Heating Simulator

click here for full screen or YouTube explanation

What is the purpose of this simulator?

What happens when we slow-down the flow rate?   What would happen if we double the radiator size?   What happens to the water temperatures if we change these things?     (See YouTube explanation if you wish)

To get a deeper and intuitive understanding of heat, water flow rates and temperatures, you can ‘play’ with this Simulator to find out.

Fault finding has changed over the years. In many ways servicing has become more efficient. Diagnosis has become very product-specific and often done to laid-down procedures.  A downside of this is that engineers are not given the opportunity to go ‘off piste’ and learn for themselves.

Further to a teaching aid, to get people thinking and talking, you could also use this as a tool to estimate how much heat your radiators could emit if working at lower (heat pump friendly) temperatures.  You just need to list out your radiator types and sizes.    [see below for suggestions]

This theory is all very well, but relating to heat pumps, it’s equally important to ensure that your system, the control, and the way it’s used, will achieve the working temperatures that you are aiming for.

The panel radiator is heated by an electric heater via a piped water circuit. You can vary the radiator size and type, the electrical power input, the flow rate, and also the surrounding air temperature     The temperatures are calculated from these inputs.    Note the room temperature simply changes the temperature that the radiator ‘experiences’. Its not a set-point.

Points to note

  1. Your system configuration and controls must be set right if you are to achieve the low operating water temperatures that you are aiming for.
  2. In practice, you can tend to get uneven temperatures (cool patches) with very big radiators. E.g., a 2.5 sq.m. radiator might act more like 2.2 sq.m. due to these ‘cool’ patches
  3. Long rectangular radiators give out a bit more heat per sqm than square ones.
  4. The values are good estimates, but check manufacturers data for more specific values.
  5. The deltaT (difference in temperature) we are considering here is the difference between flow and return temperatures.   The same term is often used to describe the difference between room temperure and radiator temperature (e.g. dt50°).  Don’t confuse the two.

Getting started

If you want suggestions to try… re-load the page to get default mid-range values.   Now observe the resulting water temperatures that we would see in real life.

Now try changing any one of the above and see what happens to the temperatures.

  • With more power input, the radiator gets hotter, and more heat is dissipated to the room.
  • If the radiator is bigger, it dissipates the same heat to the room, but at a lower temperature
  • When the water flow-rate reduces, the difference between the flow and return (deltaT, dT) increases. The flow-temperature rises and return-temperature drops. Watch the thermometers. Try increasing the heat input and also doubling the flow rate. Watch the dT.
  • If the room temperature changes, the radiator temperature changes too.  This is simply because the heat delivered depends on the difference in temperure between the radiator and the room air temperature.   So, if the temperature is fixed, from a boiler of buffer tank, the heat emitted increases if you open a window and cool the room.


If you have an existing radiator system, and are thinking of adding a heat pump, and are concerned about energy efficiency, then you may want to operate on a relatively low temperature to keep the heat pump efficient

  1. Go to each radiator and enter the approximate area and radiator type
  2. Now change the heat input (Kw) until you get an average radiator temperature of say 40°C (e.g. 42.5°C flow and 37.7° return temperatures)  (40°C = 104°F)
  3. Now adjust the flow rate until the dt is about 5 degrees.  (9f)
  4. Note down the heat (kW) and flow rate (lit/min) for this radiator
  5. Now do this for every radiator in your house
  6. Total them all up

From this you have estimated how many kW’s of heat you can ‘pump’ into your radiators so that they will operate at an energy-efficient temperature for a heat pump.

You may of course choose to run your heat pump at warmer temperatures, and accept that your COP may not be as good.

Be mindful that some heat pumps are relatively large, so not all systems would work efficiently at such low operating temperatures, due mostly to them stopping and starting a lot.

For anyone still reading.. I thought hard about the inputs and outputs   (i.e. should the inputs be area and temperature, giving kW output? or
should the inputs be area and kW heat, giving radiator temperature result.
I concluded that the 2nd option is a better way of thinking about it from a learning point of view.     e.g. the circulating water temperature is the final end-result of the various inputs (kWs, area etc)