Radiator size calculations

I am sure this has come up 100 times, but are any of the web-based radiator size calculators any good. I tried

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radiators fitted 15-20 years ago give out a lot less heat for their size?

Thanks for any info.

Mr F.

Reply to
Mr Fizzion
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Nope. Send me a mail and I'll send you a copy of the Myson one. That is no longer on their web site but works OK.

You can also use a spreadsheet if you have the U vales of the materials.

Regarding radiators, the main factors that affect output, for a given flow and return temperature are whether fins are fitted.

Reply to
Andy Hall

Not sure which question you're answering there, Andy, but are you saying that the radcalcs one is no good? I have it in my 'Favourites' and as it happens, was going to use it myself this week. What's the problem with it?!

Would appreciate a copy of your Myson prog if you wouldn't mind - my hotmail address is valid.

Thanks David

Reply to
Lobster

If still in as-new-condition & same material (normally steel) then o/p is similar, but watch for internal corrosion, silting up, blocked/reduced diameter inlets & outlets on old installations.

Basically a standard pattern panel radiator can have (a) no fins (b) fins one side (c) fins on both sides - each one adds a bit more to output, provided your CH can pump the requisite extra quantity of heat from the boiler. (It can't do that if other radiators leave no surplus heat). Then you can double (& treble) up any of the 3 panel+fin styles to make a thicker radiator etc etc. But as you add more panels & fins air circulates away less easily & so output per sq m doesn't rise quite as fast the extra fins might sugggest. eg doubling up doesn't quite double radiator maximum output (somewhere in the range 1.9 to 1.95 IIRC).

There is (or was) a Which book on plumbing/CH which had a handy rule of thumb for sizing - measure the cubic capacity of the room in cubic feet & multiply by 5 for downstairs & 4 for a cooler upstairs. The result is the radiator size in BThU.

Works very well in my experience. & provides a decent margin for boiler capacity. Remember, unless yours are special or unusual circumstances, you are only sizing to ensure your ch keeps you warm on maybe only 7 of the coldest days in the year + ensuring you'll get a decent supply of hot water on those days. Rest of the time your boiler will always be working under capacity, which it achieves by switching off for increasing time spans during the day as summer approaches.

Up the 5 & 4 to 6 & 5 if a cold old house, reduce if it is modern part L house.

All in all, IMHE it pays to be generous with boiler & radiator sizing, it doesn't cost much extra to modestly oversize, & all that happens is that your boiler stays off for longer - it doesn't mean you'll burn more fuel. So use calc progs just to check you aren't undersizing (& to make sure your ch pipes are the right diameter).

HTH

Reply to
jim_in_sussex

Last time I used it a few months ago, it was out by a factor of 3 (too high on radiator sizes) compared with the Myson one and hand calculation. It seemed to be a combination of some U values being incorrect and wrong arithmetic. I didn't investigate in detail to be honest.

On its way to you.

Reply to
Andy Hall

This is very rough and ready and hopefully is generous. There is an enormous difference in heat loss for a house with solid brick walls and single glazing vs. one with insulated cavity walls and double glazing.

It can be as much as 5:1 difference, so these approximations can really lead to tears in terms of undersizing or oversizing.

Also, it is better to stick to metric units throughout. Feet and BThUs are pretty much out of use on data sheets and it's very unwise to mix and convert because it can easily lead to errors.

The differences cn be far more than these factors suggest.

That's true enough. It's reasonable to oversize somewhat with a condensing boiler because the system can then run cooler and more efficiently for most of the time. However radiators with larger output start to cost quite a bit more and may take up more wall space.

It's better to use proper calculation in the first place, check the U values and use that for the sizing.

Reply to
Andy Hall

If the boiler is modulating will this be different? Will it simply adjust the burn rate instead of switching on and off?

Mr F

Reply to
Mr Fizzion

Best he fist cavity wall insulation.

Reply to
Doctor Drivel

Yes it will, down to the lower limit of the boiler output. For a condensing boiler, it's worth doing.

You can make a further improvement with a condensing boiler by designing the system to run the radiators at 70 degrees flow and 50 return rather than the 82 and 70 used for a conventional boiler.

However....... Look up the tables on the radiator manufacturer data sheet to determine radiator sizing.

THe default output relates to a laboratory measured radiator test method. To obtain practical outputs, a correction factor is applied. This is around 0.9 for 82/70 operation and around 0.6 for 70/50. It is based on mean water to air temperature which is basically the midpoint temperature of the radiator (76 or 60 degrees) less the room temperature. A table of correction factors for different room temperatures is usually on the data sheet.

To pick a simple example. Let's say you needed 600W of heat to overcome the room heat loss and you are going to use 70/50 degrees. Using the correction factor of 0.6, you would need a radiator of nominal 1000W output.

Reply to
Andy Hall

I didn't know that you were into that.....

I would have thought that it would have felt prickly.

Reply to
Andy Hall

in principal it makes no difference, but:

It is *not* like comparing the fuel connsumption of a 2 litre model T with a 1 litre version.

Basic scenario is [leaving aside many ifs & buts]:

  1. Your home in given external weather conditions requires a fixed amount of heat to maintain whatever temperature you select.
  2. Standard/conventional/normal [ie not heat pumps etc] UK oil & gas CH systems simply release the stored energy (commonly called the fuel's calorific value) binding the fuel's hydrocarbon molecules together by combining it with atmospheric oxygen (ie burning) to create chemicals (mostly CO2 & H2O) which require less energy to bind their molecules. The unwanted energy is released as heat.
  3. Because of the principal of conservation of energy, it doesn't matter how fast or slowly your fuel burns - a fixed amount of fuel will produce a fixed amount of heat [assuming your heating system produces/does no mechanical work]. Assuming all the fuel fed into the boiler is burnt, then the size & design of the boiler makes no difference to this fundamental fact.
  4. The heat produced warms the house via the radiators [useful heat] & warms up the flue gasses which in turn warm up your garden [wasted heat].
  5. Boiler design can only alter the balance between useful heat & waste heat. So keeping the boiler design the same, a larger boiler should use just the same amount of fuel as a smaller one of the same design - but if it burns the fuel faster, then the CH system will shut down more so that only just the amount of heat required by your home (see 1 above) is generated.

OTOH spending more on a larger boiler *could* mean you buy a boiler with a different design which proportionally pushes out more (or less) waste heat. But that makes no difference to the fundamental situation when you are comparing like with like.

  1. That's the simple picture. It gets more complicated when you don't ignore the ifs & buts & consider second order heat losses & other factors, though they won't normally make a significant difference to the overall energy picture [even so any savings thrown up are usually worth having].
  2. Car engines are different. Here, in broad terms, you have a heat engine which is subject to the second law of thermodynamics. The fuel is burnt in the engine to produce heat. When the driver calls for it (by pressing the accelerator) the engine converts some of that heat into work. ie the heat is a reservoir from which work is extracted. Otherwise the heat is wasted/lost via the radiator coolant & exhaust gas & by warming air blown over the engine by the fan.

A larger engine maintains a greater heat resource (ie uses more fuel) & can thus provide more work.

Thus, as the car engine is kept running all the time, an oversized engine, unlike an oversized CH system, will use more fuel than a smaller one to undertake the same task (eg a journey).

What happens to that wasted heat the car engine generates while it provides work? It heats up the planet....but that is where heat pumps begin.

HTH

Reply to
jim_in_sussex

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