# 30kW System Boiler Choice

My calculations are quite correct. If you need a 30kW boiler to maintain a temperature of 21C when it is -2C outside, a 15kW boiler will only be able to maintain a temperature of 9.5C, no matter how long you wait.
This might be true if all heat in a house was lost via steady state conduction, but it isn't...........

D Halisham you seem to struggle along with Simond that central heating is not designed to only reach 21 degrees when its minus 1 outside strange you both use the figure minus 2.
I'm not struggling at all. I don't know why Simond used -2C in his example; I just used it to maintain consistency. In any case using -1C is no longer recommended; the temperature varies regionally. See Whole House Boiler Size Calculator or the latest version of CE54 - Domestic Boiler Sizing Method, published by the Energy Saving Trust.

The rad sizing is to get it to 21 in a set time (this is not the maximum temp the house will get to) otherwise the heating could be on all day to eventually get it there, its in the old books that you wont find in your google search.
Firstly, I don't need to do google searches to find this out; it's simple physics. Yes, if the radiator was sized to match exactly the heat loss when the room temperature was 21C, it would take longer to reach temperature. That's why radiators are oversized by 10-15%. This is also done to allow TRVs to control the temperature better.

Radiators are sized on the heat loss, which depends on room geometry, U-values of the materials used and the maximum anticipated temperature difference. Time does not come into it. If you can prove otherwise .... .

So using your calculation in the winter of 2010 when it was minus 17 outside even with a 30kw boiler a house would only have got to only 5 degrees eh don't think so . Or in the worst places at minus 26 the houses would not have got above freezing.
The answer is air changes! Heatloss calculations allow for up to three air changes per hour, depending on the room. This can account for up to a third of the heat loss from a room. When it is very cold, the first thing anyone does is to close all windows to prevent the heat escaping. This reduces the air changes, so more heat is available.

In winter of 2010 did your own house heat up to perfectly ok
Yes, but that's because the house has been double glazed and loft and cavity insulated. It still has the original radiators which are now twice as large as necessary, so there is plenty of spare capacity for very cold days.

My calculations are quite correct. If you need a 30kW boiler to maintain a temperature of 21C when it is -2C outside, a 15kW boiler will only be able to maintain a temperature of 9.5C, no matter how long you wait.
This might be true if all heat in a house was lost via steady state conduction, but it isn't...........
Let's turn the problem around.

1. How much heat will it take to heat house A from -2C to 21C? Answer 30kW

2. How much heat will it take to heat house A from -2C to 9.5C? Answer 15kW

The heat loss per degree difference is exactly the same!

the problem is that thats how people see the function of heating...to raise the temperature...

in reality of course a good heating system maintains the temperature enabling boilers to be significantly smaller than the design equation leads us to believe...

And to add some anecdotal evidence....

Called to a property where an older couple had a Potterton Netaheat 18Kw boiler. I had been requested to provide a new boiler quote and recommended a Viessmann 19kW WB1B, having done some quick heatcalcs.

Customer said house had not got warm enough in colder weather for over 3 years. They had a BG contract, and BG had first changed the pump a year or two ago, then a new engineer had diagnosed the fault in the last 2 weeks as needing a new room stat, which he had changed but the house was still cold.

I smelt a rat, because the 18kW would have easily been enough to keep their house warm, and the circ temp on the rads was suspiciously low, so not wanting to touch the boiler*, I simply gas rated it at the meter.

And I got 9.5kW, for an 18Kw boiler that doesn't modulate. The gas working pressure was OK at the meter so it was clearly a gas valve related defect in the boiler.

The customer called out BG after suffering three cold winters and a succession of imbeciles, told them where to look (and what they thought of them) and the fault was fixed.

I wasted 2hrs of my working day and around £6 worth of diesel, because they were delighted I had found the reason, and now didn't need a new boiler.

Their boiler didn't counteract the heatloss because it was only running at 9.5kw; they can vouch for that - their house was full of electric fan heaters!

So, one must conclude;
- undersized boiler = cold house
- some BG people shouldn't be working (apologies to the good ones)

*If you are not in the trade, I'll explain that it is never a good idea to touch anyone's old heap when doing a quote -

Because you become technically liable for it's condition if it is dangerous....... and many are, or they wouldn't be considering getting it changed.

funnily enough I have anecdotal evidence to the contrary...

but I use compensation controls which temperature profile the house.

On one house I rated the boiler at 18kws when the heat loss of the house was about 28kws...

One has to understand (as I am sure you do) that compensation controls match heat generated to heat lost, either through an out door sensor or an indoor one or both..

with these you get long continuous burns at temperatures around 50c...almost all the time...

So a boiler can be smaller than the calculated size, but it must use compensation controls, no numpty on off controls...

the reason for getting away with this is because the temperature only varies about 3-4 in the house...short heat up times are energy intense...hence the need for bigger boilers...

ALEC1 wrote
in reality of course a good heating system maintains the temperature

Not every one needs it maintained.

One has to understand (as I am sure you do) that compensation controls match heat generated to heat lost, either through an out door sensor or an indoor one or both..

One also has to understand (as Im sure you don't) that mother nature is a great controller of the ambients.
So when its 12 degrees outside mother nature will help find the balance.

if you want to challenge mother nature the solution is insulation....nothing to do with heating...

I've no need to challenge mother nature and many people are the same. At least not above 10c. Not everyone is cold blooded wimps who need their heating on 24/7.
Is it any wonder we are heading into an energy crisis. and you lot with your fancy controls think you can save the world.

This is proving to be a very interesting debate.

We have now heard that weather compensating controls allow a boiler to be considerably smaller than heatloss calculations suggest.

And from a surprising source*.

I'm trying not to laugh here.

So, one must assume that there are TWO sets of heatloss calculations, one sets a suitable boiler size for on/off control, and another sizes it for weather based flow temp control.

Please point me to the second type of calculation!

*OK I'm kidding, Alec is the forum's 'weather compensation' fetishist.

one can only summize as the 2nd sizing theory remains elusive if it exists at all.
In fact the heat loss remains the same how ever you control the heating, which means that the way you control it must be significant..

if people dont want to be warm they can run their compensated heating systems at lower temperatures...its the stable temperatures that people enjoy, the swings are uncomfortable, exasperated by rapid heat ups...which dry the air...

'So a boiler can be smaller than the calculated size, but it must use compensation controls, no numpty on off controls...'

But you said above a boiler can be sized smaller with WC! Have you changed your mind?
[/i]

weather compensation IS a compensation control...

a compensation control merely matches heat generated to heat loss, and can do this in a variety of ways, one way is with an outdoor sensor...aka weather compensation...

alternatively an indoor sensor can be used...both vary the flow temperature...

Let's turn the problem around.

1. How much heat will it take to heat house A from -2C to 21C? Answer 30kW

2. How much heat will it take to heat house A from -2C to 9.5C? Answer 15kW

The heat loss per degree difference is exactly the same!

That's not turning the problem around, it's setting a different problem.

1) The amount of heat (energy) required to raise a substance (the house) by 1C is related to the specific heat capacity of the substance. Here, increase in temperature is directly proportional to energy supplied.

2) The amount of heat (energy) required to maintain the house at that temperature is dependent on the heat loss. In a house that is basically how draughty it is and how well it's insulated. Heat loss, if only considering conductive heat loss, is proportional to the temperature difference between the inside and outside of the house. But heat loss due to draughts is going to be a bit more wooly to calculate.

Now it is complicated somewhat because when you are raising the temperature of the house (problem 1) it is also losing heat (problem 2).

I suspect combining both problems to give a single solution requires integral calculus and it's too early in the morning for that.

What can be said with certainty is that a boiler capable of outputting 30kw will raise the temperature of a house quicker than a 15kw boiler. And the 30kw boiler will be 'on' less time when maintaining any temperature.

assuming of course the radiator give out that much heat... in reality you find 15kw heat output of radiators with boilers of 30kw...a complete nonsense...as the system can only dissipate heat through radiators which have to be correctly sized...

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