I am sure this has come up 100 times, but are any of the web-based radiator size calculators any good. I tried
Thanks for any info.
Mr F.
I am sure this has come up 100 times, but are any of the web-based radiator size calculators any good. I tried
Thanks for any info.
Mr F.
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.
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
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
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.
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.
If the boiler is modulating will this be different? Will it simply adjust the burn rate instead of switching on and off?
Mr F
Best he fist cavity wall insulation.
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.
I didn't know that you were into that.....
I would have thought that it would have felt prickly.
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]:
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.
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
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