2 taco pumps in series on OWB?

[MNBobcat]

Hi All,

I'm installing a Classic boiler model 6048. I just bought 300 feet of 1 1/4 thermopex. The boiler will be located 240 feet from the house. It will enter the house and go to my two off-peak electric water heaters. From there, it will run 40 feet to my furnace. Then it will run to a furnace in my garage and then return to the OWB. I'm figuring about a 500 - 550 foot loop round trip. Piping will run in series.

An 014 TACO with 1 1/4 lines is rated for 8.26 - 9.03 GPM flow for that distance. Would there be any benefit of installing a pump at the OWB and another pump in the house? If not, I'll likely install just the one pump in the house. I was thinking of putting it inside in case it ever needs to be changed out.

The entire loop described above will be using 1 1/4. However, I'm ASSUMING that when it enters the exchanger for the hot water heaters that it will be reduced to 1 inch? Anyone know if that is true? If so, do I lose the benefit of 1 1/4 tubing (because of the reductions) and would the reduction determine where I should place the pump? I can place the pump so that its the first thing in the house (first thing between the house and OWB) or I can place it after the furnace so that its pushing on the return side or doesn't it matter where the pump is located?

Thanks!

 

[jebatty]

What do you show as your pump head with 550' of 1.25"? My calculator shows 16' of head at 10 gpm, which just about matches the pump curve for the 0014 at 10 gpm. You can add another pump, but head rises rapidly as gpm increases, and you won't gain much. You likely would do better with a single 0011 or 0013. I think you're going to max out at about 12 gpm (pump head 22.7') with 1.25" pipe. Lengths of lesser diameter pipe in the loop will add even more head. 1.25" pipe generally is rated at 14 gpm max, while 1.5" is rated at 25 gpm.

FWIW, my calculator shows head of 9.1' at 12 gpm with 550' of 1.5" pipe, and 13.8' head at 15 gpm. You very long loop probably would have been better designed with 1.5" pipe.

 

[Gooserider]

My understanding is that a reduced diameter pipe anywhere in the circuit pretty much limits your flow to the volume of that section - so it's worth trying to avoid that if you can. Would it be possible to plumb the water tanks in parallel? Or possibly put a bypass on them that would allow part of the water to go around them and thus have the same effective flow restriction?

Also, when figuring your loop distances don't forget that EVERY fitting the water goes through needs to be accounted for as fittings will increase your "effective loop length" by some amount - the usual aproach is to make a list of each fitting type, look up on a table what the "equivalent pipe distance" is, multiply, and add the result to the actual pipe length - considering that it sounds like you will have a lot of fittings, and many fittings have large equivalent lengths, you may be looking at an effective loop that is closer to 7-800 feet or more....

Bottom line, you probably should have used a bigger pipe, or doubled lines, at this point I would look to see what kind of flow reductions you can do, or possibly look at some of alternative plumbing setups like mini-tube injection mixing to see if you can get the effective loop lengths down.

Also keep in mind the chronic issue with OWB systems of having to try to deal with cavitation problems due to the lack of pressure causing insufficient NPSH for the pump to work properly. In a pressured system you would normally have a "point of no pressure change" at the expansion tank, and would want to set it up to pump away from that point, but I'm not sure where that would be on an open OWB setup. I would tend to think that your best bet for a pump location will be to find the lowest point in the system (in order to maximize the atmosphere pressurization), preferably where you will have a large tank or other supply feeding into the pump intake in order to minimize the suction induced pressure drop.

Gooserider

 

[MNBobcat]

Hi Jim,

Thanks for the reply. I had heard the term "head' before but never understood what it meant. After your post, I did some reading and got quite an education. This is going to be a bit more complicated to design than I thought. Thanks for sending me in the right direction.

Thermopex doesn't make anything larger than 1 1/4 inch.

Classsic says they want a minimum 6 GPM flow rate. So I guess my goal would be to stay above that. Though it sounds like I will need to do more research into what size exchangers are available, etc, before we can figure out the correct pump size and what flow rate and head I'll be looking at.

 

[Gooserider]

Hi Jim,

Thanks for the reply. I had heard the term "head' before but never understood what it meant. After your post, I did some reading and got quite an education. This is going to be a bit more complicated to design than I thought. Thanks for sending me in the right direction.

Thermopex doesn't make anything larger than 1 1/4 inch.

Classsic says they want a minimum 6 GPM flow rate. So I guess my goal would be to stay above that. Though it sounds like I will need to do more research into what size exchangers are available, etc, before we can figure out the correct pump size and what flow rate and head I'll be looking at.

In addition to Classic's flow rate requirement, you also need to think about your loads heat demand, as that can have a big impact on your flow... Remember that the boiler water is in essence a "conveyor belt" that has the job of carrying BTU's from your boiler to the load and dumping them off... If the conveyor can carry at least as many BTU's as the load needs, everybody is happy, if it falls short, then the guy in charge of heating is going to be in trouble... The "capacity" of the conveyor is a function of two numbers - the flow rate and the temperature difference between the supply and return. These numbers are a tradeoff within certain practical limits - for a given BTU load, more flow will lower the temperature drop and vice versa, so you can lower your required flow rate by increasing the amount of temperature drop your system can take advantage of...

However, just as there are practical limits on flow rate imposed by head losses, you can only get so much temperature difference - your boiler water can't get much over 180-190* safely, and you generally don't want your return temperature to be much below 140* or so in order to prevent boiler condensation problems (plus it is harder to extract useful heat from lower temperatures)

You will need to make sure that your system is designed so that you are conveying enough BTU's at your expected flow rate. This can also be helped by doing all that you can to avoid having the BTU's getting dumped in the wrong spots, mostly by making sure your lines are REALLY well insulated...

Gooserider

 

[pybyr]

You may want to look into some of the newer-generation circulators with permanent magnet ECM motors- they're more efficient on electrical power usage relative to net pumping work-- the savings on future electric bills could add up particularly quickly in your situation, as your pump(s) will be larger than common residential zone circulators, and, it sounds, working more of the time. Someone else can probably chime in in a more informed way about makes/ models/ choices currently available.