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# Sizing the hot water circulating pump??

#1
12-26-07, 10:18 PM
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Sizing the hot water circulating pump??

Can anyone give me the formula or reference to determine the size of pump I need for a hot water boiler's circulating pump.

The pipes are about 2.5 inches in the basement and one inch by the time they reach the third floor.

The longest loop up and back is about 150 feet of pipe length with about five 90's in the loop.

I have purchased a 1/12th HP B&G pump, series 100, 1750 rpm, and only 1.75 amps current use. Then I thought, I better do a calcuation before installing.

Thank you.

#2
12-27-07, 06:24 AM
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Tom tom:
To select a pump you need two bits of information, (1) the GPM's and (2) the Head.

The picture below is taken from a manual called "The Heating Helper", produced by a reputable boiler manufacturer.

Given the longest circuit length you mention, you'll need your circulator to be able to produce about 8-to-12 ft of head (depending on how many feet of pipe smaller than 2.5-in you have in this place). This is half the battle. The other half is the GPM's. The 75 GPM's value the table shows is a "maximum"...this won't do it. You need to know the total heat loss of your building (in Btuh's)...you'll then divide the heat loss by 10,000 to get the GPM's.

Furthermore, as per B&G (from a chart not included here), a 2.5-in pipe should move NO LESS THAN 25 GPM's, else when your lines become airbound (and they will!), the circulating water won't be able to push the air for removal.

So here you have it. Once you figure out the heating load, dividing by 10,000 will give you your GPM's. This together with the 8-ft of head will give you all you need to select your pump...a procedure which is done by looking at B&G or TACO (or whichever manufacturer) catalogs of Pump Curves.

Hope this helps...

#3
12-27-07, 07:27 AM
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Further reading that explains the procedure pflor described above.

http://www.taco-hvac.com/uploads/Fil...irculators.pdf

2.5" and 1" piping sounds like it might be a converted gravity system, hence high water volume. There are recent threads here describing some of the issues involved in distributing BTUs, expansion capacity, etc. in that kind of system. But if the system has been working fine except for the pump failure, then you are probably ok.

What is the brand/model of the old pump?

#4
12-27-07, 02:38 PM
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Unless... perhaps TomTom currently is running a gravity system, and is thinking about converting to a pumped system, in which case there's more to be done than just adding a pump I believe... ?

#5
12-27-07, 09:22 PM
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Following on question of how to size the circulating pump for the hot water heat

Originally Posted by xiphias
Further reading that explains the procedure pflor described above.

http://www.taco-hvac.com/uploads/Fil...irculators.pdf

2.5" and 1" piping sounds like it might be a converted gravity system, hence high water volume. There are recent threads here describing some of the issues involved in distributing BTUs, expansion capacity, etc. in that kind of system. But if the system has been working fine except for the pump failure, then you are probably ok.

What is the brand/model of the old pump?

Thanks, You guys are generous with your experience and knowledge and great. It makes a big difference for me. The article and graph and comments were helpful and encouraging...

Acutally, there wasn't a pump failure. But the 1950 year installed boiler is being replaced with a 90 plus efficient Munchkin 140M boiler that modulates its BTU output between 30,000 and 140,000 based on algorithmic calculation.

Since there is a primary Boiler loop and a secondary circulating loop with circulating pumps on both of these loops, I am discontinuing the old 1950s circulator also. Because it is probably too large and probably uses too much electricity, and eventually I will break the secondary into about three or more zones with zone valves. {Also, I already scavenged it out and installed it in another house.}

I looked again and three radiators on my longest loop. It is basically one inch and picks up a second floor bath radiator and then two radiators on the third floor.

My concern is will my 1/12th Bell and Gosset 100 series pump placed in the 'secondary' circulating loop handle this [until I break the system apart into three zones.]?

The longest loop therefore is like this: After twenty feet feed from and 20 feet return to the boiler of 2.5 inch pipe on the basement ceiling, there is a take off of 1 inch pipe from the basement to the third, about forty feet up and forty feet back with about two tees and six ninties of 2.5 inches and about four 1 inch 45s and about 8 90's of l inch in the entire system.

The original pump was about 1/6 HP B&G 100 series. It appears it was not a gravity feed system.

[I have come across two gravity feed systems in large houses 10 years older {1897 and 1903 designs}. The pipes were sized at 6 inches and even eight inches at the boiler of one of the houses. [70,000 cubic feet of space to heat per house, compared to about 25 to 30,000 cu feet of space in the current house in question in this article.]

I am digesting all the calculations to come up with the target operating point for the system.[Thank you for the link to the Taco site.]

BUT it is more complicated, because the Munchkin boiler uses a primary loop that double tees into the closed system's secondary radiator supply and return loop as explained before.

I am surprised what the flow and pump head charactersistics are on the specified Taco Pump : Taco 007*.

The Munchkin 140M boiler modulates its BTU output from30,000 to 140,000... and therefore the gpm flow assuming 20 degree Temp drop to overcome is from 3 to 14 gpm with a head loss of the Munchkin to overcome of 2 to 10 Feet of Head as I read the friction feet of head vs Flow in galls per minute graph provided. [I don't quite understand how to read the graph. It appears the more gpm the pump causes, the greater the friction losses through the heat exchanger of the boiler??]

As I read them this afternoon, the toco pump specs are way in excess of 12 gallons per minute and developing 10 feet of head.

I don't know how to interpret the reading of these graphs perhaps. It appears the table of values forthis Taco 0010-F3-1 pump will deliver 10 gallons per minute when it is developing 4 feet of pump head pressure.

And when it can't deliver that volume, then it develops a pressure head of 20 feet.

Is there something analogous to 'mass and energy conservation and conversion here going on with pressure head developed by a pump and the amount of flow of fluid, gpm, it is able to cause?

I.E., when the pump is thwarted in delivering the fluid flow through it, it ramps up the amount of force per square inch or pressure it is causing to be added to the 'waters'.

So, in this case, the Munchkin boiler curving up crosses the Pumps arcing down at about 6 feet of head and 9.75 gallons per minute.

So it would appear this is the operating point of the pump in the system of the primary looping through the Munchkin.

And at just under 10 gpm flow, 1.25 inch pipe is chosen so that bubbles are carried out and there is not too much noise created [2fps to 4fps per the table inserted in answer to yesterday's inquiry [thank you.]]

What I don't get, if the above is correct,... will not the pump active in the primary loop cause pressure to drop across the two tees that connect it to the secondary and draw its water from the secondary?

It will cause the water velocity to pickup between the two tees as the secondary water is drawn into the primary loop to be heated and then expelled back into the secondary loop through the exiting or second tee.

[Only 6 inches of 1.25 inch pipe is specified between them]. And since the cross section area of the pipe is the same through out, as the velocity goes up, then the pressure goes down [pva=p'v'a' conintuity of flow, where p is pressure and v is velocity and a area at the two different points.]

The water diverted in from the first tee of the secondary loop will be caused to hurry up as it is impelled by the primary loop's Taco pump.

When it comes up against all of the slower moving water as it dumps again into the secondary loop, it will create a pressure head.

If so, it appears the primary pump will also add a positive pressure head to boost the flow in the secondary loop.

Therefore the secondary pump will have this pressure added in series to it, giving the circulating water a double boost as it feeds the radiators.

Any thoughts of this?

[I learned a great deal yesterday reading about the mono systems with their OS tees of the 1930's and 1940s. That logic must apply here also, but in a positive addition of pressure to the loop?]

Thank you.

TomTom

Last edited by Tom tom; 12-27-07 at 10:52 PM. Reason: Accuracy and clarity

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