# Temperatures in a central heating system

1. Jul 17, 2007

### TSN79

A central heating system has been thought to run with temperatures 80/60, and so the flow of water through the system has been calculated accordingly. Now it turnes out that it will run with 60/40 instead. In order to have the system deliver the same effect, I assume more water will need to flow through the pipes. But how do I find out how much?

2. Jul 20, 2007

### Artman

Minor difference. Since there is a 20 deg Temperature drop in both systems, and the ranges are not that different, the energy transfer is similar, with the only difference being a slight drop in the specific heat of water, usually considered negligible (a value of 1) at those temperatures.

However I would be careful returning water to the boiler at 40 deg C, there is a risk of thermal shock. You should use some sort of secondary loop at that low return temperature.

Last edited: Jul 20, 2007
3. Jul 20, 2007

### mgb_phys

I would have thought the major difference was the heat to be supplied into the room. With the radiators at 60deg the dT to the room will be 2/3 what it would be at 80deg and so to give the same heating effect you are going to have to push more water around.

4. Jul 20, 2007

### Staff: Mentor

???? the dT is 80-60=20 and 60-40=20

5. Jul 20, 2007

### mgb_phys

The dT between the radiators and the room temperature (say 20deg) is either 60 or 40.
The rate of heat transfer from the radiator to the room is proprtional to dT.

6. Jul 21, 2007

### Staff: Mentor

That's called approach temperature and though it is true that with a lower approach temperature you will get less heat transfer, he didn't say how the heat would be dissipated or why the temp would be lower, only that the delta-T is still 20. Ie, maybe they are fan coil units and in addition to lowering the boiler temp, they raised the airflow. We just don't know.

Last edited: Jul 21, 2007