circulator pump sizing

Glenn_18

Just installed a outdoor wood boiler. There is a 320 foot loop of 1" pex from the boiler to a 185,000 btu water to air heat exchanger and back to the boiler. What size pump, gpm and feet of head would be needed? Thank you.

Glenn_18

Additional info

Dont know if this would help any but these are message from Taco pumps regarding pump sizing:

Glenn, without getting too technical, based on the 185000 btu output of the heat exchanger, you will need a flow rate of 18.5 gpm with a 20 deg delta T. The formula to calculate the required flow is GPM= BTUh divided by the delta T x 500. The 009 pump is undersized based on this. However if you calculate the BTUh required to satisfy the load you may have a lower flow rate, you are also limited to the diameter pipe used. See attached document.

Glenn, we still need to know the load. According to Wirsbo design guide, 1\ pex has a max flow rate of 11 gpm

Brad White_203

More info needed

What is your design delta-T and Flow Rate? That number of BTUs is meaningless without that. It could be 18.5 GPM if you are running a 20 degree delta but that is way to high for 1" by my standards.

If you ran 40 degrees, you would be running 9.25 GPM, still pushing it but you may skate by.

If I were designing it and confident of my control and ability to deal with lower return water temperatures such as a bypass valve, a 50 or 60 degree delta-T might be considered. This will get your flow rate down to between 6.2 and 7.4 GPM, more "mainstream" if you will.

Also, is this loop glycol? That too will have an effect, both on flow rate (fewer BTUs per pound), more flow (to compensate for fewer BTUs per mass flow rate) and more pressure drop than water at any given flow.

Aside from the 320 feet of 1" pex (which one might assume has no "fitting factor" if bends are smooth), what fittings are there in the circuit? It has to start and end someplace.

Also the PD of the boiler- is it a coil/HEX or is it direct volume of the boiler itself (which may have minimal pressure drop).


Can you fill in any of the blanks?

Glenn_18

Thanks for the response. The boiler itself is 196 gallon tank above the firebox, no coils. Straight water in the system. Fittings in the loop: 2 unions, 3 full port ball valves, 2 90's and all pex bends are smooth. As far as the degign delta T, I don't know anything about that. The boiler dealer sold me a taco 009-f5 with the boiler but just asked the approx feet of the loop and nothing else. I'm getting heat but it seems it could be warmer being the boiler runs between 175 and 185.

Brad White_203

Glenn, I think you may

have an issue with your vendor or whomever recommended you use one-inch tubing. What Taco offered you is correct (ie: it agrees with me

If you run the 40 degree drop though, not 20, you can run 9.25 GPM. I do not have the Uponor pex data, but if I take it as smooth Type L copper (because I have tables for that right here), I get a drop in "feet of head per 100 feet of tubing" of about 6 feet. At 320 LF, your tubing alone would be about 19 feet of head. Throw in your valves and fittings, if that adds another foot I would be surprised.

Personally, I would have run at least 1.5" tubing. That would handle the 18.5 GPM you would want for a rational temperature drop. That circuit would, if equivalent to copper, impose about 3.15 feet of head per 100 feet of tubing. Your total 320 feet plus valves and fittings might get you to 11 feet of head.

18.5 GPM at 11 feet, that is rational in my opinion. A Taco 0012 or 0014 intersect near that operating point. I would go with the 0014 but that is not knowing anything else about the system. Just a hunch. But this is not the system you have, just what I would want it to be if it were me.

A proper circulator given what you have to work with, that 1" system, wants to flow 9.25 GPM against 20 feet of head. A Taco 011 would work, but would have a surplus of head (about 24 feet at 9.5 GPM), so your flow rate would likely balance out to an operating point a GPM or so above or below that, depending on your system curve.

If you have a 009, that peaks (zero flow pressure) at 34 feet. It will not produce anything near 9.25 GPM. Peak flow (wide open, no system resistance) is about 8 GPM at about 6 feet of head. Very steep pump curve, that one.

Glenn_18

Thanks

Brad, Thank you very much. It seems some of these outdoor boiler dealers know about the boilers but not designing a system and after excavating, installing and so fourth with what they sell you this is what people end up with.

Again, You were very helpfull and THANK YOU!

heatman327

we see this too often

Glenn,
sorry to say that this is all to often a common problem with outdoor boilers. No one properly sizes the line and they do not provide the quantity and quality of heat as advertised. There is very little regulation in this segment of the industry so the problems proliferate as more and more of these units are sold.Education is the only way to solve this issue.....

Brad White_203

You are welcome, Glenn

Sorry for the diagnosis. It is too bad. As heatman said, this part of the equation is so often ignored and to what poor ends?

If the manufacturers and the reputable dealers can get together and at least put out some guidelines.

Something along the lines of, "Boiler Size such and such puts out "X" BTU's per hour. We recommend that the flow rate be "Q GPM" and the minimum line size for that be not less than "Z" Pipe Size. Consult a qualified professional installer or engineer for specifics on your particular installation."

Not to make it a commodity of information but to avoid what are to most of us, obvious judgment lapses or at least not connecting the dots, what all the parts mean. As heatman said, that hurts us all.

Has the frost gone so deep that you cannot re-pipe? I hope that you can. In case you were thinking of it, I would abandon the old one and run one line set of appropriate size. You "might" be able to combine the current S&R line sets into one with split flow.

I forgot to mention, make sure that your circulator is bronze or stainless steel if this is an open system. An iron circulator will not last a season or worse, a season and a half...