No there isn't. They all get it wrong. Some hopelessly wrong. Even
worse are the sites and leaflets that take just the house type into
consideration and spit out a figure. I've tried loads of them and
found all to be of less use than an ashtray on a motorbike. Some even
mix metric units such as metres with deprecated imperial units such as
BTU/hr. Do this, and there is a high probability of calculation
To do the job properly, you need to calculate the heat losses from each
room first of all. This means taking each element of the construction
(walls, windows, ceiling, floor) amd calculating the heat loss through
it. That calculation is simple. You measure the area in square
metres and multiply by the temperature difference across it and a
factor called the U value. There are table of U values for different
materials. Then you add up the individual numbers. Of course,
there will be some cases where the heat loss is zero - between two
rooms at the same temperature. For heating calculations, one
normally uses -3 degrees for outside, 21 degrees for living rooms, 18
degrees for other downstairs rooms(kitchen, dining room), 22 degrees
for bathrooms and 16-18 degrees for bedrooms.
To these are added the heat loss through air changes. This is done
similarly, except that it is done on cubic metres, a volumetric
equivalent to U value and the number of air changes per hour. There
are tables of typical air change rates for different room types.
If you wanted to, all of this is very easily calculated using a pencil
and paper and calculator or a spreadsheet. Even the computer programs
require you to measure the room, which is most of the work. Some
programs help you a bit by subtracting window area from wall area for
Once you have heat loss on this basis for each room, you can apply
certain loading factors - e.g. 10% for high ceilings, exposed positions
or small usage rate during the day.
Some of the radiator manufacturers have programs to calculate this lot
on their web site. I found the Myson one is the best but is currently
not there. If you drop me an email I can send it to you as an
The objective of radiators is to compensate the heat loss and maintain
Now that you have heat loss figures in watts, you can choose the
radiators. The manufacturer data sheets publish nominal outputs for
the radiators in watts, but assuming that the heating water temperature
is 90 degrees. Conventional UK systems work with 82 degrees flow
temperature and 70 degrees return. The data sheet has a list of
correction factors based on mean water to air temperature (MWTA). This
is calculated as the average of the flow and return temperatures (76 in
this example) less the room temperature - so it would be 55 degrees in
a lounge for example. For this example, you should get a factor of
about 0.9 from the table and that should be multiplied by the radiator
output. Thus, if your room needed 900W of heat to maintain
temperature, you would need a 1000W radiator.
At this point you can make another design decision. If this is a new
system and you are going to use a condensing boiler, then you can run
it more efficiently at lower temperatures. For new designs, 70 and
50 degrees are the design figures. Of course this gives a lower MWTA
and you will have a smaller factor from the table - 0.6. The
implication of that is larger radiators or with more panels/fins.
You can trade that with cost saving on the fuel.
This second part of the calculation is not done by any of the
calculation programs or web sites AFAIK.
One final factor is that if you are planning to use radiator cabinets
or other impedimenta to radiator output, you need to factor by up to
30% for those - i.e. 1000W radiator drops to 700W.
Just wondering - are there any heating engineers out there, or has
anyone *ever* come across one, who account for even a fraction of the
parameters Andy listed? TBH most of those I've come across wouldn't
even use one of the dodgy calculators; they'd just eyeball the room and
say 'that needs a 3000 BTU'...
My Dad, who is a long standing plumbing and heating engineer, has a
calculator whch is several discs of card with a pin in the middle. On
the discs are most of the parameters Andy mentioned, I use it for
sizing rads where I can and I know he's used it a lot over the years.
Then I think that one should be seeking a heating engineer and not a
Anybody still using BTUs (let alone the correct nomenclature of
BTU/hr) should be visiting the post office on Thursdays.
There are such things as Mears calculators for doing a ready reckoner approach
On one occasion I did seek quotes from professionals for a complete
heating installation to see whether DIY would be worth it or whether it
would make more sense to outsource the work.
The results were revealing.
There were the bodgers (jobbing plumbers) who would just walk round
eyeballing the room as you say, and there were heating engineers who
did the job properly and produced calculated results.
Of the bodgers, one produced a list which was massively wrong - some
rooms oversized by 100%, others under by the same amount. The other
overdesigned everything to the level of 300% and the radiators would
have almost covered the walls.
All three heating engineers produced properly designed systems. One
did calculations on a form provided by the HVCA, another used a Mears
calculator and the third pencil and paper. The calculated ones were
the most accurate and the Mears one was oversized - i.e. about 30%
So my reaction to anybody not doing a proper job would be to show them
the door. It's the same principle as employing an electrician who
doesn't test his installation work or a gas fitter who doesn't do a
Why, as long as they only use the deprecated units, and (as you said
in your pervious reply) don't mix imperial with metric, what is
wrong - surely if they know what they are doing the calculations will
come out the same?
The chances of error are high because the numbers are odd values rather
than 10s and some manufacturers are publishing data in SI units only.
In any case, it's long past time that feet, inches and all the rest of
it were dumped for the consistent and superior metric system.
I also think that there should be ten days to a week and ten months to
a year, but the Sun won't co-operate with that. In the meantime,
we'll have to make do with the Revolutionary Calendar.
Unfortunately. you can't buy paint in 568ml tins. There are a lot of jobs
for which half a litre ain't enough, whereas a pint would have been fine.
Similarly with litres and quarts.
If only the powers that be would follow the age-old adage "If it ain't
broke, don't fix it!"
You seem keen to dismiss out of hand anything which is "deprecated" - which
simply means disapproved of (by whom?) or non-preferred.
But there are a hell of a lot of people about who still think in Imperial
units and have an instinctive feel for what 7'6" looks like, or how heavy
half a cwt feels.
Personally, I don't particularly care - being reasonably multi-lingual,
having trained in Physics in the 60's (using CGS rather than MKS or S.I.
units!), and having spent the early part of my career predicting the
accelerative performance of motor cars in fps units.
If people want to design their heating systems using British Thermal Units
and degrees Fahrenheit, good luck to them!
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