under sized boiler ???

[AWheating]

The technical guy from RM cylinders insisted we allowed 9kw allowance for a 180 unvented on a recent install to achieve recovery times specified....

a 180l cylinder will prob have a 16-18kw coil, so the manufacturer seems to suggest allowance of half of the coils max. Seems about right to me.

i think sometimes more thought has to be put into these things than adding the max loads equals. Things change, all properties and situations are different. Many new builds have amazing u values that require little heat on the heating side sometimes only a few kw's, but may have a cylinder with a 20-25kw coil.

 

[happyflyer]

a 180l cylinder will prob have a 16-18kw coil, so the manufacturer seems to suggest allowance of half of the coils max. Seems about right to me.

i think sometimes more thought has to be put into these things than adding the max loads equals. Things change, all properties and situations are different. Many new builds have amazing u values that require little heat on the heating side sometimes only a few kw's, but may have a cylinder with a 20-25kw coil.

Like you say all it takes is a bit of thought, not a lot around these days

 

[Ermintrude]

Just sized my house just ordered a new boiler . 5TW (terra watt) do you think it will cycle much? I have 4m2 ufh 1 600x600 single panel. And a 14kw coil?

 

[sheff paul]

Just sized my house just ordered a new boiler . 5TW (terra watt) do you think it will cycle much? I have 4m2 ufh 1 600x600 single panel. And a 14kw coil?

it will if its a worcestor,with a Vaillant steady as she go's

 

[Ray Stafford]

agreed maximum load always has been always will and should be..

Sorry, I just can't agree with this.

"Maximum load" results from a calculation that is full of assumptions and guesswork.

You use a figure for the u-value of a wall that is educated guesswork in the case of modern construction methods, and a rough approximation in the case of older properties.

You assume an outside temperature which would have been wildly pessismistic this year (at least where I live) and wildly optimistic last year. Incidentally, you make the same assumptions for house on a windswept hill in ***bria as we do here in a protected spot in the sunny south coast.

You assume a number of air changes per hour which is absolutely a stab in the dark.

You assume that all customers have identical lifestyles, hot water requirements and preferred room temperatures.

Then (at least in domestic work) you guess pipe-sizes, utterly ignore the effect of pipe runs and valve restrictions and even how the radiator theoretical outputs were calculated.

And after all those assumptions, guesses and approximations, "maximum load" has about as much mathematical rigour as my estimate of Pompey's chances in the FA cup next year.

At the very best, you are covering your bum against claims by being able to prove that your system was designed to best practice. That may be a worthwhile aim, but lets not pretend its engineering.

 

[happyflyer]

Sorry, I just can't agree with this.

"Maximum load" results from a calculation that is full of assumptions and guesswork.

You use a figure for the u-value of a wall that is educated guesswork in the case of modern construction methods, and a rough approximation in the case of older properties.

You assume an outside temperature which would have been wildly pessismistic this year (at least where I live) and wildly optimistic last year. Incidentally, you make the same assumptions for house on a windswept hill in ***bria as we do here in a protected spot in the sunny south coast.

You assume a number of air changes per hour which is absolutely a stab in the dark.

You assume that all customers have identical lifestyles, hot water requirements and preferred room temperatures.

Then (at least in domestic work) you guess pipe-sizes, utterly ignore the effect of pipe runs and valve restrictions and even how the radiator theoretical outputs were calculated.

And after all those assumptions, guesses and approximations, "maximum load" has about as much mathematical rigour as my estimate of Pompey's chances in the FA cup next year.

At the very best, you are covering your bum against claims by being able to prove that your system was designed to best practice. That may be a worthwhile aim, but lets not pretend its engineering.

Ray,

You are 100% right, I just couldn't be bothered to type it all out, you probably missed a few other things but I think people will get the message, guesswork guesswork guesswork.

What about this building control inspector who is looking after it all.

 

[happyflyer]

In the commercial sector a lot of over-sizing goes on, consultnats fear of PI attacks

 

[sheff paul]

Sorry, I just can't agree with this.

"Maximum load" results from a calculation that is full of assumptions and guesswork.

You use a figure for the u-value of a wall that is educated guesswork in the case of modern construction methods, and a rough approximation in the case of older properties.

You assume an outside temperature which would have been wildly pessismistic this year (at least where I live) and wildly optimistic last year. Incidentally, you make the same assumptions for house on a windswept hill in ***bria as we do here in a protected spot in the sunny south coast.

You assume a number of air changes per hour which is absolutely a stab in the dark.

You assume that all customers have identical lifestyles, hot water requirements and preferred room temperatures.

Then (at least in domestic work) you guess pipe-sizes, utterly ignore the effect of pipe runs and valve restrictions and even how the radiator theoretical outputs were calculated.

And after all those assumptions, guesses and approximations, "maximum load" has about as much mathematical rigour as my estimate of Pompey's chances in the FA cup next year.

At the very best, you are covering your bum against claims by being able to prove that your system was designed to best practice. That may be a worthwhile aim, but lets not pretend its engineering.

your on the right track,but back in the day with a 120 litre cylinder connected with a s,y plan making an allowance for the cylinder wasn't essential,now you have a cylinder double the capacity with the same reheat time,so the heat has gotta come from some where to power it,and new boilers modulate so if its marginally over sized it not gonna be a problem.Then when billy the builder comes along and knocks up a monster extension the boiler can still cut it

 

[Ermintrude]

But every single calculation is an approximation and gas assumptions. Even calculating the area if a circle is an approximation and has a been rounded up or down. Pi, is an irrational number and cannot be expressed as a fraction so nothing is 100% accurate. Fluid dynamics is based on tensor calculus which is fraught with approximations.


Heating systems can be designed very well with rule of thumb and a rudimentary model. A real model would be immense and require a super computer.? Lorenz had some very interesting thoughts on chaos. If you make a cup of tea u upset the thermal drifts and how a radiator works I some one else's house 120miles away in 3 days time!


When you have designed and installed a system you can check it's performance. Set rooms and flows so you get delta t of rads and 22c drop at boiler and cylinder on full tilt few measurements and bobs your uncle

SBEM calculations were tallied against loads of new builds and were within 5% of actual. This was done before BREEAM and it shows that calculations do work and that you should at least think about what your doing before just doing it?!!!!!

 

[Ermintrude]

In the commercial sector a lot of over-sizing goes on, consultnats fear of PI attacks

Should be worried about over sizing as that's going to be the next block of litigation as energy goes up. Exceeding the spec ?