Weak Pressure - not Bladder

[pt44]

Hi

Hope someone can pass on some help here. I've just installed a thermostatic bath/shower mixer made by Crosswater. Its one of their cheaper models. I've installed it twice before and on one job the pressure was a little bit weak through it, which I put down to the hot being gravity fed with the tank not being very high up. However, I've just installed one today in a flat which has a combi boiler and my first impression is that the pressure/flow through the mixer is pretty poor.

The cold pressure is 3 bar. The boiler is an old Potterton Puma 80. I measured the hot pressure and my gauge said it was also 3 bar - however, the flow of the hot is much less than the cold, so I don't know how its pressure can be the same? My common sense says it can't be, but that's what the pressure gauge was showing. Weird.

My presumption is that because the thermo valve cannot get enough hot water fast enough, it therefore restricts the flow of hot and cold - as they have to mix. This reduces the overall flow. Is there anything I can do about this?

Can I add a pump to the hot or both? I don't really understand how the pump would work - would it pull too much water - more than the boiler can produce. Or would the boiler simply keep up. Is the slow running hot due to anything wrong with the boiler - or is it just that the boiler is old. Would a new boiler help?

Sorry for all the nooby questions. I understand a lot of things plumbing - and I'm no idiot - but this is beyond my knowledge level. So any help would be much appreciated.

Paul

PT Home Improvement | For All Your Home Improvement Needs

 

[helpsy]

You can pump from a tank but not off the mains.

 

[bobpape]

You mentioned you can not understand how pressure can be the same but the flow different,

One way of explaining why is by looking at the issue another way.

Water pressure in a pipe will equalise upstream and downstream of an obstruction
provided the downstream end is plugged.

Remove the plug and whilst pressure will drop upstream and downstream of the obstruction the flow out of the unplugged end will only be what the obstruction will allow,
and the pressure downstream of the obstruction will be less than upstream.

Different pressure upstream and downstream of an obstruction is called "pressure differential"

Pressure and flow should be thought of as seperate entities,

Obstruction can be a valve, stopcock, boiler heat exchanger, dented pipe etc.

 

[justlead1]

Hi. Pumps must not be used on mains supplies. And as Mr Page says there is a big difference in the dynamics of water when moving against static. Pressure is the energy that pushes the water through the pipes but it is used up by the resistance of the system (walls of pipe, fitting, heat exchangers, etc) In your case the hot supply has more resistance than the cold supply. You could try and restrict the cold to the shower, in an effort to balance volume at point of delivery. Check out "Hydraulic gradient" this will help to explain the reasons these problems exist. Good Luck

 

[pt44]

Hi

Thanks for the answers so far. To clarify. No pumps on mains supply - does this also mean I cannot pump the hot feed from the combi boiler?

The pressure check I did was from a point further away than the bath. I will check today, directly off the bath hot and cold feed.

Surely turning down the cold feed will only mean a reduced total flow and in fact this is what the thermostatic mixer is doing itself internally.

Paul

 

[bobpape]

You are experiencing one of the inherent problems with Combi Boilers,
There is nothing you can do about it except change the system by installing a CWSC and a Hot Cylinder, you could retain the Combi Boiler but pipe as you would a conventional wet system,

 

[helpsy]

Hi

Thanks for the answers so far. To clarify. No pumps on mains supply - does this also mean I cannot pump the hot feed from the combi boiler?

The pressure check I did was from a point further away than the bath. I will check today, directly off the bath hot and cold feed.

Surely turning down the cold feed will only mean a reduced total flow and in fact this is what the thermostatic mixer is doing itself internally.

Paul

The hot supply is off the mains cold supply, just travelling through the heat exchanger in the boiler. So you cannot pump the hot supply.

 

[justlead1]

Hi pt44 The standing pressure will alway be the same when water is not flowing. Volume is the issue here. Combi boilers are only second generation too over the sink Ascots or Mains Sevens water heaters. Which delivered hot water albeit slow to fill a bath but for poor house holds better than boiling a kettle or firing up the copper. Much the same a combi's, however showers will work well providing they do not require a large volume of water. I am not famiuar with the mixer you have fitted, but it may be worth fitting a smaller shower rose (say from an electric shower) just to see if it mixes/ performs any better.