hot 22mm pipe

[Dannyparty92]

Had our opened vented hot water cylinder swapped for a combi boiler. The DHW pipe from the boiler is in 22mm all the way to its final draw off which is the bath. since the swap the flow at the bath hot water tap is poor. I thought it would be much better flow as the hot is now running off the mains. Thanks in advance for any anwsers.

 

[payney1974]

Depends if your mains are any good mate.

 

[payney1974]

Don't forget a combi has to heat the water as it passes through the heat exchanger, if it flows through too fast the water won't have enough time in the heat exchanger to capture enough heat.


With your old cylinder you had a mass of hot water that can flow as fast as the tap will let it.

 

[kirkgas]

The original flow was gravity and the pressure would be 0.1 bar per metre height from the water level in the loft tank down to the tap, at a temp of 60 degrees, the flow is now mains and determined by what is coming in, added to that is the flow restriction required to maintain a decent temp rise, some combis deliver 10-13 litres/min at a temp rise of 35 degrees so its easy to see where people aren't happy with the result, a point which should be fully discussed by the installer before the job is done

 

[StillLearnin]

Check the specified flow rate of the combi (in it's manual) and then check at the tap that it is somewhere around this

 

[Dannyparty92]

so its pretty much the heat ex in the combi and the oversized 22mm pipe thats causing the flow to be poor?

 

[Mike Jackson]

What size combi is it?

 

[Dannyparty92]

24kw mike.

 

[buckley plumb]

24kw mike.

most 24kw boilers are restricted to 8 litres to allow 40 degrees temp rise

 

[Dan Masey]

What do people mean by a temp rise? As in the temp of the cold mains to the temp at the hot tap?

 

[Dannyparty92]

Yes mate as it goes though the heat ex.

 

[payney1974]

Its normally a 35 degree rise i.e if cold water is at 12 degrees boiler will heat that water to 47 degrees if the flow rate and gas rate are correct

 

[Mike Jackson]

You'll never get a good flow at a bath tap with a 24Kw combi. Only really suitable if you are showering.

 

[Dannyparty92]

So the heat ex acts a flow restrictor mike?

 

[Mike Jackson]

No, there may be a flow restrictor in the inlet, depending on the boiler, but the heat input is insufficient to give a decent flow rate. 24 Kw will give about 9 l/min at a 35°C temp rise. The old gravity system would give a much higher flow rate at a temp of 60.

 

[Dannyparty92]

There must be a flow restrictor then. It's a woster 24 I junior. I understand what you are saying but if there wasn't a flow restrictor then I could get good flow but it would be cold lol. Would downsizing from 22 to 15 help?

 

[Mike Jackson]

The Worcester has a flow restrictor in. Reducing the pipe size won't help the flow rate but will mean the dribble of water will get hot quicker as you've got less cold water to push out first.

 

[Dannyparty92]

Cheers mike.

 

[buckley plumb]

you coukd try one of those combi saves things not used one myself others may advise - it restricts the flow until the boiler temp is up