Simon Barker:

I'm sure that, given a bit of time, a physicist could give you an near-exact formula to calculate the size of radiator you would need to maintain the temperature of a given room.


The trouble is, it would have many variables that you don't have the exact values for.  Plugging in wild guesses would give you a result no more accurate than the plumber's rule of thumb.


Off the top of my head, you would need to know:

The temperature of the water in the radiator (and do you want to account for the temperature drop from the inlet to the outlet?).

The thermal conductivity of the steel and paint of the radiator.

The surface area of the radiator.

The specific heat capacity of the air in the room, including any water vapour in it.

How well the air is circulated around the room - do you want to do computational fluid dynamics on that?

The thermal conductivity of the walls, floor, ceiling, windows and doors.

The temperatures on the other sides of those walls, floor, etc. (not necessarily the same as the outdoor temperature).

How much cold air leaks in around doors and windows.

What temperature that air is at.

A formula that is 90% accurate is better than a formula that is 50% accurate! In a world of rising energy prices and proactive measures to reduce CO2 emissions then plumbers have to play their part as well.


Another factor I haven't mentioned are the effects of gas fires on central heating systems. If a room has both a gas fire and a radiator then should the radiator be deliberately underpowered or not? Installing the main thermostat in the same room as a gas fire will confuse the central heating system when the fire is on and make the rest of the building colder as a result. 

Roy Bowdler:


As I said in an earlier comment, unless something like this is scaled up hugely, there just isn’t a financial or other justification for significant input from a professional (IEng/CEng type) engineer. There is another thread about consumer units. In my situation, I realised that I needed an accredited Gas Safe Plumber and Part P Electrician. I found that despite an enthusiasm to get involved, it was more sensible to just let them get on with it, once trust was established.

Are other countries ahead of the game than Britain is - unless you find a highly knowledgeable person in the HVAC trade? Other countries where central heating system components are specified to an accuracy of less than 10% using formulas taking into account the fabric of buildings that are underpinned by thermodynamics whereas Britain bodges its way to an accuracy of 50% using plumber's rules of thumb passed down from the 1950s. Every man on the street knows about how fridges sold in Germany were more energy efficient than similar models sold in Britain.


The impression I get of the IET is that it prefers to stay in its comfort zones and shies away from many engineering subjects. There was a discussion some time ago about the semiconductor industry. I would eat my hat if the IET got involved in anything to do with Power Wheels type electric vehicles for young children although there are engineers who design and develop such machines. 

Roy Bowdler:


It was my understanding that my Bosch refrigerators came of the same production line but had a different plug fitted. Have I been played for a fool? If so, is it the heat engine that is different or the insulation?  

 

David McQuiggan:

If you have kids, have them work the problem using this STEM activity at http://www.mathscareers.org.uk/wp-content/uploads/2014/05/StemCar_Keepingahousewarm.pdf, part of a Department for Education initiative to promote subject choice and careers in Science, Technology, Engineering and Maths (STEM) delivered by the Centre for Science Education at Sheffield Hallam University.

I encountered this interesting document a few months ago. It's a bit simplified because it's designed for secondary school rather than professional use but it's a darn sight better than the old school method of multiplying the volume of the room by a number then knocking off a percentage for double glazing to compute the required power output of a radiator so beloved by plumbers and the websites of companies that sell radiators. I have tried several online calculators with fairly typical living room and bedroom configurations and they have produced significantly varying results so cannot be relied on for accuracy. This is cynical, but some of the formulas behind the calculators could be fiddled in order to increase profits for the companies by encouraging customers to buy more expensive radiators than they need. 


The document is thought provoking. Does Britain need more people with degrees in STEM subjects or does Britain need more people with a better basic STEM education in occupations that traditionally do not employ people with degrees - such as plumbers and salesmen from central heating parts suppliers? Also is work experience or vocational further education better? The 16 year old who spends two years working under a time served central heating installer will have more practical skills and experience working on real systems in real buildings plus customer facing soft skills, but he might be deficient when it comes to deeper theory of thermodynamics and pick up obsolete and outdated rules of thumb for design and specification. The 16 year old who spends two years on a central heating course will lack the work experience although he will have a better knowledge of deeper theory of thermodynamics so is more capable when it comes to designing and specifying a new installation or major upgrade that is sufficiently warm and energy efficient at the same time.

Simon Barker:

I suspect most people don't care enough to perform a complicated calculation, based on parameters that are themselves only guesses.  There's no point in having a fancy formula if you don't know the correct values to plug into it.


What's the result of fitting an over-sized radiator?  It will take up a bit more wall space and cost a few pounds more to buy.  But on a cold day it will warm the room up quicker, and if it's fitted with a TRV, it will still shut off when the room is warm enough.


So if in doubt, just pick a radiator that your rule-of-thumb says is big enough for the size of the room.  And if in doubt, add a bit.

I can see where you are coming from but it brings the STEM activity document into dispute. Is it a waste of time to teach secondary school students such information if they later start a career in HVAC and are shot down in flames by an ageing plumber with his 1950s rule of thumb either because he doesn't have the knowledge to understand it or is too stubborn to change? Also, what if HVAC folk in other countries adopt more advanced methods producing more accurate results? Should the British just accept that we are laggards because of cultural reasons?

Arran Cameron:

online radiator power outlet calculators ...