Thermostatic Mixing Valves (TMVs)

11 Nov 2005
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United Kingdom
Last week I was asked to do a few jobs at a Day Care Home. One of the jobs was to look at why they can only get cold, or at the very best tepid, water from some of the hot taps. The problem taps are on 4 washbasins and a bath which I discovered are all fitted with adjustable TMVs. I think they were fitted about 5 years ago but no-one can tell me if they have ever worked correctly. They are the adjustable type which I believe don't comply with current regs for Care Homes, and should be TMV3s? The TMVs fitted to the washbasins are 15mm and the one fitted to the bath is 22mm. The one on the bath works OK until someone operates the valve on the mixer tap to divert the water to a shower head and then it goes cold, even when the shower head is no higher than the tap.
The only way of getting warm water out of the hot taps on the washbasins is to "throttle-back" the cold water supply using the isolator valve but this reduces the flow of cold water from the cold tap to a dribble and is also very noisy. I think this is what other "experts" have done in the past but it's not really an acceptable solution, is it?
The Care Home is in a hard water area so I took one of the 15mm TMVs off and cleaned the strainers but they weren't too bad but I couldn't get any further into the valve.
I am beginning to think that the TMVs have never worked correctly and the system is badly designed. The cold water feed is from the rising main and is at a lot higher pressure than the hot water, although the header tank is near the ceiling on the first floor and the TMVs are all on the ground floor so there is probably about 4 to 5 metres of "head". Nevertheless, it's as though the hot water supply is being overwhelmed by the cold water supply at the TMVs. If you think the problem is due to the cold water prressure being too high, should I fit some pressure reducing devices? Are there such things as 15mm and 22mm "in-line" pressure reducing devices? If so, do I fit them next to each TMV so that they only reduce the pressure to the TMV and not the cold water tap? This would be difficult as there's not much room, it would be a lot easier to fit them so that they reduce the cold water pressure to the TMV AND the cold water tap.
Thanks for you help in anticipation.
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I was looking at the instructions that came with a thermostatic shower the other day and it said that if the water pressure was too high, to fit a pressure reducing valve just above the rising main as no application in the home wants more than 3 bar pressure.

joe-90 said:
no application in the home wants more than 3 bar pressure.

That`s right and you don`t want disgruntled applications moaning@ you ,do you :eek: 15mm pressure reducers readily available from plumbers merchts. ;) adjustable...but pre-set @3 bar
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Unfortunately Stu you have walked into a minefield.

Reading the instructions for most of these TMV's especially the higher spec TMV3's makes it very difficult to rectify existing design faults.

It may prove very expensive for the care home manager to resolve these issues.

If you don't install these valves correctly as per MI you are liable (as is the manager) for any future consequence. You will also need flow rate pressure and temperature test gear.

Personally, I stear clear of this work unless strainers, checkvalves, servicing valves, pressures are balanced (or within limits) can be met.

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