Mixer valve faults and issues
The basics of a faulty mixer valve
If you’ve been doing boiler repairs for some time, you will no doubt have come across an issue with poor domestic hot water temperatures at the hot tap due to a faulty mixer shower. If you haven’t then read on so you know what to look for in future to save time and costly, unnecessary parts use.
Not long after my apprenticeship, I came across an old combi boiler, with poor DHW temperature at the taps, with very little system experience and without the know how to test everything thoroughly, I jumped to conclusions on what was wrong with the boiler assuming I must have been taught everything I need to know.
I could see the boiler was modulating down and assumed it was faulty sensors, after changing those there was the plate heat exchanger, a gas valve, a main heat exchanger and I’m sure the list went on. Thankfully I worked with some very helpful engineers who never hesitated to share knowledge when they could and I shortly after learned about passing mixer valves, but it took some amount of explaining before I could understand exactly what was going on, this short article is to try explain passing mixer valves visually to help new and old plumbing and heating engineers alike.
When a hot or cold tap is opened, the pressure of the water at the tap is released and decreases rapidly in the nearby pipework. The difference between this low pressure and the higher mains or tank pressure is was pushes the water through the pipework to the outlet.
You will rarely if ever have equal pressure at hot and cold taps. The reason for this is that the feed pipework to hot taps is almost inevitably more complicated and therefore more restrictive, so a greater pressure is lost across the hot pipework than the cold.
This is especially true on combi boilers, where the place heat exchanger and often flow restrictors offer a resistance to the flow of the hot water. On unvented cylinders there is a pressure reducing valve fitted to control how much pressure can be fed into the cylinder and therefore to the hot taps. These should have “Pressure balanced” feeds to the cold to give an equal water pressure to the hot and cold outlets, however often the hot is controlled and the cold is run straight from the mains to the outlets, especially in retrofits where vented cylinders have been replaced with an unvented type.
This can cause issues when mixing valves are fitted to pipework, as when fitted incorrectly or when faulty, the higher cold pressure can over power the hot pressure and feed cold water through to hot outlets giving the appearance of a boiler or cylinder fault.
Mixers in this case count as any “single” valve that mixes hot and cold before the outlet, so mixer showers are the obvious one, but any mixer tap of a type that only has one handle that you swivel one way or another to choose between hot or cold counts as well. The sort of tap with a single outlet but two separate handles (one for hot and one for cold) does not count as a mixer in this case as any fault causing one or both taps to pass and let water by, will simply present as a dripping or running tap. Lastly there is “blending” valves, which are commonly found nowadays fitted under baths to mix some cold water into the hot before it gets to the hot tap at the bath to prevent scalding.
When the hot tap is run, the lower pressure in the hot pipework is overcome by the higher pressure in the cold main. Where under normal circumstances the shower mixer would have a non-return valve preventing any reverse flow into the hot pipework, they often fail or are never fitted in the first place.
The result is that as cold water enters the hot pipework is flows towards the hot outlet causing the water to be cooler than what you would expect at the hot tap.
The effect this has varies considerably from the hot water at the tap outlet being a degree or two less than it should be to being almost completely cold water at the outlet. The fault can also be intermittent, depending on the position of the faulty mixers internals and in the case of thermostatic mixers, the temperature at the mixer can alter how much, if at all is passing back into the hot pipework.
In the first case I mentioned from just after my apprenticeship, if I had taken a temperature clamp to the hot outlet just under the boiler, and then compared that temperature to a measurement at the hot tap outlet, I would have seen that there was a very significant drop in temperature between the boiler and tap, and therefore could not have been a boiler issue originally as the boiler was producing hot water without issue, it was simply modulating down as the water in it was already hot and the flow rate through the boiler was reduced.
Another common method to proving this issue, is to simply isolate the cold water feed to the boiler while a hot tap is running. This should stop any water flow through the hot pipes to the tap, however if a mixer is passing, there is now no pressure in the hot pipe and plenty in the cold main, so cold is pushed though the mixer and to the hot tap. So if you isolate the supply to the boiler (or hot water cylinder) and water keeps flowing at the tap then you have proved there is a feed of cold to the tap.
The solution to the problem is often replacing the fitting that it passing. However the addition of “check” valve (also known as a non-return valve) to the hot pipe directly below the faulty fitting, will prevent any cold water being pushed back down the hot pipework as the valve is essentially a one way street for water. It’s often also cheaper than replacing an expensive mixer shower/tap which may be functioning perfectly otherwise.
