A walkthrough of the pipe and pump sizing of a central heating system Step by step approach to the process.

Pipe and pump sizing on a heating system

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You must now consult the pump manufacturer’s literature to select a pump and pump speed based on your calculation.

The diagram above indicates that a Grundfos UPS-15/50 pump will give 30.31kpa on either speed 2 @ 0.28l/s or speed 3 @ 0.38l/s.

Sizing the circulator pump

Before we can size the circulating pump, we must calculate the total pascals per section of pipe. This is completed by first finding out the effective length of pipe in Column 9.&nbsp; It is common practice to allow a 33% increase in the pipework to allow for the fitting’s resistance. This is a fairly simple task that involves multiplying the pipe length by 1.33, as shown in the index circuit table in the following slide.&nbsp; This will give the effective pipe length. Now, multiply the effective length by the pascals for the section.

Example Boiler to A is 12m in length x 1.33 (resistance 33%) So 12 x 1.33 = 15.96m effective length

To counteract the resistance to flow, a central heating circulating pump is used to force the water around the system.&nbsp; Resistance, measured in&nbsp;pascals, is always greater at the beginning of the circuit than at the end, as it diminishes with length.

Let us now look at the information we need to be able to calculate

In a central heating system, which part of the system pipework has the largest diameter? The first section, from the boiler to the first heat emitter.

How do we do the calculation?

Above is a rule of thumb that can be used to identify the pipe size required for pipe runs to heat emitters. this will give you an approximate pipe size that will adequately supply enough heat to travel through the pipe to heat the emitter. On the next page we will use the provisional pipe size featured above to estimate the pipe sizes required.

It is the water which transfers the heat, so the more heat we need, the more water we need. The more water we need, the bigger the pipe; therefore, the more heat we need, the bigger the pipe required.

As the heated water moves along the pipe, it will encounter frictional resistance. What adds to the frictional resistance?

Since the pump size is calculated from the index circuit, this is what we will be concentrating on. The index circuit is the longest circuit and follows the sections of pipework shown in the table below.

On the next page we will look at how to calculate, use the information provided and in which order to carry out the calculations with the information provided to us and; which reference material we may need to consult to assist us with the calculations and inputting of the data.

Notice the circuits listed and the pipe lengths, we shall look at these regularly

We have determined that 28mm looks good, but are there any alternative pipe sizes?

The 22mm pipe will also deliver the required flow rate with a velocity of 0.72m/s (nearer to the ideal 1m/s), but take a look at the pascals per metre.

280p/m is very close to the 300p/m max. There are several advantages to using 22mm, as it’s cheaper to buy and install!&nbsp; However, the pascals may present a problem later, although this will not be known until the rest of the index circuit is calculated.&nbsp; For this example, 22mm pipe will be chosen. The data can be entered onto the table.

<a href="https://www.convertunits.com/from/pascal/to/meter+of+head">Conversion site</a> Pascals to metres head of water

Our figure in pascals = 10,679.9 Using the conversion site or simply multiplying the following; 10,679.9 X 0.0001019977334 (metres head per pascal) we will find the following. 1.09 metres head

The next step is to find out if the boiler creates a significant pressure drop.&nbsp; Some low water content boilers generate high resistance to flow through the heat exchanger, which results in a drop in pressure.&nbsp; This, too, is measured in metres head.&nbsp; Consulting the boiler manufacturer’s instructions will indicate if this is the case. Any&nbsp;pressure drop at the boiler will also need to be added to the pressure drop across the index circuit.

Let us assume the following. that there is a pressure drop at the boiler of 2m head, then the total for the system will be: 2 + 1.09 = 3.09 metres head pressure drop This must be converted to kilopascals (kpa). To convert metres head to kpa simply multiply the metres head by 9.81: 3.09 x 9.81 = 30.31kpa

Find a flow rate (qm) which is as close as possible to 0.225, but not less. We can see 0.228kg/s. The velocity (c) in this case is 0.43m/s. The pressure loss per metre (∆p/l) is 82.5pa.

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Pipe Sizing information for central heating installs

Remember our flow rate is 0.225 kg/s Look down the column below for 28mm

We have looked at 'the rule of thumb' now let us look at other methods. The process is completed using a suitable table for tabulation.

Why? Because this section has to carry the most water (heat), as it has to feed all heat emitters at this point. The greatest resistance to flow will be in the longest circuit.&nbsp; If the pump used will overcome the resistance in the longest circuit, it will always circulate around any other circuit, simply because the resistance is less.&nbsp; For this reason, pump sizing is always calculated from the longest circuit.&nbsp; This is known as&nbsp;the index circuit.

The image on the left shows the pipework from the boiler to the letter A, this is the pipework that needs to carry enough heat to supply all of the system. On the previous page you will notice that all the sections are identified by a letter and sometimes a letter and a number. These are used in a table later on when calculating the pipe size.

What information do we wish to know by the end of this e-Book?

To be able to calculate central heating system requirements used in dwellings. Also select central heating system components in accordance with calculations from predetermined data.&nbsp;

Pipe sizing central heating systems is not calculated on the amount of heat required by any one room in kW. 

It is calculated on the amount of water containing heat that will flow down a pipe in kilograms per second or kg/s.

Look at section&nbsp;Boiler to A&nbsp;on the drawing.

The total boiler load is 17.968kW and so the pipe size from Boiler to A will need to carry all of that heat. A 5% margin for heat loss from the pipe is being added and the length of the pipe run is 12m. 17.968 x 1.05 (5%) = 18.866kW.

Looking at our required kW for the system, then clearly 15mm pipe would be too small. 22mm pipe will supply 12kW, we need 8.67kW, so Boiler to A would require 22mm pipe to be installed.

Section Boiler to A = add all the Watts together for every heat emitter as boiler to A is supplying them all. Downstairs - 1859w, 1127w, 1759w and 340w. = 5085 watts or 5.08kW. That is downstairs, now let us do upstairs; 904w, 456w, 475w, 753w and 1001w = 3589 watts or 3.58kW. We can now add these together to use the provisional pipe sizing as seen earlier to look at what size pipe will supply the kW required. 3589 + 5085 = 8674 watts or 8.67kW.

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