Hello all.
This is my first post but pretty sure it won't be my last.
In the process of getting my lovely wife the bathroom she wants with the help of my father in law and some 'tradesmen'. Not quite going according to plan as I work 70 miles away and also have a flat elsewhere to work on. I'm not specifically a tradesman but a bit of a jack of all (where a little knowledge is a dangerous thing) but that's what my job involves. Father in law has been fixing and fitting everything for many years. Terminology and theory may be where I fall down but here's the situation.

We have a low pressure system. Vented hw cylinder in airing cupboard next to bathroom with ch. Approx' 2420mm (from floor level) above sits our cold water storage tank with a smaller second tank (header tank for ch?) beside it. The pipe runs in the floor from the cylinder, starting at 22mm then going down to 15mm for the last short run, are approx' 2300 horizontal/950 vertical to the sink monobloc waterfall mixer ( yes- cheap Chinese rubbish but it works ok for teeth brushing and looks nice) and 3600 horizontal/800 vertical to the bath filler and shower handset (Phoenix Pu range wall mounted sequential 1/4 turn). There is a concealed thermo shower mixer (I believe it's an Ultra or Phoenix two know job (!)) above the bath fed directly from the cw storage tank in 15mm and the hot comes back up from a t in the airing cupboard across the bathroom ceiling in the loft and then drops down the wall. The drop from ceiling to shower mixer valve is 1100mm and the fixed shower head sits 830mm above this.
I mention all this because I will no doubt be returning with other questions and also because it's good to have as much information as possible.

So having struggled to find a bath filler/shower mixer that runs on our low pressure system that is wall mounted and my wife likes (and me too) I finally put it on only to have to remove it because the pressure from the hw seemed to backflow through the cw pipes all the way back to the loft tank after running the bath filler for a while then stopping it. At no point whilst testing it after installing did I get cold coming through, it was just very hot or scalding! Whilst I found signs of previous venting into the tank and disconnected overflow pipes (nice!) it was definitely backflow through the cw pipes as the toilet cistern was filling with hot water and the cold side of the basin mixer was running hot..also feeling the pipes was an obvious sign. So to remedy this I've got check valves to install - was a bit worried there wouldn't be enough pressure to operate them but just tried them over my temporary 'taps' ( a couple of isolation valves with handles ) and they seem to work. So my first question is should I bother stripping the mixer or just send it back as faulty straight away. The fact that I seemed to get no hw suggests some altering of the valve inside the mixer - do they come pre-set? Also should you be able to get backflow so dramatically after supposedly having turned the mixer off? I'm assuming it's sequential as the flow rate differs little and there is only one control know - no tilting, just turn for on and further to increase temp.

So that's an easy one for starters ! Once that's sorted I can figure out if it's an airlock or just terrible flow rate for the concealed shower mixer (supposedly a low pressure mixer too), but first things first. Thanks chaps!

Hi

If the hot and cold is both fed from the same tank , then back flow is not possible.

Sounds like and installation error.

Hot and cold to the bath should be 22mm.

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Thanks Scruff for the welcome and quick reply. I've been comingon here quite a bit for a whole host of things, both to discover solutions or to validate my own. Just with all the travelling only just started posting as I was sat comfortably in front of a computer rather than using my phone -which is what I am using now!

So - yes the pipes are 22mm with the exception of the last run up the wall. My F.i.L didn't think it would have too dramatic an effect on the pressure and it was what we had. We work differently he and I but he's been doing it longer than me and I only got married earlier this year so when we started I had no intention of arguing - I just turned up to work! Indeed the flow from the mixer, whilst not great, is not awful. It makes a decent bath in a half hour and the bath is big. The shower head is a bit pathetic but it's for hair rinsing and little else. Once sorted there's the fixed shower above and a power shower in the en-suite.

As for backflow not being possible I don't know what else to call it. I've read through threads on here and on other forums and understand that there should be equal pressure but this is exactly what happened. The feeds coming from the wall were capped off after an incident with the first mixer (which was a waterfall matched to the basin mixer) and the connection in the wall (not done by me).
Everything was working as well as could be expected (not including the shower). I fitted the bath/shower mixer without non-return valves (yes...against regs) because we had the issue of too low pressure to oprerate when I fitted them to the shower and also, as you say, both hot and cold are coming from the same tank. So I ran the mixer to check, put loo paper around connections to check for leaks as I like to do, then went to work. Only problem was no cold but figured I'd have to remove the control knob and adjust accordingly. Came back from work that night and the kids were saying that hot water was coming from the basin tap regardless of which way they turned it. Checked and it was true. Checked both pipes in the wall behind the mixer and they were both hot. Checked water entering cistern of the bathroom toilet and it was hot. Checked the cold feed coming from the storage tank in the loft and it was hot. Checked all over, isolation valves, pipes in loft, en-suite....all the same. Checked after hw was used for showers and all and cold ran cold again.Switched hw on again and turned down thermostat on cylinder (had been set quite high),waited for water to heat up then ran the mixer again. Same thing happened. Logically, working backwards, removed the last thing to be added - the bath mixer. Put an isolation valve with handle on each pipe coming out the wall so at least my misses could run a bath and no hot water from cold tap anymore...or in the cistern or pipes going to the tank.

Pipe runs were only altered in the floor...no major replumbing done. Feeds come from cylinder and tank to iso valves outside bathroom door then teed off for basin, tee for cistern then straight to bath mixer.

Regs don't say you need check valves when you have gravity hot and cold supplies as there cannot be a back flow.

As I said in my first post you have an installation fault or a possibly a faulty cylinder.

By way of an explanation, if the water in the storage tank is getting hot, then you will get hot water from the cold outlets.

The cylinder and connection is where you/we need to concentrate.

How old is the cylinder, is it in a hard or soft water area. A general pic in the cupboard may show something.

How high is the header tank water level in relation to the storage tank.

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