One thing that could cause this if the cold tap is supplied directly from the mains water and the hot tap from a storage tank (usually in the loft). Hence the two supplies would be at different pressures. With a Combi boiler it may be due to "resistance" in the heat exchanger in the boiler having the effect of providing hot water to the bath at lower pressure than the cold water.
It could be adjusted by fitting valves on the feed pipes to the taps then adjusting them until equal flow is attained. You could just put one on the cold supply but if this were then to reduce the cold flow to less than the hot when it was fully open you would then have the opposite problem to now.
It's a combi system, and I guess it would be the heat exchanger causing the difference as you say. I intend to have the boiler replaced and was wondering, being a complete rocking horse with anything tech :cry: , if flow determines in any way the boiler rating. Would actually installing a 35kw over a 30kw for example be of any value e.g. running a quicker bath?
[quote="htg engineer"]Check the boiler imnstruction it will tell you what flow rate the boiler will produce hot water at.
htg[/quote] I asked because a friend was telling me that my flow rate being what is it I would get no extra benefit from a 35kw with a 35 deg rise rate of 14 l/nin over a 30kw with a 35 deg rise rate of 12 l/min.
The boiler can only work as well as the mains to your house, if our cold mains flow rate is poor the hot water flow rate will be poor.
A 35kw boiler will have a greater flow rate than a 30kw boiler.
So 35kw boiler will give a flow rate of 14L/min - but if your cold water is only 10L/min - the boiler cannot increase that - how could it ????
You couldn't put 1 cup of water in a kettle then fill 3 cups with hot water could you !!
Don't forget, the flow rate will never be the same for hot and cold due simply to the extra distance the hot water has to travel let alone having to negotiate the boiler heat exchanger.
Obviously as Htg says, a bigger boiler will increase the flow providing your water main can shove it up the pipe. Whether ALL the difference between the rates is due to boiler capacity or simply plumbing is difficult to say. Do as Htg says and check the handbooks for the flow rates. You can download them from the manufacturers website.
The main point you have to check when buying a combi is the flow rate - for a decent shower and filling a bath.
If the flow rate isn't greater than the boiler can cope with it shouldn't take long for the water to reach temperature - normally 60-70oC is the maximum you will get - for safety reasons.
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by cercleforet » Sat Feb 21, 2009 10:23 am 
