Service and support


Why are there cold spots in my radiators?

These can be caused by any of the following:

  • the system is not balanced correctly.
  • the conveying height of the circulation pump is too low or the pump is running at too low a power setting, which causes the flow-rate in certain circuits to be too small.
  • there is too much air in the system.
  • the thermostat control is covered by an item of furniture, or curtains. This causes the temperature at the thermostat to be significantly higher than in the room itself, which in turn causes the valve to be completely or partially closed and the flow-rate to decrease.
  • the thermostat control is not being used correctly.
  • the main thermostat is being subjected to large external sources of heat, such as the sun's rays. Since the main thermostat has reached the desired temperature, the radiators in other rooms do not heat up.
  • the water temperature is too low.
  • the radiator is too long: if both the inflow and outflow connections are at the same end, the water might not be flowing through all of the radiator. For long radiators, the connections should be at opposite corners.
  • impurities in the valve unit. When the system is filled with water, suspended particles can collect at the valve and decrease the flow significantly.
  • the radiator is only partially warm: a phenomenon which is sometimes easy to explain. When the desired room temperature is reached, the valve will close, even if the radiator is not yet completely filled.

Why can I hear sounds in my radiators?

  • a clack-clack-clack sound indicates an incorrect connection with inflow and outflow exchanged.
  • a hissing sound indicates that the flow-rate is too high through the valve, caused by the pressure being too high.
  • a tick-tick-tick sound indicates there is tension in the system in the pipes, the radiator, and the wall units.

This is common in winter when temperatures outside drop to freezing. A radiator that is not protected against frost inside can freeze. This doesn’t only happen in houses still being built, it can also happen in houses that are already inhabited.

How and why do radiators freeze?

Household spaces that, in the winter, are either never or rarely heated are called "risk spaces". Examples include: verandas; garages; spare bedrooms; attics; cellars; halls; storage spaces; washing spaces and even living rooms that are not heated in the winter for budgetary reasons. (PURMO's service department has never encountered a frozen radiator in a kitchen or bathroom.)

If these spaces are not heated, the temperatures fall below freezing point and the water inside the radiator turns to ice, causing the radiator to crack, or even explode because of the enormous pressure of the frozen and expanded water (ice). This can lead to the radiator being ruined and needing to be replaced. If thawing takes place, water will spout out of the radiator causing a great deal of damage.

How can frozen radiators be avoided?

1. Always fit radiators with thermostat valves. This will guarantee that a supply of warm central heating water remains available. You must, however, always ensure that you do not fully shut the thermostat valve, but turn it to the star.

2. Do not set the night temperature too low. Most people have a room thermostat in the living room. If this is set to 15°C at night and this temperature is maintained in the room, the thermostat will send a message to the boiler to stop heating for the time being. This is dangerous for the radiators in other rooms. The temperature in the living room may stay at around +/- 15°C for a long time, whilst the temperature in the veranda falls below zero. Because the boiler has stopped heating, the radiator in the veranda will freeze.

3. When you are away from home (whether for a relatively short time or for a few days) do not change the settings of the radiators, boiler or the thermostats.

4. Do not assume that heat produced in adjacent spaces will automatically ensure that the spaces you do not heat will be kept frost-free. In other words, make sure that each space is always (at any time of the day or year) provided with the necessary minimum heating.

How can I prove that a radiator has frozen?

Nearly all people suffering a frozen radiator do not agree with the diagnosis. Often, the customer incorrectly believes that the temperature in the room did not fall to below freezing point because a) he trusts his room thermostat, or b) claims that because the other rooms are heated it is not possible for anywhere else in his house to freeze.

There are, of course, ways to prove that a radiator has indeed frozen:

  1. the upper water channel will have been deformed by the expansion process of the change from water into ice. The water channel will be wavy and crooked, usually along the whole length of the radiator.
  2. the steel between the weld points will have bulged as a result of the expansion of the ice. This bulging of the steel between the weld points will be worse at the top and less serious at the bottom.
  3. the steel will be cracked, usually at the top weld points or at the connection box.

There is no doubt that a radiator which displays this degree of deformity has frozen. The pressure exerted on a radiator when freezing takes place is +/- 20 bar. PURMO conducts regular tests on radiators to test their pressure resistance. At +/- 20 bar (the normal test pressure for all radiators amounts to 8 bar, and the guaranteed working pressure is 6 bar) the radiator will start to display exactly the same characteristics as a frozen radiator. This pressure cannot be caused by the central heating water itself, the poor quality of the steel or by another manufacturing fault.

A frozen radiator is always a minor disaster for the customer as they are responsible for the water damage and must pay the costs themselves. It is therefore understandable that they will argue against the conclusion that the radiator froze. Unfortunately, this brings a climate of dissatisfaction and bad publicity for the radiator. Therefore, to prevent such negative occurrences, all the necessary measures need to be taken before and during installation, and the customer is also to be informed about the consequences of spaces that are barely heated or not at all.

When do I need to use Low Surface Temperature radiators?

LST radiators are an ideal choice wherever high efficiency heating is required, yet where safety demands a low surface temperature, ie. where people may injure themselves if left in contact with a standard radiator. The PURMO Safety Radiator never runs above a maximum 43°C and is ideal for hospitals, nurseries and residential homes, as well as children’s bedrooms.

What is the difference between a direct and an indirect system?

A direct system is one where the tap water actually mixes with the water in the radiators. An indirect system separates the tap water from the radiator water, and can also be called a “closed” system. PURMO radiators can only be fitted to an indirect system.

Why do I need to bleed my radiators, how do I do it, and when is the best time of year to do so? (Applicable to vented systems only)

You need to bleed your radiators in order for them to stay efficient. When the weather is warmer, this is the ideal time to think about this very necessary, if mundane task, to ensure you won’t be left in the cold next winter. Over time, air builds up inside a heating system, and creates a bubble inside the pipes, blocking the flow of hot water that keeps radiators warm. This trapped air makes radiators inefficient, so unless you feel like bringing a touch of winter indoors, it pays to periodically ‘bleed’ your radiators and towel warmers.

If you’re a complete novice at this, here is a five-step guide:

  1. Turn your thermostat up and touch all the radiators and towel rails in your house to find those which are not working properly.
  2. Turn your boiler unit off. Locate the inlet valve and open it. If the valve was the problem, the radiator will now start to heat up properly.
  3. If that's no help, locate the bleeder valve at the other end of the radiator and use a standard air key or screwdriver to open it up, turning anti-clockwise.
  4. Once all the air is released, scalding water will spurt out, so be prepared and have a small bucket or pan handy to catch it.
  5. As soon as you see water, shut the valve tightly, although take care not to over tighten it. Job well done!

How do I calculate the heating requirements of a house/room?

Calculations for room heat requirements are quite complex, and in order to establish the radiator output and size required, both fabric heat loss and ventilation heat loss need to be calculated. For details, follow the explanation below.

Fabric heat loss

The difference in temperature between the inside and outside of a building causes heat to flow through the property towards the lower of the two temperatures, and subsequently, be lost. It is transferred through walls, floors and roofs, and it is vital that we know how much heat is lost through each surface so that calculations can be made to heat a building to the ideal temperature.

The transfer of heat is calculated using “U” Values - the rate of loss of heat in Watts per square metre that element per degree centigrade temperature difference across that element. Complete tables of pre calculated "U" Values, are published by British Standards Institute and can be found online at

“U” values are then fed into the following formula:

Fabric heat loss (Watts) = U value (W/m² °C) x area (m²) x temperature difference (°C) i.e. the difference between the room temperature and the outside air temperature at winter design conditions.

Ventilation heat loss

Heat is also lost through ventilation. Air flowing through a property loses heat, and we calculate the rate of this by dividing the volume of air moving through the room per hour, by the volume of the room itself. This will give us the air changes per hour. Fortunately, recommended figures are available and can be found online. A typical living room and dining room have an air change rate of approximately 1.5, whereas a kitchen, bathroom and cloakroom have an air change rate of approximately 2.0. Rooms with open flumes have an air change of approximately 4.0. Of course, this figure does depend on what kind of property is being evaluated. A leaky, period property will have higher air change rates than a more modern home.

The ventilation factor is taken as the specific heat of air which is 0.33 W / m³ °C and is used to calculate the heat loss to the air changing within the rooms due to infiltration or mechanical ventilation.

To calculate ventilation heat loss, the above information is fed into the following formula:

Ventilation Heat Loss (Watts) = Room Volume (m³) x air change rate (qty) x temperature difference (°C) x ventilation factor (W/m³°C)

Total room heat loss

The total heat loss from the room (Watts) is calculated by: fabric heat loss + ventilation heat loss. This indicates the amount of heat which needs to be delivered to the room to keep it at its ideal design temperature. This figure can then be used to determine the size of the radiator needed in the room. The process must be repeated for every room and heated space within the property.

However, these days it is quite commonplace to carry out heat loss calculations by using special software (PURMO offer their own heat loss manager, for details please contact [email protected]). The designer will feed the building dimensional and fabric type information into the software to obtain the total amount of heat under design conditions needed to heat the room or property. If the designer overestimates, the room may exceed the design temperature and the heating may be less economical to run. If the designer underestimates, the room will not reach the required temperature in the depths of winter, and the house will be under heated.

Can I curve or bend radiators to fit into a bay window?

PURMO Column radiators can be curved to fit most applications or alternatively, use smaller radiators to make up the total length of the bay. We do not recommend curving or angling standard Compact radiators.

What are the benefits of a ‘clean-air system’?

Stuffy rooms without proper ventilation can be a breeding ground for toxic mould, which can lead to a number of allergic reactions including asthma. As we spend the majority of our time indoors, it is vital that heating methods are improved in order to ensure a healthier living environment. If opening a window to let fresh air in is not an option, then installing a clean-air ventilation system is an alternative way to improve air quality, while heating the home at the same time.

These devices are designed to look like a normal panel radiator, but also deliver filtered and heated fresh air directly from outdoors. The ‘used’ air is extracted through an exhaust air vent and fan, creating the suction needed to circulate the clean air throughout the room.

Such an air system can also have a positive affect on damp problems within homes. Excess moisture in the internal airspace can lead to ceiling and wall damp, and the steady degradation of the building itself. With an air ventilation system, the damp caused by condensation is either removed or reduced, and the air is dryer and cleaner. Moisture damage to our homes can be costly, but an air ventilation system which prevents or lessens the consequences of damp is a sensible investment for the future. These clean-air systems are also suitable for commercial premises because, due to ventilation being demand controlled, they are extremely energy efficient.

The PURMO Air radiator is a proven cost effective solution in Finland, with issues of mould growth and airborne bacteria now resolved across the country.

Will old twin entry radiator valves fit new radiators?

These older valves are identified by having both pipe connections in one radiator valve at one side of the radiator. They are not suitable for modern radiators. If new radiators are being fitted, it may be advisable to also renew the radiator valves.

What is the benefit of fitting a thermostatic radiator valve to the radiator?

The fitting of a thermostatic radiator valve (TRV) to a radiator gives room temperature comfort control and can help to avoid overheating if a radiator is too large for the room. This piece of equipment provides an extra fine tuning control to central heating, by allowing individual room temperatures to be maintained by sensing room temperature and automatically adjusting the flow of hot water into the radiator. Much a designer item as a functional control; the right valve will give that perfect finishing touch for a modern or traditional scheme. Beware of shielding TRVs with drapes or furniture, as the valve will not be in a position to efficiently measure the room temperature. If trapped in a warm spot, it can cause the radiator to shut down too soon, resulting in a room temperature below the desired level. Likewise, if a TRV is in a constant cold spot, perhaps being influenced by a draught from doors, windows, airbricks etc, the room may overheat at times. Of course, in an apartment or flat, it is almost impossible to account for a neighbour’s over or under heating next door, above or below. This can make a big difference.

I’m fed up of tucking my curtains behind my radiator – what’s the solution?

Stylish, floor length curtains are an aspiration that many homeowners have for their living room, but a radiator situated underneath a window can hamper this simple but very complimentary touch. How many of us have to resort to hanging short curtains, or, even worse, tucking our curtains behind our radiator so the heat goes into the room rather than out the window? This can look clumsy and ruin the appearance of the room. However, PURMO’s radiators can be fixed to the floor by a bracket, rather than to the wall, enabling curtains to hang neatly behind. This simple alternative can make a big difference in a living room or conservatory.

Can I use my towel warmer in the summer too?

Yes, if you buy a dual fuel unit. This comes with an electric radiator kit, so that in the summer when your heating is switched off, the electric element can be used to heat the unit, meaning you can still enjoy the comfort and convenience of warm, dry towels.

How should I position my radiators?

The best place for any radiator is in the coldest part of the room, which is usually under a window. Also, using several small radiators in a large room, rather than one bulky radiator, will ensure better heat distribution.

The physical size of a radiator is also important, as it can affect the balance of a room. There is nothing worse than having a small but high output radiator placed against a large blank wall. In terms of room aesthetics, it is better to pick a larger dimensioned radiator with a lower output per square metre.