Vaillant ecoTEC 824, 831 or 837 for small Victorian house?

[DavidSK]

...and is it inefficient to over-specify a combi boiler?

I've read the FAQ No 34, which implies either the 831 or the 837 would work, but which would work best?

We are replacing the two rather old boilers (separate heating and hot-water) with a single combi boiler. I've settled on a Vaillant ecoTEC Plus boiler with a VRC430. However, I would sincerely appreciate advice on which ecoTEC Plus boiler is most appropriate/efficient for my house. The Energy Saving Trust's online calculator gave a figure of 18kW as the requirement for the boiler, which appears to sit within the ranges of all three of the ecoTEC Plus boilers:
824: 6.7-19.0kW
831: 8.7-24.0kW
837: 12.0-28.0kW

We've been told by a family friend (who is a retired plumber and couldn't comment on the latest Vaillant boilers) that we want a hot-water flow rate of at least 11 litres per minute, which discounts the 824 (@9.4 L/m). And given that there's only about £30 difference between the 831 and the 837 I would instinctively go for the higher rated model. Is this sensible? Or would that not leave the boiler enough “flexibility” to cope with anything but the coldest days and be less efficient?

As you can tell, I know very little about CH (this being my first house – very steep learning curve). Ultimately I will defer to my plumber as to which model is best, but I would like to have a (semi) informed idea as to what I want/need and why.

Thanks in advance for any help you can give.

Regards,
Dave

House details:
-Built in 1900
-End terrace
-solid brick walls
-2.5 bedrooms
-Small two-up two-down layout with small rear-extension
-Slate roof(s) with aprox 70mm insulation
-Sash windows
-6 radiators
-Thermostatic valves on all radiators
-1 bathroom (bath and separate shower [not power shower]) on ground-floor. Not unusual for both the shower and the bath to be run at the same time.
-2 occupants (happy with their first house, even though they need to replace the boilers and possibly rewire the entire house)

-Old heating boiler: Ideal W2000
-Old hot-water boiler: Main Medway

 

[simond]

Not unusual for both the shower and the bath to be run at the same time.

If you intend to run the bath and shower at the same time, you can rule out any of the boilers you have mentioned. Only a fandango combi like the ACV Heatmaster could manage this.

There is no substitute for getting a learned heating specialist round for advice. Your research has sent you down a blind alley I'm afraid.

 

[DavidSK]

[If you intend to run the bath and shower at the same time, you can rule out any of the boilers you have mentioned. Only a fandango combi like the ACV Heatmaster could manage this.

There is no substitute for getting a learned heating specialist round for advice. Your research has sent you down a blind alley I'm afraid.

Thank you. I now have another avenue to research. I've had two CORGI registered plumber quote so far, one specified a Worcester Bosh Greenstar 28i Junior, the other specified "a Vaillant combi". Our present decrepit boiler (situated in the bathroom) almost copes with both bath and shower being run at the same time, I had therefore - mistakenly - assumed that a modern unit would cope easily. This isn't a facetious question, I have never had to think about CH/DHW before; but does this mean that the Vaillant ecoTEC Plus boilers (and presumably the equivalent from other manufacturers) are only ever capable of supplying one hot-tap at any one time?

 

[JohnD]

does your old system have a cylinder?

If you are thinking of getting a combi, put a bucket under the cold tap at the kitchen sink and see how many litres per minute it delivers. this is the maximum flow (hot and cold combined) that you can get from a combi since it has no storage and relies on the water flow coming from the main. If you try and divide that between a bath and shower, you will see it is quite a meagre flow.

 

[simond]

I'm sorry, I did not see that you had an Ideal W2000 AND a Main Medway.

A Main Medway will not run two items successfully together.

I guess it is all down to customer perception. Most customers would argue that a Main Medway is not suitable for two HW appliances together, you can of course get them both to 'work' by turning the taps right down.

If you have been happy with a Medway, then an ecoTEC 831 or 837 will give a slightly enhanced performance to this, subject to your water supply being up to it. The 837 is a fair bit more than a £30 step though.....

 

[Deleted member 105431]

Personally I'd say get the Vaillant you can afford, the higher output the better. Give the Worcester a wide birth as they are having big probs with the aluminium heat exchangers at the mo.

 

[gas4you]

The 837 is a fair bit more than a £30 step though.....

And about 3" deeper off the wall, so may not suit all installation positions.