Vaillant Boiler Size

[prs999]

Hi,

We have an ecotech plus 837 in our house that was originally over rated on installation.
It was installed based on the hot water flow rate and told that the heating would modulate itself down accordingly.
This is the case in very cold weather but not when its a bit warmer.

It is now having a few starting issues which will need repairing so i'm wondering if now is the time that we get a smaller boiler.

My question is What boilers if any are physically a direct replacement for the boiler? It is on a standard top plumbing bracket on the wall.
I would like not to have to re-plumb any of the pipework and ideally not the flue as its in a cupboard in a finished kitchen.
Are there any models that will just lift on the existing installation?

Thanks

 

[gmartine]

None of the large ecotech combis have very good modulation, I would get the right boiler/engineer first.

Edit: not true, the Ecotech Exclusive has a modulation of around 7:1 so min load of around 4kw, although it states in the literature that mod is 10:1. I just looked at min and maximum heating loads.

 

[John.g]

Someone on here, @ShaunCorbs?, pointed out some well known (combi) model that will modulate from something like 36kw right down to < 2 kw.

 

[gmartine]

That'd be me John.g, the Viessmann 200W has a mod of 17:1. Too big and expensive although I'd recommend it.

 

[John.g]

The "normal" Vaillant of min output 5/6kw seem to have difficulty getting away again during cycling as they hold the firing at 70%/80% for 60 secs on start up leading to water temp exceeding Target+5C (and only on target+3C with one poster on here) which results in boiler shut down again or else the deltaT exceeds 30C again leading to shut down, don't know if this is common on other makes. I can certainly vouch that a relation's Vokera Vision 20S, min output 5.5kw, will cycle quite happily with a demand as low as 1.5kw (one rad), it just has a anti cycle time of 1 minute and will then run for ~ 5 minutes before repeating the cycle. This boiler auto range rates to 75% of max for 10 minutes after firing so 15kw in this case but the boiler still has no control problems on cycling.

 

[prs999]

Thanks for your replies.

The sizing of the boiler will be done by an engineer if it happens.
I am more interested in the pipes lining up. are they the same spacings for the entire ecotech range?
I would like the engineer to not have to re plumb as its all behind kitchen cupboards.
If the pipes do not line up then we'll probably have stick with the larger oversized boiler for now

Thanks

 

[Scott_D]

You will need a new flue regardless of boiler choice

 

[EvilDrPorkChop]

You could have a Vaillant Green IQ, same pipe centres, 1:10 modulation range although tbh i'd stick with the 838 Plus. The GreenIQ range has been problematic. The 835 actually outputs more on CH so you'd be daft to go down to a 835 if you're bothered about modulation range.

Your best bet would be to use the Vaillant controls that set the flow temperature, the SensoHome or SensoComfort.

If you wait a bit longer they're launching a new Plus later this year/early next.

 

[king of pipes]

Another boiler of a smaller output will not line up but the alterations should be able to be done inside the unit , another 837 would probably fit with minor alterations it is possible to range rate the heating load and alter the cycling time but 5-6 kw is about as low as your gonna get , other than that it might be best to use Valliant controls and weather compensation as recommended above . Kop

 

[John.g]

Even with range rating the minimum output of the 837 is still 12 kw? and it can't be range rated below this.

 

[EvilDrPorkChop]

Even with range rating the minimum output of the 837 is still 12 kw? and it can't be range rated below this.

An 838 will go down to 7.1kw. The 837 is the old model, so the OP may find the newer 838 is better for his system.