Gradual low hot pressure on new bath mixer

[RogerTheDodger]

Newbie here with some newbie questions. Not a plumber by trade but I do enjoy tinkering and learning the hard way.

Just moved into a new house and I've replaced the old bath taps as they were well past looking their best and the hot water tap pressure was woeful. The hot water basin tap nearby was fine and hot water throughout the house also fine, so I put it down to the bath tap itself.

I've replaced the two individual bath taps with a cheapo bath mixer (no shower mixer) from B&Q, which I believe is rated down to 0.2 bar. Replaced the tap connector rubber washers with new fibre washers, tightened up and everything was groovy. Hot water pressure brilliant, just like rest of house.

Then gradually over the last two weeks, the bath hot water pressure has very gradually reduced to the point where there is barely anything coming out. What does come out is still hot, but it's even worse than the original hot tap was. All the other hot water taps in the house are OK. I've also noticed that if I run the mains-fed cold water on the mixer, it comes out nice and powerfully, but as soon as I open the hot mixer tap, the cold reduces to a trickle as well.

From what I've read it could be an airlock as I did drain the hot water cylinder before I replaced the taps. If this is the case, can it be resolved by draining the system again or am I going to have to force the air out somehow?

I've also since read that I've possibly made a glaring error in fitting a mixer tap to a mains-fed cold and gravity-fed hot water system. I've read that I am potentially contaminating the mains cold with water from the cylinder. Is this an unacceptable setup, even with no shower mixer? Like I say I am something of a newbie so if I'm going to have to fit valves or anything I might just go back to having two separate taps.

 

[Paulus]

Welcome Roger.

It does sound like you may have an airlock. Try turning on both hot and cold taps on the mixer whilst holding your hand over the spout. This should force water back up through the hot water pipework and clear any air.

If it isn't air, you may have disturbed muck in the bottom of the cylinder which has found its way into the pipework or tap cartridge so the next thing I would try would be to take out the cartridge and give it a clean.

You shouldn't need to drain the cylinder to work on the hot tap, there should be a gate valve between header tank and cylinder. Turn this off and let your hot taps drain.

To put your mind at rest, there is no problem with having mains fed cold and tank fed hot to a mixer tap.

 

[Leemiller]

Hi def sounds like and air lock if you kitchen tap is a mixer try this
1) put you hand over the kitchen tap spout
2) slowly open the tap and keep your hand on the spout
3) slowly move the tap over to the hot side and the cold mains should force the water up the hot supply
4) this may take a few attempts
you should hear air bubbling up in the cold water storage tank this is the air being forced out ?

 

[RogerTheDodger]

Thank you both for the advice.

I have tried the thumb over the tap trick a few times but not having much success as the water just tends to spray everywhere, no matter how hard I try to seal it. Tried it at both the kitchen and bath mixer with the same results.

I can sometimes just about get it to work on the bath mixer, but it makes an almighty vibrating noise that doesn't sound too good and usually attracts the attention of the wife! However it does seems to temporarily improve the flow.

Any other ways of doing this? Maybe some sort of attachment I can securely fit over the tap to prevent leaks?

 

[Leemiller]

Don't worry about the noise that's normal it will work i am pretty sure

 

[RogerTheDodger]

Don't worry about the noise that's normal it will work i am pretty sure

Thanks, should the noise stop when the airlock has cleared or will it make this noise regardless?

 

[Leemiller]

It's only because you for sending water the wrong way you want cause any damage as its an open vented system the only thing what may happen is the overflow on the cold water tank may overflow

 

[RogerTheDodger]

I've tried this thumb trick a few times now, to the point water is overflowing from the overflow pipe, but it makes no difference, hot water pressure on mixer still awful. Also if I open hot water tap when the cold is already flowing (literally blasting out), pressure drops significantly on the cold too.

Any ideas or is it back to separate taps for me?