are you treating the 340 ft.and 300ft. pex loops as separate loops, not counting the head loss with fittings, valves. Are you saying an 009 or 013 will take care of the run from the boiler to house loop then an 008 will take care of the slab circuits. Is it normal to put a bypass loop on an outdoor wood boiler 170' from the house.

The in-slab loops are not part of the boiler-to-house circuit, and yes I'm saying a 008, or similar, or smaller even will move plenty of water for the slabs.

How much head there will be in the boiler-to-house circuit depends on the gpm in the circuit, approximately according to the table listed above. If we look up the pump curves for the 009 and the 013 pumps, for instance, we can see that the 009 develops 22 ft of head at 6 gpm where it matches up with the table, and that the 013 develops about 29 feet of head a 7 gpm where the 013 pump curve matches up with the table. Note in order to add 1 gpm of flow we need to go from a 1.4 ampere pump to a 2.0 ampere pump.

Instead of eyeballing the table you can plot the points from the table onto a printout of the pump curve graph provided by Taco:

(broken link removed to http://www.taco-hvac.com/uploads/FileLibrary/101-064.pdf)

Where the load curve that you plotted intersects and particular pump curve is what the gpm and head would be for that pump in your circuit.

The point is that as we increase gpm, head goes up in proportion to gpm to the 1.85 power. If gpm doubles, head goes up by a factor of 3.6. At some point it does not matter what pump we use -- or how many pumps we put is series -- to get more pressure, to get more flow, because the amount of power it would take would not be not be affordable. I'm not saying that the 009 or 013 will pump enough water to meet your needs, I'm saying that with 340 ft of 1 inch PEX the 009 or 013 will be about as big a pump as would likely be affordable in terms of the amount of water pumped vs. the amount of electricity consumed.

In order to maintain minimum return water temperature to the boiler it is normal to have a near-boiler loop. Some use a thermostatic mixing valve.

Another common option would be to have a simple loop (without a thermostatic mixing valve) with its own dinky pump and with an aquastat that would disable the boiler-to-house loop when return temperature falls below 145 degF (for instance) and then re-enables the boiler-to-house loop when return temperature rised above 150 degF (for instance). Meanwhile the near-boiler loop pump runs constantly and the aquastat can see when return temperature has risen enough to enable the boiler-to-house loop.