Radiator sizes

[Keith Dunbar]

Is there a good website to help calculate optimum radiator sizes for rooms?

Keith

[Andy Hall]

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 errors.

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 example.

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 attachment.

The objective of radiators is to compensate the heat loss and maintain the temperature.

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.

[Lobster]

[snip]

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'...

David

[Lurch]

On Mon, 02 Jul 2007 07:54:06 GMT, Lobster mused:

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.

[Andy Hall]

Then I think that one should be seeking a heating engineer and not a dodgy plumber.

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% sandbagging.

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 drop test.

[dennis]

I did the full calculation on my house when I put the heating in.. then screwed it all up the following year by having cavity wall insulation installed so I had rads that were 40% too big. It didn't matter much as I have zone valves and stats in each room and don't rely on external influences to maintain the temps I want.

[:Jerry:]

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?

[Andy Hall]

At least it was in the right direction though.

If you have moved or are planning to move to a condensing boiler, this is convenient as well.

[Roger]

The message from Lobster contains these words:

And in most cases they will get away with it. Only if it is undersize will there be any comeback. Stick TRVs on all the radiators and the 'heating engineer' aka coyboy plumber doesn't even need to balance the system, and if he has bothered to fit a room stat it usually won't cause such a upset that the householder actually notices the fight between the stat and the adjacent TRV.

Oh yes and don't use Imperial units in the presence of Andy. It is likely to start him foaming at the mouth. :-)

[Keith Dunbar]

Andy - tried to e-mail you as invited but bounced.

[Andy Hall]

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.

[Andy Hall]

On its way

[The Medway Handyman]

I still maintain that those BTU's were a lot warmer than these new fangled Calories - and less fattening. I've put on weight since they came out so it must be true.

[:Jerry:]

Not so far out that it will matter, in this subject, and as long as any error is positive rather than of a negative influence,

I agree, but unless you are going to enter into brain wiping and thought control it will never happen.

[Andy Hall]

The trouble is that at least one conversion is about a factor of 3 (ignoring decimal places) (BTU/hr W)

That may go unnoticed but create a real problem.

[Roger]

The message from "The Medway Handyman" contains these words:

I don't think you understand ISO metric. Calories have been consigned to the dustbin of history along with centimetres, dynes, ergs, etc.

[The Medway Handyman]

Arrrgh!! Now what are we supposed to use? I've only just mastered centimetres!

[Andy Hall]

Building Industry. Millimetres.

However the sizes are ridiculous. 2440 x 1220 for a sheet of ply, yet bizarrely it is 18mm thick

[:Jerry:]

Err??????!!!!!

What else should it be, would it really be better to quote 2m 440mm x

1m 220mm, that (to me) is as bad as mixing centimetres and millimetres!

[<me9]

ITYM 8' x 4' x 3/4"