Design constraints for water central heating systems

Limiting factor 1 - the temperature of the water from the boiler.
Limiting factor 2 - temperature drop around the circuit.
Limiting factor 3 - the speed the water circulates.
Limiting factor 4 - the flow verses pipe diameter.
Limiting factor 5 - the length of pipe of a given size.
Limiting factor 6 - the effect of fittings.

The idea of the pages covering the design of central heating systems is not just to give you the tools you need to design a good serviceable system but also to give you an understanding of the underlying principles.

This pages explains why certain limiting factors are applied to the design of water central heating systems.

Limiting factor 1 - the temperature of the water from the boiler.

The hotter the circulating water is, the greater heat it can hold and emit through the radiators (or other emitters). However water turns to steam at 100 °C (212 °F), and when it does, it dramatically increases in volume and in the restricted volume of the circulating pipework/boiler can cause excess pressures. To prevent the water from boiling, the boiler thermostat in a good design of system is usually set to switch off at about 80 °C (180 °F).

Limiting factor 2 - temperature drop around the circuit.

As heated water cools, the amount of heat it transfer to the surrounding air decreases. The higher the temperature difference, the higher the rate of transfer and once the water has cooled to the air temperature, no further transfer of heat is possible.

The heated water from the boiler is circulated around the system and is relatively cool when it arrives back at the boiler. The bigger the temperature difference between boiler outlet and inlet, the harder the boiler needs to work to reheat the water.

In practice, both the above contribute to a need to minimise the temperature drop around the circuit. A temperature drop of 12 °C (20 °F) is a good target to aim at in a design as this will give effective transfer of heat from the radiators while not putting too much demand upon the boiler.

Limiting factor 3 - the speed the water circulates.

From the detailed design of a central heating systems, the rate of thermal transfer required will be established (either as kW or BTU). The need to keep the temperature drop around the circuit within limiting factor 2, will determine the volume of water needed to pass around the system.

To achieve the required temperature drop, the circulating water needs to give up a certain amount of heat. For a 12 °C temperature drop, 1 litre of water gives up about 14 watts (or imperial, for 20 °F temperature drop, 1 gallon of water gives up 200 BTU).

With the example house, the total heat requirement equates to 7.25 kW (24,700 BTU), so to achieve this with a 12 °C temperature drop, a total of 519 litres of water per hour, (or imperial, 124 gallons per hour). These volumes are the flow through the boiler, the actual water will be circulated many times in the hour.

The larger the volume of water, the faster it needs to move and with any liquid, the faster it moves, the more friction is generated and there is increased risk of internal truculence causing vibration and noise.

Experience has shown that a speed of about 90 cm/second (3 ft/second) is the optimum maximum to ensure efficient operation.

Limiting factor 4 - the flow verses pipe diameter.

The speed of water flow in limiting factor 3 above is not a real problem as the size of water pipe directly affect the speed for a given water volume, if to achieve the required water flow the speed is in excess of 90 cm/second in a given size of pipe, increasing the pipe size will reduce the speed.

As a guide, the speeds for 500 litres per hour flow are calculated as:

15 mm dia. pipe - 148 cm/second
22 mm dia. Pipe - 55 cm/second
28 mm dia. Pipe - 31cm/second

In imperial, the speeds for 100 gal per hour flow are calculated as:

15 mm dia. Pipe - 53 inch/second
22 mm dia. Pipe - 20 inch/second
28 mm dia. Pipe - 12 inch/second

Going back to the example, the required flow was calculated above as 519 litres/hour, so :

Using 15 mm dia pipe, the requirement of 519 litres/hours gives a flow of 154 cm/second (i.e. 148 * (519/500)) - this exceeds the maximum of 90 cm/second so 15 mm pipe is not suitable.
Using 22 mm dia pipe, the same total flow requires a flow of 57 cm/second (i.e. 55* (519/500)) - this is well within the maximum speed so 22 mm dia pipe can be used.

To make the calculations easy, the following are the maximum watts and BTU which can be carried by the different pipe sizes based upon a litre of water giving up 1.4 watts (12 degrees C) (or one gallon giving up 200 BTU (20 degrees F)) while not exceeding the flow rate given in limiting factor 3 above.

15mm dia	4.25kW	13,620 BTU
22mm dia	11.5kW	36,630 BTU
28mm dia	20.5kW	65,240 BTU

Limiting factor 5 - the length of pipe of a given size.

While limiting factor 4 above showed how to overcome the maximum flow rate for a given size of pipe, the length of pipe also has an effect on the force required to circulate the water. The longer the length of pipe, the greater the pressure drop for a given flow rate. This table gives the pressure drop per unit length for various flow rates and pipe sizes.

Pump performance curve The pressure drop is important to know for setting up the circulating pump. Every pump has a performance curve (example to the right) which defines its capability to handle various flow rates and pressure (also referred to as 'head loss'). Providing that when the calculated flow and pressure are plotted on the graph, they met under the curve (i.e. in the coloured area), the pump is adequate for the installation. There's not much that can be done to reduce the water flow in a system, however increasing the size of pipes will reduce the head loss.

Limiting factor 6 - the effect of fittings.

Limiting factor 5 above raised the subject of head loss due to the length of the pipes, fittings and valves also contribute to head loss. This is one reason to avoid connectors if the actual pipe can be bent as this has a negotiable effect. A compression fitting in pipe has the same effect as approximately 60 cm (2 ft) of pipe run, solder connectors have a slightly smaller effect.

Unless the design is working on the limits of the flow rate for a particular pipe size, a simple percentage allowance can be used rather than calculating the head loss for each connector. For compression joints (or a mix of compression/solder), use a 35% allowance, where only solder connectors are used, allow 30%. To use this, if the pipe run is 10 metres (say 30 ft), use an equivalent length of 13.5 metres (40.5 ft) for compression joints or 13 metres (39 ft) where only solder connectors are used.