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Water Temperature Differential, What Is It? and Why Is It A Problem for District Heating Systems?

In an ideal world hot water would be supplied to the back of every outlet across the complete building or in the case of a district heating system across the whole estate, at a stable and reliable 60°C and all TMV mixing valves would work perfectly providing a safe 41°C at every shower and tap. I think we can confidently say that this utopian view is seldom realized, with many hot water systems struggling the achieve a basic supply temp of 42°C. The problems of this all too common dilemma are often not realized or under-appreciated and present as the hot water system not working correctly.

Mora One Shower Kit

What is Water Temperature Differential?

Even the technical departments of some of the countries best-known shower mixer manufacturers give the wrong answer to this question. They will often quote that their mixing valves will produce a stable temperature + or minus 1°C as long as hot and cold water pressures are equal. This is not the water temperature differential requirement for their mixer but the outlet temperature guarantee from what you as the user have set as the required temperature. The water temperature differential is the difference between the hot water supply temperature at the back of the mixer and the mixed water outlet temperature from the mixing valve. This doesn’t sound important but it is.


Why is WaterTemperature Differential Important?

In a perfect world hot water supply temperature would be 60°C at the back of the mixer but in the real world, hot water supply temperature at the back of the mixer can be as little as 42°C. This means that if the user selects a temperature of 41°C on the mixing valve the resultant 1°C difference in supply and outlet temperature will mean that only a very small amount of cold water will be required to be added to the hot water supply to bring the temp down. This results in many thermostatic cartridges reacting as if the cold supply had failed and as such shut down the mixing valve or at the very least destabilize as it tries to compensate. In real terms this means that the hot water supply temperature to the back of the mixer must be a minimum of between 5°C- 10°C hotter than the set outlet temperature for the mixer to function properly i.e. Mixer set at Outlet temp 41°C may require a hot water supply at minimum temp of 51°C at the back of the mixer to function safely, satisfactorily and provide a stable temperature. Simply put a mixer with a High working water temp differential requirement is bad and a mixer with a Low working water temp differential requirement is good.


What Symptoms Will My Mixing Valve Display?

If your mixing valve is not working as you would expect, ie running hot then cold, is unable to maintain a constant and stable outlet temperature or shuts off unexpectedly, then the most likely cause is that your mixing valve is unable to cope with the challenges of a low water temperature differential.


What is a Standard Water Temperature Differential Requirement?

The vast majority of manufacturers quote a minimum water temperature differential requirement of between 5°C- 10°C with many being in excess of that. The higher the mixing valves temperature differential requirement the more likely the unit is to destabilize. The lower the temperature differential requirement of the mixing valve the more stable and safer the mixer is, being very able to produce a consistent outlet temperature even when the hot water supply temp 44°C is only marginally hotter than the required outlet temp 43°C set on the mixer or 1°C differential.


What Is The Water Temperature Differential Requirement of FM Mattsson Units?

FM Mattsson thermostatic and pressure balanced mixing valves due to their unique technical advantage can easily cope with a temperature differential of as low as 0°C. This means that if the hot water temperature is 41°C than the outlet temperature can be set at 41°C and still maintain a constant stable temperature and flow. In real terms, our mixing valves can be relied upon to perform at 100% over the broadest range of conditions outperforming any mixer currently available and as such are an ideal unit for use in all applications including the most demanding of district heating systems.


How Can I Avoid This Risk?

Buy a mixer with a low-temperature differential requirement is the simplest and cheapest option then you have the best chance of maintaining a stable and safe temp at all you outlets across your estate. The other alternative is to optimize your hot water system ensuring that all hot water returns are working safely and efficiently providing hot water to all outlets at a temp in excess of 10°C above required outlet temperature.

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Europower House, Lower Road, Cookham, Maidenhead SL6 9EH, UK

01628 529024

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