Why won’t a shower pump work as a whole house pump?

[JKBDIY]

Curious to know why a single shower pump can’t be installed at the hw cylinder (in a conventional/vented system?

I have a header tank in loft, cylinder on mezzanine floor and I’m installing a new mixer shower on ground floor but am I not as well boosting the whole house?

 

[muggles]

It can, if you buy a decent one. I recommend Stuart Turner Monsoon. Only thing to really think about is that it will run at night if someone gets up to the loo and uses hot water to wash their hands.

 

[Madrab]

A lot of pumps, especially the plastic shower pumps, aren't continuous use rated and have a specific duty cycle and they also can't handle one end of the pump running against a closed system, i.e if just a hot or cold tap was opened then the other end would then overheat and damage the impeller.
Similarly even though a lot of the brass impeller housing are continuously rated if it's a dual pump then it needs to be specifically suited as a whole house to ensure it doesn't damage itself where one side could be running against a closed head for extended periods of time, in that instance then 2 separate pumps are usually employed.

Some pumps have a crossover/bypass arrangement that allow a twin pump to run single ends but I believe that is specific to salamander.