Confused over boiler flow rates

[oldskoolskater]

I have almost decided that a Potterton Powermax HE will get fitted in my house, but then something struck me..... The boiler stats brag about delivering 48 l/min until the store is depleted so the 150l store would be emptied in 3 mins. Isn't this flow rate totally dependent upon the flow rate capacity of the 15mm pipes and the restrictions through the taps?? If the taps could only pass 30 l/min for example, would pressure not just build up in the pipes?

I'm not looking to start a war about conventional vs combi because I just don't have the space, so please don't tell me how daft I am for getting a combi.

Cheers

Steve

[Christian McArdle]

Yes. Your maximum flow rate will be determined by the plumbing and potential flow rate available from your mains water supply. If the mains supply, 15mm pipework and taps only pass 30lpm, then you will only get 30lpm from the system. You would, at least, get it for about 5 minutes, though.

I don't know what you mean by pressure building up in the pipes. The pressure in the pipes will be the full static pressure when the tap is off. It will drop when a tap is opened. The more it is opened, and the worse your mains supply and plumbing is, the more the pressure will drop.

Christian.

[Doctor Drivel]

It delivers what the mains can provide. If you have 48 litres/min then the bath will be filled in 3 mins. Best have a dedicated 22mm pipe from the stopcock (even if it is a 15mm mains pipe) to the Powermax. Just before the Powermax tee off for the cold to the showers(s). One line per shower. Replace the stop c*ck with a larger bore to reduce restrictions and/or renew the mains pipe to 25mm MDPE.

[oldskoolskater]

I replaced the mains into the house with 25m MDPE earlier this year for a different project and the cold flow out of the taps is phenominal now

• the current combi cannot keep up with hot water. Everything from the stopcock is 15mm though at present.

Regarding pressure in the system: My guess was that the Powermax delivers 48l/min by means of a pump (it cannot use gravity). I guess that this pump would not be running until hot water is drawn off and out of the store, so if the boiler was delivering 48 and the taps could only pass 30, there would be a build up of pressure in the system? Or am I talking cobblers? :)

Cheers

Steve

[Christian McArdle]

I'm afraid so!

No pump, at least on the DHW side. The water gets pushed through the mains, through the combi, through the pipes and through the taps. It will come as fast as it can be pushed. If the mains and plumbing could push 100 litres a minute, it would do so. The boiler's flow rate is just saying how much you can push through it before the water gets too cold to be usable. Some boilers actually had an electrically controlled valve to prevent the water going too cold by restricting the flow, although this is rare. I don't know about the Powermax.

Christian.

[Doctor Drivel]

As I said, have a dedicated 22mm pipe from the stopcock to the Powermax, and the only tee offs are the cold supplies to the showers just before the Powermax. At the stopcock tee off and have a line only to the cold outlets.

I can't think now, but the Powermax may have "balanced" a cold water oulet feed taken from the Powermax.

[Dave]

Doctor Drivel wrote: [...]

:-)

[Doctor Drivel]

Sweet FA was written

Shame. He went all shy.