Viessman any good

I have heard on here that an overrated boiler can give trouble what with it keep switching on and off but I could be wrong.
 
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Viessmann technical haven't seen your gas run. Ask your installer for their justification in wanting to increase the size of the boiler, when the present one does the job just fine and the new one will be more efficient. If they can't give you a sensible answer, find a new installer
 
Valid points, thanks.
Did email the co querying the 19kw and they just came back saying that's the size the installer recommended without any reasons why
 
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Thanks

Have also noticed that installers try and push u to get a combi boiler , going from an old vented system to a system boiler or combi, surely similar in terms of work involved
 
Mottie said:
I have heard on here that an overrated boiler can give trouble what with it keep switching on and off but I could be wrong.
Click to expand...
Info gleened from elsewhere

If a boiler is oversized it will ‘cycle’ on and off excessively, heating up too quickly to only run for a short period, only to turn itself off because it’s too hot. Each fire up is something close to full capacity which dumps a huge amount of heat into the system and overheats the radiators. This will end up decreasing the energy efficiency and creating more stress on the components, increasing wear and tear and shortening the lifespan of the boiler. A boiler is at its most efficient when it can run for long periods at a lower output, so this is recommended to ensure you are not paying more money in the long run
 
Interesting , thanks, as regards oversizing boiler , can't the thermostat just be turned down on the boiler
 
rhinosaw said:
Interesting , thanks, as regards oversizing boiler , can't the thermostat just be turned down on the boiler
Click to expand...

Not really, the boiler will always want to go straight to full power until a certain temperature is reached only then will it modulate down to a low flame to keep the temps constant.

With an oversized boiler the water temp rises to quick and causes the boiler to cycle.

By range rating the boiler you put a physical limit on the maximum flame size in terms of kw.
 
Exturbo
Can't have open vent boiler , installer didn't reccomend pressurising vented/heat only boiler, he said system boiler best option. Not that I necessarily agree with that but who am I to disagree
 
rhinosaw said:
Exturbo
Can't have open vent boiler , installer didn't reccomend pressurising vented/heat only boiler, he said system boiler best option. Not that I necessarily agree with that but who am I to disagree
Click to expand...

Oh ok you have a system boiler already, I was under the impression it was currently open vent


I think you should get some heat loss calcs done as already advised, but based on the basic info provided the 11kw would likely suit your requirements much better than the 19kw will.
 
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No it's open vent but I can't have another, there is no head of water between f&e tank and boiler as it's in a flat.
Not sure how this has worked ok for the past 40 years, installed when flats built so ancient
 
sxturbo said:
Not really, the boiler will always want to go straight to full power until a certain temperature is reached only then will it modulate down to a low flame to keep the temps constant.
Click to expand...
On my Vaillant Ecotec plus 18Kw heat only boiler I can set the maximum output. I have it set on 15Kw in winter when I have the heating on but at the moment, only using it to heat the water, I think it’s set on 10 or 12Kw.
 
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