under sized boiler ???

[sheff paul]

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?!!!!!

Bugger me Erm have you been knocking about with Chris Watkins

 

[AWheating]

if you look at the domestic heating design guide as a general guide as many do in the industry you will find a table that they have recommended allowances for certain sized cylinders.

for example's

120l cylinder requires aprox 7.2kw to heat up in one hour (10-60) and suggests an allowance of 2.0kw when sizing your boiler.

225l cylinder requires aprox 13.0kw to heat up in one hour (10-60) and suggests an allowance of 3kw when sizing your boiler.

we all understand that modern cylinders have high recovery/high kw coils which can re heat the cylinders in 15 mins. But i would think that if your cylinder needs to recover in this time frame its prob undersized or not correct for use. I would say that if you wanted to be over cautious you may make an allowance for about half the cylinder rated kw in your calculations as suggested to shef by RM. But this could still be over the top really.

 

[AWheating]

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?!!!!!

after my head hurt trying to understand your post it all boils down to the bit in red

 

[sheff paul]

if you look at the domestic heating design guide as a general guide as many do in the industry you will find a table that they have recommended allowances for certain sized cylinders.

for example's

120l cylinder requires aprox 7.2kw to heat up in one hour (10-60) and suggests an allowance of 2.0kw when sizing your boiler.

225l cylinder requires aprox 13.0kw to heat up in one hour (10-60) and suggests an allowance of 3kw when sizing your boiler.

we all understand that modern cylinders have high recovery/high kw coils which can re heat the cylinders in 15 mins. But i would think that if your cylinder needs to recover in this time frame its prob undersized or not correct for use. I would say that if you wanted to be over cautious you may make an allowance for about half the cylinder rated kw in your calculations as suggested to shef by RM. But this could still be over the top really.

9kw for a 180 may be on the high side,i can see what where they are coming from

 

[Ray Stafford]

So if I went and turned off your central heating, and switched the entire output of your boiler to hot water,without touching the rads, how many minutes would it be before you started to notice purely from detecting the drop in air temperature?

20mins? 30mins? Well, blow me, the cylinder is fully recovered, so lets heat those rads back up.

And that would only be an issue when it was freezing outside and someone had drained your entire 210 litre cylinder.

Under more realistic conditions, the cylinder would have recovered in less time than it would take to brew up a cuppa.

 

[happyflyer]

goooo on Ray hit um !!! you must be my age, well thereabouts

 

[SimonG]

Thought pi was 22/7 as a fraction

 

[Ermintrude]

PI is a non reoccurring number http://en.m.wikipedia.org/wiki/Pi

22/7 is a simplification . But it would take longer for you to heat a 210 cylinder than realise it didn't actually matter!

 

[doitmyself]

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.

Reminds me of the way the Gas Board (as it was pre privatisation), when they had the monopoly, used to advertise heating systems with "Guaranteed Temperatures". All they did was install oversized rads and an oversized boiler. No heatloss calculations - just a rule of thumb, which some online calculators still use: room volume in cu ft x 5 (if I remember correctly) gave you the BTU required. I remember picking up a radiator manufacturer's leaflet at B &Q in the late 80's which had this formula on it.

 

[SimonG]

PI is a non reoccurring number Pi - Wikipedia, the free encyclopedia

22/7 is a simplification . But it would take longer for you to heat a 210 cylinder than realise it didn't actually matter!

Bit like this whole thread then.