Water pressure pump

[geordysurfer]

A recently installed Vailant gas water heater has highlighted that our water pressure reduces during the day. The water temperature reduces (even with the machine at its maximum). I am unable to fit a header tank so think I'm going to have to install a pump to control the pressure. Can anyone advise the type of pump to fit?

[gasman]

I do not know of any pump that can be fitted to the mains water. They have to be fed via a storage tank.

[buildersmate]

To me, your diagnosis of the 'problem' that concludes in your mind that you want to install a pump is not correct.
You seem to be saying the temperature reduces (during the day) when the mains water pressure reduces. Generally the reverse would be expected to occur. If this is a mains-pressure supplied water heater, a cooler temperature is received at the output when the pressure is higher (because the same amount of the gas energy is being expended to heat a greater volume of water passing through. Please explain further if you want more advice.