Gas Boiler Two bed room Flat

[mickyg]

oh dear you still don't get it. May I suggest you stop posting on the technical operation of boilers, clearly you can't see past what you read and I assume, probably rightly that your not an RGI.
I understand perfectly how vaillant combis work ta. If you were an engineer and you'd worked with them so would you.

 

[D_Hailsham]

oh dear you still don't get it.

Please tell me what do I still not get? You can make it as simple or as technical as you like.

 

[mickyg]

right ok, lets go back to the start.

You said

The problem I can see is that the heating output will be too large for a two bed flat, which will probably require no more than 8-9kW max. The 831 has a range of 8.7-24kW and the Pro28 9-24kW. This means that, except when the outside temperature is below freezing, the boiler will be running in on-off mode as its lower modulation limit is too high.

now tell me again, why do you think this. be as technical or as simple as you like.

 

[D_Hailsham]

You said

The problem I can see is that the heating output will be too large for a two bed flat, which will probably require no more than 8-9kW max. The 831 has a range of 8.7-24kW and the Pro28 9-24kW. This means that, except when the outside temperature is below freezing, the boiler will be running in on-off mode as its lower modulation limit is too high.

now tell me again, why do you think this.

OK, it's back to school

Let's assume the flat only needs 12kW to heat it and the boiler can modulate between 20kW and 8kW. It's the depth of winter, temperature outside is -1°C, so the radiators are churning out 12kW to keep the house at 19°C (chosen to make the maths easier) and the boiler control is modulating the gas supply to provide the 12kW required.

The outside temperature rises to 4°C so the boiler now needs to provide only 9kW. That's OK it can modulate down to 8kW.

It gets warmer and the outside temperature rises to 9°C. The boiler now needs to provide only 6kW to keep the house at 19°C, but it can only modulate down to 8kW, which it does. But that means the water is heating up faster than the rads can emit the heat; result is the return water is higher than it should be, so the boiler shuts down until the flow or return temperature has dropped, at which point the boiler starts up again and the on-off cycle starts again.

 

[gas4you]

It gets warmer and the outside temperature rises to 9°C. The boiler now needs to provide only 6kW to keep the house at 19°C, but it can only modulate down to 8kW, which it does. But that means the water is heating up faster than the rads can emit the heat; result is the return water is higher than it should be, so the boiler shuts down until the flow or return temperature has dropped, at which point the boiler starts up again and the on-off cycle starts again.

That could be any system/boiler that is fitted as all boilers are set up for the coldest design scenario.

Personally I can't see a problem with it.

This is where the benefit of weather comp comes in.

 

[D_Hailsham]

That could be any system/boiler that is fitted as all boilers are set up for the coldest design scenario.

I agree that is how all boilers are specified.

Personally I can't see a problem with it.

The problem is that a boiler running in on/off mode is not so efficient as one which is running continuously. That's why modulating burners were invented.

This is where the benefit of weather comp comes in.

But all weather compensation does is adjust the flow temperature of the boiler; and if the boiler cannot modulate low enough to maintain this temperature, it will revert to on/off mode.

 

[Tipper]

Whilst not wishing to open another can of worms.....I still can't see the justification for 'range rating' a boiler that automatically modulates its output to meet the CH demands.

It's constantly being mentioned on this forum but can anyone explain the need? The MIs for my Vaillant say don't bother but why do RGIs say you need to? Is it a requirement of older less sophisticated boilers?

 

[D_Hailsham]

Whilst not wishing to open another can of worms.....I still can't see the justification for 'range rating' a boiler that automatically modulates its output to meet the CH demands.

In theory there is no need. But say you have a combi boiler which produces 30kW for hot water and 10-28kW for heating, but your house only needs 14kW for heating. We can forget about the HW side and concentrate on the heating side. When the boiler ignites it automatically goes to max output of 28kW . The water heats up so quickly that the return temperature is above the limit set by the mfr, so the boiler shuts off it does not modulate down as you would expect. By range rating the boiler (setting the maximum output) you can stop this happening.

The problem would not occur if burners started off at low output and increased output to the required level.

 

[David937]

When our boiler was at 28kw the only difference was that the house warmed up quicker, the boiler starts at a lower output and then after a minute or two you hear it ramping up the output to maximum, as the flow temperature reaches the set point you hear it modulate down. The flow temperature was set at 65C.

There is no cycling unless the flow temperature is set lower than usual.

 

[gas4you]

It is especially important for new Vaillants as strangely the heat exchanger don't like heat, well not too much of it anyway.

Most boiler will fire up at max output then as the return temperature rises, will modulate down as needed.

By range rating they cannot fire up at max to more than the system needs.

Top end boilers, such as ATAGs, fire at the lowest rate, then ramp up to the power needed for the system, are different and do not need range rating as they will never fire at more than the system needs.

 

[David937]

It is especially important for new Vaillants as strangely the heat exchanger don't like heat, well not too much of it anyway.

By too much do you mean more than the system actually needs?

What could happen to the heat exchanger?

 

[gas4you]

Yes, if Vaillants are set at max when the system requires a lot less there can be problems with the HE.

Could be attributed to some of the seal failures as well, IMHO, as well as lack of proper servicing.

If there is a basic problem with an item such as the seal and the boiler is running too powerful, then that problem is going to be made worse.