Discuss Something wrong with the bathtub mixer tap in the General DIY Plumbing Forum area at Plumbers Forums

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Hi All,
I hope you are doing well. I have this old-school bathtub mixer tub.

I think there is something wrong with it.
When I pour cold water then I have to wait for hot water twice or 3 times longer like cold water (higher pressure) forces itself into the hot water pipe otherwise how to explain this 20-30 seconds lag?
Please advise
 
Anyone got any advice to offer?

We had one of these in when I was growing up, it was always really temperamental!
 
thank you Lou. This is quite strange. I don 't know if this is typical for these older mixer taps or it is broken in some way.
I reckon it's to do with much higher cold water pressure comparing to hot water.
This lag is too long so it must be cold water pushes itself into hot water pipe. I think the lag is well over 30 seconds.
 
Mains pressure on the cold side ?
 
I think it's probably a design flaw; the outlet path isn't large enough to allow unimpeded flow from both H & C.

For example, if the maximum flow from the outlet is 10 litre per minute (lpm) and the supplies can individually deliver 10 lpm then when hot an cold are mixed 50:50 to provide "warm" each will be contributing 5 lpm to the total. This means it will take twice as long before the outlet temperature gets warm.

You can test this theory by measuring the flow (with a jug and stopwatch) from the tap when running (a) hot only, (b) cold only and (c) hot and cold together.

The workaround is to fill a bath by opening just the hot valve first, waiting until it starts running hot (ca 60°C) and then opening the cold valve enough to mix the outlet temperature down to ca 40°.
 
Try stopping the flow with the palm of your hand
If you can't, then it probably mains.
If cold is mains (high pressure) and the hot is gravity (low pressure) then this could be the cause.
A more modern mixer should fix this.
Otherwise, make both supplied the same pressure.
 
Try stopping the flow with the palm of your hand
If you can't, then it probably mains.
If cold is mains (high pressure) and the hot is gravity (low pressure) then this could be the cause.
A more modern mixer should fix this.
Otherwise, make both supplied the same pressure.
right so I change to a more modern mixer otherwise I would need a water pump to boost the hot water pressure.
 
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