Radiators and Heat Transfer

Radiators are devices that transfer heat into the home. Ideally, they should transfer heat at the same speed as the heat leaves the home through the building fabric – floors, walls, roof, doors, and windows.

The speed at which heat leaves the home is dependent on the temperature difference between the room temperature and the outside temperature. Just as a ball rolls quicker down a slope if the height difference is greater, heat also moves quicker if the temperature difference is greater.

For example, the temperature difference between a chilly -3°C outside temperature and a home being maintained at a toasty 21°C is 24°. If it was 9°C outside and 21°C indoors, then the temperature difference is now 12°, which is half the difference. Heat would now move half as quickly through the fabric of the same home in this second case.

The Speed of Heat from Radiators

The example above illustrates how heat moves quicker if there is a greater difference between the inside and outside temperatures. The speed of heat into the home from the radiators is also dependent on temperature difference: the difference between the water temperature inside the radiator and the room temperature.

Which radiator is moving heat into the room quicker in the table below?

The speed of heat into a home can be changed using compensation controls which automatically adjust the flow temperature of water from the heat source to the radiator.

The 1960s Homes Which Overheated

Let’s use a leaky home built in the 1960s to explain how radiators would often need low water temperature inside them to move heat into the home at the correct speed.

Home heating is designed to keep people warm on the average coldest day of winter. In the UK, this is -3°C. This is known as the Outside Design Temperature (ODT) and is not to be confused with the average temperature of winter, which is around 7°C – 9°C.

Some of the homes built in the 1960s would lose 10,000 joules of heat per second through the fabric when it was -3°C outside and 21°C inside, a difference of 24°.

Therefore, the single panel radiators used back in the 1960s were sized so they could push 10,000 joules of heat per second into the home.

To move heat into the home at this speed from single panel radiators, there needed to be a large temperature difference between the water inside the radiators and the 21°C room temperature. Remember, heat moves quicker if there is a larger temperature difference.

This was achieved with a water temperature of 80°C flowing from the boiler (something like a Potterton Diplomat) to the radiators. This provided 10,000 J/s.

However, most of winter is never -3°C!

If the radiators are always receiving water at 80°C from the boiler, then for most of winter they are pushing heat into the home far too quickly at the rate of 10,000 joules per second. The home overheats.

In fact, if the outside temperature was 9°C (with just a 12° difference between inside and outside temperature), then the home now only loses 5000 J/s through the fabric.

Therefore, the radiators only have to push 5000 J/s into the home. This can be achieved with a flow temperature of approximately 53°C.

Don’t corrode my boiler, please!

Even back in the 1960s, homes could be heated with low-temperature water in the radiators for most days of the winter. However, this strategy was not used because condensing boilers had not yet been invented.

An oil or gas flame in a boiler is well over 1000 degrees Celsius and contains water vapor as well as carbon dioxide. If the flame touches a cold metal surface, the water vapor condenses back into a liquid. This liquid is slightly acidic and corrosive, and it can rot the metal water jacket (heat exchanger) within the boiler. 

54 degrees Celsius may be hot to human skin but it is cold to a 1000 degrees Celsius flame and is the temperature at which condensation starts to form on the boiler heat exchanger.

This is why the temperature of water flowing from and back to the boiler was kept very high to prevent condensate from corroding boilers. It wasn’t homes which needed high temps, but the boilers.

How did people stop getting too hot in their homes?

There were a few things a homeowner could do back in the 1960s to stop being overheated.

1. Open windows. When cold, shut windows. Repeat as many times as necessary throughout the day to maintain comfort.
2. Turn radiators off in individual rooms. Turn them back on again when cold. Repeat as many times as necessary throughout the day to maintain comfort.
3. Leave the armchair, walk to the boiler and turn off. Get up out of the chair and turn on again when cold. Repeat many times throughout the day to maintain comfort.

What did people really do?

People did employ a little bit of the first strategy with windows. 

Thermostatic radiator valves could be used (expensive back then) but mainly wall thermostats prevented overheating.

Wall thermostats automatically shut off the boiler and then switched it back on again when the inside temperature dropped. This is not a great control strategy, but it is still being used today despite condensing boilers coming to the European market in 1981.

In fact, many of the commentators in the heating industry and even controls manufacturers themselves have forgotten why wall thermostats were introduced into domestic heating. They were not introduced as energy efficiency devices like many think, they were introduced to prevent homes from overheating.

Thankfully, boilers and heat pumps can be controlled with compensation controls which constantly change the temperature of the water to the radiators.

The problem is that many people, including heating installers, have grown up thinking that radiators always need very hot water in them. 

It is a mindset which we somehow have to change while remaining considerate of home owners’ needs.