Poor HW flow from Combi boiler

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We’ve got a Vaillant 37kW Combi which I choose to supply a good shower performance but have not been impressed especially as we’ve converted the loft into a master bedroom with en-suite shower.

When we had the boiler installed the mains flow was around 20 l/min but recently the Water Board has done some major mains pipe work in our area and since that was completed the flow has dropped to about 13 l/min at the tap close to the boiler.

I’ve measured the hot water flow at the tap closest to the boiler and that is only 8 l/min. This flow reduces to under 5 l/min. in the en-suite which gives a very poor shower.

Should the hot water flow reduce by 5 l/min through the heat exchanger for the hot tap which is only 2 metres from the Combi? Do I have a problem? BTW, the heater plate was replaced six months ago.

Should the flow to the loft also reduce by the amount measured?

Richard
 
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It will reduce depending on the shower output capabilities. Is there a kink or restriction in the hose or feed to the hose?
 
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The flow measurements I’ve taken have all been from taps.

Richard
 
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Two things to check : constrictions and height. Make sure all stopcocks, isolating valves and taps are full flow. Many isolationg ballvalves ( quarter turn) have a smaller hole than the 15mm pipe. There is your main stopcock,maybe another isolating valve at the boiler and then another before the tap. Maybe even your footpath valve or meter has not been fully opened. Asfor height, there is bound to be a drop of pressure and flow in your loft. 1 bar pressure is 33ft of water (10 m actually) and mains pressure is typically 2.1 bar so you may well be halving the pressure in you loft. However you can improve flow by ironing out the constrictions and bends.
 
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Yes, I have been working round the pipes to ensure all valves etc are fully open.

Take your point about water head. The shower is probably about 20ft above the boiler outlet so I could be losing about 0.6 bar but how does pressure affect flow?

But should I lose almost 5 l/min flow through the heat exchanger of my combo boiler? Note; this is being measured at the sink taps which are within a metre of the boiler.

I have checked other hot taps downstairs and they show the same flow and also the same figure in the bathroom on the first floor.
 
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below the boiler you have two plastic valve controls that you open and close when topping up the pressure, remove the screw in the centre of the cold water inlet one and use a spanner or wrench to make sure it is indeed closed as the plastic swells and even though it says closed it might not be properly closed
 
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Checked and fully closed. How would that affect the flow?

Richard
 
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Checked and fully closed. How would that affect the flow?

Richard
Because it diverts the water that comes out of your tap to fill the boiler, if it is not closed properly it would restrict the flow of water entering the plate heat exchanger
 
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where that valve is, the fitting before that is a double check valve, it could be that when they were working on the external water mains some debis has got into your supply and is partially obstructing the check valve
 
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The external water mains work was done when the previous boiler was in situ. Changed to the Vaillant about 4 years later.

Richard
 

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Did they renew the filling loop?

We’ve got a Vaillant 37kW Combi which I choose to supply a good shower performance but have not been impressed especially as we’ve converted the loft into a master bedroom with en-suite shower.

When we had the boiler installed the mains flow was around 20 l/min but recently the Water Board has done some major mains pipe work in our area and since that was completed the flow has dropped to about 13 l/min at the tap close to the boiler.

Information is misleading, as you didn’t indicate when the boiler was installed in relation to the water mains repairs.
 
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How long has this problem been occurring, and how long has this boiler been installed?
Has anything else changed, been worked on, replaced or altered?

but recently the Water Board has done some major mains pipe work in our area

Recently or many years ago?

The external water mains work was done when the previous boiler was in situ. Changed to the Vaillant about 4 years later.
 
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Sorry, you were asking about the major work carried out in our area by the Water Board. This was done about two years ago. This was when we started to notice the drop in flow from taps and showers. I remember having a water problem about six years ago can’t remember what it was all about but I do remember that they tested the water flow and it was about 19 l/min. Now I only measure about 13 l/min, which I guess is not bad it’s just the 5 l/min drop across the heat exchanger which was replaced last year.

Richard
 
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