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under sized boiler ???

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Discuss under sized boiler ??? in the Boilers area at Plumbers Forums

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Ermintrude said:
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?!!!!!
Click to expand...

Bugger me Erm have you been knocking about with Chris Watkins
 
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.
 
Ermintrude said:
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?!!!!!
Click to expand...

after my head hurt trying to understand your post it all boils down to the bit in red :)
 
AWheating said:
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.
Click to expand...

9kw for a 180 may be on the high side,i can see what where they are coming from
 
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.
 
Ray Stafford said:
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.
Click to expand...
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.
 
doitmyself said:
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.
Click to expand...

Thats an over simplified version of the finger in the air guess simplified version. Different multipliers based on the type of room. My old gaffer used to use that for his rad sizing. But those were the days when i was still at college and having to calculate heat losses using the r values of materials to get the u value. He was always near enough that it didnt matter.
 
from the horses mouth - megaflow tech

The coil ratings in the Megaflo eco can vary from approx. 15 to 24 kW. In a standard domestic installation it is usually considered unnecessary to allow these high figures when calculating the boiler size. For the Megaflo we usually suggest adding 15,000 BTU = approx. 4.5KW to the boiler rating over & above the heating requirement.
(I have known for installers to allow just 3kW as this is usually the same rating as the back up immersion should it have to be used.)

For the maximum coil outputs of the Megaflo eco coil please refer to page 14 of the attached brochure.
 
Imagine adding total cylinder loading to a boiler size if you fitted a 300l ACV bet part of 60kW IIRC:17:
 
It pretty obvious what is going to happen if you undersized a boiler, its going to take long to get everything hot from cold, the insulation on a Mega Flow is pretty good, I doubt if any MF on a domestic installation ever starts off from cold i.e 10 Deg C. Does anyone ever work it all out, I think not, you are doing what you always did, some will worked it out. It is a very complex subject that not everyone totally agrees with one another.
 
AWheating said:
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.
Click to expand...

Spot on!
 
happyflyer said:
Jezz,

Does it get that cold up where you live -298 is that C or kelvin...looks like hell can freeze over after all....:32:
Click to expand...

0k = -298c point at which all molecules stop moving so no thermal energy. Theoretically coldest temp possible
 
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