under sized boiler ???

[cr0ft]

Our cheap skate new home builders installed a Potterton Promax 15 SL 15KW system boiler onto our system. It has an unvented hot water cylinder with an 18.3KW heat exchanger. There are just under 15KW of radiators in our house.

You can get away with sizing a boiler to suit the heating requirement only in a house like Ray says. The rads will not be heated for the half hour or so a modern unvented cylinder takes to recharge, in practise it will hardly be noticed.

I caveat this with there are just 2 of us in the house and we use showers only most of the time.

Our house is a 5 bedroom detached house. I would say the boiler is undersized and when we change it I will fit a 24KW system boiler but it does the job.

 

[Nostrum]

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.

 

[Whyme]

Suppose its like everything, make do and change settings or get it changed..also being there and knowing if their bath people or shower, and when through the day are people using hw....

 

[Killy Bing]

the heating stops when hw is heating up.

Lockshield valve on the coil, balance it and job done?

 

[Whyme]

im assuming its balanced. i still believe it to be undersized but hay ho.

 

[Killy Bing]

My parents house needs 29kw heat, it has two unventeds, a 250 and 300 litre.

boiler is 32kw output. Coils are balanced as is the system and it runs it more than fine.

it has solar but thats usually pretty irrelevant when the heating is needed.

if you cant see a lockshield valve on the coil then its the path of least resistance for the system to pump to.

simple valve and extend hot water periods.

all it needs

 

[Whyme]

Mmm all i was saying is worst case demand is the key to see your kw output.

 

[Bronze_tap]

adjust the clock so the hot water comes on at a time the heating is not required.

Explain to the customer that that system is not designed to work with the Randal-style clock. Use the heating system in the Canada mode, not in the Cornwall 2hours day mode.
Remove the clock on the cylinder and put a thermostat instead the clock on the heating in the coldest room.
Make sure that the heating IS ON FOR 24/7 in THERMOSTAT-controlled mode.
Water should be in 24/7 on cyl thermostat.
Buy some programmed valves, to cut out unused rooms. Get the house PROPERLY insulated.

I remeber heating the 4room semi house with 32KW ex-combi boiler modulated down to ~8KW when the outside was -7 C. inside was 19 C. Solid uninsulated walls + loft insulation.

PS: You are not going to save much with well insulated unvented cyllinder kept on the timer. So ditch it for the cyl.

PPS: Do check the condition of the boiler's heatex (esp if the boiler is older than 5-20 years, and was serviced last time 5-10 years ago), Make sure it is clean. Use a spatula/stiff brush/stick for clening the crud out of it.

 

[Mike Jackson]

Lockshield valve on the coil, balance it and job done?

I was told that under part L we should no longer fit balancing valves to cylinders. The reason being all systems should be fitted with a programmer capable of separate times for heating and hot water and this enables the full load of the boiler to be sent to the cylinder before the heating comes on. Doing this means that the boiler can be accurately sized for the heat requirements of the property and it will remain in condensing mode for longer.

 

[SafeGasInstall]

Not a very good install, but could get around it by setting the hot water and heating times separately so they aren't on at the same time.

top post Matt ,

have water to heat at early morning and then late afternoon will sort it out

 

[happyflyer]

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!!!

 

[sheff paul]

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.

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

 

[Whyme]

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!!!

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

 

[Mike Jackson]

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.

 

[happyflyer]

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

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...

 

[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 ?