My boiler hot water flow rate is slow

[Justice League]

Vaillant Ecotec Pro 28

Running 7 radiators.

Outside Tap Cold water flow rate = 12l/s (it took 5s to fill 1l pot)

Kitchen Tap - just running cold water = 4 l/s

Bristan Shower Mixer - cold water rate = 12 l/s Hot water rate is really low

The boiler heats up the water fine -its nice and hot but the flow rate is really poor - so I don't know whether that is a fault with the taps or the boiler.

The radiators heat up just fine and within 10-15 mins are toasting hot.

So can I increase the flow rate on the boiler or is this some problem with it?

 

[John.g]

You should get ~ 11.5LPM at 45C assuming cold mains at 10C with a 28kw combi.
Can you/did you measure this flowrate. (should get ~ 8LPM at 60C.)
You could also switch off the boiler, turn the shower mixer to max temp and see what the flow is and also a hot tap on its own.

 

[Justice League]

You should get ~ 11.5LPM at 45C assuming cold mains at 10C with a 28kw combi.
Can you/did you measure this flowrate. (should get ~ 8LPM at 60C.)
You could also switch off the boiler, turn the shower mixer to max temp and see what the flow is and also a hot tap on its own.

I am going to put the shower thermostat to 60 and put it on full and measure the flow rate.

If the problem is the boiler, what could it be?

 

[Justice League]

Are my filling loops in the correct position?

They are both closed position.

 

[John.g]

I am going to put the shower thermostat to 60 and put it on full and measure the flow rate.

If the problem is the boiler, what could it be?

If you have poorish mains pressure then the best way to get the max flow out of the shower mixer is to have the HW set to ~60C you will then be mixing water at 60C with water at 10C (to give 11.5LPM at 45C) from two different pipes (even though both are fed initially from the mains). If you have the HW temperature set to 40/45C then all the HW is from the combi and the flow may be lower.

Both filling valves should be closed after boiler top up, pressure should be ~ 1.5bar when cold.

 

[Ric2013]

What strikes me is that cold kitchen tap is 4l/m but outside tap is 12l/m (I'm assuming 'l/s' is a typo).

Could there be a restriction on the cold between the main stopcock and the kitchen, thus limiting flow to kitchen tap (and boiler inlet and hence hot water output if also in kitchen)? Perhaps an isolation valve or a blocked inline filter?

 

[Justice League]

What strikes me is that cold kitchen tap is 4l/m but outside tap is 12l/m (I'm assuming 'l/s' is a typo).

Could there be a restriction on the cold between the main stopcock and the kitchen, thus limiting flow to kitchen tap (and boiler inlet and hence hot water output if also in kitchen)? Perhaps an isolation valve or a blocked inline filter?

Yes its a typo. Its 12l/m.

I am going to look where the outside tap for the cold water gets its feed from - I think its from the same pipe that feeds the kitchen sink as the outside tap is on the kitchen wall.

When I turned the hot water on for 5 minutes the flow pipe from the bottom of the boiler got hot - but this was the pipe as it connects to the boiler - a foot further down the pipe was cold. I assume the top of the pipe was just hot from the residual heat from within boiler?

My radiators did not get warm or should I run the hot water for longer to see if they do?

 

[Ric2013]

Yes its a typo. Its 12l/m.

I am going to look where the outside tap for the cold water gets its feed from - I think its from the same pipe that feeds the kitchen sink as the outside tap is on the kitchen wall.

When I turned the hot water on for 5 minutes the flow pipe from the bottom of the boiler got hot - but this was the pipe as it connects to the boiler - a foot further down the pipe was cold. I assume the top of the pipe was just hot from the residual heat from within boiler?

My radiators did not get warm or should I run the hot water for longer to see if they do?

Thermal expansion as you are heating the primary, after all? <1' of pipe? I don't think this is related to the poor DHW flow.

Yes, thanks, see what's between the outside tap (which has reasonable flow), the boiler, and the kitchen sink.

Is the flow from the kitchen tap that poor because of the design of the tap itself, I wonder? Seems unlikely - I'd be expecting 4l/m from a kitchen mixer if it were running from a gravity hot system, so to have 4l on the cold (mains) side is surprisingly low.

 

[Justice League]

Thermal expansion as you are heating the primary, after all? <1' of pipe? I don't think this is related to the poor DHW flow.

Yes, thanks, see what's between the outside tap (which has reasonable flow), the boiler, and the kitchen sink.

Is the flow from the kitchen tap that poor because of the design of the tap itself, I wonder? Seems unlikely - I'd be expecting 4l/m from a kitchen mixer if it were running from a gravity hot system, so to have 4l on the cold (mains) side is surprisingly low.

There is nothing between the outside tap and the kitchen tap - its the same cold feed which is about 8m run from the boiler.

So is it best to assume the kitchen tap needs replacing?

 

[Ric2013]

I'm wondering if you have very poor mains pressure and it isn't enough to force water through a (presumably) restrictive modern tap designs/the boiler secondary heat exchanger? The flow sounds okay (albeit not great), but perhaps as soon as it hits a bit of resistance, the flow drops because of a lack of pressure. But that is inconsistent with having a pretty decent cold flow through your shower.

Could be a partial blockage on the pipework going to your boiler and the cold kitchen tap, or two separate obstructions (in the tap and in the boiler heat exchanger?). But, at this point, this is only speculation and if I were there I'd want to test these theories, otherwise it's just clutching at straws.