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Preventing Coldspots

Cold spots are among the most common faults in central heating systems. If caught in time, they are straightforward to fix. However, they can cause considerable damage to the system if left untreated.

What causes cold spots?

Cold spots at the top of a radiator are caused by air entering the system or hydrogen gas forming as a result of corrosion. Radiators in this state require frequent venting.
When cold spots occur in the middle and lower parts of a radiator, they stem from a build-up of magnetite (iron oxide), which appears as a black sludge. This forms as a result of electrolytic corrosion (a chemical reaction between metals). Cold spots that occur throughout the system may be the result of bacterial contamination.

What should be done?
After turning on the central heating system and venting all radiators, apply Fernox Cleaner F5 or Cleaner F3 via the feed and expansion tank alternatively Fernox Express Cleaner F5 can be dosed directly into the radiator. Circulate for at least an hour at the normal operating temperature. To remove hardened iron oxides and limescale, the cleaning time may need to be extended up to one week.

If an individual radiator still has a cold spot after one hour’s circulation, increase the flow by closing the valves of all other radiators. Additional doses of Fernox Cleaner F5 or Cleaner F3 may be necessary. Drain and flush thoroughly until the water runs clear. More than one complete change of water may be required.

Once the system is cleaned and flushed, use any Fernox Protector F1 to prevent sludge and limescale from forming. Inhibiting will improve energy efficiency and extend the life of the system by stopping the pumps from sticking and remove the need for frequent venting.

Aeration of circulating water leads to rapid corrosion. In almost all untreated central heating systems hydrogen gas is produced because of corrosion. However, gas should not be confused with air. Using any Fernox Protector F1 prevents corrosion and the formation of both hydrogen and bacterial gases such as methane. Aeration can take place at the same time as gas formation, but this fault cannot be corrected by corrosion prevention alone.

What should be done?
Here are some common causes of aeration and ways to correct them:

Pumping over
An incorrect system design layout can lead to a state of hydraulic imbalance, which will adversely affect the efficiency of the system. Water treatment alone will not address poor design. Consult a reputable installer for advice.

An incorrect position of the cold feed pipe in relation to the pump and open vent pipe can allow the water level in the open vent pipe to be drawn to a lower point when the pump is engaged. Each time the pump is switched off, re-oxygenated water enters the circulating system. On passing through the boiler, dissolved air is expelled from the solution, forming air blisters which usually collect in the first radiator on the flow side or in the towel rail in the bathroom.

Water treatment alone will not address poor design. Consult a reputable installer for advice.