Heating Simulator

This is fantastic! I have wanted to do this sort of thing for ages, and now thanks to my niece Ellie who constructed it, it’s working!

Click here for a full screen version of the Simulator below

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 and the delta if we change these things?

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 and compare temperatures they would operate at given different heat outputs

So, there you have it.   If you wish, you can simply change the input values and see what happens.

Points to note

  • Your system configuration and controls must be set right if you are to achieve the low operating water temperatures that you are aiming for.
  • In practice, you might 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
  • Long rectangular radiators give out a bit more heat per than square ones.
    (say 5-10% variation??)
  • The values are good non-optimistic estimates, but check manufacturers data for more specific values.
  • The delta (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. dt 50°).  Don’t confuse the two.


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 (delta, 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 temperature between the radiator and the room air temperature.   So, if the water temperature is fixed, from a boiler of buffer tank, the heat emitted increases slightly 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

  • Go to each radiator and enter the approximate area and radiator type
  • 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)
  • Now adjust the flow rate until the dt is about 5 degrees  (9F)
  • Note down the heat (kW) and flow rate (lit/min) for this radiator
  • Now do this for every radiator in your house
  • 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 might want to use the radiator type-in-the-numbers version)

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 on the simulator.  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)

5 thoughts on “Heating Simulator”

  1. Hi John,

    Excellent website and videos. Thanks for your effort.

    When selecting radiators their power is often quoted as “X”kW at dT50.

    Is this dT referring to the flow and return temperatures or the dT between the radiator and the ambient temperature of the room?

    1. Hello Jonathan. Here on the heating simulator I am talking about the flow-return dt. Within the first minute of this video, ( https://youtu.be/u3qN-YqB46I ) I define the two dt’s .. the average (mean) radiator to room temp dt, and also the flow-return dt

  2. Thank you for this visualisation John. I’m trying to back solve an equation…and your comment “Next one on the drawing board – pressure drops and pipe sizes to play with.” got me excited! I am trying to design an ASHP system based on the existing pipework size of plumbing system. The property is circa 400sqm, doubled glazed, well insulated (retrofitted), the pipework distribution to existing fin convector radiators is 22mm O/D (galvanised steel), heat source is an ancient gas boiler. I want to replace it with an ASHP ( either low or high water temperature). Am considering whether to adopt a hybrid system (part gas/part ASHP) for UFH/ radiator system. I’m prepared to take a view on the system variables to avoid ripping out a plumbing system which seems to be fine. Any advice or direction would be grateful received. Thank you. JJ

  3. This is one of the most useful heat pump-related data visualisations I’ve seen on the web (and it’s not trying to sell me something based on the output either!) I like the way you can play around with the variables to get a feel for how different design decisions affect the result. Very well done! It’s also an interesting visual to create from a web technologies perspective and has inspired me to tinker with SVG for my own illustrations in future. Thanks for sharing.

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