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Am I missing something here, should not a boiler be sized for the maximum load, if it is not then you need a priority system one way or another, HWS or Heating. A bit like buying a car that does 130 mph but only downhill,on a good day,with a tailwind. I suppose most system are a compromise, if the Mega flow has a demand of say 20 Kw and the heating system is 15 kw then around 35 kw would be correct, anything less then there is a compromise, and has safegasinstall says stagger the loading, sorted!!! savvy!!!
 
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Nostrum said:
There is no requirement to allow both the heating and hot water to be run at the same time under full load. Remember, the heat loss calculated will be at -1,-3 or whatever ie 1-2 weeks a year.

The rating of the coil is irrelevant really as long as heat up time complies with part L. In reality who's checking? How often will you need to heat the entire cylinder from cold?

if you are adding the coil size to the heat loss then your massively over sizing boilers.

Sounds like this boiler was sized perfectly which makes a change.

Also, who's to say the radiators aren't over sized? Sizing a boiler should be done by heat loss of the building not radiator sizes.

if they are running out of water, the cylinder is undersized not the boiler.
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The technical guy from RM cylinders insisted we allowed 9kw allowance for a 180 unvented on a recent install to achieve recovery times specified....
 
happyflyer said:
Am I missing something here, should not a boiler be sized for the maximum load, if it is not then you need a priority system one way or another, HWS or Heating. A bit like buying a car that does 130 mph but only downhill,on a good day,with a tailwind. I suppose most system are a compromise, if the Mega flow has a demand of say 20 Kw and the heating system is 15 kw then around 35 kw would be correct, anything less then there is a compromise, and has safegasinstall says stagger the loading, sorted!!! savvy!!!
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agreed maximum load always has been always will and should be..
 
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I hope that you guys that are advocating sizing the boilers for the combined heating and hot water loads are also correctly sizing the primaries. I'm expecting to see a lot more 28mm primaries to cylindersin the future then.
 
Whyme said:
agreed maximum load always has been always will and should be..
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Wyhme that should be the end of this thread but it won't be, but it won't be. People will go on about, turn this, tweak that, fit this, fit that, you are spot on, if the total loads add up to 30kW then it should be that plus some for intermittent use, yes we forgot that, intermittent use another 10% or maybe a bit more. Skimping all that time cut the cost of the job...
 
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sheff paul said:
The technical guy from RM cylinders insisted we allowed 9kw allowance for a 180 unvented on a recent install to achieve recovery times specified....
Click to expand...

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.
 
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AWheating said:
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.
Click to expand...

Like you say all it takes is a bit of thought, not a lot around these days
 
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?
 
Ermintrude said:
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?
Click to expand...

it will if its a worcestor,with a Vaillant steady as she go's
 
Whyme said:
agreed maximum load always has been always will and should be..
Click to expand...

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.
 
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Ray Stafford said:
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.
Click to expand...

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.
 
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In the commercial sector a lot of over-sizing goes on, consultnats fear of PI attacks
 
Ray Stafford said:
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.
Click to expand...

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
 
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?!!!!!
 
happyflyer said:
In the commercial sector a lot of over-sizing goes on, consultnats fear of PI attacks
Click to expand...

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