I have just fitted a brand new Worcester 24 Si combi boiler, and the
CH seems to work fine, but the DHW seems to have a 'fixed' flow rate
of about 7.7 L/min, whether the output temp chosen is 40c or 60c, and
whether the incoming supply temp is 15c or 20c (haven't had cold
enough weather to test it at say 5c yet); in other words, if the
incoming water temp is 15c and i request DHW at 60c, it flows at 7.7
L/min, and if the incoming water temp is say 20c and i request DHW at
only 40c, it flows at............you guessed it 7.7 L/min !!...so to
recap, whether the boiler has to raise the temp by 45c or 20c, it
always seems to flow at the same rate (7.7 L/min).
by the way, this is the first time i've fitted central heating, so i'm
still learning and would appreciate all the help i can get
many thanks to anyone who can help.
The "rate" of flow is dependant upon the incoming pressure and the flow
restrictor (usually in the cold inlet line at the flow detector switch.
Plus any other restriction due to pipe lengths, type of tap in use,
stop-tap opening etc. Your boiler temperature setting causes the flame to
modulate up (to a maximum) or down from there to give the temperature you
have asked for at the same flow rate for each. Are you perhaps thinking it
is an electric shower where a higher flow gives a lower temperature?
On Sun, 10 Aug 2003 12:13:44 +0000 (UTC), "John"
i suppose what i thought was that if i asked for a low(ish)
temperature output of say 40c on a hot day (with, say a cold input of
20c), then the boiler would respond by 'upping' the flow rate as the
requested temperature rise is easily achieved. (my incoming pressure
gives 24L/min, by the way).
but i take it the boiler just doesn't have the capability to vary the
flow rate (other than to restrict it (to 7.7L/min in my case)), which
brings me to the question: why do boiler manufacturers even mention
the different flow rates a particular model could handle, if the
restrictor is not adjustable (apart from removing/replacing it)
The restrictor is only one of a number of resistances to the flow all of
which are in series. The size of the pipe from the mains is number one, the
degree of opening of the stop tap is number two, the size and length of the
pipe and number of bends between the stop tap and the boiler is number
three, the setting of any service isolation valve on the boiler supply is
number four, the flow restrictor is number five, the size and length of the
discharge pipe between the boiler and the tap is number six, the size and
setting of the tap is number seven. In conjunction with these resistances
you must consider the pressure from the mains under dynamic (i.e. running)
conditions to result in an actual flow rate. I suspect that since you say
you can get 24litres/minute from the supply this may be at a tap near to
the stoptap without the house pipework restrictions from number three
onwards. Of course it might just be that some mains borne crud has become
lodged in the flow restrictor or a strainer near it and all it needs is
taking out and cleaning.
Look at the specs. I think this combi raises the incoming main cold water
by 35C to give aprox 10-11 litres/min. The instructions should indicate
where the flow restrictor is to up the flow.
BTW, the mains temp is still low despite the hot weather. It takes about 3
months for the air temp to affect the ground temp at 700mm and below the
It is true that the flow rate is pretty well set for the combi, with the
only factor affecting it is the pressure of the mains itself.
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