Shower Running Cold (hidden unit)

[Hoddo]

I replaced the cartridge and the shower now produces lovely hot water, however, the shower does not appear to trigger the boiler.

When the shower is turned on it runs cold until the on-suite sink hot tap has been run for 10 seconds, after this the shower will run hot until you either touch the temperature knob or turn the flow off. Its as if the boiler doesn't know the shower is there and demanding hot water.

Any idea why it may not be triggering the boiler?

[herewego]

It must be because the cartridge is not opening the hot water valve. Looking at N24s question it seems the cartridge needs to be correctly aligned so I suggest the cartridge is not in correctly. Did it come with instructions?

[mmcd87]

I had a similar problem with a combi boiler and my old mixer tap shower where it would run cold unless like you said, I ran the hot tap in the sink. In my case the flow rate was too poor for the boiler and I had to change the shower head for a bigger one. Try unscrewing the shower head and see if you get hot water that way. May be related to the problem you are having.