Combi - time for HW to reach taps

[D_Hailsham]

There's usually no need to run the tap until hot water comes out as it will be too hot to use without mixing with cold.

Put the plug in before turning on the tap. Then you don't waste any water.

 

[EFLImpudence]

Thanks chaps but I'm still none the wiser.

What am I missing here?

It doesn't matter if you have a (relatively) small boiler or a large one or a nuclear reactor.

You have to run the cold water out of the pipe before the heated water can get there.

If that takes thirty seconds then it will take thirty seconds.

 

[shray]

Surely the quickest way to get "heated"water to the tap is to run the tap on full to clear the standing cold water in the run from the boiler to said tap ?!

Granted the faster the flow of the water, the cooler the water will be so I'm going to throw it out there that the the boiler KWage has very little relevance to how quick the "heated" water gets to the tap. It does however have relevance to the fact that the water can be run at a greater flow and produce hotter water.

 

[Alfie25]

Indeed, I know that you've got to drain off all the cold water sitting in the pipes and that the boiler size doesnt make any difference to that. My question is more to do with after the cold has been drawn off - assuming the tap is half or 3/4 open, does a higher kW boiler provide hotter water faster than a lower kW boiler? E.g. would a lower kW boiler provide lukewarm water for longer than a higher kW boiler?

 

[DP]

Alfie, the system is set up to provide designed flow rate for a specified size of boiler if the system set up is as specified in the manual.

You need to be more precise with your question as your setting of half flow at the tap makes no sense. What setting is the HW thermostat at? What flow rate have you got at the tap (remember half open tap for you might be a trickle for me when my tap is half open)? What is the cold water temperature? Is your boiler set up correctly? is the burner pressure correct? Is the flow rate as specified in the manual? TOO MANY questions to give you an answer.

All that can be said is, after the cold water has been purged, regardless of what the flow rate is upto the maximum rate of delivery specified for that boiler, HW thermostat determines what temperature is delivered at the tap maximum being 35 degrees plus the temperature of cold water at book value flow rate for that boiler.

 

[onlyfitidealboilers]

But that is not totally true either!

Most combi boilers have an inbuilt flow restrictor to ensure the flow does not become greater that the boiler can heat.

So in many cases the flow rate is higher with a larger combi and so the cold slug of water in the pipes will be cleared sooner.

But engaging pre heat if available will significantly reduce the waiting time!

Tony

Wat I find on my manufacturer calls is when a customer complains of dhw problems, I most always blame pipe runs stroke pipe volume.

How I explain it is, it is quite normal for the moment you turn on the hot tap to get a long slug of cool water then it will get warm to hot as this is the pre heated water from the boiler, then it will drop back to cool again as this is the slug of water that has passed through the boiler without being heated, and then finally you will get the hot water through


This hole process can take up to 30 secs or so.

 

[onlyfitidealboilers]

Surely the quickest way to get "heated"water to the tap is to run the tap on full to clear the standing cold water in the run from the boiler to said tap ?!

Granted the faster the flow of the water, the cooler the water will be so I'm going to throw it out there that the the boiler KWage has very little relevance to how quick the "heated" water gets to the tap. It does however have relevance to the fact that the water can be run at a greater flow and produce hotter water.

Spot on well said!

 

[onlyfitidealboilers]

Although the water does not get any hotter the higher the KW rating
As this is only set by the boiler thermostat and flow rates. Ranging from 40c to 65c

 

[dapoole]

Alfie, the system is set up to provide designed flow rate for a specified size of boiler if the system set up is as specified in the manual.

You need to be more precise with your question as your setting of half flow at the tap makes no sense. What setting is the HW thermostat at? What flow rate have you got at the tap (remember half open tap for you might be a trickle for me when my tap is half open)? What is the cold water temperature? Is your boiler set up correctly? is the burner pressure correct? Is the flow rate as specified in the manual? TOO MANY questions to give you an answer.

All that can be said is, after the cold water has been purged, regardless of what the flow rate is upto the maximum rate of delivery specified for that boiler, HW thermostat determines what temperature is delivered at the tap maximum being 35 degrees plus the temperature of cold water at book value flow rate for that boiler.

Sorry DP to hijack the thread however I've noticed that you've worked on Vokera Linea boilers before and was wondering if you'd be able to help me as you're in the Glasgow area. I've sent you a Friends request as I cannot message you otherwise. I've already had an engineer out to check our boiler but I'm unsure that he knows how to correctly diagnose and resolve the problem and he's now suggesting I now get the PCB changed. This after he's already changed the main knob after originally saying that this was the problem. Please can you message me? Thanks.