Pumped shower problems.

10 Feb 2014
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United Kingdom
I have a shower mixer that is above the water level in our cold water tank. This is in a loft conversion. Cold water is fed to to a twin booster pump fitted below the cold water tank. The hot water is fed to the booster pump from a point at the top of the hot water tank through a special flange. This flange is fitted above the inlet to booster pump.

When in operation the shower water rapidly runs cold (2-3 minutes) and the hot water pipes from the pump also go cold. The thermostatic shower mixer valve has been replaced without change.

Any ideas?
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It sounds like your running out of hot water, how is your cylinder heated electric immersion heater or boiler ?
It sounds like your running out of hot water, how is your cylinder heated electric immersion heater or boiler ?

The tank is indirectly heated from a gas boiler. The input side of the pump remains hot. It is only the shower head side of the pump that goes cold. Although both input sides of pump (hot & cold) are under the same head (the cold water tank). I believe there is a differential pressure difference at the mixer that causes cold water to go back into the hot side of the pump.

What do you think?


John Dent
What is the make of the shower valve.
What pump is it make and model, is it a positive or negative head pump.
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What is the make of the shower valve.
What pump is it make and model, is it a positive or negative head pump.

The pump is a QT80 supplied by Techflow Products that is I believe part of Stuart Turner Pumps.
The Shower Mixer is is from Armitage Shankd a Trevi Thermostatic Shower Mixer No 320. I have already changed the mixer valve in case this was causing a problem.
A simplstic view is that the pump suppliers water from the same head but goes different routes. Can these routes create a pressure differential that is causing this problem. Tests show that the hight of the shower head is critical. When at the same level as the mixer valve there is no loss of temperature.


John Dent
You have the wrong pump fitted.

Your pump is a positive head pump, it will not pump water above the top of the water level in the cold water roof tank.

You need to change the pump for a negative head pump

this is the replacement for yours


This is the pump I fit


The reason the shower is going cold is because there is a built in bypass in your existing pump, as a positive head pump will not pump above the height of the cold water level, cold water is passing through the bypass into the hot supply.
There is probably more restriction on the hot pipework than the cold and water will find the easiest route.

Hence when you lower the shower head below the water level you get hot water.

Hope this helps Steve
Hi Steve

Many thanks for your help. I understand what you are say but I understood that the QT80 was a Negative Head pump.

I have had rthis system since 2005 and it has never worked properly. I have always had the same problem. Hot for 3 to 4 minutes.

I have checked back to my original oder in 2005 and it says QT80NH. The Techflow documentation says QT80NH. However how do I check that this is the right QT80 pump. There is no PHYSICAL connection (that I can see) between the hot side of the pump and the cold side. There are electronic control circuits that connect the shower side of both hot and cold to the pump internals. During installation a Techflow plumber changed the circuit boards more than once.

They are still making this pump. The one on your diagram has a bell shaped device above the middle of the pump. Mine dpoes not have this.

It is possible that they supplied me with the wrong pump.

I am goint to e-mail them again and pose the question.
Hi Steve

I have just sent a picture of the pump QT80 together with the original delivery note that has QT80NH on it, to Techflow Products. I will let you know what I get back.


John Dent

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