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Pumps & Valves: Getting the balance right

Making sure that a domestic central heating system provides optimum efficiency and comfort is not just about the hardware - it is as much about how that hardware is used and balanced says Bjorn Sejr Nielsen, marketing director, Danfoss Randall.
Pumps & Valves: Getting the balance right
So much emphasis has been given to the current high levels of technology serving the domestic heating trade that it would be easy to think that just by installing a high-efficiency boiler and some relevant advanced controls the perfect system could be instantly achieved. Unfortunately, it is not that simple.

Each and every radiator in a central heating system needs to be capable of generating its designed heat output if the entire system is to operate at peak efficiency. Achieving this has more to do with system balance than the selection of boilers or controls.

A heating system, therefore, needs to be balanced to ensure correct flow through all the radiators and, thus, achieve optimum efficiency. In a system that is not balanced, certain radiators are likely to overheat while others might provide poor heat input; and there will be significant difference in temperature between radiators during heating up.

In the interests of fuel economy, system efficiency and home comfort, it is important to ensure that all radiators in a system are providing their correct designed heat input.

Hydronic balancing

Hydronic balancing is the process of optimising the distribution of water in a building's heating or cooling system so that it provides the intended indoor climate at optimum energy efficiency and minimum operating cost.

In an unbalanced system there will always be circuits that receive more water than they require. These favoured circuits, usually those closest to the boiler, tend to withhold flow from other more distant circuits that are not then able to provide the comfort required. Radiator thermostatic valves (TRVs) may help temporarily by gradually reducing the flow in favoured circuits but during high-load periods all the TRVs operate in a near-permanent open position. As a result the distant circuits still receive too little flow.

Balancing limits the flow in favoured circuits and forces water through others. As a result the required design flows are available throughout the system, which can then provide the required indoor climate. Also, avoiding overflows means the pump is not doing unnecessary work which saves energy, reduces operating cost and can reduce the size of the pump required.

Balancing central heating rads

System balancing is normally carried out by using the lockshield valve to regulate the flow of water to each radiator. By partial closing of these valves, the radiators nearest the pump can be restricted more than those that are further away. The best way to get this right is by using radiator thermometers, which can simply be clipped to the flow and return pipe at either end and measuring the temperature differential. Experienced installers may find they can manage without the thermometers, simply relying on the 'feel' of the radiators. However, for those less confident, the basic balancing procedure is as follows.

· Start by switching off the system and letting the water cool down
· Open both the lockshield and TRVs on all radiators
· Fit the radiator thermometers to the inlet and outlet pipes of the nearest radiator to the boiler. Do not fit them to the main flow and return pipes
· Switch on the central heating system
· Close the lockshield valve on the first radiator to almost closed, as the temperature of the systems comes up, gradually open up the valve until the temperature difference between the two thermometers is about 11°C
· Move the thermometers to the next radiator away from the boiler. Close down the lockshield valve and adjust it until the temperature difference increases to about 11°C (the temperature difference will probably start at less than this as both valves are fully open)
· Work along the rest of the radiators until they have all been balanced.

Having done this, the lock shields should then have any final fine adjustments carried out and their covers fitted.

Presetting valves

Some people believe that having TRVs in a heating system will bring about automatic balancing. This is not so. Simply installing TRVs will not provide hydronic balance as the system will need to settle before any effect can be felt.

However, for larger installations, special presetting TRVs are available that greatly simplify the process and enable accurate system balancing.
Valves with presetting allow the commissioning engineer precisely to set the calculated maximum flow of water to individual radiators without the need for costly and inaccurate temperature measurements.

Danfoss Randall valves for this purpose have a separate presetting device integrated into the valve body that does not interfere with the degree of opening of the valve cone, as seen with some other presetting systems. Presetting is achieved without special tools by means of a setting collar, which is covered once the radiator thermostat is mounted.
8 August 2010

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