Viessmann boiler

[BingoBongo]

just realised the op said system boiler.

surely they mean the combi

also the wc for the 200 (£150) comes with the outside sensor

 

[D_Hailsham]

just realised the op said system boiler.

surely they mean the combi

Maybe the OP does mean a combi, but he is keeping very quiet at the moment. On the other hand he could have an installer who decides what boiler you need by the number and makes of cars parked outside the house.

 

[JohnD]

is the new 100 35kW? It used to be 24 max (non-combi).

Edit: my mistake, yes, the range for 100W is
System: 26kW, 30kW & 35kW
Combi: 26kW, 30kW & 35kW

But the heat-only is still 13 kW, 18 kW & 24 kW

 

[Agile]

To the installers, the new 100 system boiler has a built in diverter valve and a seperate temperature control for the HW mode.

Most people like that and it does give better efficiency on HW.

The slight downside perhaps is that it will not run the CH while its heating HW.

A slight upside is that it gives priority to heating the HW.

Tony

 

[firekill]

sorry been away , yes i did mean a system boiler mainly because we have two bathrooms and may add a on suite in future .got 38kw of rads so i thought we may need 35kw system .do you think i should cough up the extra and go for the 200 , i am a bit confused , ps thanks for all the posts and advice so far

 

[Agile]

If you are heating a cylinder the boiler with the built in diverter valve and a seperate temperature control for HW will be more efficient.

The 100 is good but the 200 has a more sophisticated control built in.

In all probability those with the 200 dont use it to their full capacity.

A friend of mine will fit a 200 for his clients in Hampstead even if they are old and dont have an internet connection! He is so unsophisticated that he cannot use texts, email or even know who his missed call was from ! Yet he now fits up market boilers from Viessmann. I think they should reward me for the extra sales I generate for them! They can do it through Andy!

Tony

 

[glasgowgas]

A friend of mine will fit a 200 for his clients in Hampstead even if they are old and dont have an internet connection! He is so unsophisticated that he cannot use texts, email or even who his missed call was from ! Yet he fits up market boilers from Viessmann. I think they should reward me for the extra sales I generate for them! They can do it through Andy!

Wot Da Funk?

 

[D_Hailsham]

got 38kw of rads

How BIG is your house?

Just because you have 38kW of rads does not mean that you need a 38kW boiler.

* The original rads may not have been sized correctly
* Insulation may have been installed since the rads were installed
* You may now have double glazed windows and draught insulation

Use the Boiler Sizing Wizard to find out what size boiler you really need.

 

[bengasman]

got 38kw of rads

Just because you have 38kW of rads does not mean that you need a 38kW boiler.

Damn site cheaper to spend a few quid more and get a bigger boiler, then to replace all the rads, not to mention the fact that oversized rads give a quicker response time and better efficiency.

* Insulation may have been installed since the rads were installed
* You may now have double glazed windows and draught insulation

That would not alter the working of the rads.

 

[D_Hailsham]

Damn site cheaper to spend a few quid more and get a bigger boiler, then to replace all the rads, not to mention the fact that oversized rads give a quicker response time and better efficiency.

I was not suggesting replacing the rads. As you say oversized rads will give a quicker heating up time and improved efficiency. However, you still need the boiler to be the correct size for the heat loss. Your radiators will give out the required heat, but with the boiler running at a much lower flow temperature, maybe 40-50°C, which means your boiler will be in condensing mode all the time and your gas bills will be lower

* Insulation may have been installed since the rads were installed
* You may now have double glazed windows and draught insulation

That would not alter the working of the rads.

Did not say it would. Just that a smaller boiler would be needed.

Don't forget that a radiator is not like an electric fire, which pumps out a constant amount of heat. The heat output of a radiator varies with three temperatures: water entering the rad; water leaving the rad; surrounding air temperature. Change any one and you change the output of the radiator.

 

[bengasman]

It is perfectly fine to install a boiler that is capable of producing the power required to feed the rads to their designed output as it will modulate down to a fraction of its maximum output when less than full is required.

Albeit the difference will be minimal, a larger fan working at a slower speed will force the poc trough the larger hex at a slower speed, which could well increase the heat absorption by a few percent. This would only apply to premix of course.

 

[mehran]

I don't think it would work like that...

it is always best to size the boiler to match what is needed. a larger boiler will not be able to modulate down as much as a smaller boiler. for example

the viessmann 200w 35kw system boiler can modulate down to 8.8kw were as the 19kw system boiler can go down to 4.8kw.

so if the house only needs for example 5kw to keep the same room temp why put in 8.8kw and force the boiler to cycle?

now I am not saying this guy needs a 19kw boiler but only that he should get a boiler that best matches his homes heating and hot water needs

 

[bengasman]

....that he should get a boiler that best matches his homes heating and hot water needs

That is the part everyone agrees on.

The question is what is "best" if a house has possibly oversized rads (whether that is the case with op or not is fairly irrelevant).

House has 38 kW output rads, but 30 kW heat loss.
Do you:
A, stick a 30 kW boiler in to match the heatloss, and ignore the fact that the boiler will not be able to heat the rads fully?
Or:
B, stick a 37 kW boiler in that can heat the rads fully, and ignore the fact that the house only has a 30 kW heatloss?

This of course assumes that it is:
A, a conventional boiler,
B, the smaller boiler will have a lower minimum rate than the bigger model,
C, that the house will actually need less than the minimum rate of the boiler,
D, it will never get colder than the design temperature out side,
E, the occupant is not, and never will be, Jamaican and likes the house warmer than the standard design temperature,
F, at not time in the future will there be an extension built, a loft converted, or any other need for more output,
G, that it is actually a demonstrable disadvantage to have the boiler cycling during some periods, rather than a theoretical issue,
H, the owner believes that the demonstrable comfort of over capacity does not outweigh the theoretical downside of cycling.

That's a lot of assumptions.

 

[mehran]

all very good points and I would always go for the safe side.

if the OP does pony up the cash for the 200w and advanced weather comp then modulation range would come become even more important.

most of the year the boiler will be in its low to mid modulation range and only for the coldest days of the year would be at max or close to max output.

but rather a customer warm and losing a few % efficiency than cold

 

[D_Hailsham]

The question is what is "best" if a house has possibly oversized rads (whether that is the case with op or not is fairly irrelevant).

House has 38 kW output rads, but 30 kW heat loss.
Do you:
A, stick a 30 kW boiler in to match the heatloss, and ignore the fact that the boiler will not be able to heat the rads fully?
B, stick a 37 kW boiler in that can heat the rads fully, and ignore the fact that the house only has a 30 kW heatloss?

Heating the rads fully is irrelevant, its heating the house which is the important thing. If you stick a 30kW boiler on to 38kW of rads, the house will still reach the required temperature, but the radiator temperatures will be lower - flow 72°C return 52°C.

But say you only need a 20kW boiler and have 38kW rads. Then the temperatures would be 61°C and 42°C.

This of course assumes that it is:
A, a conventional boiler,

And there have been several topics about OPs with problems when the heating output of a combi boiler has not been range rated.

B, the smaller boiler will have a lower minimum rate than the bigger model,
C, that the house will actually need less than the minimum rate of the boiler,

The second (C) is easy to determine as the heat required is proportional to the outside temperature. If you need 22kw to raise the temperature from -1°C to 21°C then you will need 11kw to raise the temp from 10°C to 21°C and 5kW to raise the temp from 16°C to 21°C.
Item B can be resolved by checking the manufacturers specifications and selecting the most appropriate boiler - though installer's preferences may distort this.

D, it will never get colder than the design temperature outside,

Then you discuss this with the client and get him to agree what temperatures to use.

E, the occupant is not, and never will be, Jamaican and likes the house warmer than the standard design temperature,

You install a system to meet the current residents requirements, not some hypothetical future requirement.

F, at not time in the future will there be an extension built, a loft converted, or any other need for more output,

Another case where you discuss this with the client before making your recommendation.

G, that it is actually a demonstrable disadvantage to have the boiler cycling during some periods, rather than a theoretical issue,

Loss of unburnt gas prior to ignition every time the boiler comes on?

H, the owner believes that the demonstrable comfort of over capacity does not outweigh the theoretical downside of cycling.

This reminds me of the early days of central heating when the sales pitch was "guaranteed temperatures". This was achieved by oversizing everything. Fine when gas was cheap and the problem of global warming had not appeared over the horizon.