BTU figures for rads?

[doblo7]

Hi,
I have noticed in the trade catalogues that BTU figures are given next to the radiator sizes. I understand BTU figures for boilers but what meaning do they have with radiators, as similar sizes have different BTU figures. Does that mean the higher the figure the more heat they will out put? or Heat they need from boiler? Does the cheapest like for like mean poorer quality or will they be similar if british standard approved?
Finally anyone have comment on a VOLKERA 25 HE condensing boiler before I purchase?
You help is very much appreciated!

 

[ChrisR]

the higher the figure the more heat they will out put

Yes.

But also, read the small print - the energy output depends on the difference between the temperature of the radiator and that of the room. It's usually written as Delta T, with a triangle for the Delta. It may be say 50 or 60 degrees. Only compare like with like!

The boiler doesn't have an L in it (a strangely common error!!). I guess it's a fairly typical boiler, neither fab nor bad.

 

[doblo7]

Thanks for your reply, It proved really helpful

 

[lorraine]

Hi,

I think they should quote the values in terms of thermal resistances because the heat generated is a function of the thermal resistance, temperature difference and area of the radiator:

heat out = area x U value x temp difference

I believe its more efficient from a total energy point of view to have a low temperature radiator but very large. I know thats not alyways practical but just a thought.

hope that helps

Lorraine

 

[chrishutt]

I believe its more efficient from a total energy point of view to have a low temperature radiator but very large.

For someone about to start a plumbing course, that's a pretty astute observation. That's why underfloor heating is so good - it's a giant , low temperature radiator. The low temp's are also helpful in relation to condensing boilers being able to achieve maximum efficiency.

I think they should quote the values in terms of thermal resistances because the heat generated is a function of the thermal resistance, temperature difference and area of the radiator

Not quite so smart, since this doesn't take account of the effect of fins, which most radiators have these days. Quoting the output is best, since that's what people want to know, but it must be for a standardised delta T (temp. differential rad:room).

 

[lorraine]

Hi Everyone

I believe the total thermal resistance does take into account the resitance of the fins. On the non fin side you've simply got

kdT/dx where x is the thickness of the metal, and k is thermal conductivity, area is constant throughout

on the finned side you've got kdT/dx then another term for the fin area which is going to be kAdT/dx. because the area term is different

add up the thermal resistances, those in series

R1 + R2 = Rtotal

for the ones in parallel (finned side)

1/R1 + 1/R2 = 1/R Total

Then that enables you to calculate the U value which I *believe* is a manipulation of the thermal resistance (Rt).

Although I may be wrong and this may not be the correct application, i got it out of this book. apparently its the de facto standard on heat transfer!

Lorraine

 

[chrishutt]

Lorraine, you're overlooking an important element in the calculation of thermal conductivity of something like a steel radiator wall.

The resistance of the wall thickness is not the only factor to take account of. There is also the resistance of the wall surface boundaries (water to steel and then steel to air).

Since the steel wall is so thin and steel is such a good conductor, the boundary resistances are likely to be relatively high, making changes in wall thickness relatively insignificant. I'm not sure if I've got all the terminology right, but I'm sure you'll find it in your book.

As for the fins, because they only make contact with the radiator wall at certain points, they will not have a uniform temperature and therefore any heat loss calculation might be rather more complicated than you're suggesting.

I'm beginning to feel sorry for your future plumbing teacher.

 

[lorraine]

hi chris,

Apologies I forgot to put in the boundary layer resitance which according to the book is

q = h x A x deltaT

and you can work out the h (convective heat transfer coefficient) from the nusselt number which is

Nusselt Number = h x L / k

was discussing with hubby and he says, this type of calcuation really relies on assumptions and you can never get a totally accurate answer because the material and heat flow through the room are never perfect, you can also never model the boundary conditions totally accurately.

Why do you feel sorry for my plumbing teacher?

Lorraine

 

[supermario]

Hi,
I have noticed in the trade catalogues that BTU figures are given next to the radiator sizes. I understand BTU figures for boilers but what meaning do they have with radiators, as similar sizes have different BTU figures. Does that mean the higher the figure the more heat they will out put? or Heat they need from boiler? Does the cheapest like for like mean poorer quality or will they be similar if british standard approved?
Finally anyone have comment on a VOLKERA 25 HE condensing boiler before I purchase?
You help is very much appreciated!

bugger the rest of 'em they are getting a bit anal the U value you need is for the room itself calculated usuing area and materials of construction and position of the room in the house ie no. of outside walls windows type of wall etc etc the factors from these can be found in plumbing text books not diy and enable you to size the radaitor in terms of btu after that style size etc are personal taste and budjet

_________________________________
moderator

edited to correct quote

 

[lorraine]

Hi

Read this

http://www.plumbingpages.com/featurepages/Heatloss.cfm

The U value is actually based on thermal resistance

Lorraine

 

[ChrisR]

Lorraine it's always worth doing sums in plumbing to see what's really going on, but there are sometimes a lot more parameters than may appear at first.
You can look up the thermal conductivity of the steel, (once you've found its composition!), and the paint (many times more resistive) for which you'll need the thickness. Then you need the area of the radiator, but that will depend on the size and a geometric factor according to the pressing shape.
The paint thickness will vary, according to the position on the radiator and a statistical variation in the manufacturing process. The temperature of the radiator will vary considerably across the area, so your sums will have to be an integral of all the areas and paint thicknesses. But then some bits of radiator will conduct to the adjacent air better than others - because of more or less laminar/turbulent flow.

Am I getting across? It's impossible to calculate! Much easier for the manufacturer to make the radiators and test them in a standard way, and give us a number which we use as a rough guide. We can only use it in a rough manner because we don't have the same temperature distribution across the radiators. For example there's top in, bottom opposite out, or bottom in, bottom out, connections.

The sort of sums you're looking at are a bit useful if you need to work out a U value for some wall say, where you have different thicknesses than are used in standard charts, or an extra layer of something different.

The US way of doing things for buildings is easier than ours - we actually work with conductivities and they work in resistances. So if their building "code" says R10 they just add layers of say 5, 3 and 2 to get it.

If you're of a perverse turn of mind to work out really interesting things, check how the transfer of heat into a HW cylinder depends on the thickness, and length, of the copper in the coil..


A man in a moon rocket is accellerated at 3g - that's about all he can stand for an extended period. If we're going to the stars we'll have to go jolly fast. Say half the speed of light. OK, so how long would it take to get to half the speed of light, if he accellerates from 0 at 3g?

 

[supermario]

Hi

Read this

http://www.plumbingpages.com/featurepages/Heatloss.cfm

The U value is actually based on thermal resistance

Lorraine

Lorraine you obviously know your onions but the u value of different materials used in the construction of the room found in tables such as the chartered institute of builbing services allow you to work out a heat loss value for a particular room. General regard is that you need to beat the heat loss in watts for a room by about 15% to heat the room up. convert this to btus tell you the rating you need for particular room then in terms of choice of radiator with that rating the world is your oyster

 

[chrishutt]

Why do you feel sorry for my plumbing teacher?

Because teachers don't like smart arse students. He (presumably he) will be trying to teach a bunch of not exactly academic students some rather basic principles, and you will keep throwing in extraneous formulae that even the teacher doesn't understand half the time.

Sometimes it's best to keep schtum (Google it). For example, on this forum you've now aroused the antarctic leviathan (ChrisR) who leaves us all feeling rather intellectually challenged.

 

[just-thinking]

why is a simple matter of btu's made so complicated by some...time for a strong ciggi and a sleep.

 

[chrishutt]

Because we want to show off.