Thermostatic shower mixer - low flow

30 Nov 2011
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United Kingdom

First-time poster but found plenty of useful info. here before. Apologies for the length of this post but I thought the more info. I gave the less time people would waste asking for info. I should have given.

I've recently added a shower (plate and thermostatic mixer bar not cubicle) to one of our bathrooms but have ended up with a very low flow rate.

The set-up is: cold tank in loft, hot cylinder at same level as the new shower. There's a vertical of around 1m 25 cm between the cold tank outlet and the shower. The pipe run from the hot cylinder to the shower is pretty much horizontal and is only about 40cm (the airing cupboard backs onto the bathroom).

The flow when set to medium temperature is as follows:
With full set-up (mixer bar, shower hose, shower head): 2.5 litres per min. this just trickles out and dribbles down the wall.
with shower head removed: 3.6 litres per min.
with shower hose removed: 3.6 litres per min.
with mixer bar removed: 6 litres per min (hot) 6 litres per min (cold) - I only ran one at a time so 12 litres per min total.

Obviously, the shower head is reducing the flow so I can get one designed for low-pressure systems but the main problem is the thermostatic mixer bar. Would the mixer bar usually reduce the flow by this much (from 12 litres per min to 3.6) in order to maintain an even temperature?

The current thermostatic mixer bar is a toolsation cheapo one. Would a more expensive bar allow a greater flow?

There don't seem to be any removable limiters within the bar. I removed the joint where the mixer bar attaches to the plate and inside there is a spring-loaded "gate" which allows the water in. In case the pressure wasn't enough to open these properly, I wedged them fully open with broken matchsticks but this didn't make any difference. The bore of these plastic gates is pretty small but I don't really want to force them out (it looks like they'd have to be broken to get them out) if they're vital to the proper operation of the bar! Any idea if the the bar will still work if I haul them out? Or are they designed to stop water that's been hanging around in the shower hose getting back into the system?

I've tried a different shower head and this makes it tolerable but still not very good so one alternative (but not my preference) would be to go for a manual mixer bar.

Thanks, in advance, for any advice.
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The current thermostatic mixer bar is a toolsation cheapo one. Would a more expensive bar allow a greater flow?

That's probably the problem. Which model is it? I can't see any thermostatic mixers on toolstation that are both cheap (£50 - £80) and suitable for gravity feeds.

You need not necessarily a super-expensive one, but a mixer, hose and head which are designed for low pressure systems. Some mixers will have ceramic valves which are quite restrictive.
22mm pipework for hot and cold as far as possible will help also.

Those spring loaded gates are check valves which are not strictly required with a gravity-fed shower, but would be necessary with other types of feed.

For what it's worth, I found this to work reasonably well with a gravity supply.
A non-thermostatic one may offer higher flow.

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