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# WATER FLOW RATE IN HEATING SYSTEM

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Member Posts: 15
I have a Taco circulator 007-F5 on my boiler. How do you determine the actual water flow rate in gallons per minute. According to the Taco website, it could be anywhere from 0-23 gpm (dated June 2015).

New baseboard heat emitters have Btu ratings for 1 gpm and 4 gpm. I am trying to determine how many feet of replacement emitters I need. Thanks

• Member Posts: 22,467
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You would need a flow meter, or some pressure gauges installed on both sides of the circulator, to get an actual gpm number.

As far as new emitters, those should size to the heat load of the various rooms in the building.

Here is a free load calculator to start a room by room load calculation.

The next step is to chose the type of emitter and the temperature you intend to run the system at. The fin tube or other emitters will have a chart showing output at various temperature and flow rates. Match the room load number to the outputs of the heaters.

https://www.slantfin.com/slantfin-heat-loss-calculator/
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 7,605
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The flow rate becomes a bigger factor when you are running the radiators in series. As the water temps drop, the required linear footage of baseboard increases.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 8,166
edited August 2020
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There is a rule of thumb. 1GPM = 10,000 BTU it comes from a calculation that assumes a 20° temperature change thru the system.

One BTU is the amount of heat needed to raise the temperature of 1 pound of water 1°F

1 gallon of water = 8.33 lbs. at room temperature

1 hour = 60 minutes

temperature change = 20

so... 8.33 x 20 x 60 = 9.996 or about 10,000

To determine the flow rate of your pump you can use the math backward.

1. With all the valves and radiator loops full open, operate the boiler for 5 minutes to get it up to the normal operating condition.
2. Check the BTU output of the boiler from the rating plate.
3. Measure the temperature at the boiler outlet (Supply) pipe and the inlet (Return) pipe.
4. Time how long the burner operates and how long it is off (If Any) for 1 full hour (or at least 15 minutes then x by 4)
5. Record all your findings every minute to get the average of the actual operating conditions
6. Turn down the thermostat and Make your wife a frozen pina colada for putting up with your experiment in the heat of the summer.

Now plug in the numbers to the equation.

For example: If the average temperature difference is 18°F and the BTU rating is 85,000 BTU and the burner operated for 10 minutes of the total 15-minute test then your output is 66.66% of the rating

85000 x .6666 = 56,661 BTU for 1 hour

8.33 x 18 x 60 = 8996.4 or 9000 BTU = 1 gallon per minute

56,661 ÷ 9000 = 6.29 gallons per minute

These numbers are approximate based on your actual experience. if the BTU rating is 85,000 output and the gas pressure is not set correctly then the actual output will be off. If the thermometers you use for the input and output are not properly calibrated then the temperature difference will not be accurate. BUT don't worry, this is not rocket science and I believe you don't actually need the Exact GPH of your pump. You just need to know approximately within a reasonable tolerance of 1/2 GPM give or take.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 8,166
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Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 8,166
edited August 2020
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OOPS I see why you need the GPM now

Are you in need of the GPH in order to select a replacement pump? Then you will not be able to do the test. My recommendation is to use a Taco 007-F5 (or equal) if your boiler rating is 175,000 BTU or lower. If you are replacing an old expensive B&G 100 or Taco 110 pump that has the 3 oil cups to maintain regularly, then the smaller more efficient "Wet Rotor" pumps are usually a good choice.

If you have something larger than say 185,000 BTU boiler and there are unusual piping configurations, then you need to do more research. Let the WALL know if you need more info.

Use the 4 GPM number at a temperature of 160°F (if that is available) then add a foot or two for good measure

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 185
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Maybe I'm missing something here, but you are wanting to determine the length of replacement heat emitters? Did the present heat emitters perform satisfactorily? If so, install replacements of comparable heat rating and flow. If not, then adjust the replacement emitters accordingly. Computing the flow rate throughout the various parts of a hydronic system can be very complicated and is seldom done for residential systems. Simple rules of thumb are usually satisfactory.
• Member Posts: 15
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Thanks everyone for your insightful answers. I think I will go with the 4 GPM ratings in finding replacement baseboard emitters. I have 1948 and 1953 catalogs for the C A Dunham baseboard emitters I want to replace. These catalogs list BTU ratings for various water temperatures.
• Member Posts: 2,509
edited August 2020
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If you have one zone, that's all dandy. If you have multiple zones that operate together, then things change.

The shorter less restrictive of the muliple zones will get the most flow and lessening of the flow up to the most restrictive circuit.

Balancing valves will balance all the circuits so the flow would be equal in all of the zones. But, thermostat controlled zones don't always come on at the same time except on the coldest day in winter. There are many things that can influence the heat in a zone's operation. Solar gain, cold side of the house ver warm side, bottom floor ver top floor, etc.

Many boilers come with the Taco 007 pump from the manufacturer. Whether that is the correct pump for your sys is an assumption. The way a pump is chosen is to determine the head energy loss for the most restrictive zone and then figure how many gal of flow you want and look for a pump curve that puts your needs in the center 1/3 of that curve. A multiple speed pump or an ECM pump might suit your needs better, depending on your sys.

It's not rocket science, but it's complicated.