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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Plumbing Questions >> Subject: Mixer Shower Running Cold When Hot Water https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1483472323 Message started by DC4_Plumbing on Jan 3^rd, 2017, 7:38pm

Title: Subject: Mixer Shower Running Cold When Hot Water
Post by DC4_Plumbing on Jan 3^rd, 2017, 7:38pm

Help!

Again we are having issues with our Mixer Shower, how can we fix it?

By lowering the Hose Shower Head the Shower Pump starts and once the cold water in the hot pipes has passed the water temperature selected is pumped out ok, at the Varispeed selected.

Unfortunately after an unmeasured short time the sound of the pump changes and the water being pumped out is just the cold from the Gravity Fed Tank.

Again after waiting an unmeasured amount of time the Shower Pump may again pump the water at the temperature selected.

• Is this a common fault with this type of shower?
• Is this a fault that is easily resolved?
• Is it simply time again for a new Shower Pump?

DIY Installation Details:
• Mixer Shower DIY Installation Date: November 2011
Upon installation the original EasiBoost Water Pump worked straight away without having to lower the Hose Shower Head.
• Bristan Varispeed Water Pump Installation Date: June 2013

• Shower: TRITON, Capella, Thermostatic Mixer Shower.
• Pump: Bristan Varispeed Water Pump 50.
• Water Level within the Header Tank is more than 230mm.

• Further information available upon request.

Kind Regards,

Dan

Title: Re: Subject: Mixer Shower Running Cold When Hot Water
Post by CWatters on Jan 8^th, 2017, 7:57am

I'm not an expert on these but since you haven't had a reply so...

Having to lower the shower head suggests there is an issue with the flow sensor that starts the pump. These typically need a minimum flow rate to be achieved before they switch on. Perhaps it was marginal to start with and scale has reduced it to where it's not quite adequate? Lowering the head increases the flow rate just enough to trigger the sensor and the pump starts.

Double check the physical layout/height of the shower head, hot and cold tank and pump meet the specification for the pump. Ditto pipe diameters and lengths.

In some situations you need a "negative head pump".

Title: Re: Subject: Mixer Shower Running Cold When Hot Water
Post by DC4_Plumbing on Jan 8^th, 2017, 5:42pm

Hi,

Thank you for your previous feedback.

As far as I remember the Hot Water Tank is an indirect type with a Pipe Coil within it, located within the airing cupboard behind the Shower at the same floor level.

The Cold Water Tank feeding the Hot Water Tank and the Cold Water Feed for the Mixer Shower is within the loft immediately above the Shower.

The distance between the highest part the Shower and the water level in the Cold Water Tank is approximately 390mm.
110mm From the top of the shower to the ceiling.
130mm From ceiling to the bottom of the cold water tank.
150mm From ceiling to the bottom of the cold water tank. (I decided to measure it).

On several occasions I have thought that we may need a ‘Negative Head’ Shower Pump but when reading the installation manual the ‘Negative Head’ is only required when the level of the cold water tank is less than 230mm to the top of the Shower.

Unfortunately the water pressure from the Cold Water Tank in the Loft and Hot Water Tank isn’t that good.

From what you have said can I assume that this will have an effect on the flow sensor of the Water Pump?

The Bristan Varispeed Water Pump 50, has a 15mm Feed on the Cold and 15mm Feed Reduced from the 25mm Pipe from the Hot Water Tank.
The Varispeed dial on the top of the Pump is currently set to less than half.

If this issue is likely to be resolved using a Mixer Shower Pump with a ‘Negative Head’ are you able to recommend a pump?

When searching online I have found them to be very very expensive.

Please list any further questions and I will endeavour to get you the answers.

Thank you in advance for your feedback.

Title: Re: Subject: Mixer Shower Running Cold When Hot Water
Post by CWatters on Jan 9^th, 2017, 5:31pm

The Bristan manual says the flow sensor needs a flow rate of at least 1.5L/min to start the pump. Check this is being achieved at all mixer temperature settings with the shower head on the hook (eg so the pump doesn't start).

If it's on the low side check/clean the inlet filters on the pump and try again. Perhaps do this anyway?

If it's over 1.5L/min and the pump doesn't start reliably then it could be a faulty flow sensor on the pump.

Edit: Have also seen a suggestion that if the hot water temperature is too hot it can cause cavitation in the pump (and lack of hot water delivery). Perhaps check it's not more than 60C.

Title: Re: Subject: Mixer Shower Running Cold When Hot Water
Post by DC4_Plumbing on Jan 9^th, 2017, 9:14pm

Thank you for your feedback and support again.

I should be ok checking the Filters.

But not being a Plumber not that I need to be a Plumber I am not sure how to test the Flow is at least 1.5L/Min?

With regards to the Temperature I can run some water off in to a container and measure the temperature with a thermometer.

Title: Re: Subject: Mixer Shower Running Cold When Hot Water
Post by CWatters on Jan 10^th, 2017, 11:23am

Quote:

But not being a Plumber not that I need to be a Plumber I am not sure how to test the Flow is at least 1.5L/Min?

I would set the mixer to full hot. Collect the water from the shower head in a kitchen measuring jug. Lets say you collect 2L in 30 seconds. That would mean the flow rate was 4L per min. Repeat with it on full cold.

You want to do this while it's demonstrating the fault condition (eg the shower head is on the hook and the pump isn't running).

Quote:

With regards to the Temperature I can run some water off in to a container and measure the temperature with a thermometer.

Start by checking what the thermostat on the hot tank is set to.