Worcester Bosch Combi

[Pete]

I have one, and I find it quite difficult to get the flow right to give optimum amount of hot water.

Would it be worth putting an in-line valve on the cold inlet to the boiler and then adjusting it such that I could set it to the flow which would be optimum temperature when taps turned on full? ie it would limit the flow to a max value which would produce hottest water.

If yes - could I adjust by trial and error or could I adjust and set to a known litres/minute to get the optimum flow. How could I find out what that optimum flow would be - I guess it would be part of the spec of the boiler?

[Andy]

Won't work. The groundwater changes temperature over the year by >10C. Why don't you just adjust the DHW temperature dial on the combi on an occasional basis - I adjust it every so often during the year on my Vaillant to keep the water around the 50C mark.

Andy.

[Pete]

But if it was done in the winter then this would be worse case scenario wouldn't it?

[John Rumm]

Yes this can work (although obviously you need to adjust the flow restriction as the groud water warms up in the summer). You may find the boiler already has a service valve on the cold inlet that you can partially turn off.

You could do either.

Look up the power of the boiler, and see if that is input power or output. If it is input then look up the efficency of it on the SEBUK database and reduce the actual power by the precentage indicated.

That will give you a number of kW - or 1000s of Joules per second that the boiler can put into the water. Divide that by 4200 and by the desired temperature rise, to get the number of kg (i.e. Litres) per sec the boiler can heat to that temp, then multiply by 60 to get litres per min. You can then set the flow rate with a bucket and stopwatch.

(personally I would go with the twiddle until about right technique!)

Note that if you set the flow a little too low it does not matter too much since the water will be hotter, and you just add more cold instead.

[<me]

Excellent - good man, I will give it a go.

[Ed Sirett]

The first things to remember is that the DHW temp control on a combi is a MAXIMUM HW temp. If you draw water faster than it can heat you simply get more lukewarm water. If this is your case consider yourself lucky, this situation can be fixed, the opposite problem is a PITA to overcome.

Ideally you want to set things so that turning any HW tap on gives the right sort of response. This may mean having to restrict the flow especially on the kitchen sink HW tap. You will probably find that the bath needs the highest flow/lowest temp so any restriction you add must not over do it for that tap.