# Rinnai RC98HPi



## Asparta (Mar 12, 2010)

Hello everyone!
Need some help with something. I have never sized a pump in my life. However, I need to size a couple now. Customer has in floor heating, 6 loops, about 1200 sq. ft. of slab. 1/2" tubing used. Has a Rinnai RC98HPi unit that he is looking at getting. I will plumb to run his domestic hot water, and then tee off a primary loop feeding a plate heat exchanger with a pump on it. Other side of heat exchanger, a second pump with all necessary trim feeding the in floor tubing. How do I size these pumps? And what is everyone's opinion on how I want to plumb this for this guy?

Thanks!


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## OldSchool (Jan 30, 2010)

You have to figure out your head.... Wirsbo has some great books with calculations for pump sizes... if you can not get it online then I will figure it out from my books...let me know


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## Asparta (Mar 12, 2010)

Ok. thanks! That is the info that i am missing. i am not sure how many feet of tubing he ran, so as to calculate the feet of head needed to overcome. Based on 12" spacing, at say 1200 sq. feet, that would equal 1200 feet of tubing right? Then i would divide that by 100, and then use the factor per 100 feet of tubing and multiply right? I just need to know that factor, as i can't seem to find my literature right now. 
What about the primary loop on the heater side of heat exchanger? How would i size that? Would only be about 5 feet of copper tube, 3/4". I know it will be a small pump, but should i size that for the minimum flow rate of the heating unit? 

The factor for 1/2" wirsbo would be great!

Thanks!


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## Asparta (Mar 12, 2010)

can someo help me with the head factor every 100 feet for 1/2" pex?
need this ASAP!
thanks


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## OldSchool (Jan 30, 2010)

Each Taco circulator pump has several important features, that have to taken into consideration prior to making a purchase.

These features include the material of construction (cast iron, bronze, stainless steel), flow rate (GPM, or Gallons per Minute), head loss (in ft of head), horsepower (HP), power supply (V) and conection type / size (flanged, threaded, sweat).


Most common type of circulator pumps used for closed loop radiant or hydronic heating applications are cast iron flanged circulators.
Stainless Steel and Bronze circulators are typical for open loop hydronic/radiant heating and domestic hot water recirculation applications. Threaded and sweat connection types are particulary common for the later application type, making it easier to connect the circulator in line with an existing water line.
By default, all Taco circulators are 115 Volts, 60Hz. All other models, such as with 230V power supply or with an outlet plug are usually custom ordered.

All circulator pumps are typically sized based on the heat load and head loss (pressure drop) for a given zone.

Knowing the heat load (in BTU's) for a given zone, allows to calculate the required circulator pump's flow rate in gallons per minute (GPM).

For hot water hydronic or radiant heating applications, the following equation can be used:

GPM = 0.002*BTU/(Temperature Drop),


where Temperature Drop is the difference between supply and return temperatures in the system and GPM is the amount of flow the circulator must produce.

Since most of the radiant heating systems utilize a 20F temperature drop, the formula can be changed to:

1 GPM = 10,000 BTU/hr,


meaning that for every 10,000 BTU's of heat load the circulator must output a 1 gallon per minute flow.

Assuming that system calls for 100,000 BTU/hr, a circulator pump should have a minimum 10 Gallons Per Minute flow rate at a given pressure drop.
For snow melt systems with a 50/50 mix of glycol and water, the above equasion is slightly different:

1 GPM = 11,000 BTU/hr
The next step is to calculate the head loss, or pressure drop in the system.
Head loss is associated with friction of the water against the internal surface of the pipes/tubing in the hydronic or radiant heating system and restricts flow rate a circulator can produce.

Although radiant heat manifold and PEX tubing sizing are a different topic, let's assume, for example, that a manifold has 8 outlets with 1/2" PEX tubing installed at 300ft length per loop and the system calls for 72,000 BTU's.

Using the formula above, we can determine the flow rate required for our given zone: 72,000 / 10,000 = 7.2 GPM.
Flow rate through every selected circuit of the manifold equals Flow Rate divided by number of Circuits:
7.2 GPM / 8 circuits = 0.9 GPM per circuit (assuming that the circuits are equally balanced).
Using a Pressre Drop Table or Pressure Drop Chart, supplied by the PEX tubing manufacturer, a pressure drop per ft of tubing can be calculated at a given GPM flow rate.

NOTE: Pressure drop data supplied by manufacturers may be available both in PSI (lbs per square inch) and in foot (ft) of head.
For conversion, use the following equation: 1 psi = 0.434 ft of head (for fresh water).

In this example, pressure drop per 1 ft of 1/2" PEX tubing at 0.75 GPM flow rate would be approximately 0.03 ft of head).
Considering that each individual PEX tubing circuit is 300 ft long, pressure drop per circuit would be 0.03 x 300 = 9.0 ft of head.

Since PEX tubing circuits are in parallel to each other, pressure drop per circuit is always the same as the total zone pressure drop. So, the total pressure drop is: 9.0 ft of head.


We now have the complete specification for the circulator pump available: 7.2 GPM flow at 9.0 ft of head pressure drop.
It is important to understand that other components installed within a given zone (such as the radiant heat manifold itself, fittings, check valves, mixing valves, balancing valves, heat exchangers, etc.) also have to be considered when sizing a circulator pump. Pressure drop information is usually available in a form of technical specifications or submittal sheet supplied by the manufacturer.
Given real conditions, we may add extra 2 ft of head just in case, making pressure drop 11ft of head.

The next and final step, is to match the obtained data with a correct pump on a Circulator Pump Curve Chart (see below).


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## OldSchool (Jan 30, 2010)

Chart


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