Slow start for hot water in mixer

[strubbly]

Hi All

I've just changed my separate bathroom taps for a mixer.

The HW is gravity fed and the cold is mains so I guess I could have expected problems...

But actually it works OK, except for one thing. When I first turn the hot on, sometimes I need to wait for ages (several minutes) before the hot starts. Once it does, it's ok.

Is this a common problem? Have I done something wrong? Is there an easy fix? Is it a problem with the tap?

The tap is specced for low pressure, down to 0.1bar, and, as I say, once the hot starts to flow, the flow rate is OK, nothing special, but acceptable.

 

[stevetheplumber]

im suprised you ever get hot water the cold main is pushing back up to the tanks stoping the flow of hot.The answer is to put a preasure reducer on the cold supply

 

[strubbly]

im suprised you ever get hot water the cold main is pushing back up to the tanks stoping the flow of hot.The answer is to put a preasure reducer on the cold supply

Thanks for replying.

I guess you're right - I'll have to do that.

I was just surprised that
a) the hot water eventually starts and is fine once it does.
b) I still get the problem even when the mixer is on hot only

 

[strubbly]

im suprised you ever get hot water the cold main is pushing back up to the tanks stoping the flow of hot.The answer is to put a preasure reducer on the cold supply

No, this isn't the problem.

I've tried the tap now with the cold water isolated, so it's hot only.

And the problem still happens. Sometimes it's fine. Other times the hot water takes several minutes to start.

I don't get it...

 

[jobsrus]

hi, does the hot water work every time when then isolator for the cold is off completley, if so it is not the mixer tap,but if you turn the isolator on just a bit thinking it will reduce the pressure it won't, it only reduces the flow which will cause the same problem as described by stevetheplumber.
the other reason for the problem maybe the flow may have been greater through the old taps than the new mixer!

 

[Bernie2]

Have you checked for an air lock on the HW supply? You can have part air locks that slow the flow as well as those that stop it all together.

To clear an air lock on a gravity system can be a bit involved, without knowing your system layout. Usually you can clear them by linking the HW supply to the cold main on either the washbasin taps or sink unit taps and blowing it out with mains pressure making sure the mains can't be contaminated.

Which, if its a new house should have enough nrv's on to stop it. If its an old house make sure you remove the link as quickly as possible.

But as usually there are some catches, the hot taps may not allow back flow to take place if they have loose jumpers.

You may just pump over into the HW cold feed cistern, instead of clearing the cylinder cold feed if the stop valves on the cistern or cylinder are screw down and not full bore gate valves.

You could always shut off all the hot taps and blow down the HW expansion/vent pipe over the storage cistern.

Its a kinda make it up as you go along job.

Are the mixers bi-flows or do you have cross flow problems?

 

[strubbly]

hi, does the hot water work every time when then isolator for the cold is off completley, if so it is not the mixer tap,but if you turn the isolator on just a bit thinking it will reduce the pressure it won't, it only reduces the flow which will cause the same problem as described by stevetheplumber.
the other reason for the problem maybe the flow may have been greater through the old taps than the new mixer!

Yes I've tried it with the cold off completely. There is not a lot of head on the hot so the flow is not great but, as I say, once it starts the flow is OK. Similar to the old tap, in fact.

Have you checked for an air lock on the HW supply? You can have part air locks that slow the flow as well as those that stop it all together.

Yes, I thought it might be something like that and I did try blowing it out but maybe I was not thorough enough...

Are the mixers bi-flows or do you have cross flow problems?

It may not be bi-flow. But I'm still getting it with the cold isolated completely so I don't think that's the problem.

It's like the HW pressure is occasionally going negative and air is going back in through the mixer. But I can't imagine why that would happen.