How Many BTUs Or KWatts Can A 15mm Heating Circuit Take?

[bengasman]

Methinks the sludge problem can be ignored IF the system is installed correctly and maintained properly.

But the OP said he has a botched heating installation

Well, there is that.

What nobody had mentioned, and I have just realized, is that this is another case of a combi possibly not being appropriate. The OP currently has 3.8kW of rads, while the 30cxi can deliver 4.95-22.4kW for heating. So the boiler would not be able to modulate low enough and would be running in on/off mode virtually all the time. Adding the extra 5.7kw of rads would bring the total up to 9.5kW, which is better but still less than half the boiler's max output.

Combis are sized for dhw-demand, not for heating. Not much point in limiting the ch output as the boiler will modulate down when reaching target temperature. Range rating is far overrated, as it should allow for harshest conditions which means 360 days per year it will cycle. So what.

 

[Deleted member 105431]

bengasman i disagree about range rating; it can make a big difference to efficency, reliability, noise, scale formation, and longevity of boilers especially where large output combi's are fitted to smaller houses.

 

[bengasman]

I found that reducing the maximum temperature has more effect than range rating, especially with steamers.
This is in relation to the vast majority of users that still set the trv on max because it is so cold outside, despite the fact that you explained at the last 6 visits that they don't make the rad hotter. Dito people turning the boiler lower because the cylinder gets too hot
Boilers and the stuff around them seem to come from some part of the galaxy that is impossible for people to understand, including those regarded by society as really well educated and highly intelligent.
And then there is the friend of the neighbour of the colleague whose father is a retired builder, so he knows all about boilers, and he said........

 

[Deleted member 105431]

I found that reducing the maximum temperature has more effect than range rating, especially with steamers.
This is in relation to the vast majority of users that still set the trv on max because it is so cold outside, despite the fact that you explained at the last 6 visits that they don't make the rad hotter. Dito people turning the boiler lower because the cylinder gets too hot
Boilers and the stuff around them seem to come from some part of the galaxy that is impossible for people to understand, including those regarded by society as really well educated and highly intelligent.
And then there is the friend of the neighbour of the colleague whose father is a retired builder, so he knows all about boilers, and he said........

yes the flow temp is important too, but as important is range rating imo. The vaillants allow you to range rate in 1kw increments and limit the flow temp in 1 deg at a time too. I usually limit the flow to 70 and set up for a 15-20 deg c diff.

 

[D_Hailsham]

Combis are sized for dhw-demand, not for heating.

I realize that. The problem is that the OP needs 30kW to give the required hot water temperature rise at the desired flow rate, but only 10kW to heat the house.

Not much point in limiting the ch output as the boiler will modulate down when reaching target temperature. Range rating is far overrated, as it should allow for harshest conditions which means 360 days per year it will cycle. So what.

If the boiler is cycling it will start up each time at max output, drop down quickly to minimum; find that is still too high, so it turns off until the water temperature has dropped then repeat the cycle. The efficiency of the boiler will drop because of the wasted gas each time the boiler lights and the boiler does not have time to adjust so the return temperature is low enough to provide condensing. The efficiency therefore drops considerably.

If the boiler is cycling 360 days of the year it's definitely oversized. Taking the OP's boiler as an example, if approx 10kW is required when the outside temp is -1C, then the boiler will run in cycling mode when the outside temp rises above about 9C. This is because the boiler can only modulate down to 5kW. I don't believe that the external temperature is above 10C for 360 days of the year.

 

[Legion]

Shouldn`t this be a subject for the combustion chamber? members of the public and children may get the wrong idea and start getting interested in installation/breakdown/servicing rather than how to turn it on.


Remember kids, Gas is Dangerous, stay in school.

 

[D_Hailsham]

Shouldn't this be a subject for the combustion chamber?

Are you saying that everything to do with gas boilers should be discussed in the CC as it is, however indirectly, gas related?

If so the vast majority of queries would have to be rejected by the mods, as most of the people asking the questions are not eligible for entry to the CC.

 

[Legion]

Are you saying that everything to do with gas boilers should be discussed in the CC as it is, however indirectly, gas related?

I am saying that children or members of the public by reading these posts may go around to their neighbours with a spanner or suchlike and attempt to disassemble various gas related components, i.e. boiler, gas fire etc.

If so the vast majority of queries would have to be rejected by the mods, as most of the people asking the questions are not eligible for entry to the CC.

That is a very good point and, in fact I have now in my hand a petition to close the Forum to stupid members of the public. It`s a test you have to pass whereby we stick their fingers into an electrical socket and ask wether they think that is dangerous.
The one`s that survive and answer `dangerous`are allowed on to the Forum.

 

[2point8ghiax]

Shouldn`t this be a subject for the combustion chamber? members of the public and children may get the wrong idea and start getting interested in installation/breakdown/servicing rather than how to turn it on.


Remember kids, Gas is Dangerous, stay in school.

Don't worry about retreating to the safety of the combustion chamber, you lost me way back.
Range rating, modulating, cycling????? All Greek to me.

Still wouldn't mind knowing what they meant, if they could be explained slowly, in simple words.

Thanks for your help guys, much appreciated.

 

[bengasman]

Don't worry about legion's comments, he is a troll.
Not a lot to worry about the discussion about modulating, cycling and the rest as it is fairly academic and predominantly of interest when designing a system. Once you have what you have, there is little you can do about it unless you are willing to tear your system apart for the sake so following one train of thought or the other.
It also depends a lot on which boiler you have; some of the points DH mentions are disputably correct, but only for SOME boilers.
For more understanding you can read through wiki and faq

 

[Legion]

Don't worry about legion's comments, he is a troll

.

Think it would be fair to tell him what a `Troll` is.

Not a lot to worry about the discussion about modulating, cycling and the rest as it is fairly academic and predominantly of interest when designing a system.

Nothing to with the design of the system, more to do with what happens within the boiler.

Once you have what you have, there is little you can do about it unless you are willing to tear your system apart for the sake so following one train of thought or the other.

In other words ` you are stupid, put up with it`.

For more understanding you can read through wiki and faq

Because I don`t have the time to explain to a moron.

 

[glasgowgas]

as Mr Tony Glazier would say.........................

if you are confused, sir, may i suggest that you have neither the knowledge or, as is probable, the required certification (from a notaried certification pre requisite body).
Why don`t you post this in the Combustion Chamber?

 

[D_Hailsham]

Range rating, modulating, cycling????? All Greek to me.

Cycling Back in the old days a boiler could be either on or off. So it went: on-off-on-off etc. It cycled between on and off

Modulation It was realised that, when the house was cold, the on period would be fairly long as the house heated up, and the off period short. But as the house reached temperature the on period would get shorted and the off period longer. Someone then came up with the idea of keeping the boiler running all the time, but adjusting its output to meet the immediate requirement - a bit like the accelerator of a car. This is modulation.

The only problem with this is that the modulation does not go down to zero output. So there comes a time when, even at the lowest output, the boiler would still be producing more heat than necessary. At this point it changes to cycling.

To give an example. Say your house requires 10kw to raise temperature from -1°C to 21°C. So you buy a 12kw boiler, (a bit extra in case it gets colder ). Lets say the boiler will modulate down to 5kW output. Now, as the required boiler output is directly proportional to the temperature rise, a 5kW boiler will raise the temperature from 10°C to 21°C. (10°C is halfway between -1°C and 21°C; 5kw is half of 10kw)

As long as the outside temperature does not rise above 10°C the boiler will be able to adjust its output to meet requirements. When the outside temperature rises above 10°C the boiler, the boiler will start cycling because it cannot modulate low enough.

Range rating The installer can sometimes adjust the maximum output of a boiler to allow for the different requirement of houses. That's range rating.