Cold water supply to WC and basin

[garfield59]

I am in the process of fitting a WC and small corner basin in what was an unused alcove upstairs in our house. I intend to use a small instantaneous water heater over the sink for hot and cold water. The electrics are being sorted by someone with the appropriate qualifications so no problem there, but he pointed out that according to the installation instructions, it is preferable for the mains cold water supply pipework to the heater unit to have nothing else on it or else it could cause a pressure drop and stop the heater working properly.
For example, at the moment, if someone uses the WC and then washes their hands, I assume the water pressure to the basin could be affected by the WC cistern refilling or will the fact that it is mains supply keep the pressure correct in both places?
To avoid this problem, could I use two separate feeds from the rising main in the loft i.e. one to the WC and one to the water heater?
Any advice welcome!
Garfield

[plumbbob]

If you have good water pressure in the rising main then it is doubtful the use of a toilet would reduce the flow to a significant degree. Any fluctuation in flow rates is going to be caused by a restricted incoming main so having a separate feed will make minimal improvement.

If you have an electric shower somewhere in the house try seeing how the flow changes if a toilet or tap is opened elsewhere.

Frankly, I would suck it and see.

[garfield59]

Thanks for the prompt response and apologies for not reading it sooner!

We don't have an electric shower and there is currently no water being used upstairs - kitchen, bathroom and WC are all downstairs.

The rising main goes up into the void below the bedroom floors and drops down into the bathroom and then on to kitchen. A separate leg goes up to the cold water tank in the loft.

Having read your reply I tried to see how good the existing flow was and so I left the kitchen sink cold tap running and then ran the bathroom cold tap, which is "upstream" i.e. nearer the main stop-cock to the house. There did not seem to be any difference in flow at the sink so I think I may be okay.

As you say plumbbob, suck it and see! Will let you know what happens.

Garfield

[garfield59]

An Update!

Well the WC has been in use for some time now and the possible flow issues I was worried about have not been an issue. There is no problem with flow to the basin when the cistern is refilling. Thanks plumbob!

However, not sure if it's a coincidence but since the new WC has been in place I have experienced water hammer throughout the house at different times. For instance, in the bathroom downstairs, the plumbing "bangs" once the cistern has refilled UNLESS I have a basin tap running at the same time. The same goes for the washing machine filling which never used to cause a noise.

Today I went up into the loft to see if the noise was coming from the new plumbing but all is well up there. It sounds like two pipes are knocking together in the void between the first and ground floors. I did wonder if I have reset the mains pressure too high when I put the water back on after doing the upstairs plumbing but I'm sure I made a note of the number of turns of the stopcock etc. I have heard of an anti water hammer valve that could be installed but it needs to be near the source of the noise and if that's somewhere between the floors that could be a problem.

Any advice welcome please!

Garfield