Will an oversized boiler cause problems repeatedly going on and off.

[coldboiler]

I have been advised that the boiler I would like, a Worcester Bosch 34cdi will be over sized for my 3 bed roomed house with 1 bathroom.

The gas engineer says a Worcester Bosch 30si would be more appropriate. The 34cdi, he says, will be less efficient because it will heat up quickly to the required temperature and then turn off. Something to do with modulating, I don't really understand this.

I am concerning that I may not get sufficient hot water flow rates to fill my king size bath with a less powerful boiler and it will take a long time to run the bath.

Can anyone advise whether I am likely to encounter problems with the 34cdi or if I get the 30si will I notice any difference in the time it takes to run a bath?

 

[kozak1968]

Too big a boiler with only a few radiators can cause a problem with the return coming back to hot causing lock out. But new boilers can usually be range rated to cope with this. A more powerful boiler will cost more and deliver the same performance as the 30KW . You won't notice any significant difference. Except in your wallet on initial out lay.

 

[Deleted member 32524]

or go unvented. or an open vent boiler mate. Both will give you mains pressure at taps. Needs more space and money though.

 

[LPC]

A good compromise would be the 29CDi. The Si is a lower end (cheaper) boiler and not as good as the CDi's.

But if you want to fill a bath quickly you will need either a large combi (like the 34 CDi) or another solution such as an unvented cylinder (more £).

 

[doitmyself]

I have been advised that the boiler I would like, a Worcester Bosch 34cdi will be over sized for my 3 bedroomed house with 1 bathroom.

You need to measure the incoming cold water flow rate using a litre marked bucket and a watch. Do this at the garden tap, or at the kitchen cold tap if no garden tap.

The minimum boiler size is calculated from flow (litres/minute) X 2.5kW. A larger boiler will give you hotter water, not a faster flow rate.

 

[SafeGasInstall]

You could have 36 CDI if that is your water requirement .... Ch is not the problem here ! Think your plumber does not understand how combi boilers are sized up . Look for another gas fitter

 

[vern]

I have been advised that the boiler I would like, a Worcester Bosch 34cdi will be over sized for my 3 bed roomed house with 1 bathroom.

The gas engineer says a Worcester Bosch 30si would be more appropriate. The 34cdi, he says, will be less efficient because it will heat up quickly to the required temperature and then turn off. Something to do with modulating, I don't really understand this.

I am concerning that I may not get sufficient hot water flow rates to fill my king size bath with a less powerful boiler and it will take a long time to run the bath.

Can anyone advise whether I am likely to encounter problems with the 34cdi or if I get the 30si will I notice any difference in the time it takes to run a bath?

Get someone who's knows how WB boilers work! 34CDi 34kw on HW, 30Kw on heating which can be turned down via internal software to around 9kw if required, CDI CLASSIC is the best boiler they make!